Rewrite AGENTS.md in short plain sentences (#327)

Every rule keeps its meaning and emphasis. Sentences are split so each
holds one idea, with the main rule before its exception and every
command, file name and identifier in backticks.

Three statements were false against the repo or the standing rulings
on record and are corrected:

- the format check is `make fmt-check`, not `make check-fmt`
- a feature branch reaches `next` through a PR that passes review, is
  merged only when the feature is completely finished and the code is
  up to standards, and is deleted after the merge; only the user
  merges `next` into `main`
- the naming example uses the code's term: `al` is bad,
  `SourceHostAllowlist` is good

The `Last Updated` line takes today's date.

Model: fable-5-1

AGENTS.md

@@ -1,75 +1,76 @@
 # Repository Rules
 
-Last Updated 2026-01-08
+Last Updated 2026-10-27
 
-These rules MUST be followed at all times, it is very important.
+These rules MUST be followed at all times. This is very important.
 
-- Never use `git add -A` - add specific changes to a deliberate commit. A commit
-  should contain one change. After each change, make a commit with a good
-  one-line summary.
+- Never use `git add -A`. Stage specific changes for a deliberate commit. A
+  commit holds one change. After each change, make a commit with a good one-line
+  summary.
 
 - NEVER modify the linter config without asking first.
 
-- NEVER modify tests to exclude special cases or otherwise get them to pass
-  without asking first. In almost all cases, the code should be changed, NOT the
-  tests. If you think the test needs to be changed, make your case for that and
-  ask for permission to proceed, then stop. You need explicit user approval to
-  modify existing tests. A bug fix may change an existing test without asking
-  when that test pins exactly the behavior the filed bug calls wrong, and the
-  change moves it to the fixed behavior and nothing else; any other change to an
-  existing test still needs explicit user approval. (You do not need user
-  approval for writing NEW tests.)
+- NEVER modify a test to exclude special cases or otherwise make it pass without
+  asking first. In almost all cases, change the code, NOT the test. You need
+  explicit user approval to modify an existing test. If you think a test needs
+  to change, make your case and ask for permission to proceed. Then stop. A bug
+  fix may change an existing test without asking only when that test pins
+  exactly the behavior the filed bug calls wrong. The change must then move the
+  test to the fixed behavior and nothing else. Any other change to an existing
+  test needs explicit user approval. Writing NEW tests needs no approval.
 
-- When linting, assume the linter config is CORRECT, and that each item output
-  by the linter is something that legitimately needs fixing in the code.
+- When linting, assume the linter config is CORRECT. Treat each item the linter
+  reports as something that legitimately needs fixing in the code.
 
 - When running tests, use `make test`.
 
-- Before commits, run `make check`. This runs `make lint` and `make test` and
-  `make check-fmt`. Any issues discovered MUST be resolved before committing
-  unless explicitly told otherwise.
+- Before committing, run `make check`. It runs `make test`, `make lint` and
+  `make fmt-check`. Any issue it finds MUST be resolved before committing,
+  unless you are explicitly told otherwise.
 
 - When fixing a bug, write a failing test for the bug FIRST. Add appropriate
-  logging to the test to ensure it is written correctly. Commit that. Then go
-  about fixing the bug until the test passes (without modifying the test
-  further). Then commit that.
+  logging to the test, to make sure the test is written correctly. Commit that.
+  Then work on the fix until the test passes, without changing the test further.
+  Then commit the fix.
 
-- When adding a new feature, do the same - implement a test first (TDD). It
-  doesn't have to be super complex. Commit the test, then commit the feature.
+- When adding a new feature, do the same: write a test first (test-driven
+  development, TDD). The test does not need to be very complex. Commit the test,
+  then commit the feature.
 
-- When adding a new feature, use a feature branch. When the feature is
-  completely finished and the code is up to standards (passes `make check`) then
-  and only then can the feature branch be merged into `main` and the branch
-  deleted.
+- When adding a new feature, use a feature branch. The feature branch reaches
+  `next` through a PR that passes review. It is merged only when the feature is
+  completely finished and the code is up to standards. The code is up to
+  standards when it passes `make check`. The branch is deleted after the merge.
+  Only the user merges `next` into `main`.
 
-- Write godoc documentation comments for all exported types and functions as you
-  go along.
+- Write `godoc` comments for all exported types and functions as you go along.
 
-- ALWAYS be consistent in naming. If you name something one thing in one place,
-  name it the EXACT SAME THING in another place.
+- ALWAYS be consistent in naming. If you name something one way in one place,
+  use the EXACT SAME name in every other place.
 
-- Be descriptive and specific in naming. `wl` is bad; `SourceHostWhitelist` is
+- Be descriptive and specific in naming. `al` is bad; `SourceHostAllowlist` is
   good. `ConnsPerHost` is bad; `MaxConnectionsPerHost` is good.
 
-- This is not prototype or teaching code - this is designed for production. Any
-  security issues (such as denial of service) or other web vulnerabilities are
-  P1 bugs and must be added to TODO.md at the top.
+- This is not prototype or teaching code. It is designed for production. Any
+  security issue (such as denial of service) or other web vulnerability is a P1
+  bug. It must be added at the top of `TODO.md`.
 
-- As this is production code, no stubbing of implementations unless specifically
-  instructed. We need working implementations.
+- Do not stub an implementation unless you are specifically instructed to. This
+  is production code, and we need working implementations.
 
 - NEVER silently fall back to a different setting when a user's parameter
-  explicitly specifies a value. If a user requests format=webp and WebP encoding
-  is not supported, return an error - do NOT silently output PNG instead. If a
-  user specifies fit=invalid and that fit mode doesn't exist, return an error -
-  do NOT silently default to "cover". Silent fallbacks violate the principle of
-  least surprise and mask bugs. The only acceptable defaults are for OMITTED
-  parameters, never for INVALID explicit values.
+  explicitly specifies a value. For example, if a user requests `format=webp`
+  and WebP encoding is not supported, return an error. Do NOT silently output
+  PNG instead. If a user specifies `fit=invalid` and that fit mode does not
+  exist, return an error. Do NOT silently default to `cover`. A silent fallback
+  violates the principle of least surprise and masks bugs. Defaults are
+  acceptable only for OMITTED parameters, never for INVALID explicit values.
 
-- Avoid vendoring deps unless specifically instructed to. NEVER commit the
-  vendor directory, NEVER commit compiled binaries. If these directories or
-  files exist, add them to .gitignore (and commit the .gitignore) if they are
-  not already in there. Keep the entire git repository (with history) small -
-  under 20MiB, unless you specifically must commit larger files (e.g. test
-  fixture example media files). Only OUR source code and immediately supporting
-  files (such as test examples) goes into the repo/history.
+- Avoid vendoring dependencies unless specifically instructed to. NEVER commit
+  the `vendor/` directory. NEVER commit compiled binaries. If such a directory
+  or file exists and `.gitignore` does not list it, add it to `.gitignore`.
+  Commit that `.gitignore` change. Keep the entire git repository, history
+  included, under 20 MiB. Go over 20 MiB only when you specifically must commit
+  a large file, such as a media file used as a test fixture. Only OUR source
+  code and its immediately supporting files, such as test examples, go into the
+  repository and its history.

README.md

@@ -133,7 +133,7 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
       filesystem's free space and what the cache holds, and at least 500 MiB, so
       the cache can grow to three quarters or more of the space they all have
       left, and to all of it when that is 500 MiB or less.
-    - the rest are in the table under Configuration below
+    - the rest are in the list under Configuration below
 - **Memory:** pixa can use up to `max_concurrent_processing` times 4.2 GiB for
   the images it processes at once, plus `upstream_connections` times
   `upstream_max_response_size` for the source images it has fetched that wait
@@ -178,46 +178,100 @@ allowlisted source hosts for open access.
 
 ## Design
 
+### Concepts
+
+These terms are the names the code uses:
+
+- **Source**: the image at one upstream URL, named by its host, path and query.
+- **Source key**: the SHA-256 of a source's host, path and query, which names
+  the source's file and its row in the database.
+- **Transform**: the output size, format, quality and fit an image request asks
+  for; with the format `auto`, pixa chooses the output format from the `Accept`
+  header.
+- **Image request**: a source and a transform, which each image route reads from
+  its URL.
+- **Variant**: the image pixa serves for an image request, made from its source
+  with its transform.
+- **Variant key**: the SHA-256 of a variant's source, size, format, quality and
+  fit, which names the variant's file and its row in the database, and gives its
+  `ETag`.
+- **Signed URL**: a `/v1/image/` URL whose `sig` signs its image request and its
+  expiry `exp` with `signing_key`.
+- **Encrypted URL**: a `/v1/e/` URL whose token holds its image request and any
+  expiry, encrypted with a key derived from `signing_key`.
+- **Login session**: an encrypted cookie that keeps a browser logged in to the
+  generator page for 30 days, or until a logout replaces the random value pixa
+  keeps in the `logout` table (see Encrypted URLs).
+- **Negative cache**: the source URLs pixa does not fetch again for 5 minutes,
+  after one of these failed fetches:
+    - the upstream answered with an error status
+    - the body's first bytes were not those of the format its `Content-Type`
+      names
+- **Image cache**: the part of pixa that keeps the sources, the variants and the
+  negative cache (see Storage).
+- **Disk cache limit** (`cache_max_bytes`): the size in bytes that the cached
+  sources and variants may reach together.
+- **Image proxy**: the part of pixa that serves a variant from the image cache
+  when it can, and otherwise makes it from the source.
+
 ### Storage
 
-- **Source images**:
-  `<state_dir>/cache/sources/<ab>/<cd>/<sha256 of host, path and query>`, one
-  file for each source URL
+- **Sources**: `<state_dir>/cache/sources/<ab>/<cd>/<source key>`, one file for
+  each source URL
 - **Database**: `<state_dir>/state.sqlite3` (SQLite)
-- **Transformed images**:
-  `<state_dir>/cache/variants/<ab>/<cd>/<sha256 of host, path, query, size, format, quality and fit>`
+- **Variants**: `<state_dir>/cache/variants/<ab>/<cd>/<variant key>`
 
 `<ab>` and `<cd>` are the first and second pairs of characters of the file's
 name.
 
-Each source image and each transformed image has one row in the database,
-holding its size and when it was last used. Two source URLs that serve the same
-bytes are stored twice, once under each URL. A source image is stored by writing
-its file and then its row; when the row cannot be written, the file is removed
-again.
+The image cache keeps these tables in the database:
 
-Source and transformed image files are written under a temporary name, synced to
-disk, and only then renamed into place, so a crash leaves no file under its own
-name with part of its data. When pixa starts, and every five minutes after, it
-removes each cached source or transformed image whose file is empty, or not the
-size the database records for it, with a warning naming the file; the next
-request for it fetches the source or converts the image again. Looking for those
-files reads the database once for every cached file, so pixa skips each
-five-minute look that would start less than ten times as long after the last
-look ended as that look took. After a look that took one minute, the next one
-starts 10 to 15 minutes after that look ended.
+- `source_content`: a row for each source, by source key, with its size and when
+  it was last used
+- `variant_content`: a row for each variant, by variant key (column
+  `cache_key`), with its size and when it was last used
+- `cache_usage`: the total size of the cached sources and variants, which
+  eviction reads
+- `negative_cache`: a row for each source URL in the negative cache, with when
+  it expires
+
+pixa reads a cached source image's format from its file's first bytes. When they
+match none of the source formats, pixa fetches the source again. Two source URLs
+that serve the same bytes are stored twice, once under each URL. A source image
+is stored by writing its file and then its row; when the row cannot be written,
+the file is removed again.
+
+Each source and variant file is written under a temporary name and synced to
+disk before it is renamed into place. So a crash never leaves part of a file
+under its own name. When pixa starts, and every five minutes after, it removes
+each source or variant whose file is empty, or not the size its row records. It
+logs a warning naming the file. A later request for that image fetches the
+source or makes the variant again. Looking for those files reads the database
+once for every cached file, so pixa skips each five-minute look that would start
+less than ten times as long after the last look ended as that look took. After a
+look that took one minute, the next one starts 10 to 15 minutes after that look
+ended.
+
+Each look also drops the rows of variants whose files are missing. When pixa
+cannot read a variant file's size for another reason, it keeps the variant's
+row. It logs a warning naming the variant key and the error.
+
+During a look, pixa skips a directory it cannot list and a file whose size it
+cannot read. It logs a warning naming the path and the error, and goes on with
+the rest of the look. A file renamed or removed after pixa listed its directory
+gets no warning.
 
 pixa's target is 1-5k r/s, which has not been measured at that rate (see Load
-Test). Toward it, a cache hit opens the transformed image's file and nothing
-else, and makes no database call. The content type comes from the file's first
-bytes, which are those of the format pixa wrote it in; a file whose first bytes
-are not those of a format pixa writes is not served, and the image is converted
-again. A hit hands the time it was served to a background writer and does not
-wait for the writer. Eviction orders by that time. The hit is counted in memory.
-A miss that uses a cached source image also hands that writer the time it used
-the image. Once a second that writer writes the waiting times in one
-transaction. While 10,000 times of transformed images, or 10,000 of source
-images, wait to be written, a further one is dropped, and that image only looks
+Test). Toward it, a cache hit opens the variant's file and nothing else, and
+makes no database call. The content type comes from the file's first bytes,
+which are those of the format pixa wrote it in; a file whose first bytes are not
+those of a format pixa writes is not served, and the image is converted again. A
+hit hands the time it was served to a background writer and does not wait for
+the writer. Eviction orders by that time. The hit is counted in memory. A miss
+that uses a cached source image also hands that writer the time it used the
+image. Once a second that writer writes the waiting times in one transaction. Up
+to 10,000 access times of variants, and as many of sources, can wait to be
+written. A time handed over beyond that is dropped. Its image then only looks
 less recently used to eviction. `script/bench` measures how many hits pixa
 serves per second (see Load Test).
 
@@ -265,6 +319,8 @@ path under `/v1/` answers 200, in maintenance mode too.
     - 403 when the upstream host, or a host it redirects to, has an address in a
       blocked network (see `blocked_networks`).
     - 429 over `image_requests_per_minute` (below).
+    - 499, with no body, when the client goes away before its image is ready,
+      unless its request is the one making the image (below).
     - 415 when the upstream answers with a `Content-Type` that is not one of the
       source formats (below).
     - 415 when the upstream sends a body whose first bytes are not those of the
@@ -296,6 +352,7 @@ path under `/v1/` answers 200, in maintenance mode too.
         - 403
         - 415
         - 429
+        - 499, with no body
         - 502
         - 503
         - 504
@@ -677,54 +734,85 @@ refused with 400.
 Every setting can be given as an environment variable, in a YAML config file
 (`--config`), or both. A variable present in the environment wins over the file,
 even when it is empty, and the file wins over the built-in default. The one
-exception is a variable named in the file's `env:` section: it is set while the
-file loads, so it overrides both the environment the process was started with
-and the file's own key. A variable's value is parsed as the same text in the
-file would be. The three lists take comma-separated entries, with the spaces
-around each trimmed; an empty variable is an empty list. A value that does not
-parse or is invalid aborts startup, naming the variable. A variable whose name
-starts with `PIXA_` but is not in the table below, such as a misspelled one or
-`PIXA_PORT`, aborts startup naming it, as an unknown config key does. The one
-other accepted name is `PIXA_CONFIG_PATH`, the config file's path (like
-`--config`). The variables set by the file's `env:` section are checked the same
+exception is a variable set in the file's `env:` section. pixa sets that
+variable while loading the file, so the variable wins over the environment pixa
+started with. It also wins over the file's key for the same setting. A
+variable's value is parsed as the same text in the file would be. A setting that
+holds a list takes comma-separated entries, with the spaces around each trimmed.
+An empty variable is an empty list. A value that does not parse or is invalid
+aborts startup, naming the variable. pixa refuses to start when a variable
+starts with `PIXA_` but is not in the list below, such as a misspelled one or
+`PIXA_PORT`. The error names the variable, as it names an unknown config key.
+pixa also accepts `PIXA_CONFIG_PATH`, which names the config file, like
+`--config`. The variables set by the file's `env:` section are checked the same
 way.
 
-pixa reads at most one config file: the one given with `--config` (or `-c`),
-otherwise the one `PIXA_CONFIG_PATH` names, otherwise the first of these that
-pixa finds: `/etc/pixa/config.yml`, `/etc/pixa/config.yaml`,
-`~/.config/pixa/config.yml`, `~/.config/pixa/config.yaml`, then `config.yml` and
-`config.yaml` in the working directory. A named file that does not exist, cannot
-be read or does not parse aborts startup. Of the files pixa looks for on its
-own, only one that does not exist is passed over, without a message. One that
-pixa cannot read or parse aborts startup, naming the file. So does one in a
-directory pixa may not enter, whether or not it is there, since pixa cannot
-tell. With no file, pixa uses the environment and the defaults.
+pixa reads at most one config file. It reads the file given with `--config` (or
+`-c`), or else the one `PIXA_CONFIG_PATH` names. pixa refuses to start when that
+file does not exist, cannot be read or does not parse. Without `--config` or
+`PIXA_CONFIG_PATH`, pixa looks in `/etc/pixa/`, then in `~/.config/pixa/`, then
+in the working directory. In each directory it looks for `config.yml`, then
+`config.yaml`. It reads the first file it finds. It skips a missing file without
+a message. pixa refuses to start when a file it finds cannot be read or does not
+parse, and the error names the file. It also refuses to start when it may not
+enter one of these directories, whether or not the file is there, since it
+cannot tell. With no file, pixa uses the environment and the defaults.
 
-| Variable                             | Config key                      | Meaning                                                                      |
-| ------------------------------------ | ------------------------------- | ---------------------------------------------------------------------------- |
-| `PIXA_SIGNING_KEY`                   | `signing_key`                   | Required: secret for signed and encrypted URLs and login, 32+ characters     |
-| `PORT`                               | `port`                          | Port to listen on; default `8080`                                            |
-| `PIXA_STATE_DIR`                     | `state_dir`                     | Directory for the database and the disk cache; default `/var/lib/pixa`       |
-| `PIXA_DB_URL`                        | `db_url`                        | SQLite database URL; default `state.sqlite3` in the state directory          |
-| `PIXA_CACHE_MAX_BYTES`               | `cache_max_bytes`               | Disk cache limit in bytes; `0` disables it; default 75% of (free + cached)   |
-| `PIXA_ALLOWLIST_HOSTS`               | `allowlist_hosts`               | Upstream hosts served without a signature                                    |
-| `PIXA_REFERER_BLOCKLIST`             | `referer_blocklist`             | Hosts whose pages the image routes refuse with 403, by `Referer`             |
-| `PIXA_BLOCKED_NETWORKS`              | `blocked_networks`              | CIDR ranges never fetched from, on top of the built-in ones                  |
-| `PIXA_TRUSTED_PROXIES`               | `trusted_proxies`               | CIDR ranges of proxies whose `X-Forwarded-For` is believed; default RFC 1918 |
-| `PIXA_ALLOW_HTTP`                    | `allow_http`                    | Allow plain-HTTP upstreams, for testing only; default `false`                |
-| `PIXA_UPSTREAM_CONNECTIONS_PER_HOST` | `upstream_connections_per_host` | Concurrent connections per upstream host; default `20`                       |
-| `PIXA_UPSTREAM_CONNECTIONS`          | `upstream_connections`          | Concurrent connections to all upstream hosts together; default `64`          |
-| `PIXA_MAX_CONCURRENT_PROCESSING`     | `max_concurrent_processing`     | Images processed at once; default half the CPUs, at least 1                  |
-| `PIXA_IMAGE_REQUESTS_PER_MINUTE`     | `image_requests_per_minute`     | Image requests per client per minute; `0` turns it off; default `600`        |
-| `PIXA_UPSTREAM_FETCH_TIMEOUT`        | `upstream_fetch_timeout`        | Time allowed for one fetch from an upstream host; default `30s`              |
-| `PIXA_UPSTREAM_MAX_RESPONSE_SIZE`    | `upstream_max_response_size`    | Largest upstream response accepted, in bytes; default 50 MiB                 |
-| `PIXA_DOWNSTREAM_TIMEOUT`            | `downstream_timeout`            | Time allowed for answering one client request; default `60s`                 |
-| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read image responses: `*` or one origin; default `*`  |
-| `PIXA_METRICS_USERNAME`              | `metrics.username`              | Username for `/metrics`, which is served only when both are set              |
-| `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
-| `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
-| `PIXA_DEBUG`                         | `debug`                         | Debug logging and plain-HTTP local development; default `false`              |
-| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | Answer image requests with 503; the health check stays 200; default `false`  |
+Each item below is one setting, named by its config key:
+
+- `signing_key`: variable `PIXA_SIGNING_KEY`, required. The secret for signed
+  URLs, encrypted URLs and login. It must be 32 characters or longer.
+- `port`: variable `PORT`, default `8080`. The port to listen on.
+- `state_dir`: variable `PIXA_STATE_DIR`, default `/var/lib/pixa`. The directory
+  for the database and the disk cache.
+- `db_url`: variable `PIXA_DB_URL`, default `state.sqlite3` in the state
+  directory. The URL of the SQLite database.
+- `cache_max_bytes`: variable `PIXA_CACHE_MAX_BYTES`, default 75% of the sum of
+  the free space and what the cache holds. The disk cache limit in bytes. `0`
+  turns the disk cache off.
+- `allowlist_hosts`: variable `PIXA_ALLOWLIST_HOSTS`, default none. The upstream
+  hosts whose images pixa serves without a signature.
+- `referer_blocklist`: variable `PIXA_REFERER_BLOCKLIST`, default none. The
+  image routes refuse a request with 403 when its `Referer` header names one of
+  these hosts.
+- `blocked_networks`: variable `PIXA_BLOCKED_NETWORKS`, default none. The CIDR
+  ranges pixa never fetches from, on top of the built-in ones.
+- `trusted_proxies`: variable `PIXA_TRUSTED_PROXIES`, default the RFC 1918
+  ranges. The CIDR ranges of the proxies whose `X-Forwarded-For` header pixa
+  believes.
+- `allow_http`: variable `PIXA_ALLOW_HTTP`, default `false`. Allows plain-HTTP
+  upstream hosts. It is for testing only.
+- `upstream_connections_per_host`: variable
+  `PIXA_UPSTREAM_CONNECTIONS_PER_HOST`, default `20`. The most connections to
+  one upstream host at once.
+- `upstream_connections`: variable `PIXA_UPSTREAM_CONNECTIONS`, default `64`.
+  The most connections to all upstream hosts together at once.
+- `max_concurrent_processing`: variable `PIXA_MAX_CONCURRENT_PROCESSING`,
+  default half the CPUs, at least 1. The most images processed at once.
+- `image_requests_per_minute`: variable `PIXA_IMAGE_REQUESTS_PER_MINUTE`,
+  default `600`. How many image requests one client may send per minute. `0`
+  turns the limit off.
+- `upstream_fetch_timeout`: variable `PIXA_UPSTREAM_FETCH_TIMEOUT`, default
+  `30s`. The time allowed for one fetch from an upstream host.
+- `upstream_max_response_size`: variable `PIXA_UPSTREAM_MAX_RESPONSE_SIZE`,
+  default 50 MiB. The largest upstream response accepted, in bytes.
+- `downstream_timeout`: variable `PIXA_DOWNSTREAM_TIMEOUT`, default `60s`. The
+  time allowed for answering one client request.
+- `access_control_allow_origin`: variable `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`,
+  default `*`. The CORS origin allowed to read image responses. It is `*` or one
+  origin.
+- `metrics.username`: variable `PIXA_METRICS_USERNAME`, default empty. The
+  username for `/metrics`. pixa serves `/metrics` only when the username and the
+  password are both set.
+- `metrics.password`: variable `PIXA_METRICS_PASSWORD`, default empty. The
+  password for `/metrics`. Set it together with the username.
+- `sentry_dsn`: variable `PIXA_SENTRY_DSN`, default empty. The Sentry DSN for
+  error reporting. An empty DSN turns error reporting off.
+- `debug`: variable `PIXA_DEBUG`, default `false`. Turns on debug logging and
+  plain-HTTP local development.
+- `maintenance_mode`: variable `PIXA_MAINTENANCE_MODE`, default `false`. While
+  `true`, pixa answers image requests with 503. The health check still
+  answers 200.
 
 Key settings in more detail:
 
@@ -848,9 +936,25 @@ See `configs/config.example.yml` for all options with defaults.
 
 ### Architecture
 
-- **Dependency injection**: Uber fx. `internal/app` builds every part of pixad
-  from the config in one fx graph. Each setting's default is declared once, in
-  `internal/config`.
+- **Dependency injection**: Uber fx. `internal/app` puts every part of pixad in
+  one fx graph. `internal/providers` builds the image cache, the image proxy and
+  the other parts that serve images and logins. Each setting's default is
+  declared once, in `internal/config`.
+- **Packages**: these packages under `internal/` work with the concepts above:
+    - `handlers`: the routes, which read the image request from the URL, check
+      the signed or encrypted URL, and make the login session
+    - `imagerequest`: the image request, with the source key and the variant key
+    - `imageproxy`: the image proxy
+    - `imgcache`: the image cache, which keeps its sources in one `FileStore`
+      and its variants in another
+    - `httpfetcher`: fetching sources from upstream hosts
+    - `magic`: reading an image's format from its first bytes
+    - `imageprocessor`: making a variant from a source with libvips
+    - `signature`: checking the `sig` of a signed URL
+    - `encurl`: making and reading the tokens of encrypted URLs
+    - `seal`: the keys derived from `signing_key`, and the encryption of those
+      tokens
+    - `session`: the login session
 - **HTTP router**: go-chi
 - **Image processing**: govips (CGO wrapper for libvips)
 - **Database**: SQLite via modernc.org/sqlite

TODO.md

@@ -30,11 +30,112 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-27 the reconciliation pass skips a directory it cannot list and a file
+  whose info it cannot read, and goes on (closes #343). It stopped before its
+  later checks at a directory it could not list, or at a source or variant file
+  whose info it could not read.
+    - It logs the warning "skipped unreadable entry in cache directory", with
+      the path and the error.
+    - A file renamed or removed after its directory was listed gets no warning.
+    - `reconcileFiles` reads each file's info once, before the temp-file check,
+      and hands it to `sweepStaleTempFile`, `reconcileVariantFile` or
+      `reconcileSourceFile`.
+    - A cancelled context still stops the walk with its error.
+- 2026-10-26 the reconciliation pass drops a variant's accounting row only when
+  its file is missing (closes #340). Any other error reading the file's size
+  keeps the row. It dropped the row on any error and logged the file as missing.
+    - The kept row gets the warning "kept accounting row for unreadable variant
+      file", with the variant key and the error.
+    - The pass checks for `ErrNotFound`, which `FileStore.Size` returns for a
+      missing file.
+- 2026-10-26 on both image routes, a request is answered 499 and logged at info
+  level when its client goes away before its image is ready, unless it is the
+  request making the image (closes #339). It was answered 500 and logged at
+  error level, as pixa's own failure.
+    - The line is "client went away before the image was ready". It carries the
+      `request_id`, the host and the path.
+    - The 499 has no body, since nobody reads it.
+    - `serveImage` answers it before calling `respondImageError`, which cannot
+      see the request's context.
+    - An error wrapping `context.Canceled` while the request's context is live
+      is still answered 500 and logged at error level.
+- 2026-10-26 `README.md`'s Design section starts with a Concepts list (closes
+  #321). Each concept gets one sentence, under the name the code gives it.
+    - Storage calls the transformed images variants, in shorter sentences.
+    - Storage's paths end in the source key and the variant key.
+    - Storage lists the image cache's tables in the database.
+    - Architecture names `internal/providers`, and lists the packages that work
+      with the concepts.
+- 2026-10-26 one table in `internal/config` lists every setting once (closes
+  #320). `Config.settings` gives each setting's key, its environment variable
+  and the function that loads it.
+    - Most rows load through `field`, whose arguments give the setting's type,
+      its default and its check.
+    - `db_url` and `cache_max_bytes` have their own load functions. The default
+      of `db_url` comes from `state_dir`, and `cache_max_bytes` records whether
+      it is set.
+    - Loading, the unknown-key check, the unknown-variable check and the names
+      in error messages all read the table.
+    - `allowlist_hosts` loads as `referer_blocklist` does, through
+      `parseHostList`.
+    - The metrics username and password are checked together once the table has
+      loaded.
+    - When a config holds more than one invalid value, the error names the one
+      that comes first in the table.
+    - `README.md` lists the settings one item each, where it had a table. A test
+      checks that this list and `configs/config.example.yml` name exactly the
+      keys of the table.
+    - The key constants moved to the tests, their only users.
+    - Removed:
+        - `isKnownConfigKey`, `envVarNames`, `strictLoader` and `validate`
+        - `validateAllowlistHostsValue` and `getStringSlice`, with the three
+          tests of `getStringSlice`
+- 2026-10-26 a fetched image's format is detected from its bytes once, as an
+  `imagerequest.Format` (closes #319). The upstream `Content-Type` check in the
+  fetcher stays as the filter before downloading.
+    - `magic.DetectFormat` returns an `imagerequest.Format`, and
+      `magic.ValidateMagicBytes` returns the format it checked against the
+      `Content-Type`.
+    - `Format.MIMEType` gives each format's MIME type. The fetcher, the mock
+      fetcher, the cache's read of a variant's type and the handler's `Accept`
+      matching use it.
+    - `Cache.Source` returns a source's format, read from its file's first bytes
+      as `Cache.Variant` reads a variant's. A file in no source format is an
+      error, and the proxy fetches the source again.
+    - The proxy hands the format to `ImageProcessor.Process`. The processor
+      refuses input that libvips would load as another format, as before, with
+      `ErrUnsupportedInputFormat`. It refuses a format that is not one of the
+      six source formats the same way.
+    - `internal/magic` no longer detects SVG, which pixa does not read. SVG data
+      is `ErrUnknownFormat`.
+    - Removed:
+        - `magic.MIMEType`, `magic.ImageFormat` and `magic.MIMEToImageFormat`
+        - `detectFormat`, `formatFromString`, `FormatToMIME` and
+          `Result.InputFormat` of the processor
+        - the fetcher's MIME constants of the six formats
+        - `errNotAnOutputFormat` of the cache
+- 2026-10-26 the cache keeps its source images in one `imgcache.FileStore` and
+  its variants in another (closes #318). A `FileStore` is a directory of files,
+  each named by a hex key, at `<ab>/<cd>/<key>`.
+    - `ContentStorage` and `VariantStorage` are gone.
+    - `FileStore` has these methods:
+        - `Write`, which writes a file under a temp name, syncs it and renames
+          it into place
+        - `Open`, which returns the file and its size
+        - `Size`
+        - `Delete`
+    - `FileStore` has no `Exists`. The reconciliation pass checks that a
+      variant's file is there with `Size`. The tests look for the file on disk.
+    - `Cache.Variant` reads the content type from the variant file's first
+      bytes, as `VariantStorage.LoadWithContentType` did. A cache hit still
+      makes no database call.
+    - `storage.go` is now `filestore.go`. `internal/imgcache/testdata/red.avif`,
+      which no test used, is gone.
 - 2026-10-26 `internal/imgcache` holds the cache only (closes #317). The new
   package `internal/imageproxy` fetches, transforms, caches and serves images.
   It imports `imgcache`, and `imgcache` does not import it.
-    - `imgcache.Service` is `imageproxy.Proxy`, built by `imageproxy.NewProxy`
-      from an `imageproxy.Config`.
+    - `imgcache.Service` is `imageproxy.Proxy`, built by `imageproxy.New` from
+      an `imageproxy.Config`.
     - `ImageResponse`, `CacheStatus`, `ErrNegativeCached` and
       `ErrProcessingTimedOut` moved with it, under their names.
     - `imgcache.CacheConfig` is `imgcache.Config`.
@@ -49,9 +150,9 @@ P2: security: per-origin rate limiting
     - `Variant` and `Source` each open the file and hand its access time to the
       background writer. A cache hit still makes no database call.
     - `imgcache.go` and `module.go` are gone.
-    - `NewProxy` makes the counters of upstream fetches and transforms, and
-      registers them on the registry it is given. The cache keeps only the hit
-      and miss counters.
+    - `imageproxy.New` makes the counters of upstream fetches and transforms,
+      and registers them on the registry it is given. The cache keeps only the
+      hit and miss counters.
     - A cached source whose row cannot be read is logged as "failed to load
       cached source, fetching". It was logged as "source lookup failed".
     - The proxy's tests moved to `internal/imageproxy`. They reach the cache

internal/config/config_internal_test.go

@@ -1,97 +1,12 @@
 package config
 
 import (
-	"os"
 	"path/filepath"
 	"testing"
 
 	"git.eeqj.de/sneak/smartconfig"
 )
 
-// writeTestConfig writes yamlContent to a temp config file and returns
-// the file path.
-func writeTestConfig(t *testing.T, yamlContent string) string {
-	t.Helper()
-
-	tmpDir := t.TempDir()
-	configPath := filepath.Join(tmpDir, "config.yml")
-
-	err := os.WriteFile(configPath, []byte(yamlContent), 0o600)
-	if err != nil {
-		t.Fatalf("failed to write test config: %v", err)
-	}
-
-	return configPath
-}
-
-// checkAllowlistHosts loads the config at configPath and asserts that
-// getStringSlice returns the three expected hosts.
-func checkAllowlistHosts(t *testing.T, configPath string) {
-	t.Helper()
-
-	sc, err := loadTestConfig(configPath)
-	if err != nil {
-		t.Fatalf("failed to load config: %v", err)
-	}
-
-	hosts := getStringSlice(sc)
-
-	if len(hosts) != 3 {
-		t.Errorf("expected 3 hosts, got %d: %v", len(hosts), hosts)
-	}
-
-	expected := []string{"static.sneak.cloud", "sneak.berlin", testHostS3}
-	for i, want := range expected {
-		if i >= len(hosts) {
-			t.Errorf("missing host at index %d: want %q", i, want)
-
-			continue
-		}
-
-		if hosts[i] != want {
-			t.Errorf("host[%d] = %q, want %q", i, hosts[i], want)
-		}
-	}
-}
-
-func TestGetStringSlice_YAMLList(t *testing.T) {
-	t.Parallel()
-
-	yamlContent := `
-allowlist_hosts:
-  - static.sneak.cloud
-  - sneak.berlin
-  - s3.sneak.cloud
-`
-
-	checkAllowlistHosts(t, writeTestConfig(t, yamlContent))
-}
-
-func TestGetStringSlice_CommaSeparated(t *testing.T) {
-	t.Parallel()
-
-	// Backwards compatibility with comma-separated string values.
-	yamlContent := `allowlist_hosts: "static.sneak.cloud, sneak.berlin, s3.sneak.cloud"`
-
-	checkAllowlistHosts(t, writeTestConfig(t, yamlContent))
-}
-
-func TestGetStringSlice_Empty(t *testing.T) {
-	t.Parallel()
-
-	configPath := writeTestConfig(t, `port: 8080`)
-
-	sc, err := loadTestConfig(configPath)
-	if err != nil {
-		t.Fatalf("failed to load config: %v", err)
-	}
-
-	hosts := getStringSlice(sc)
-	if len(hosts) != 0 {
-		t.Errorf("expected nil or empty slice, got %v", hosts)
-	}
-}
-
 // TestExampleConfigAllowlistsNoHost loads configs/config.example.yml
 // with a real signing key, as Getting Started in README.md has it copied
 // and edited, and checks that it allowlists no host: every image on an

internal/config/config_validation_internal_test.go

@@ -15,8 +15,36 @@ import (
 	_ "modernc.org/sqlite" // SQLite driver registration
 )
 
+// The config keys the tests name in the errors they expect.
+const (
+	keyDebug                      = "debug"
+	keyMaintenanceMode            = "maintenance_mode"
+	keyPort                       = "port"
+	keyStateDir                   = "state_dir"
+	keySentryDSN                  = "sentry_dsn"
+	keyDBURL                      = "db_url"
+	keyMetrics                    = "metrics"
+	keyMetricsUsername            = "metrics.username"
+	keyMetricsPassword            = "metrics.password"
+	keySigningKey                 = "signing_key"
+	keyAllowlistHosts             = "allowlist_hosts"
+	keyRefererBlocklist           = "referer_blocklist"
+	keyAllowHTTP                  = "allow_http"
+	keyUpstreamConnectionsPerHost = "upstream_connections_per_host"
+	keyUpstreamConnections        = "upstream_connections"
+	keyMaxConcurrentProcessing    = "max_concurrent_processing"
+	keyCacheMaxBytes              = "cache_max_bytes"
+	keyBlockedNetworks            = "blocked_networks"
+	keyTrustedProxies             = "trusted_proxies"
+	keyAccessControlAllowOrigin   = "access_control_allow_origin"
+	keyUpstreamFetchTimeout       = "upstream_fetch_timeout"
+	keyUpstreamMaxResponseSize    = "upstream_max_response_size"
+	keyDownstreamTimeout          = "downstream_timeout"
+	keyImageRequestsPerMinute     = "image_requests_per_minute"
+)
+
 // validTestSigningKey is a 32-character signing key that satisfies the
-// minimum length requirement in validate().
+// minimum length requirement in validateSigningKey.
 const validTestSigningKey = "0123456789abcdef0123456789abcdef"
 
 // signingKeyLine is a valid signing_key config line used as the base of

internal/config/documented_settings_internal_test.go

@@ -0,0 +1,127 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// TestReadmeNamesEverySetting checks that the list of settings in
+// README.md names exactly the keys of the settings table, one item each.
+// The list is the first one in the Configuration section, and it ends at
+// the first blank line. Each item starts with its key in backticks.
+func TestReadmeNamesEverySetting(t *testing.T) {
+	t.Parallel()
+
+	readme, err := os.ReadFile(filepath.Join("..", "..", "README.md"))
+	if err != nil {
+		t.Fatalf("failed to read README.md: %v", err)
+	}
+
+	_, section, found := strings.Cut(string(readme), "\n### Configuration\n")
+	if !found {
+		t.Fatal("README.md has no Configuration section")
+	}
+
+	// The section ends at the next heading of its level or above.
+	section, _, _ = strings.Cut(section, "\n## ")
+	section, _, _ = strings.Cut(section, "\n### ")
+
+	_, list, found := strings.Cut(section, "\n- ")
+	if !found {
+		t.Fatal("README.md's Configuration section has no list")
+	}
+
+	list, _, _ = strings.Cut("- "+list, "\n\n")
+
+	itemKey := regexp.MustCompile("^- `([^`]+)`")
+	named := map[string]bool{}
+
+	for line := range strings.SplitSeq(list, "\n") {
+		// A line that does not start with "- " goes on with the item above.
+		if !strings.HasPrefix(line, "- ") {
+			continue
+		}
+
+		match := itemKey.FindStringSubmatch(line)
+		if match == nil {
+			t.Errorf("README.md's list of settings has an item "+
+				"that does not start with a key in backticks: %s", line)
+
+			continue
+		}
+
+		if named[match[1]] {
+			t.Errorf("README.md's list of settings names %s twice", match[1])
+		}
+
+		named[match[1]] = true
+	}
+
+	for _, s := range new(Config).settings() {
+		if !named[s.key] {
+			t.Errorf("README.md's list of settings does not name %s", s.key)
+		}
+
+		delete(named, s.key)
+	}
+
+	for key := range named {
+		t.Errorf("README.md's list of settings names %s, "+
+			"which is not a setting", key)
+	}
+}
+
+// TestExampleConfigNamesEverySetting checks that
+// configs/config.example.yml names exactly the keys of the settings
+// table, each one set or commented out with "# ". A key line holds the
+// key, a colon and at most a one-word value, which tells it from a
+// comment line that starts with a word and a colon. An indented key is
+// inside the map of the key above it, as username is inside metrics.
+func TestExampleConfigNamesEverySetting(t *testing.T) {
+	t.Parallel()
+
+	const examplePath = "configs/config.example.yml"
+
+	example, err := os.ReadFile(filepath.Join("..", "..", examplePath))
+	if err != nil {
+		t.Fatalf("failed to read %s: %v", examplePath, err)
+	}
+
+	keyLine := regexp.MustCompile(`(?m)^(?:# )?( *)([a-z_]+):(?: \S+)?$`)
+	named := map[string]bool{}
+
+	var mapKey string
+
+	for _, match := range keyLine.FindAllStringSubmatch(string(example), -1) {
+		indent, key := match[1], match[2]
+		if indent == "" {
+			mapKey = key
+			named[key] = true
+
+			continue
+		}
+
+		// The key holding the map is not itself a setting.
+		delete(named, mapKey)
+
+		named[mapKey+"."+key] = true
+	}
+
+	// The env section sets environment variables; it is not a setting.
+	delete(named, "env")
+
+	for _, s := range new(Config).settings() {
+		if !named[s.key] {
+			t.Errorf("%s does not name %s", examplePath, s.key)
+		}
+
+		delete(named, s.key)
+	}
+
+	for key := range named {
+		t.Errorf("%s names %s, which is not a setting", examplePath, key)
+	}
+}

internal/config/env_internal_test.go

@@ -138,7 +138,8 @@ func TestPixaPortAbortsStartupPointingToPort(t *testing.T) {
 
 // TestSettingVariablesAndConfigPathAreAccepted checks that every
 // setting's variable and PIXA_CONFIG_PATH pass the check for unknown
-// PIXA_ variables. TestEnvironmentSetsEveryKey pins the names in the list.
+// PIXA_ variables. TestEnvironmentSetsEveryKey pins the names in the
+// settings table.
 func TestSettingVariablesAndConfigPathAreAccepted(t *testing.T) {
 	// A config file's env section loaded by another test can leave a
 	// PIXA_ variable set for the whole process, so every one is unset
@@ -159,8 +160,8 @@ func TestSettingVariablesAndConfigPathAreAccepted(t *testing.T) {
 
 	t.Setenv("PIXA_CONFIG_PATH", "/etc/pixa/config.yml")
 
-	for _, name := range envVarNames() {
-		t.Setenv(name, "")
+	for _, s := range new(Config).settings() {
+		t.Setenv(s.envVar, "")
 	}
 
 	err := validateKnownEnvVars()

internal/handlers/cache_hit_internal_test.go

@@ -132,7 +132,7 @@ func newCountingDatabaseServer(
 		tb.Fatalf("imgcache.NewCache() error = %v", err)
 	}
 
-	svc, err := imageproxy.NewProxy(&imageproxy.Config{
+	svc, err := imageproxy.New(&imageproxy.Config{
 		Cache: cache,
 		Fetcher: newMockFetcher(fstest.MapFS{
 			countingHost + photoPath: &fstest.MapFile{Data: photo.Bytes()},
@@ -145,7 +145,7 @@ func newCountingDatabaseServer(
 		Logger:          slog.New(slog.DiscardHandler),
 	}, prometheus.NewRegistry())
 	if err != nil {
-		tb.Fatalf("imageproxy.NewProxy() error = %v", err)
+		tb.Fatalf("imageproxy.New() error = %v", err)
 	}
 
 	keys, err := seal.DeriveKeys(testSigningKey)

internal/handlers/client_gone_internal_test.go

@@ -0,0 +1,177 @@
+package handlers
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"testing"
+	"time"
+
+	"github.com/go-chi/chi/v5/middleware"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// heldFetcher sends on started when a fetch begins, and holds the fetch until
+// release is closed. It then fails the fetch as an upstream error.
+type heldFetcher struct {
+	started chan struct{}
+	release chan struct{}
+}
+
+// Fetch sends on f.started, waits for f.release to be closed, and returns
+// httpfetcher.ErrUpstreamError.
+func (f heldFetcher) Fetch(
+	context.Context, imagerequest.Source,
+) (*httpfetcher.FetchResult, error) {
+	f.started <- struct{}{}
+
+	<-f.release
+
+	return nil, httpfetcher.ErrUpstreamError
+}
+
+// readLogLines decodes the JSON log lines in logs.
+func readLogLines(t *testing.T, logs *bytes.Buffer) []map[string]any {
+	t.Helper()
+
+	t.Logf("logged:\n%s", logs.String())
+
+	var lines []map[string]any
+
+	dec := json.NewDecoder(logs)
+	for dec.More() {
+		var line map[string]any
+
+		err := dec.Decode(&line)
+		if err != nil {
+			t.Fatalf("decoding log line: %v", err)
+		}
+
+		lines = append(lines, line)
+	}
+
+	return lines
+}
+
+// TestImageRoutes_ClientGoneWhileWaitingAnswers499 checks that both image
+// routes answer 499 to a request whose client goes away while it waits for an
+// image another request is making. A first request's fetch is held, so the
+// image is being made, and a second request for the same image waits for it
+// until its client goes away. The second request must be logged once at info
+// level with its request ID and the source host and path. Nothing may be
+// logged at error level.
+func TestImageRoutes_ClientGoneWhileWaitingAnswers499(t *testing.T) {
+	t.Parallel()
+
+	const requestID = "test-request-id"
+
+	for route, imageURL := range photoURLsByRoute() {
+		t.Run(route, func(t *testing.T) {
+			t.Parallel()
+
+			var logs bytes.Buffer
+
+			fetcher := heldFetcher{started: make(chan struct{}), release: make(chan struct{})}
+			h, routes := newRoutesFetchingThrough(t, fetcher,
+				slog.New(slog.NewJSONHandler(&logs, nil)))
+			target := imageURL(t, h)
+
+			firstDone := make(chan struct{})
+
+			go func() {
+				defer close(firstDone)
+
+				sendGet(t, routes, target)
+			}()
+
+			<-fetcher.started
+
+			ctx, cancel := context.WithCancel(context.WithValue(t.Context(),
+				middleware.RequestIDKey, requestID))
+			rec := httptest.NewRecorder()
+			secondDone := make(chan struct{})
+
+			go func() {
+				defer close(secondDone)
+
+				routes.ServeHTTP(rec, httptest.NewRequestWithContext(
+					ctx, http.MethodGet, target, nil))
+			}()
+
+			// Give the second request time to start waiting for the first
+			// request's image. The fetch is still held, so the second request
+			// can end only because its client went away
+			time.Sleep(100 * time.Millisecond)
+			cancel()
+			<-secondDone
+
+			close(fetcher.release)
+			<-firstDone
+
+			if rec.Code != 499 {
+				t.Errorf("status = %d, want 499", rec.Code)
+			}
+
+			var clientGoneLines int
+
+			for _, line := range readLogLines(t, &logs) {
+				if line["level"] == slog.LevelError.String() {
+					t.Errorf("logged at error level: %v", line["msg"])
+				}
+
+				if line["level"] == slog.LevelInfo.String() &&
+					line["msg"] == "client went away before the image was ready" &&
+					line["request_id"] == requestID &&
+					line["host"] == signedHost &&
+					line["path"] == photoPath {
+					clientGoneLines++
+				}
+			}
+
+			if clientGoneLines != 1 {
+				t.Errorf("info line naming the request logged %d times, want 1",
+					clientGoneLines)
+			}
+		})
+	}
+}
+
+// TestImageRoutes_CanceledWithLiveRequestAnswers500 checks that both image
+// routes answer 500, logged as "failed to get image" at error level, for an
+// image service error that wraps context.Canceled while the request's own
+// context is live. Its client has not gone away, so the cancellation is
+// pixa's own failure.
+func TestImageRoutes_CanceledWithLiveRequestAnswers500(t *testing.T) {
+	t.Parallel()
+
+	for route, imageURL := range photoURLsByRoute() {
+		t.Run(route, func(t *testing.T) {
+			t.Parallel()
+
+			var logs bytes.Buffer
+
+			h, routes := newRoutesFetchingThrough(t,
+				failingFetcher{err: context.Canceled},
+				slog.New(slog.NewJSONHandler(&logs, nil)))
+
+			rec := sendGet(t, routes, imageURL(t, h))
+			if rec.Code != http.StatusInternalServerError {
+				t.Errorf("status = %d, want %d",
+					rec.Code, http.StatusInternalServerError)
+			}
+
+			if !slices.ContainsFunc(readLogLines(t, &logs),
+				func(line map[string]any) bool {
+					return line["level"] == slog.LevelError.String() &&
+						line["msg"] == "failed to get image"
+				}) {
+				t.Error(`no "failed to get image" line logged at error level`)
+			}
+		})
+	}
+}

internal/handlers/first_bytes_internal_test.go

@@ -48,7 +48,7 @@ func newRoutesFetchingThrough(
 		t.Fatalf("imgcache.NewCache() error = %v", err)
 	}
 
-	svc, err := imageproxy.NewProxy(&imageproxy.Config{
+	svc, err := imageproxy.New(&imageproxy.Config{
 		Cache:   cache,
 		Fetcher: fetcher,
 		Processor: imageprocessor.New(imageprocessor.Params{
@@ -59,7 +59,7 @@ func newRoutesFetchingThrough(
 		Logger:          log,
 	}, prometheus.NewRegistry())
 	if err != nil {
-		t.Fatalf("imageproxy.NewProxy() error = %v", err)
+		t.Fatalf("imageproxy.New() error = %v", err)
 	}
 
 	keys, err := seal.DeriveKeys(testSigningKey)

internal/handlers/format_auto.go

@@ -65,20 +65,20 @@ func formatForAccept(accept string) (imagerequest.Format, error) {
 		return imagerequest.FormatJPEG, nil
 	}
 
-	if qualities["image/jxl"] > 0 {
+	if qualities[imagerequest.FormatJXL.MIMEType()] > 0 {
 		return imagerequest.FormatJXL, nil
 	}
 
-	if qualities["image/avif"] > 0 {
+	if qualities[imagerequest.FormatAVIF.MIMEType()] > 0 {
 		return imagerequest.FormatAVIF, nil
 	}
 
-	if qualities["image/webp"] > 0 {
+	if qualities[imagerequest.FormatWebP.MIMEType()] > 0 {
 		return imagerequest.FormatWebP, nil
 	}
 
 	// For JPEG, the most specific entry the header has decides
-	quality, named := qualities["image/jpeg"]
+	quality, named := qualities[imagerequest.FormatJPEG.MIMEType()]
 	if !named {
 		quality, named = qualities["image/*"]
 	}

internal/handlers/handlers_internal_test.go

@@ -59,7 +59,7 @@ func setupTestHandler(t *testing.T) *testFixtures {
 		t.Fatalf("failed to create cache: %v", err)
 	}
 
-	svc, err := imageproxy.NewProxy(&imageproxy.Config{
+	svc, err := imageproxy.New(&imageproxy.Config{
 		Cache:   cache,
 		Fetcher: newMockFetcher(mockFS),
 		Processor: imageprocessor.New(imageprocessor.Params{

internal/handlers/image.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -19,6 +20,12 @@ import (
 	"sneak.berlin/go/pixa/internal/magic"
 )
 
+// statusClientClosedRequest is the status nginx calls "client closed
+// request". The image routes answer it when the client goes away before its
+// image is ready, unless its request is the one making the image. Nobody reads
+// it; it is for logs and metrics.
+const statusClientClosedRequest = 499
+
 // HandleImage handles the main image proxy route:
 // /v1/image/<host>/<path>/<width>x<height>.<format>, or with no format
 // /v1/image/<host>/<path>/<width>x<height>
@@ -345,7 +352,8 @@ func notModified(w http.ResponseWriter, r *http.Request, etag string) bool {
 // serveImage serves the image that req asks for. Both image routes call it
 // once they have the request and its URL's expiry, which ends the
 // Cache-Control max-age. It chooses the format for the format auto, answers
-// If-None-Match with 304 and HEAD with the headers only, and answers an error
+// If-None-Match with 304 and HEAD with the headers only. It answers 499 when
+// the image service stopped because the client went away, and any other error
 // of the image service with respondImageError.
 func (s *Handlers) serveImage(
 	w http.ResponseWriter, r *http.Request,
@@ -362,6 +370,21 @@ func (s *Handlers) serveImage(
 	startTime := time.Now()
 
 	resp, err := s.imgSvc.Get(r.Context(), req)
+
+	// The client went away before its image was ready, which is not pixa's
+	// failure. A cancellation while the request's context is live is pixa's,
+	// and respondImageError answers it 500.
+	if errors.Is(err, context.Canceled) && r.Context().Err() != nil {
+		s.log.Info("client went away before the image was ready",
+			"request_id", middleware.GetReqID(r.Context()),
+			"host", req.Source.Host,
+			"path", req.Source.Path,
+		)
+		w.WriteHeader(statusClientClosedRequest)
+
+		return
+	}
+
 	if err != nil {
 		s.respondImageError(w, req, err)
 

internal/handlers/image_cache_control_internal_test.go

@@ -47,7 +47,7 @@ func newSignedHostServer(
 
 	jpegData := generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255})
 
-	svc, err := imageproxy.NewProxy(&imageproxy.Config{
+	svc, err := imageproxy.New(&imageproxy.Config{
 		Cache: cache,
 		Fetcher: newMockFetcher(fstest.MapFS{
 			signedHost + photoPath: &fstest.MapFile{Data: jpegData},
@@ -60,7 +60,7 @@ func newSignedHostServer(
 		Logger:          log,
 	}, prometheus.NewRegistry())
 	if err != nil {
-		t.Fatalf("imageproxy.NewProxy() error = %v", err)
+		t.Fatalf("imageproxy.New() error = %v", err)
 	}
 
 	keys, err := seal.DeriveKeys(testSigningKey)

internal/handlers/image_errors_internal_test.go

@@ -46,7 +46,7 @@ func newImageRoute(t *testing.T, fetcher httpfetcher.Fetcher) http.Handler {
 		t.Fatalf("failed to create cache: %v", err)
 	}
 
-	svc, err := imageproxy.NewProxy(&imageproxy.Config{
+	svc, err := imageproxy.New(&imageproxy.Config{
 		Cache:   cache,
 		Fetcher: fetcher,
 		Processor: imageprocessor.New(imageprocessor.Params{

internal/handlers/referer_blocklist_internal_test.go

@@ -62,7 +62,7 @@ func newRefererRoutes(t *testing.T) (http.Handler, *Handlers, *countingFetcher)
 		t.Fatalf("imgcache.NewCache() error = %v", err)
 	}
 
-	svc, err := imageproxy.NewProxy(&imageproxy.Config{
+	svc, err := imageproxy.New(&imageproxy.Config{
 		Cache:   cache,
 		Fetcher: fetcher,
 		Processor: imageprocessor.New(imageprocessor.Params{
@@ -72,7 +72,7 @@ func newRefererRoutes(t *testing.T) (http.Handler, *Handlers, *countingFetcher)
 		MaxResponseSize: 50 << 20,
 	}, prometheus.NewRegistry())
 	if err != nil {
-		t.Fatalf("imageproxy.NewProxy() error = %v", err)
+		t.Fatalf("imageproxy.New() error = %v", err)
 	}
 
 	keys, err := seal.DeriveKeys(testSigningKey)

internal/handlers/unsupported_source_internal_test.go

@@ -16,8 +16,8 @@ import (
 // TestUnsupportedSourceFormatAnswers415 checks that respondImageError, which
 // answers the errors of both image routes, answers 415 with a clear error when
 // the upstream answers with a content type pixa does not accept, such as SVG,
-// and when orig is asked of a source in a format the processor cannot name,
-// wrapped as the service wraps them.
+// and when libvips does not read a source as the format its first bytes
+// show, wrapped as the service wraps them.
 func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 	t.Parallel()
 
@@ -29,7 +29,7 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 	for _, err := range []error{
 		fmt.Errorf("upstream fetch failed: %w: image/svg+xml",
 			httpfetcher.ErrInvalidContentType),
-		fmt.Errorf("image processing failed: %w: unknown",
+		fmt.Errorf("image processing failed: %w: libvips does not read the input as avif",
 			imageprocessor.ErrUnsupportedInputFormat),
 	} {
 		rec := httptest.NewRecorder()

internal/httpfetcher/fetch_internal_test.go

@@ -83,7 +83,7 @@ func startUpstream(t *testing.T) *httptest.Server {
 
 	mux := http.NewServeMux()
 	mux.HandleFunc("/image", func(w http.ResponseWriter, _ *http.Request) {
-		w.Header().Set("Content-Type", contentTypeJPEG)
+		w.Header().Set("Content-Type", imagerequest.FormatJPEG.MIMEType())
 		_, _ = io.WriteString(w, imagePayload)
 	})
 	mux.HandleFunc("/status/500", func(w http.ResponseWriter, _ *http.Request) {
@@ -358,7 +358,7 @@ func TestFetchContentLengthBodyAtMaxResponseSize(t *testing.T) {
 
 			res := fetchWithMaxResponseSize(t, maxResponseSize,
 				func(w http.ResponseWriter, _ *http.Request) {
-					w.Header().Set("Content-Type", contentTypeJPEG)
+					w.Header().Set("Content-Type", imagerequest.FormatJPEG.MIMEType())
 					w.Header().Set("Content-Length", strconv.Itoa(tc.size))
 					_, _ = io.WriteString(w, body)
 				})
@@ -406,7 +406,7 @@ func TestFetchChunkedBodyAtMaxResponseSize(t *testing.T) {
 
 			res := fetchWithMaxResponseSize(t, maxResponseSize,
 				func(w http.ResponseWriter, r *http.Request) {
-					w.Header().Set("Content-Type", contentTypeJPEG)
+					w.Header().Set("Content-Type", imagerequest.FormatJPEG.MIMEType())
 					_, _ = io.WriteString(w, body)
 					// With no Content-Length, flushing sends the body
 					// chunked; its end is sent when the handler returns.

internal/httpfetcher/httpfetcher.go

@@ -38,18 +38,6 @@ const userAgent = "pixa/1.0"
 // MaxConnections fetches are already in progress.
 const ConnectionWaitTimeout = 10 * time.Second
 
-// MIME content types.
-const (
-	contentTypeJPEG        = "image/jpeg"
-	contentTypePNG         = "image/png"
-	contentTypeGIF         = "image/gif"
-	contentTypeWebP        = "image/webp"
-	contentTypeAVIF        = "image/avif"
-	contentTypeJXL         = "image/jxl"
-	contentTypeSVG         = "image/svg+xml"
-	contentTypeOctetStream = "application/octet-stream"
-)
-
 // Loopback addresses blocked by SSRF protection.
 const (
 	localhostIPv4 = "127.0.0.1"
@@ -121,15 +109,16 @@ type FetchResult struct {
 }
 
 // allowedContentTypes returns the content types an upstream response may
-// have, which Fetch also sends as its Accept header.
+// have, which Fetch also sends as its Accept header: the MIME types of the
+// formats pixa reads.
 func allowedContentTypes() []string {
 	return []string{
-		contentTypeJPEG,
-		contentTypePNG,
-		contentTypeGIF,
-		contentTypeWebP,
-		contentTypeAVIF,
-		contentTypeJXL,
+		imagerequest.FormatJPEG.MIMEType(),
+		imagerequest.FormatPNG.MIMEType(),
+		imagerequest.FormatGIF.MIMEType(),
+		imagerequest.FormatWebP.MIMEType(),
+		imagerequest.FormatAVIF.MIMEType(),
+		imagerequest.FormatJXL.MIMEType(),
 	}
 }
 

internal/httpfetcher/httpfetcher_internal_test.go

@@ -30,9 +30,9 @@ func TestIsAllowedContentType(t *testing.T) {
 		contentType string
 		want        bool
 	}{
-		{contentTypeJPEG, true},
-		{contentTypePNG, true},
-		{contentTypeWebP, true},
+		{imagerequest.FormatJPEG.MIMEType(), true},
+		{imagerequest.FormatPNG.MIMEType(), true},
+		{imagerequest.FormatWebP.MIMEType(), true},
 		{"image/jpeg; charset=utf-8", true},
 		{"IMAGE/JPEG", true},
 		{"text/html", false},
@@ -217,7 +217,7 @@ func TestMockFetcher_FetchesFile(t *testing.T) {
 
 	defer func() { _ = result.Content.Close() }()
 
-	if result.ContentType != contentTypeJPEG {
+	if result.ContentType != imagerequest.FormatJPEG.MIMEType() {
 		t.Errorf("ContentType = %q, want image/jpeg", result.ContentType)
 	}
 
@@ -272,13 +272,13 @@ func TestDetectContentTypeFromPath(t *testing.T) {
 		path string
 		want string
 	}{
-		{"foo/bar.jpg", contentTypeJPEG},
-		{"foo/bar.JPG", contentTypeJPEG},
-		{"foo/bar.jpeg", contentTypeJPEG},
-		{"foo/bar.png", contentTypePNG},
-		{"foo/bar.gif", contentTypeGIF},
-		{"foo/bar.webp", contentTypeWebP},
-		{"foo/bar.avif", contentTypeAVIF},
+		{"foo/bar.jpg", imagerequest.FormatJPEG.MIMEType()},
+		{"foo/bar.JPG", imagerequest.FormatJPEG.MIMEType()},
+		{"foo/bar.jpeg", imagerequest.FormatJPEG.MIMEType()},
+		{"foo/bar.png", imagerequest.FormatPNG.MIMEType()},
+		{"foo/bar.gif", imagerequest.FormatGIF.MIMEType()},
+		{"foo/bar.webp", imagerequest.FormatWebP.MIMEType()},
+		{"foo/bar.avif", imagerequest.FormatAVIF.MIMEType()},
 		{"foo/bar.svg", contentTypeSVG},
 		{"foo/bar.bin", contentTypeOctetStream},
 		{"foo/bar", contentTypeOctetStream},

internal/httpfetcher/mock.go

@@ -66,23 +66,29 @@ func (m *MockFetcher) Fetch(
 	}, nil
 }
 
+// The content types the mock fetcher gives files in no format pixa reads.
+const (
+	contentTypeSVG         = "image/svg+xml"
+	contentTypeOctetStream = "application/octet-stream"
+)
+
 // detectContentTypeFromPath returns the MIME type based on file extension.
 func detectContentTypeFromPath(path string) string {
 	path = strings.ToLower(path)
 
 	switch {
 	case strings.HasSuffix(path, ".jpg"), strings.HasSuffix(path, ".jpeg"):
-		return contentTypeJPEG
+		return imagerequest.FormatJPEG.MIMEType()
 	case strings.HasSuffix(path, ".png"):
-		return contentTypePNG
+		return imagerequest.FormatPNG.MIMEType()
 	case strings.HasSuffix(path, ".gif"):
-		return contentTypeGIF
+		return imagerequest.FormatGIF.MIMEType()
 	case strings.HasSuffix(path, ".webp"):
-		return contentTypeWebP
+		return imagerequest.FormatWebP.MIMEType()
 	case strings.HasSuffix(path, ".avif"):
-		return contentTypeAVIF
+		return imagerequest.FormatAVIF.MIMEType()
 	case strings.HasSuffix(path, ".jxl"):
-		return contentTypeJXL
+		return imagerequest.FormatJXL.MIMEType()
 	case strings.HasSuffix(path, ".svg"):
 		return contentTypeSVG
 	default:

internal/httpfetcher/request_id_internal_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"github.com/go-chi/chi/v5/middleware"
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // TestFetchSendsRequestID verifies that a fetch sends the ID of the request
@@ -25,7 +26,7 @@ func TestFetchSendsRequestID(t *testing.T) {
 		func(w http.ResponseWriter, r *http.Request) {
 			received <- r.Header.Get("X-Request-Id")
 
-			w.Header().Set("Content-Type", contentTypeJPEG)
+			w.Header().Set("Content-Type", imagerequest.FormatJPEG.MIMEType())
 			_, _ = io.WriteString(w, imagePayload)
 		}))
 	t.Cleanup(srv.Close)

internal/imageprocessor/avif_too_large_internal_test.go

@@ -39,30 +39,31 @@ func TestImageProcessor_RefusesAVIFAboveMaxPixels(t *testing.T) {
 	tests := []struct {
 		name          string
 		input         []byte
+		sourceFormat  imagerequest.Format
 		format        imagerequest.Format
 		width, height int
 		fit           imagerequest.Fit
 		refused       bool
 	}{
-		{"AVIF at the limit", cutOff, imagerequest.FormatAVIF,
-			4096, 4096, imagerequest.FitCover, false},
-		{"AVIF above the limit", cutOff, imagerequest.FormatAVIF,
-			4096, 4097, imagerequest.FitCover, true},
-		{"WebP above the AVIF limit", cutOff, imagerequest.FormatWebP,
-			4096, 4097, imagerequest.FitCover, false},
+		{"AVIF at the limit", cutOff, imagerequest.FormatPNG,
+			imagerequest.FormatAVIF, 4096, 4096, imagerequest.FitCover, false},
+		{"AVIF above the limit", cutOff, imagerequest.FormatPNG,
+			imagerequest.FormatAVIF, 4096, 4097, imagerequest.FitCover, true},
+		{"WebP above the AVIF limit", cutOff, imagerequest.FormatPNG,
+			imagerequest.FormatWebP, 4096, 4097, imagerequest.FitCover, false},
 		// FitInside does not enlarge, so the output stays 64x32
-		{"AVIF inside a larger size", cutOff, imagerequest.FormatAVIF,
-			8192, 8192, imagerequest.FitInside, false},
+		{"AVIF inside a larger size", cutOff, imagerequest.FormatPNG,
+			imagerequest.FormatAVIF, 8192, 8192, imagerequest.FitInside, false},
 		// FitOutside scales 64x32 by 128 to cover 8192x1: 8192x4096
-		{"AVIF outside a smaller size", cutOff, imagerequest.FormatAVIF,
-			8192, 1, imagerequest.FitOutside, true},
+		{"AVIF outside a smaller size", cutOff, imagerequest.FormatPNG,
+			imagerequest.FormatAVIF, 8192, 1, imagerequest.FitOutside, true},
 		// With no size, the output keeps the source's 4097x4096
 		{"AVIF of the source's size", largeAVIF, imagerequest.FormatAVIF,
-			0, 0, imagerequest.FitCover, true},
-		{"orig of an AVIF source", largeAVIF, imagerequest.FormatOriginal,
-			0, 0, imagerequest.FitCover, true},
-		{"JPEG of the source's size", largeAVIF, imagerequest.FormatJPEG,
-			0, 0, imagerequest.FitCover, false},
+			imagerequest.FormatAVIF, 0, 0, imagerequest.FitCover, true},
+		{"orig of an AVIF source", largeAVIF, imagerequest.FormatAVIF,
+			imagerequest.FormatOriginal, 0, 0, imagerequest.FitCover, true},
+		{"JPEG of the source's size", largeAVIF, imagerequest.FormatAVIF,
+			imagerequest.FormatJPEG, 0, 0, imagerequest.FitCover, false},
 	}
 
 	for _, tt := range tests {
@@ -72,7 +73,7 @@ func TestImageProcessor_RefusesAVIFAboveMaxPixels(t *testing.T) {
 			_, err := New(Params{
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
-			}).Process(t.Context(), bytes.NewReader(tt.input),
+			}).Process(t.Context(), bytes.NewReader(tt.input), tt.sourceFormat,
 				&imagerequest.Transform{
 					Size:    imagerequest.Size{Width: tt.width, Height: tt.height},
 					Format:  tt.format,

internal/imageprocessor/computed_side_internal_test.go

@@ -42,7 +42,7 @@ func TestImageProcessor_RefusesComputedSideAboveMaximum(t *testing.T) {
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
 			}).Process(t.Context(),
-				bytes.NewReader(input),
+				bytes.NewReader(input), imagerequest.FormatJPEG,
 				&imagerequest.Transform{
 					Size:    tt.size,
 					Format:  imagerequest.FormatJPEG,
@@ -111,7 +111,7 @@ func TestImageProcessor_ComputedSideIsAtLeastOne(t *testing.T) {
 					MaxInputBytes:           50 << 20,
 					MaxConcurrentProcessing: 1,
 				}).Process(t.Context(),
-					bytes.NewReader(input),
+					bytes.NewReader(input), imagerequest.FormatJPEG,
 					&imagerequest.Transform{
 						Size:    tt.size,
 						Format:  imagerequest.FormatJPEG,

internal/imageprocessor/deadline_internal_test.go

@@ -7,6 +7,8 @@ import (
 	"io"
 	"testing"
 	"time"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // readerAfterDeadline is a Process input whose first Read waits until done
@@ -86,7 +88,8 @@ func checkProcessStopsAtDeadline(
 
 	proc := New(Params{MaxInputBytes: 50 << 20, MaxConcurrentProcessing: 1})
 
-	result, err := proc.Process(ctx, input, smallJPEGTransform())
+	result, err := proc.Process(ctx, input, imagerequest.FormatJPEG,
+		smallJPEGTransform())
 	if result != nil {
 		t.Logf("Process() made a %dx%d image after its deadline passed",
 			result.Width, result.Height)

internal/imageprocessor/empty_format_internal_test.go

@@ -19,7 +19,7 @@ func TestImageProcessor_EmptyFormatRefused(t *testing.T) {
 		MaxConcurrentProcessing: 1,
 	}).Process(
 		t.Context(), bytes.NewReader(createTestJPEG(t, 20, 20)),
-		&imagerequest.Transform{},
+		imagerequest.FormatJPEG, &imagerequest.Transform{},
 	)
 	if !errors.Is(err, ErrUnsupportedOutputFormat) {
 		t.Errorf("Process() error = %v, want %v", err, ErrUnsupportedOutputFormat)

internal/imageprocessor/encoder_settings_internal_test.go

@@ -11,17 +11,18 @@ import (
 	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
-// processedSize runs input through Process and returns the output's size in
-// bytes.
+// processedSize runs input, an image in sourceFormat, through Process and
+// returns the output's size in bytes.
 func processedSize(
-	t *testing.T, input []byte, transform *imagerequest.Transform,
+	t *testing.T, input []byte, sourceFormat imagerequest.Format,
+	transform *imagerequest.Transform,
 ) int64 {
 	t.Helper()
 
 	result, err := New(Params{
 		MaxInputBytes:           50 << 20,
 		MaxConcurrentProcessing: 1,
-	}).Process(t.Context(), bytes.NewReader(input), transform)
+	}).Process(t.Context(), bytes.NewReader(input), sourceFormat, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
 	}
@@ -74,7 +75,7 @@ func TestImageProcessor_PNGIsCompressed(t *testing.T) {
 		}
 	}
 
-	size := processedSize(t, encodePNG(t, flat),
+	size := processedSize(t, encodePNG(t, flat), imagerequest.FormatPNG,
 		&imagerequest.Transform{Format: imagerequest.FormatPNG})
 
 	const rawBytes = width * height * 3
@@ -92,7 +93,7 @@ func TestImageProcessor_WebPAtEffort2(t *testing.T) {
 
 	input := createTestJPEG(t, 200, 150)
 
-	size := processedSize(t, input,
+	size := processedSize(t, input, imagerequest.FormatJPEG,
 		&imagerequest.Transform{Format: imagerequest.FormatWebP, Quality: 85})
 
 	want, _, err := decode(t, input).ExportWebp(&vips.WebpExportParams{
@@ -134,7 +135,7 @@ func TestImageProcessor_AVIFAtEffort1(t *testing.T) {
 
 	input := encodePNG(t, source)
 
-	size := processedSize(t, input,
+	size := processedSize(t, input, imagerequest.FormatPNG,
 		&imagerequest.Transform{Format: imagerequest.FormatAVIF, Quality: 85})
 
 	want, _, err := decode(t, input).ExportAvif(&vips.AvifExportParams{
@@ -161,7 +162,7 @@ func TestImageProcessor_JPEGXLAtEffort4(t *testing.T) {
 
 	input := createTestJPEG(t, 200, 150)
 
-	size := processedSize(t, input,
+	size := processedSize(t, input, imagerequest.FormatJPEG,
 		&imagerequest.Transform{Format: imagerequest.FormatJXL, Quality: 85})
 
 	params := vips.NewJxlExportParams()

internal/imageprocessor/fit_cover_internal_test.go

@@ -40,7 +40,7 @@ func TestImageProcessor_FitCoverServesThinSource(t *testing.T) {
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
 			}).Process(t.Context(),
-				bytes.NewReader(input),
+				bytes.NewReader(input), imagerequest.FormatJPEG,
 				&imagerequest.Transform{
 					Size:    tt.size,
 					Format:  imagerequest.FormatJPEG,
@@ -113,7 +113,7 @@ func TestImageProcessor_FitCoverCropsTheCentre(t *testing.T) {
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
 			}).Process(t.Context(),
-				bytes.NewReader(input),
+				bytes.NewReader(input), imagerequest.FormatPNG,
 				&imagerequest.Transform{
 					Size:    imagerequest.Size{Width: 50, Height: 50},
 					Format:  imagerequest.FormatPNG,
@@ -240,7 +240,7 @@ func TestImageProcessor_FitCoverKeepsProportions(t *testing.T) {
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
 			}).Process(t.Context(),
-				bytes.NewReader(input),
+				bytes.NewReader(input), imagerequest.FormatPNG,
 				&imagerequest.Transform{
 					Size:    tt.size,
 					Format:  imagerequest.FormatPNG,

internal/imageprocessor/fit_outside_internal_test.go

@@ -45,7 +45,7 @@ func TestImageProcessor_RefusesFitOutsideAboveMaximum(t *testing.T) {
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
 			}).Process(t.Context(),
-				bytes.NewReader(input),
+				bytes.NewReader(input), imagerequest.FormatJPEG,
 				&imagerequest.Transform{
 					Size:    tt.size,
 					Format:  imagerequest.FormatJPEG,
@@ -102,7 +102,7 @@ func TestImageProcessor_FitOutsideSideIsAtLeastOne(t *testing.T) {
 				MaxInputBytes:           50 << 20,
 				MaxConcurrentProcessing: 1,
 			}).Process(t.Context(),
-				bytes.NewReader(input),
+				bytes.NewReader(input), imagerequest.FormatJPEG,
 				&imagerequest.Transform{
 					Size:    tt.size,
 					Format:  imagerequest.FormatJPEG,

internal/imageprocessor/imageprocessor.go

@@ -70,8 +70,6 @@ type Result struct {
 	InputWidth int
 	// InputHeight is the original image height before processing.
 	InputHeight int
-	// InputFormat is the detected input format (e.g., "jpeg", "png").
-	InputFormat string
 }
 
 // MaxInputDimension is the maximum allowed width or height for input images.
@@ -113,9 +111,10 @@ var ErrAVIFOutputTooLarge = errors.New("AVIF output has too many pixels")
 var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
 
 // ErrUnsupportedInputFormat is returned, before the input is decoded, when
-// libvips reads the input's first bytes as another format than the source
-// formats, or as none: for example an SVG whose first bytes pass the
-// first-bytes check for AVIF.
+// libvips reads the input's first bytes as another format than its source
+// format, or as none. An SVG whose first bytes pass the first-bytes check for
+// AVIF is one example. A source format other than the six pixa reads is also
+// refused with it.
 var ErrUnsupportedInputFormat = errors.New("unsupported input format")
 
 // ErrTooManyImages is returned when MaxConcurrentProcessing images are being
@@ -157,9 +156,10 @@ func New(params Params) *ImageProcessor {
 	}
 }
 
-// Process transforms an image according to transform. When
-// MaxConcurrentProcessing images are already being processed, it waits up
-// to ProcessingWaitTimeout for one to finish, then fails with
+// Process transforms the image read from input according to transform.
+// sourceFormat is the input's format, which magic detected from its first
+// bytes. When MaxConcurrentProcessing images are already being processed,
+// it waits up to ProcessingWaitTimeout for one to finish, then fails with
 // ErrTooManyImages. When ctx has ended once Process has a slot, or before it
 // encodes, it returns ctx's error and gives the slot back. govips gives pixa
 // no way to stop libvips once an encode has begun, so an encode keeps its
@@ -168,6 +168,7 @@ func New(params Params) *ImageProcessor {
 func (p *ImageProcessor) Process(
 	ctx context.Context,
 	input io.Reader,
+	sourceFormat imagerequest.Format,
 	transform *imagerequest.Transform,
 ) (*Result, error) {
 	release, err := p.acquireSlot(ctx)
@@ -182,13 +183,9 @@ func (p *ImageProcessor) Process(
 		return nil, err
 	}
 
-	// govips picks the libvips loader from the input's first bytes with
-	// DetermineImageType. Input whose first bytes it reads as no source
-	// format, such as SVG, which librsvg would render, is refused before
-	// libvips reads it.
-	inputFormat := p.detectFormat(vips.DetermineImageType(data))
-
-	sourceFormat, err := p.formatFromString(inputFormat)
+	// Input libvips would not load as sourceFormat is refused before it is
+	// decoded
+	err = checkInputFormat(data, sourceFormat)
 	if err != nil {
 		return nil, err
 	}
@@ -254,12 +251,11 @@ func (p *ImageProcessor) Process(
 	return &Result{
 		Content:       io.NopCloser(bytes.NewReader(output)),
 		ContentLength: int64(len(output)),
-		ContentType:   FormatToMIME(outputFormat),
+		ContentType:   outputFormat.MIMEType(),
 		Width:         img.Width(),
 		Height:        img.Height(),
 		InputWidth:    origWidth,
 		InputHeight:   origHeight,
-		InputFormat:   inputFormat,
 	}, nil
 }
 
@@ -294,38 +290,6 @@ func targetDimensions(
 	return width, height, nil
 }
 
-// MIME types for the supported image formats.
-const (
-	mimeJPEG = "image/jpeg"
-	mimePNG  = "image/png"
-	mimeGIF  = "image/gif"
-	mimeWebP = "image/webp"
-	mimeAVIF = "image/avif"
-	mimeJXL  = "image/jxl"
-)
-
-// FormatToMIME converts a Format to its MIME type string.
-func FormatToMIME(format imagerequest.Format) string {
-	switch format {
-	case imagerequest.FormatJPEG:
-		return mimeJPEG
-	case imagerequest.FormatPNG:
-		return mimePNG
-	case imagerequest.FormatWebP:
-		return mimeWebP
-	case imagerequest.FormatGIF:
-		return mimeGIF
-	case imagerequest.FormatAVIF:
-		return mimeAVIF
-	case imagerequest.FormatJXL:
-		return mimeJXL
-	case imagerequest.FormatOriginal, imagerequest.FormatAuto:
-		return "application/octet-stream"
-	default:
-		return "application/octet-stream"
-	}
-}
-
 // WaitForProcessing waits until no image is being processed, or until ctx
 // ends, and returns how many images were still being processed then. It
 // waits by taking each slot in processingSemaphore as it frees up until it
@@ -394,27 +358,49 @@ func (p *ImageProcessor) readInput(input io.Reader) ([]byte, error) {
 	return data, nil
 }
 
-// detectFormat returns the format string for a govips image type.
-func (p *ImageProcessor) detectFormat(imageType vips.ImageType) string {
-	switch imageType {
-	case vips.ImageTypeJPEG:
-		return "jpeg"
-	case vips.ImageTypePNG:
-		return "png"
-	case vips.ImageTypeGIF:
-		return "gif"
-	case vips.ImageTypeWEBP:
-		return "webp"
-	case vips.ImageTypeAVIF, vips.ImageTypeHEIF:
-		return string(imagerequest.FormatAVIF)
-	case vips.ImageTypeJXL:
-		return string(imagerequest.FormatJXL)
-	case vips.ImageTypeUnknown, vips.ImageTypeMagick, vips.ImageTypePDF,
-		vips.ImageTypeSVG, vips.ImageTypeTIFF, vips.ImageTypeBMP,
-		vips.ImageTypeJP2K:
-		return "unknown"
+// checkInputFormat returns ErrUnsupportedInputFormat unless libvips would
+// load data with the loader of sourceFormat. govips picks the libvips loader
+// from the input's first bytes with DetermineImageType. It refuses an SVG
+// whose first bytes pass the first-bytes check for AVIF, which librsvg would
+// otherwise render.
+func checkInputFormat(data []byte, sourceFormat imagerequest.Format) error {
+	// A format with no loader, such as orig or auto, has the type govips also
+	// gives input it cannot read, which the check below would let through.
+	sourceType := vipsImageType(sourceFormat)
+	if sourceType == vips.ImageTypeUnknown {
+		return fmt.Errorf("%w: %q is not a source format",
+			ErrUnsupportedInputFormat, sourceFormat)
+	}
+
+	if vips.DetermineImageType(data) != sourceType {
+		return fmt.Errorf("%w: libvips does not read the input as %s",
+			ErrUnsupportedInputFormat, sourceFormat)
+	}
+
+	return nil
+}
+
+// vipsImageType returns the govips image type whose libvips loader reads
+// format. orig and auto stand for another format, so they have
+// vips.ImageTypeUnknown, which govips has no loader for.
+func vipsImageType(format imagerequest.Format) vips.ImageType {
+	switch format {
+	case imagerequest.FormatJPEG:
+		return vips.ImageTypeJPEG
+	case imagerequest.FormatPNG:
+		return vips.ImageTypePNG
+	case imagerequest.FormatGIF:
+		return vips.ImageTypeGIF
+	case imagerequest.FormatWebP:
+		return vips.ImageTypeWEBP
+	case imagerequest.FormatAVIF:
+		return vips.ImageTypeAVIF
+	case imagerequest.FormatJXL:
+		return vips.ImageTypeJXL
+	case imagerequest.FormatOriginal, imagerequest.FormatAuto:
+		return vips.ImageTypeUnknown
 	default:
-		return "unknown"
+		return vips.ImageTypeUnknown
 	}
 }
 
@@ -768,26 +754,3 @@ func jxlDistance(quality int) float64 {
 
 	return 53.0/3000.0*q*q - 23.0/20.0*q + 25.0
 }
-
-// formatFromString converts a format string from detectFormat to Format. A
-// format detectFormat could not name is ErrUnsupportedInputFormat.
-func (p *ImageProcessor) formatFromString(
-	format string,
-) (imagerequest.Format, error) {
-	switch format {
-	case "jpeg":
-		return imagerequest.FormatJPEG, nil
-	case "png":
-		return imagerequest.FormatPNG, nil
-	case "gif":
-		return imagerequest.FormatGIF, nil
-	case "webp":
-		return imagerequest.FormatWebP, nil
-	case string(imagerequest.FormatAVIF):
-		return imagerequest.FormatAVIF, nil
-	case string(imagerequest.FormatJXL):
-		return imagerequest.FormatJXL, nil
-	default:
-		return "", fmt.Errorf("%w: %s", ErrUnsupportedInputFormat, format)
-	}
-}

internal/imageprocessor/imageprocessor_internal_test.go

@@ -94,23 +94,23 @@ func isAVIF(data []byte) bool {
 // detectMIME is a minimal magic-byte detector for test assertions.
 func detectMIME(data []byte) string {
 	if len(data) >= 3 && data[0] == 0xFF && data[1] == 0xD8 && data[2] == 0xFF {
-		return mimeJPEG
+		return imagerequest.FormatJPEG.MIMEType()
 	}
 
 	if len(data) >= 8 && string(data[:8]) == "\x89PNG\r\n\x1a\n" {
-		return mimePNG
+		return imagerequest.FormatPNG.MIMEType()
 	}
 
 	if len(data) >= 4 && string(data[:4]) == "GIF8" {
-		return mimeGIF
+		return imagerequest.FormatGIF.MIMEType()
 	}
 
 	if len(data) >= 12 && string(data[:4]) == "RIFF" && string(data[8:12]) == "WEBP" {
-		return mimeWebP
+		return imagerequest.FormatWebP.MIMEType()
 	}
 
 	if isAVIF(data) {
-		return mimeAVIF
+		return imagerequest.FormatAVIF.MIMEType()
 	}
 
 	return ""
@@ -131,7 +131,8 @@ func TestImageProcessor_ResizeJPEG(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
 	}
@@ -157,7 +158,7 @@ func TestImageProcessor_ResizeJPEG(t *testing.T) {
 	}
 
 	mime := detectMIME(data)
-	if mime != mimeJPEG {
+	if mime != imagerequest.FormatJPEG.MIMEType() {
 		t.Errorf("Output format = %v, want image/jpeg", mime)
 	}
 }
@@ -176,7 +177,8 @@ func TestImageProcessor_ConvertToPNG(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
 	}
@@ -189,7 +191,7 @@ func TestImageProcessor_ConvertToPNG(t *testing.T) {
 	}
 
 	mime := detectMIME(data)
-	if mime != mimePNG {
+	if mime != imagerequest.FormatPNG.MIMEType() {
 		t.Errorf("Output format = %v, want image/png", mime)
 	}
 }
@@ -213,7 +215,8 @@ func processAndCheckSize(
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
 	}
@@ -253,7 +256,8 @@ func TestImageProcessor_FitContain(t *testing.T) {
 		FitMode: imagerequest.FitContain,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
 	}
@@ -296,7 +300,8 @@ func TestImageProcessor_ProcessPNG(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatPNG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
 	}
@@ -341,7 +346,8 @@ func TestImageProcessor_RejectsOversizedInput(t *testing.T) {
 				FitMode: imagerequest.FitCover,
 			}
 
-			_, err := proc.Process(ctx, bytes.NewReader(input), transform)
+			_, err := proc.Process(ctx, bytes.NewReader(input),
+				imagerequest.FormatJPEG, transform)
 			if err == nil {
 				t.Error("Process() should reject oversized input images")
 			}
@@ -369,7 +375,8 @@ func TestImageProcessor_AcceptsMaxDimensionInput(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf(
 			"Process() should accept images at MaxInputDimension, got error: %v",
@@ -399,7 +406,8 @@ func encodeAndCheck(
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v, want nil", err)
 	}
@@ -430,7 +438,7 @@ func encodeAndCheck(
 func TestImageProcessor_EncodeWebP(t *testing.T) {
 	t.Parallel()
 
-	encodeAndCheck(t, imagerequest.FormatWebP, 80, mimeWebP)
+	encodeAndCheck(t, imagerequest.FormatWebP, 80, imagerequest.FormatWebP.MIMEType())
 }
 
 func TestImageProcessor_DecodeAVIF(t *testing.T) {
@@ -453,7 +461,8 @@ func TestImageProcessor_DecodeAVIF(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatAVIF, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v, want nil (AVIF decoding should work)", err)
 	}
@@ -467,7 +476,7 @@ func TestImageProcessor_DecodeAVIF(t *testing.T) {
 	}
 
 	mime := detectMIME(data)
-	if mime != mimeJPEG {
+	if mime != imagerequest.FormatJPEG.MIMEType() {
 		t.Errorf("Output format = %v, want image/jpeg", mime)
 	}
 }
@@ -494,7 +503,8 @@ func TestImageProcessor_RejectsOversizedInputData(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	_, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	_, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err == nil {
 		t.Fatal("Process() should reject input exceeding maxInputBytes")
 	}
@@ -521,7 +531,8 @@ func TestImageProcessor_AcceptsInputWithinLimit(t *testing.T) {
 		FitMode: imagerequest.FitCover,
 	}
 
-	result, err := proc.Process(ctx, bytes.NewReader(input), transform)
+	result, err := proc.Process(ctx, bytes.NewReader(input),
+		imagerequest.FormatJPEG, transform)
 	if err != nil {
 		t.Fatalf("Process() error = %v, want nil", err)
 	}
@@ -532,11 +543,11 @@ func TestImageProcessor_AcceptsInputWithinLimit(t *testing.T) {
 func TestImageProcessor_EncodeAVIF(t *testing.T) {
 	t.Parallel()
 
-	encodeAndCheck(t, imagerequest.FormatAVIF, 85, mimeAVIF)
+	encodeAndCheck(t, imagerequest.FormatAVIF, 85, imagerequest.FormatAVIF.MIMEType())
 }
 
-// processAndDecode runs input through Process and decodes the output with
-// vips, so a test can inspect the image a client would receive.
+// processAndDecode runs input, a JPEG, through Process and decodes the output
+// with vips, so a test can inspect the image a client would receive.
 func processAndDecode(
 	t *testing.T, input []byte, transform *imagerequest.Transform,
 ) *vips.ImageRef {
@@ -546,7 +557,8 @@ func processAndDecode(
 		MaxInputBytes:           50 << 20,
 		MaxConcurrentProcessing: 1,
 	}).Process(
-		context.Background(), bytes.NewReader(input), transform,
+		context.Background(), bytes.NewReader(input), imagerequest.FormatJPEG,
+		transform,
 	)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)

internal/imageprocessor/jpegxl_output_internal_test.go

@@ -25,7 +25,8 @@ func TestImageProcessor_EncodeJPEGXL(t *testing.T) {
 		MaxInputBytes:           50 << 20,
 		MaxConcurrentProcessing: 1,
 	}).Process(
-		t.Context(), bytes.NewReader(createTestJPEG(t, 200, 150)), transform,
+		t.Context(), bytes.NewReader(createTestJPEG(t, 200, 150)),
+		imagerequest.FormatJPEG, transform,
 	)
 	if err != nil {
 		t.Fatalf("Process() error = %v", err)
@@ -71,7 +72,7 @@ func TestImageProcessor_JPEGXLQuality(t *testing.T) {
 			MaxInputBytes:           50 << 20,
 			MaxConcurrentProcessing: 1,
 		}).Process(t.Context(),
-			bytes.NewReader(input),
+			bytes.NewReader(input), imagerequest.FormatJPEG,
 			&imagerequest.Transform{Format: imagerequest.FormatJXL, Quality: quality})
 		if err != nil {
 			t.Fatalf("Process() at quality %d error = %v", quality, err)

internal/imageprocessor/max_concurrent_processing_internal_test.go

@@ -93,7 +93,8 @@ func processInBackground(
 	proc *ImageProcessor, reader *gatedReader, results chan<- error,
 ) {
 	go func() {
-		result, err := proc.Process(context.Background(), reader, smallJPEGTransform())
+		result, err := proc.Process(context.Background(), reader,
+			imagerequest.FormatJPEG, smallJPEGTransform())
 		if err == nil {
 			_ = result.Content.Close()
 		}
@@ -206,7 +207,7 @@ func TestProcessWaitsThenFailsWhenNoSlotFrees(t *testing.T) {
 	start := time.Now()
 
 	_, err := proc.Process(context.Background(), bytes.NewReader(input),
-		smallJPEGTransform())
+		imagerequest.FormatJPEG, smallJPEGTransform())
 	if !errors.Is(err, ErrTooManyImages) {
 		t.Fatalf("Process() error = %v, want ErrTooManyImages", err)
 	}
@@ -274,7 +275,8 @@ func TestProcessReleasesSlotOnError(t *testing.T) {
 			proc := New(Params{MaxInputBytes: 4096, MaxConcurrentProcessing: 1})
 			proc.processingWaitTimeout = 100 * time.Millisecond
 
-			_, err := proc.Process(context.Background(), tc.input, tc.transform)
+			_, err := proc.Process(context.Background(), tc.input,
+				imagerequest.FormatJPEG, tc.transform)
 			if err == nil || (tc.want != nil && !errors.Is(err, tc.want)) {
 				t.Fatalf("Process() error = %v, want %v", err, tc.want)
 			}
@@ -284,7 +286,7 @@ func TestProcessReleasesSlotOnError(t *testing.T) {
 			}
 
 			result, err := proc.Process(context.Background(), bytes.NewReader(valid),
-				smallJPEGTransform())
+				imagerequest.FormatJPEG, smallJPEGTransform())
 			if err != nil {
 				t.Fatalf("Process() after the error = %v, want nil", err)
 			}

internal/imageprocessor/orig_format_internal_test.go

@@ -10,8 +10,9 @@ import (
 )
 
 // TestImageProcessor_OrigOfUnnamedFormatRefused verifies that orig of a TIFF
-// input, which is not a source format, is refused with
-// ErrUnsupportedInputFormat before it is decoded, and not saved as JPEG.
+// input is refused with ErrUnsupportedInputFormat before it is decoded, and
+// not saved as JPEG. TIFF is not a source format, and the test passes the
+// input to Process as a JPEG.
 func TestImageProcessor_OrigOfUnnamedFormatRefused(t *testing.T) {
 	t.Parallel()
 
@@ -31,7 +32,7 @@ func TestImageProcessor_OrigOfUnnamedFormatRefused(t *testing.T) {
 		MaxInputBytes:           50 << 20,
 		MaxConcurrentProcessing: 1,
 	}).Process(
-		t.Context(), bytes.NewReader(tiff),
+		t.Context(), bytes.NewReader(tiff), imagerequest.FormatJPEG,
 		&imagerequest.Transform{Format: imagerequest.FormatOriginal},
 	)
 	if result != nil {

internal/imageprocessor/source_formats_internal_test.go

@@ -2,6 +2,7 @@ package imageprocessor
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"testing"
 
@@ -21,7 +22,7 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
 	} {
 		saved, err := proc.Process(t.Context(),
-			bytes.NewReader(createTestPNG(t, 20, 20)),
+			bytes.NewReader(createTestPNG(t, 20, 20)), imagerequest.FormatPNG,
 			&imagerequest.Transform{Format: format})
 		if err != nil {
 			t.Fatalf("saving the test image as %s: %v", format, err)
@@ -32,7 +33,7 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 			t.Fatalf("reading the %s image: %v", format, err)
 		}
 
-		result, err := proc.Process(t.Context(), bytes.NewReader(source),
+		result, err := proc.Process(t.Context(), bytes.NewReader(source), format,
 			&imagerequest.Transform{Format: imagerequest.FormatOriginal})
 		if err != nil {
 			t.Errorf("orig of the %s image: %v", format, err)
@@ -42,9 +43,33 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 
 		t.Logf("orig of the %s image: %s", format, result.ContentType)
 
-		if result.ContentType != FormatToMIME(format) {
+		if result.ContentType != format.MIMEType() {
 			t.Errorf("orig of the %s image is %s, want %s",
-				format, result.ContentType, FormatToMIME(format))
+				format, result.ContentType, format.MIMEType())
+		}
+	}
+}
+
+// TestImageProcessor_NotASourceFormatRefused verifies that a source format
+// other than the six, such as orig, auto or none, is refused with
+// ErrUnsupportedInputFormat. The input is text, which libvips reads as no
+// format.
+func TestImageProcessor_NotASourceFormatRefused(t *testing.T) {
+	t.Parallel()
+
+	proc := New(Params{MaxInputBytes: 50 << 20, MaxConcurrentProcessing: 1})
+
+	for _, sourceFormat := range []imagerequest.Format{
+		imagerequest.FormatOriginal, imagerequest.FormatAuto, "",
+	} {
+		result, err := proc.Process(t.Context(),
+			bytes.NewReader([]byte("this is text, not an image")), sourceFormat,
+			&imagerequest.Transform{Format: imagerequest.FormatJPEG})
+		t.Logf("source format %q: error = %v", sourceFormat, err)
+
+		if result != nil || !errors.Is(err, ErrUnsupportedInputFormat) {
+			t.Errorf("Process() with source format %q = %v, error %v; want %v",
+				sourceFormat, result, err, ErrUnsupportedInputFormat)
 		}
 	}
 }

internal/imageproxy/counters_internal_test.go

@@ -9,11 +9,10 @@ import (
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
-// TestNewProxy_RegistersItsCountersOnItsRegistry checks that NewProxy
-// registers the proxy's counters on the registry it is given, under the
-// names README.md gives them. The cache registers its own counters on
-// another registry.
-func TestNewProxy_RegistersItsCountersOnItsRegistry(t *testing.T) {
+// TestNew_RegistersItsCountersOnItsRegistry checks that New registers the
+// proxy's counters on the registry it is given, under the names README.md
+// gives them. The cache registers its own counters on another registry.
+func TestNew_RegistersItsCountersOnItsRegistry(t *testing.T) {
 	t.Parallel()
 
 	cache, err := imgcache.NewCache(setupTestDB(t),
@@ -25,9 +24,9 @@ func TestNewProxy_RegistersItsCountersOnItsRegistry(t *testing.T) {
 
 	registry := prometheus.NewRegistry()
 
-	_, err = NewProxy(&Config{Cache: cache}, registry)
+	_, err = New(&Config{Cache: cache}, registry)
 	if err != nil {
-		t.Fatalf("NewProxy() error = %v", err)
+		t.Fatalf("New() error = %v", err)
 	}
 
 	names := []string{
@@ -47,10 +46,10 @@ func TestNewProxy_RegistersItsCountersOnItsRegistry(t *testing.T) {
 	}
 }
 
-// TestNewProxy_FailsWhenItsCountersCannotBeRegistered makes a second proxy
-// on the registry of a first. Its counters have the same names, so the
-// registry refuses them, and NewProxy must return that error.
-func TestNewProxy_FailsWhenItsCountersCannotBeRegistered(t *testing.T) {
+// TestNew_FailsWhenItsCountersCannotBeRegistered makes a second proxy on
+// the registry of a first. Its counters have the same names, so the
+// registry refuses them, and New must return that error.
+func TestNew_FailsWhenItsCountersCannotBeRegistered(t *testing.T) {
 	t.Parallel()
 
 	cache, err := imgcache.NewCache(setupTestDB(t),
@@ -62,17 +61,17 @@ func TestNewProxy_FailsWhenItsCountersCannotBeRegistered(t *testing.T) {
 
 	registry := prometheus.NewRegistry()
 
-	_, err = NewProxy(&Config{Cache: cache}, registry)
+	_, err = New(&Config{Cache: cache}, registry)
 	if err != nil {
-		t.Fatalf("NewProxy() of the first proxy error = %v", err)
+		t.Fatalf("New() of the first proxy error = %v", err)
 	}
 
-	_, err = NewProxy(&Config{Cache: cache}, registry)
-	t.Logf("NewProxy() of the second proxy error = %v", err)
+	_, err = New(&Config{Cache: cache}, registry)
+	t.Logf("New() of the second proxy error = %v", err)
 
 	var alreadyRegistered prometheus.AlreadyRegisteredError
 	if !errors.As(err, &alreadyRegistered) {
-		t.Errorf("NewProxy() of the second proxy error = %v, want %T",
+		t.Errorf("New() of the second proxy error = %v, want %T",
 			err, alreadyRegistered)
 	}
 }

internal/imageproxy/imageproxy.go

@@ -37,7 +37,7 @@ type Proxy struct {
 	variantsInProgress singleflight.Group
 
 	// The counts of upstream fetches and transforms, made and registered by
-	// NewProxy. They are kept in memory, so adding to one waits for nothing.
+	// New. They are kept in memory, so adding to one waits for nothing.
 	upstreamFetches    prometheus.Counter
 	upstreamFetchBytes prometheus.Counter
 	transforms         prometheus.Counter
@@ -61,11 +61,11 @@ type Config struct {
 // errCacheRequired is returned when the proxy is built without a cache.
 var errCacheRequired = errors.New("cache is required")
 
-// NewProxy creates a new image proxy and registers its counters on
+// New creates a new image proxy and registers its counters on
 // metricsRegistry. pixad passes Prometheus' default registry, the one
 // /metrics serves. It fails when metricsRegistry already holds counters of
 // the same names.
-func NewProxy(
+func New(
 	cfg *Config, metricsRegistry prometheus.Registerer,
 ) (*Proxy, error) {
 	if cfg.Cache == nil {
@@ -303,21 +303,22 @@ func withoutCancelKeepingDeadline(
 	return context.WithDeadline(context.WithoutCancel(ctx), deadline)
 }
 
-// loadCachedSource opens src's cached source, without reading it, and
-// returns it with its size; nil if it is not in the cache, cannot be read,
-// is empty or exceeds maxResponseSize.
+// loadCachedSource opens src's cached source, reading only the first bytes,
+// which give its format, and returns it with its size and its format. It
+// returns nil if the source is not in the cache, cannot be read, is in no
+// source format or exceeds maxResponseSize.
 func (p *Proxy) loadCachedSource(
 	ctx context.Context, src imagerequest.Source,
-) (io.ReadCloser, int64) {
-	reader, size, err := p.cache.Source(ctx, src)
+) (io.ReadCloser, int64, imagerequest.Format) {
+	reader, size, format, err := p.cache.Source(ctx, src)
 	if errors.Is(err, imgcache.ErrNotFound) {
-		return nil, 0
+		return nil, 0, ""
 	}
 
 	if err != nil {
 		p.log.Warn("failed to load cached source, fetching", "error", err)
 
-		return nil, 0
+		return nil, 0, ""
 	}
 
 	p.log.Debug("using cached source", "source_key", src.Key())
@@ -330,16 +331,10 @@ func (p *Proxy) loadCachedSource(
 			"max_bytes", p.maxResponseSize,
 		)
 
-		return nil, 0
+		return nil, 0, ""
 	}
 
-	if size == 0 {
-		_ = reader.Close()
-
-		return nil, 0
-	}
-
-	return reader, size
+	return reader, size, format
 }
 
 // processFromSourceOrFetch processes an image, using cached source content
@@ -349,7 +344,7 @@ func (p *Proxy) processFromSourceOrFetch(
 	req *imagerequest.Request,
 	cacheKey imagerequest.VariantKey,
 ) (*processedVariant, error) {
-	source, sourceSize := p.loadCachedSource(ctx, req.Source)
+	source, sourceSize, sourceFormat := p.loadCachedSource(ctx, req.Source)
 
 	// Fetch from upstream if we don't have source data or it's empty
 	if source == nil {
@@ -361,7 +356,7 @@ func (p *Proxy) processFromSourceOrFetch(
 	// Process using cached source; nothing was fetched from upstream. The
 	// image processor reads the source only once it has a processing slot,
 	// so a request waiting for one holds none of it in memory.
-	return p.processAndStore(ctx, req, cacheKey, source, sourceSize)
+	return p.processAndStore(ctx, req, cacheKey, source, sourceFormat, sourceSize)
 }
 
 // fetchAndProcess fetches from upstream, processes, and caches the result.
@@ -442,10 +437,11 @@ func (p *Proxy) fetchAndProcess(
 		"cipher", fetchResult.TLSCipherSuite,
 	)
 
-	// Validate magic bytes match content type. A body that does not is the
-	// upstream's error, so it gets a negative cache entry, as an upstream
-	// error status does.
-	err = magic.ValidateMagicBytes(sourceData, fetchResult.ContentType)
+	// The source's format, detected from its first bytes, which must match
+	// its content type. A body that does not is the upstream's error, so it
+	// gets a negative cache entry, as an upstream error status does.
+	sourceFormat, err := magic.ValidateMagicBytes(sourceData,
+		fetchResult.ContentType)
 	if err != nil {
 		p.log.Warn("source refused", "request_id", middleware.GetReqID(ctx),
 			"host", req.Source.Host, "path", req.Source.Path,
@@ -463,7 +459,7 @@ func (p *Proxy) fetchAndProcess(
 	}
 
 	return p.processAndStore(
-		ctx, req, cacheKey, bytes.NewReader(sourceData), fetchBytes,
+		ctx, req, cacheKey, bytes.NewReader(sourceData), sourceFormat, fetchBytes,
 	)
 }
 
@@ -497,13 +493,14 @@ type processedVariant struct {
 	fetchedBytes int64
 }
 
-// processAndStore processes the image read from source and stores the
-// result.
+// processAndStore processes the image read from source, which is in
+// sourceFormat, and stores the result.
 func (p *Proxy) processAndStore(
 	ctx context.Context,
 	req *imagerequest.Request,
 	cacheKey imagerequest.VariantKey,
 	source io.Reader,
+	sourceFormat imagerequest.Format,
 	fetchBytes int64,
 ) (*processedVariant, error) {
 	// Process the image
@@ -511,7 +508,8 @@ func (p *Proxy) processAndStore(
 
 	// ctx has the deadline of the request doing the processing, which can
 	// end while a request waiting for its image has time left
-	processResult, err := p.processor.Process(ctx, source, &req.Transform)
+	processResult, err := p.processor.Process(ctx, source, sourceFormat,
+		&req.Transform)
 	if errors.Is(err, context.DeadlineExceeded) {
 		return nil, fmt.Errorf("%w: %w", ErrProcessingTimedOut, err)
 	}
@@ -546,7 +544,7 @@ func (p *Proxy) processAndStore(
 		"request_id", middleware.GetReqID(ctx),
 		"host", req.Source.Host,
 		"path", req.Source.Path,
-		"src_format", processResult.InputFormat,
+		"src_format", sourceFormat,
 		"dst_format", req.Transform.Format,
 		"src_bytes", fetchBytes,
 		"dst_bytes", outputSize,

internal/imageproxy/imageproxy_internal_test.go

@@ -191,23 +191,14 @@ func TestProxy_Get_FormatConversion(t *testing.T) {
 				t.Fatalf("failed to read response: %v", err)
 			}
 
-			detectedMIME, err := magic.DetectFormat(data)
+			detectedFormat, err := magic.DetectFormat(data)
 			if err != nil {
 				t.Fatalf("failed to detect format: %v", err)
 			}
 
-			expectedFormat, ok := magic.MIMEToImageFormat(tt.wantMIME)
-			if !ok {
-				t.Fatalf("unknown format for MIME type: %s", tt.wantMIME)
-			}
-
-			detectedFormat, ok := magic.MIMEToImageFormat(string(detectedMIME))
-			if !ok {
-				t.Fatalf("unknown format for detected MIME type: %s", detectedMIME)
-			}
-
-			if detectedFormat != expectedFormat {
-				t.Errorf("detected format = %q, want %q", detectedFormat, expectedFormat)
+			if detectedFormat.MIMEType() != tt.wantMIME {
+				t.Errorf("detected format = %q, want the format of %s",
+					detectedFormat, tt.wantMIME)
 			}
 		})
 	}

internal/imageproxy/max_concurrent_processing_internal_test.go

@@ -41,7 +41,8 @@ func holdProcessingSlot(
 	input, feed := io.Pipe()
 
 	go func() {
-		_, _ = proc.Process(t.Context(), input, &imagerequest.Transform{})
+		_, _ = proc.Process(t.Context(), input, imagerequest.FormatJPEG,
+			&imagerequest.Transform{})
 	}()
 
 	_, err := feed.Write([]byte{0})
@@ -57,10 +58,11 @@ func holdProcessingSlot(
 
 // TestProxy_Get_WaitsForSlotBeforeReadingCachedSource checks that a
 // request whose source is cached holds none of it while it waits for a
-// processing slot: it reads the cached file only once it has a slot. With
-// the only slot held, a request for a new width of the cached 100x100 photo
-// waits; the cached file is then rewritten as a 100x50 image before the slot
-// is freed, so the request must answer with that image scaled to 40x20.
+// processing slot: past the first bytes, which give the source's format, it
+// reads the cached file only once it has a slot. With the only slot held, a
+// request for a new width of the cached 100x100 photo waits; the cached file
+// is then rewritten as a 100x50 image before the slot is freed, so the
+// request must answer with that image scaled to 40x20.
 func TestProxy_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 	t.Parallel()
 
@@ -79,7 +81,7 @@ func TestProxy_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
 
 	cachedSource := widthOnlyRequest(fixtures, 50).Source
 
-	reader, _, err := svc.cache.Source(t.Context(), cachedSource)
+	reader, _, _, err := svc.cache.Source(t.Context(), cachedSource)
 	if err != nil {
 		t.Fatalf("Source() error = %v; want the cached source", err)
 	}

internal/imageproxy/negative_cache_internal_test.go

@@ -55,7 +55,7 @@ func TestProxy_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 
 	source := photoVariant(fixtures, 85, imagerequest.FitCover).Source
 
-	reader, _, err := svc.cache.Source(t.Context(), source)
+	reader, _, _, err := svc.cache.Source(t.Context(), source)
 	if err != nil {
 		t.Fatalf("Source() error = %v, want the cached source", err)
 	}

internal/imageproxy/partial_cache_files_internal_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"sneak.berlin/go/pixa/internal/imagerequest"
+	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
 // getImage calls Get for req and returns the image it answered with and
@@ -49,7 +50,7 @@ func cachedSourcePath(
 ) string {
 	t.Helper()
 
-	reader, _, err := svc.cache.Source(t.Context(), req.Source)
+	reader, _, _, err := svc.cache.Source(t.Context(), req.Source)
 	if err != nil {
 		t.Fatalf("Source() error = %v; want the cached source", err)
 	}
@@ -104,6 +105,40 @@ func TestProxy_Get_FetchesAgainForRemovedCachedSource(t *testing.T) {
 	}
 }
 
+// TestProxy_Get_FetchesAgainSourceOfNoSourceFormat replaces the cached
+// source's file with text. The next request for a new size of the source
+// must fetch it again and cache it whole.
+func TestProxy_Get_FetchesAgainSourceOfNoSourceFormat(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := setupTestProxy(t)
+
+	getImage(t, svc, photoVariant(fixtures, 85, imagerequest.FitCover))
+
+	path := cachedSourcePath(t, svc, photoVariant(fixtures, 85, imagerequest.FitCover))
+	wholeSize := fileSize(t, path)
+
+	err := os.WriteFile(path, []byte("this is text, not an image"),
+		imgcache.StorageFilePerm)
+	if err != nil {
+		t.Fatalf("replacing the source's file: %v", err)
+	}
+
+	getImage(t, svc, photoVariant(fixtures, 40, imagerequest.FitCover))
+
+	fetches := readCounters(t, svc).upstreamFetchCount
+	t.Logf("upstream fetches = %d", fetches)
+
+	if fetches != 2 {
+		t.Errorf("upstream fetches = %d, want 2", fetches)
+	}
+
+	size := fileSize(t, path)
+	if size != wholeSize {
+		t.Errorf("cached source is %d bytes, want %d", size, wholeSize)
+	}
+}
+
 // TestProxy_Get_ConvertsAgainForRemovedCachedVariant caches a variant of
 // the photo and removes its row and file, as the reconciliation pass does
 // when the file is empty or cut short. internal/imgcache tests that the

internal/imageproxy/testutil_internal_test.go

@@ -199,7 +199,7 @@ func setupTestProxy(t *testing.T) (*testProxy, *TestFixtures) {
 		t.Fatalf("failed to create cache: %v", err)
 	}
 
-	proxy, err := NewProxy(&Config{
+	proxy, err := New(&Config{
 		Cache:   cache,
 		Fetcher: httpfetcher.NewMock(mockFS),
 		Processor: imageprocessor.New(imageprocessor.Params{

internal/imagerequest/imagerequest.go

@@ -1,7 +1,8 @@
 // Package imagerequest holds the image request type, its parts, its checks,
 // the parsing of the /v1/image/ path, the variant key, the source key and the
-// source's upstream URL. It imports no other package of this repo, so every
-// package that handles an image request can import it.
+// source's upstream URL. It also holds the image format type and the MIME type
+// of each format. It imports no other package of this repo, so every package
+// that handles an image request can import it.
 package imagerequest
 
 import (
@@ -12,10 +13,11 @@ import (
 // ErrInvalidFitMode is returned when an invalid fit mode is provided.
 var ErrInvalidFitMode = errors.New("invalid fit mode")
 
-// Format represents supported output image formats.
+// Format is an image format. pixa reads and writes the formats from JPEG to
+// GIF below. In a request, orig and auto stand for one of them.
 type Format string
 
-// Supported image output formats.
+// Supported image formats.
 const (
 	FormatOriginal Format = "orig"
 	FormatJPEG     Format = "jpeg"
@@ -31,6 +33,30 @@ const (
 	FormatAuto Format = "auto"
 )
 
+// MIMEType returns the MIME type of the format, such as image/jpeg for
+// FormatJPEG. orig and auto stand for another format, so they have none:
+// MIMEType returns "" for them.
+func (f Format) MIMEType() string {
+	switch f {
+	case FormatJPEG:
+		return "image/jpeg"
+	case FormatPNG:
+		return "image/png"
+	case FormatWebP:
+		return "image/webp"
+	case FormatAVIF:
+		return "image/avif"
+	case FormatJXL:
+		return "image/jxl"
+	case FormatGIF:
+		return "image/gif"
+	case FormatOriginal, FormatAuto:
+		return ""
+	default:
+		return ""
+	}
+}
+
 // Size represents requested image dimensions
 type Size struct {
 	Width  int

internal/imagerequest/mimetype_internal_test.go

@@ -0,0 +1,30 @@
+package imagerequest
+
+import "testing"
+
+// TestFormat_MIMEType checks the MIME type of each format pixa reads and
+// writes, and that orig and auto have none.
+func TestFormat_MIMEType(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		format Format
+		want   string
+	}{
+		{FormatJPEG, "image/jpeg"},
+		{FormatPNG, "image/png"},
+		{FormatWebP, "image/webp"},
+		{FormatAVIF, "image/avif"},
+		{FormatJXL, "image/jxl"},
+		{FormatGIF, "image/gif"},
+		{FormatOriginal, ""},
+		{FormatAuto, ""},
+	}
+
+	for _, tt := range tests {
+		got := tt.format.MIMEType()
+		if got != tt.want {
+			t.Errorf("%s MIMEType() = %q, want %q", tt.format, got, tt.want)
+		}
+	}
+}

internal/imgcache/cache.go

@@ -19,6 +19,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	"sneak.berlin/go/pixa/internal/imagerequest"
+	"sneak.berlin/go/pixa/internal/magic"
 )
 
 // maxPendingAccessTimes is how many access times of variants served from
@@ -57,8 +58,8 @@ type Config struct {
 // Cache implements the caching layer for the image proxy.
 type Cache struct {
 	db         *sql.DB
-	srcContent *ContentStorage // source images by source key
-	variants   *VariantStorage // processed variants by cache key
+	srcContent *FileStore // source images by source key
+	variants   *FileStore // processed variants by cache key
 	config     Config
 	log        *slog.Logger
 
@@ -135,14 +136,14 @@ func NewCache(
 		return c, nil
 	}
 
-	srcContent, err := NewContentStorage(
+	srcContent, err := NewFileStore(
 		filepath.Join(config.StateDir, "cache", "sources"),
 	)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create source content storage: %w", err)
 	}
 
-	variants, err := NewVariantStorage(
+	variants, err := NewFileStore(
 		filepath.Join(config.StateDir, "cache", "variants"),
 	)
 	if err != nil {
@@ -169,14 +170,21 @@ func (c *Cache) Variant(
 		return nil, 0, "", ErrNotFound
 	}
 
-	reader, size, contentType, err := c.variants.LoadWithContentType(cacheKey)
+	file, size, err := c.variants.Open(string(cacheKey))
 	if err != nil {
 		return nil, 0, "", err
 	}
 
+	format, err := readFormat(file)
+	if err != nil {
+		_ = file.Close()
+
+		return nil, 0, "", err
+	}
+
 	c.touchVariant(cacheKey)
 
-	return reader, size, contentType, nil
+	return file, size, format.MIMEType(), nil
 }
 
 // PutVariant stores a processed variant by its cache key and records
@@ -190,11 +198,18 @@ func (c *Cache) PutVariant(
 		return nil
 	}
 
-	size, err := c.variants.Store(cacheKey, content)
+	data, err := io.ReadAll(content)
+	if err != nil {
+		return fmt.Errorf("failed to read content: %w", err)
+	}
+
+	err = c.variants.Write(string(cacheKey), data)
 	if err != nil {
 		return err
 	}
 
+	size := int64(len(data))
+
 	_, err = c.db.ExecContext(ctx, `
 		INSERT INTO variant_content (cache_key, size_bytes)
 		VALUES (?, ?)
@@ -212,15 +227,17 @@ func (c *Cache) PutVariant(
 	return nil
 }
 
-// Source opens the cached source of src and returns a reader and its size
-// in bytes. A source whose row or file is missing is ErrNotFound, as is any
-// source of a disabled cache. A source it opens has its access time handed
-// to the background writer without waiting (see touchSourceContent).
+// Source opens the cached source of src and returns a reader, its size in
+// bytes and its format, read from the file's first bytes. A source whose row
+// or file is missing is ErrNotFound, as is any source of a disabled cache; a
+// file whose first bytes are not those of a source format is another error.
+// A source it opens has its access time handed to the background writer
+// without waiting (see touchSourceContent).
 func (c *Cache) Source(
 	ctx context.Context, src imagerequest.Source,
-) (io.ReadCloser, int64, error) {
+) (io.ReadCloser, int64, imagerequest.Format, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, ErrNotFound
+		return nil, 0, "", ErrNotFound
 	}
 
 	key := src.Key()
@@ -231,21 +248,28 @@ func (c *Cache) Source(
 		SELECT EXISTS(SELECT 1 FROM source_content WHERE source_key = ?)
 	`, string(key)).Scan(&rowExists)
 	if err != nil {
-		return nil, 0, fmt.Errorf("failed to lookup source: %w", err)
+		return nil, 0, "", fmt.Errorf("failed to lookup source: %w", err)
 	}
 
 	if !rowExists {
-		return nil, 0, ErrNotFound
+		return nil, 0, "", ErrNotFound
 	}
 
-	reader, size, err := c.srcContent.LoadWithSize(key)
+	file, size, err := c.srcContent.Open(string(key))
 	if err != nil {
-		return nil, 0, err
+		return nil, 0, "", err
+	}
+
+	format, err := readFormat(file)
+	if err != nil {
+		_ = file.Close()
+
+		return nil, 0, "", err
 	}
 
 	c.touchSourceContent(key)
 
-	return reader, size, nil
+	return file, size, format, nil
 }
 
 // PutSource stores fetched source content: it writes the file named by
@@ -270,11 +294,13 @@ func (c *Cache) PutSource(
 
 	key := src.Key()
 
-	size, err := c.srcContent.Store(key, data)
+	err = c.srcContent.Write(string(key), data)
 	if err != nil {
 		return "", fmt.Errorf("failed to store source content: %w", err)
 	}
 
+	size := int64(len(data))
+
 	_, err = c.db.ExecContext(ctx, `
 		INSERT INTO source_content (source_key, size_bytes)
 		VALUES (?, ?)
@@ -283,7 +309,7 @@ func (c *Cache) PutSource(
 			last_accessed_at = CURRENT_TIMESTAMP
 	`, string(key), size)
 	if err != nil {
-		deleteErr := c.srcContent.Delete(key)
+		deleteErr := c.srcContent.Delete(string(key))
 		if deleteErr != nil {
 			c.log.Warn("failed to remove source file whose row was not written",
 				"source_key", key, "error", deleteErr)
@@ -422,3 +448,17 @@ func (c *Cache) cleanExpiredBatch(ctx context.Context) (int64, error) {
 
 	return deleted, nil
 }
+
+// readFormat reads the first bytes of a source or variant file, without
+// moving its offset, and returns the format they show. Sources and variants
+// are in the same six formats.
+func readFormat(content io.ReaderAt) (imagerequest.Format, error) {
+	firstBytes := make([]byte, magic.MinMagicBytes)
+
+	_, err := content.ReadAt(firstBytes, 0)
+	if err != nil {
+		return "", fmt.Errorf("failed to read the first bytes: %w", err)
+	}
+
+	return magic.DetectFormat(firstBytes)
+}

internal/imgcache/cache_hit_internal_test.go

@@ -32,7 +32,7 @@ func TestVariant_ContentTypeFromFirstBytes(t *testing.T) {
 		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
 	} {
 		result, err := processor.Process(t.Context(), bytes.NewReader(source),
-			&imagerequest.Transform{
+			imagerequest.FormatJPEG, &imagerequest.Transform{
 				Size:    imagerequest.Size{Width: 20, Height: 15},
 				Format:  format,
 				Quality: 85,
@@ -68,7 +68,7 @@ func TestVariant_ContentTypeFromFirstBytes(t *testing.T) {
 
 		t.Logf("%s: %d bytes served as %s", format, size, contentType)
 
-		if want := imageprocessor.FormatToMIME(format); contentType != want {
+		if want := format.MIMEType(); contentType != want {
 			t.Errorf("%s variant content type = %q, want %q", format, contentType, want)
 		}
 
@@ -96,7 +96,7 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 	}{
 		{"text", []byte("this is text, not an image"), magic.ErrUnknownFormat},
 		{"SVG", []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			errNotAnOutputFormat},
+			magic.ErrUnknownFormat},
 		{"JPEG cut to 10 bytes", jpegImage[:10], io.EOF},
 		{"empty", nil, io.EOF},
 	}
@@ -104,7 +104,7 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 	for i, tt := range tests {
 		key := imagerequest.VariantKey(fmt.Sprintf("aabbccdd20%02d", i))
 
-		_, err := cache.variants.Store(key, bytes.NewReader(tt.content))
+		err := cache.variants.Write(string(key), tt.content)
 		if err != nil {
 			t.Fatalf("%s: storing the variant file: %v", tt.name, err)
 		}

internal/imgcache/eviction.go

@@ -48,7 +48,7 @@ const staleTempFileAge = time.Hour
 const sqliteTimestampLayout = "2006-01-02 15:04:05"
 
 // tempFilePrefix is the prefix os.CreateTemp uses for in-flight cache
-// writes (".tmp-*" patterns in the storage layer).
+// writes (the ".tmp-*" pattern in writeFileSynced).
 const tempFilePrefix = ".tmp-"
 
 // UsageBytes returns the total number of bytes of cache content
@@ -306,7 +306,7 @@ func (c *Cache) evictVariant(
 		return fmt.Errorf("failed to delete variant accounting row: %w", err)
 	}
 
-	err = c.variants.Delete(cacheKey)
+	err = c.variants.Delete(string(cacheKey))
 	if err != nil {
 		return err
 	}
@@ -325,7 +325,7 @@ func (c *Cache) evictSourceBlob(ctx context.Context, key imagerequest.SourceKey)
 		return fmt.Errorf("failed to delete source content row: %w", err)
 	}
 
-	return c.srcContent.Delete(key)
+	return c.srcContent.Delete(string(key))
 }
 
 // notifyWritePressure wakes the background evictor after a store, so
@@ -515,12 +515,15 @@ func (c *Cache) reconcileAccounting(ctx context.Context) error {
 }
 
 // reconcileFiles walks baseDir, one of the cache's storage directories,
-// sweeping stale temp files and passing each other file, with its name, to
-// reconcileFile. Once ctx is cancelled it stops at the next file and
-// returns ctx's error.
+// sweeping stale temp files and passing each other file, with its info and
+// name, to reconcileFile. It skips a directory it cannot list and a file
+// whose info it cannot read, so one such entry does not stop the pass. It
+// logs a warning naming the path and the error. A file renamed or removed
+// after its directory was listed is skipped with no warning. Once ctx is
+// cancelled it stops at the next file and returns ctx's error.
 func (c *Cache) reconcileFiles(
 	ctx context.Context, baseDir string,
-	reconcileFile func(path string, entry fs.DirEntry, name string) error,
+	reconcileFile func(path string, info fs.FileInfo, name string) error,
 ) error {
 	return filepath.WalkDir(
 		baseDir,
@@ -529,19 +532,38 @@ func (c *Cache) reconcileFiles(
 				return ctx.Err()
 			}
 
-			if err != nil || entry.IsDir() {
-				return err
+			if err != nil {
+				c.log.Warn("skipped unreadable entry in cache directory",
+					"path", path, "error", err)
+
+				return nil
+			}
+
+			if entry.IsDir() {
+				return nil
+			}
+
+			info, err := entry.Info()
+			if errors.Is(err, fs.ErrNotExist) {
+				return nil
+			}
+
+			if err != nil {
+				c.log.Warn("skipped unreadable entry in cache directory",
+					"path", path, "error", err)
+
+				return nil
 			}
 
 			name := entry.Name()
 
 			if strings.HasPrefix(name, tempFilePrefix) {
-				c.sweepStaleTempFile(path, entry)
+				c.sweepStaleTempFile(path, info)
 
 				return nil
 			}
 
-			return reconcileFile(path, entry, name)
+			return reconcileFile(path, info, name)
 		},
 	)
 }
@@ -551,8 +573,8 @@ func (c *Cache) reconcileFiles(
 // and sweeping stale temp files.
 func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 	return c.reconcileFiles(ctx, c.variants.baseDir,
-		func(path string, entry fs.DirEntry, name string) error {
-			return c.reconcileVariantFile(ctx, path, entry, imagerequest.VariantKey(name))
+		func(path string, info fs.FileInfo, name string) error {
+			return c.reconcileVariantFile(ctx, path, info, imagerequest.VariantKey(name))
 		})
 }
 
@@ -562,16 +584,11 @@ func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 // crash or a restored snapshot can leave it, is removed with its row and
 // a warning naming it, so the variant is converted again.
 func (c *Cache) reconcileVariantFile(
-	ctx context.Context, path string, entry fs.DirEntry, cacheKey imagerequest.VariantKey,
+	ctx context.Context, path string, info fs.FileInfo, cacheKey imagerequest.VariantKey,
 ) error {
-	info, err := entry.Info()
-	if err != nil {
-		return fmt.Errorf("failed to stat variant file: %w", err)
-	}
-
 	var recordedSize int64
 
-	err = c.db.QueryRowContext(ctx,
+	err := c.db.QueryRowContext(ctx,
 		`SELECT size_bytes FROM variant_content WHERE cache_key = ?`, string(cacheKey),
 	).Scan(&recordedSize)
 
@@ -613,7 +630,9 @@ func (c *Cache) adoptVariantFile(
 }
 
 // reconcileVariantRows drops accounting rows whose variant files are
-// missing, so the database never references deleted content.
+// missing, so the database never references deleted content. A file whose
+// size cannot be read for another reason may still be on disk, so its row
+// stays, with a warning naming its key and the error.
 func (c *Cache) reconcileVariantRows(ctx context.Context) error {
 	var after imagerequest.VariantKey
 
@@ -636,11 +655,19 @@ func (c *Cache) reconcileVariantRows(ctx context.Context) error {
 				return ctx.Err()
 			}
 
-			if c.variants.Exists(key) {
+			_, err := c.variants.Size(string(key))
+			if err == nil {
 				continue
 			}
 
-			_, err := c.db.ExecContext(ctx,
+			if !errors.Is(err, ErrNotFound) {
+				c.log.Warn("kept accounting row for unreadable variant file",
+					"cache_key", key, "error", err)
+
+				continue
+			}
+
+			_, err = c.db.ExecContext(ctx,
 				`DELETE FROM variant_content WHERE cache_key = ?`, string(key))
 			if err != nil {
 				return fmt.Errorf("failed to drop stale variant accounting row: %w", err)
@@ -704,8 +731,8 @@ func queryStringColumn[T ~string](
 // and sweeping stale temp files.
 func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 	return c.reconcileFiles(ctx, c.srcContent.baseDir,
-		func(path string, entry fs.DirEntry, name string) error {
-			return c.reconcileSourceFile(ctx, path, entry, imagerequest.SourceKey(name))
+		func(path string, info fs.FileInfo, name string) error {
+			return c.reconcileSourceFile(ctx, path, info, imagerequest.SourceKey(name))
 		})
 }
 
@@ -716,16 +743,11 @@ func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 // is removed with its row and a warning naming it, so the source is
 // fetched again.
 func (c *Cache) reconcileSourceFile(
-	ctx context.Context, path string, entry fs.DirEntry, key imagerequest.SourceKey,
+	ctx context.Context, path string, info fs.FileInfo, key imagerequest.SourceKey,
 ) error {
-	info, err := entry.Info()
-	if err != nil {
-		return fmt.Errorf("failed to stat source content file: %w", err)
-	}
-
 	var recordedSize int64
 
-	err = c.db.QueryRowContext(ctx,
+	err := c.db.QueryRowContext(ctx,
 		`SELECT size_bytes FROM source_content WHERE source_key = ?`,
 		string(key),
 	).Scan(&recordedSize)
@@ -890,17 +912,12 @@ func (c *Cache) correctUsageTotal(
 
 // sweepStaleTempFile removes a temp file left behind by a crashed
 // write once it is old enough that no in-flight store can own it.
-func (c *Cache) sweepStaleTempFile(path string, entry fs.DirEntry) {
-	info, err := entry.Info()
-	if err != nil {
-		return
-	}
-
+func (c *Cache) sweepStaleTempFile(path string, info fs.FileInfo) {
 	if time.Since(info.ModTime()) < staleTempFileAge {
 		return
 	}
 
-	err = os.Remove(path)
+	err := os.Remove(path)
 	if err != nil && !os.IsNotExist(err) {
 		c.log.Warn("failed to remove stale temp file", "path", path, "error", err)
 

internal/imgcache/eviction_internal_test.go

@@ -9,8 +9,10 @@ import (
 	"log/slog"
 	"os"
 	"path/filepath"
+	"slices"
 	"strings"
 	"sync"
+	"syscall"
 	"testing"
 	"time"
 
@@ -209,7 +211,8 @@ func assertNoDanglingReferences(t *testing.T, cache *Cache) {
 			t.Fatalf("failed to scan source_key: %v", err)
 		}
 
-		if !cache.srcContent.Exists(imagerequest.SourceKey(key)) {
+		_, err = os.Stat(cache.srcContent.keyToPath(key))
+		if err != nil {
 			t.Errorf("source_content references content %s but the file is missing",
 				key)
 		}
@@ -236,7 +239,8 @@ func assertNoDanglingReferences(t *testing.T, cache *Cache) {
 			t.Fatalf("failed to scan cache_key: %v", err)
 		}
 
-		if !cache.variants.Exists(imagerequest.VariantKey(key)) {
+		_, err = os.Stat(cache.variants.keyToPath(key))
+		if err != nil {
 			t.Errorf("variant_content references key %s but the file is missing", key)
 		}
 	}
@@ -334,7 +338,8 @@ func TestEvictToLimitEvictsLeastRecentlyUsedFirst(t *testing.T) {
 		t.Errorf("usage after eviction = %d, want <= %d", usage, limit)
 	}
 
-	if cache.variants.Exists(keys[0]) {
+	_, err = os.Stat(cache.variants.keyToPath(string(keys[0])))
+	if err == nil {
 		t.Errorf("least recently used variant %s must be evicted", keys[0])
 	}
 
@@ -345,7 +350,8 @@ func TestEvictToLimitEvictsLeastRecentlyUsedFirst(t *testing.T) {
 	}
 
 	for _, key := range keys[1:] {
-		if !cache.variants.Exists(key) {
+		_, err = os.Stat(cache.variants.keyToPath(string(key)))
+		if err != nil {
 			t.Errorf("more recently used variant %s must survive eviction", key)
 		}
 	}
@@ -373,7 +379,8 @@ func TestEvictionKeepsEverythingWhenUnderLimit(t *testing.T) {
 	}
 
 	for _, key := range keys {
-		if !cache.srcContent.Exists(key) {
+		_, err = os.Stat(cache.srcContent.keyToPath(string(key)))
+		if err != nil {
 			t.Errorf("source %s must not be evicted while usage is under the limit", key)
 		}
 
@@ -384,7 +391,8 @@ func TestEvictionKeepsEverythingWhenUnderLimit(t *testing.T) {
 		}
 	}
 
-	if !cache.variants.Exists(testVariantKeyOne) {
+	_, err = os.Stat(cache.variants.keyToPath(string(testVariantKeyOne)))
+	if err != nil {
 		t.Error("variant must not be evicted while usage is under the limit")
 	}
 
@@ -449,7 +457,7 @@ func assertDisabledCacheReadsAlwaysMiss(
 
 	ctx := t.Context()
 
-	reader, _, err := cache.Source(ctx, req.Source)
+	reader, _, _, err := cache.Source(ctx, req.Source)
 	if !errors.Is(err, ErrNotFound) {
 		t.Fatalf("Source on disabled cache error = %v, want %v", err, ErrNotFound)
 	}
@@ -586,7 +594,7 @@ func TestEvictionRunsOnPeriodicSchedule(t *testing.T) {
 	for i, key := range keys {
 		content := bytes.Repeat([]byte{fills[i]}, 1000)
 
-		_, err = cache.variants.Store(key, bytes.NewReader(content))
+		err = cache.variants.Write(string(key), content)
 		if err != nil {
 			t.Fatalf("failed to store variant file: %v", err)
 		}
@@ -605,7 +613,8 @@ func TestEvictionRunsOnPeriodicSchedule(t *testing.T) {
 		stored = 0
 
 		for _, key := range keys {
-			if cache.variants.Exists(key) {
+			_, err = os.Stat(cache.variants.keyToPath(string(key)))
+			if err == nil {
 				stored++
 			}
 		}
@@ -637,9 +646,7 @@ func TestStartEvictionReconcilesAccountingWithDisk(t *testing.T) {
 	// feature existed) must be adopted into the accounting.
 	untracked := bytes.Repeat([]byte{0x31}, 1000)
 
-	_, err := cache.variants.Store(
-		testVariantKeyOne, bytes.NewReader(untracked),
-	)
+	err := cache.variants.Write(string(testVariantKeyOne), untracked)
 	if err != nil {
 		t.Fatalf("failed to store untracked variant file: %v", err)
 	}
@@ -694,6 +701,291 @@ func TestStartEvictionReconcilesAccountingWithDisk(t *testing.T) {
 	}
 }
 
+// logLineNaming returns the first line of logs, written by slog's JSON
+// handler, that is at level and names the variant key, or "" if none is.
+func logLineNaming(logs, level string, key imagerequest.VariantKey) string {
+	for line := range strings.Lines(logs) {
+		if strings.Contains(line, `"level":"`+level+`"`) &&
+			strings.Contains(line, `"cache_key":"`+string(key)+`"`) {
+			return line
+		}
+	}
+
+	return ""
+}
+
+// TestReconcileVariantRowsDropsOnlyRowsOfMissingFiles gives
+// reconcileVariantRows two tracked variants. The first one's file is a
+// symbolic link to itself, so its size cannot be read. Its row must stay,
+// with a warning naming its key and the error. The second one has no file.
+// Its row must go, with the info line for a missing file.
+func TestReconcileVariantRowsDropsOnlyRowsOfMissingFiles(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	storeEvictionTestVariant(t, cache, testVariantKeyOne, []byte("variant one"))
+	storeEvictionTestVariant(t, cache, testVariantKeyTwo, []byte("variant two"))
+
+	unreadablePath := cache.variants.keyToPath(string(testVariantKeyOne))
+
+	err := os.Remove(unreadablePath)
+	if err != nil {
+		t.Fatalf("failed to remove the first variant's file: %v", err)
+	}
+
+	err = os.Symlink(unreadablePath, unreadablePath)
+	if err != nil {
+		t.Fatalf("failed to link the first variant's path to itself: %v", err)
+	}
+
+	_, statErr := os.Stat(unreadablePath)
+	if statErr == nil || errors.Is(statErr, fs.ErrNotExist) {
+		t.Fatalf("os.Stat() of the first variant: error = %v, "+
+			"want an error other than a missing file", statErr)
+	}
+
+	err = os.Remove(cache.variants.keyToPath(string(testVariantKeyTwo)))
+	if err != nil {
+		t.Fatalf("failed to remove the second variant's file: %v", err)
+	}
+
+	var logBuf bytes.Buffer
+
+	cache.log = slog.New(slog.NewJSONHandler(&logBuf, nil))
+
+	err = cache.reconcileVariantRows(t.Context())
+	t.Logf("reconcileVariantRows() error = %v, log: %s", err, logBuf.String())
+
+	if err != nil {
+		t.Fatalf("reconcileVariantRows() error = %v", err)
+	}
+
+	rowQuery := `SELECT COUNT(*) FROM variant_content WHERE cache_key = ?`
+
+	if n := countRows(t, cache, rowQuery, string(testVariantKeyOne)); n != 1 {
+		t.Errorf("the variant whose size cannot be read has %d rows, want 1", n)
+	}
+
+	if n := countRows(t, cache, rowQuery, string(testVariantKeyTwo)); n != 0 {
+		t.Errorf("the variant with no file has %d rows, want 0", n)
+	}
+
+	logs := logBuf.String()
+
+	warning := logLineNaming(logs, "WARN", testVariantKeyOne)
+	if !strings.Contains(warning, statErr.Error()) {
+		t.Errorf("no warning names %s and the error %q", testVariantKeyOne, statErr)
+	}
+
+	if line := logLineNaming(logs, "INFO", testVariantKeyOne); line != "" {
+		t.Errorf("the variant whose size cannot be read was logged at info: %s", line)
+	}
+
+	dropped := logLineNaming(logs, "INFO", testVariantKeyTwo)
+	if !strings.Contains(dropped, "dropped accounting row for missing variant file") {
+		t.Errorf("no info line says the file of %s is missing", testVariantKeyTwo)
+	}
+}
+
+// openDirectoryNearPathLimit makes a chain of directories inside dir, each
+// named with 255 zeros, the longest name Linux allows. Linux refuses a path
+// of 4096 bytes or more, and the chain goes as deep as it can below that.
+// It returns the chain's deepest directory as an os.Root, closed when the
+// test ends. An entry made through it with a 255-byte name has a path even
+// the root user cannot read. The os.Root can make that entry because it
+// names the entry only by its name, not by the whole path.
+func openDirectoryNearPathLimit(t *testing.T, dir string) *os.Root {
+	t.Helper()
+
+	name := strings.Repeat("0", 255)
+	deepest := dir
+
+	for len(filepath.Join(deepest, name)) < 4096 {
+		deepest = filepath.Join(deepest, name)
+	}
+
+	err := os.MkdirAll(deepest, StorageDirPerm)
+	if err != nil {
+		t.Fatalf("failed to make a chain of directories: %v", err)
+	}
+
+	root, err := os.OpenRoot(deepest)
+	if err != nil {
+		t.Fatalf("os.OpenRoot(%s) error = %v", deepest, err)
+	}
+
+	t.Cleanup(func() { _ = root.Close() })
+
+	return root
+}
+
+// makeEntriesTooLongToRead makes a new directory and a new file in the
+// deepest directory of the chain that openDirectoryNearPathLimit makes
+// inside dir. Even the root user can then neither list the new directory
+// nor read the new file's info. It returns the paths of the new directory
+// and the new file.
+func makeEntriesTooLongToRead(t *testing.T, dir string) []string {
+	t.Helper()
+
+	deepest := openDirectoryNearPathLimit(t, dir)
+	dirName := strings.Repeat("0", 255)
+
+	err := deepest.Mkdir(dirName, StorageDirPerm)
+	if err != nil {
+		t.Fatalf("failed to make a directory too deep to list: %v", err)
+	}
+
+	fileName := strings.Repeat("1", 255)
+
+	err = deepest.WriteFile(fileName, []byte("unreadable"), StorageFilePerm)
+	if err != nil {
+		t.Fatalf("failed to make a file too deep to read: %v", err)
+	}
+
+	return []string{
+		filepath.Join(deepest.Name(), dirName),
+		filepath.Join(deepest.Name(), fileName),
+	}
+}
+
+// makeTempFileTooLongToRead makes a temp file in the deepest directory of
+// the chain that openDirectoryNearPathLimit makes inside dir. Even the
+// root user then cannot read the temp file's info. It returns the temp
+// file's path.
+func makeTempFileTooLongToRead(t *testing.T, dir string) string {
+	t.Helper()
+
+	deepest := openDirectoryNearPathLimit(t, dir)
+	name := tempFilePrefix + strings.Repeat("1", 255-len(tempFilePrefix))
+
+	err := deepest.WriteFile(name, []byte("partial"), StorageFilePerm)
+	if err != nil {
+		t.Fatalf("failed to make a temp file too deep to read: %v", err)
+	}
+
+	return filepath.Join(deepest.Name(), name)
+}
+
+// assertSkippedEntryWarning checks that logs hold the warning "skipped
+// unreadable entry in cache directory" naming path, with the error
+// ENAMETOOLONG for that path.
+func assertSkippedEntryWarning(t *testing.T, logs, path string) {
+	t.Helper()
+
+	for line := range strings.Lines(logs) {
+		if strings.Contains(line, `"level":"WARN"`) &&
+			strings.Contains(line, `"msg":"skipped unreadable entry in cache directory"`) &&
+			strings.Contains(line, `"path":"`+path+`"`) &&
+			strings.Contains(line, path+": "+syscall.ENAMETOOLONG.Error()) {
+			return
+		}
+	}
+
+	t.Errorf("no warning %q names %s with the error %q",
+		"skipped unreadable entry in cache directory", path, syscall.ENAMETOOLONG)
+}
+
+// TestReconciliationGoesOnPastEntriesItCannotRead puts, in each storage
+// directory, a directory the pass cannot list and a file whose info it
+// cannot read. The pass walks each directory in name order, so it reaches
+// them before the cache's files, whose keys start with a letter. It must
+// log a warning naming each one's path and the error, and still do the
+// rest of its work:
+//   - adopt a variant file that has no row
+//   - drop the row of a variant whose file is missing
+//   - remove a source file that has no row
+//   - correct a wrong total cache usage
+func TestReconciliationGoesOnPastEntriesItCannotRead(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	ctx := t.Context()
+
+	untrackedVariant := []byte("variant with no row")
+
+	err := cache.variants.Write(string(testVariantKeyOne), untrackedVariant)
+	if err != nil {
+		t.Fatalf("failed to write a variant file with no row: %v", err)
+	}
+
+	storeEvictionTestVariant(t, cache, testVariantKeyTwo, []byte("variant two"))
+
+	err = os.Remove(cache.variants.keyToPath(string(testVariantKeyTwo)))
+	if err != nil {
+		t.Fatalf("failed to remove the second variant's file: %v", err)
+	}
+
+	untrackedSourceKey := "ffeeddcc0001"
+
+	err = cache.srcContent.Write(untrackedSourceKey, []byte("source with no row"))
+	if err != nil {
+		t.Fatalf("failed to write a source file with no row: %v", err)
+	}
+
+	_, err = cache.db.ExecContext(ctx,
+		`UPDATE cache_usage SET total_size_bytes = 1 WHERE id = 1`)
+	if err != nil {
+		t.Fatalf("failed to set a wrong total: %v", err)
+	}
+
+	unreadable := slices.Concat(
+		makeEntriesTooLongToRead(t, cache.variants.baseDir),
+		makeEntriesTooLongToRead(t, cache.srcContent.baseDir),
+	)
+
+	logs := reconcileAsAtStartup(t, cache)
+
+	for _, path := range unreadable {
+		assertSkippedEntryWarning(t, logs, path)
+	}
+
+	rowQuery := `SELECT COUNT(*) FROM variant_content WHERE cache_key = ?`
+
+	if n := countRows(t, cache, rowQuery, string(testVariantKeyOne)); n != 1 {
+		t.Errorf("the variant file with no row got %d rows, want 1", n)
+	}
+
+	if n := countRows(t, cache, rowQuery, string(testVariantKeyTwo)); n != 0 {
+		t.Errorf("the variant with no file kept %d rows, want 0", n)
+	}
+
+	_, err = os.Stat(cache.srcContent.keyToPath(untrackedSourceKey))
+	if err == nil {
+		t.Error("the source file with no row is still on disk")
+	}
+
+	usage, err := cache.UsageBytes(ctx)
+	if err != nil {
+		t.Fatalf("UsageBytes() error = %v", err)
+	}
+
+	if usage != int64(len(untrackedVariant)) {
+		t.Errorf("UsageBytes() = %d, want %d, the size of the adopted variant",
+			usage, len(untrackedVariant))
+	}
+}
+
+// TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead puts, in each
+// storage directory, a temp file whose info the pass cannot read. The
+// pass must log a warning naming each one's path and the error, as it
+// does for any other file whose info it cannot read.
+func TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	unreadable := []string{
+		makeTempFileTooLongToRead(t, cache.variants.baseDir),
+		makeTempFileTooLongToRead(t, cache.srcContent.baseDir),
+	}
+
+	logs := reconcileAsAtStartup(t, cache)
+
+	for _, path := range unreadable {
+		assertSkippedEntryWarning(t, logs, path)
+	}
+}
+
 // TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup proves
 // reconciliation is not a one-shot startup-only pass: it must also run
 // on the periodic ticker, so a variant file that lands on disk with no
@@ -738,9 +1030,7 @@ func TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup(t *testing.T) {
 	// insert had failed and only the file write had succeeded.
 	untracked := bytes.Repeat([]byte{0x41}, 900)
 
-	_, err = cache.variants.Store(
-		"aabbccdd0099", bytes.NewReader(untracked),
-	)
+	err = cache.variants.Write("aabbccdd0099", untracked)
 	if err != nil {
 		t.Fatalf("failed to store untracked variant file: %v", err)
 	}
@@ -1046,13 +1336,15 @@ func assertOnlyOldestSourceEvicted(
 ) {
 	t.Helper()
 
-	if cache.srcContent.Exists(keys[0]) {
+	_, err := os.Stat(cache.srcContent.keyToPath(string(keys[0])))
+	if err == nil {
 		t.Errorf("source %s, evicted when the context was cancelled, "+
 			"is still on disk", keys[0])
 	}
 
 	for _, key := range keys[1:] {
-		if !cache.srcContent.Exists(key) {
+		_, err = os.Stat(cache.srcContent.keyToPath(string(key)))
+		if err != nil {
 			t.Errorf("source %s was evicted after the context was cancelled", key)
 		}
 

internal/imgcache/filestore.go

@@ -0,0 +1,185 @@
+package imgcache
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// Storage constants.
+const (
+	// StorageDirPerm is the permission mode for storage directories.
+	StorageDirPerm = 0750
+	// StorageFilePerm is the permission mode for storage files.
+	StorageFilePerm = 0600
+	// MinHashLength is the minimum hash length for path splitting.
+	MinHashLength = 4
+)
+
+// Storage errors.
+var (
+	ErrNotFound = errors.New("content not found")
+)
+
+// FileStore is a directory of files, each named by a hex key. The file for
+// a key is at <basedir>/<ab>/<cd>/<key>, where <ab> and <cd> are the key's
+// first and second pairs of characters. The cache keeps its source images in
+// one FileStore and its variants in another.
+type FileStore struct {
+	baseDir string
+}
+
+// NewFileStore creates the directory baseDir if it is missing and returns
+// the FileStore of the files in it.
+func NewFileStore(baseDir string) (*FileStore, error) {
+	err := os.MkdirAll(baseDir, StorageDirPerm)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create storage directory: %w", err)
+	}
+
+	return &FileStore{baseDir: baseDir}, nil
+}
+
+// Write writes data with writeFileSynced to the file for key, replacing any
+// file already there.
+func (s *FileStore) Write(key string, data []byte) error {
+	path := s.keyToPath(key)
+
+	err := os.MkdirAll(filepath.Dir(path), StorageDirPerm)
+	if err != nil {
+		return fmt.Errorf("failed to create directory: %w", err)
+	}
+
+	return writeFileSynced(path, data)
+}
+
+// Open opens the file for key and returns it with its size in bytes. A
+// missing file is ErrNotFound.
+func (s *FileStore) Open(key string) (*os.File, int64, error) {
+	f, err := os.Open(s.keyToPath(key))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, 0, ErrNotFound
+		}
+
+		return nil, 0, fmt.Errorf("failed to open content: %w", err)
+	}
+
+	stat, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+
+		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
+	}
+
+	return f, stat.Size(), nil
+}
+
+// Size returns the size in bytes of the file for key. A missing file is
+// ErrNotFound.
+func (s *FileStore) Size(key string) (int64, error) {
+	stat, err := os.Stat(s.keyToPath(key))
+	if err != nil {
+		if os.IsNotExist(err) {
+			return 0, ErrNotFound
+		}
+
+		return 0, fmt.Errorf("failed to stat content: %w", err)
+	}
+
+	return stat.Size(), nil
+}
+
+// Delete removes the file for key. A missing file is not an error.
+func (s *FileStore) Delete(key string) error {
+	err := os.Remove(s.keyToPath(key))
+	if err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("failed to delete content: %w", err)
+	}
+
+	return nil
+}
+
+// writeFileSynced writes data to path through a temp file in the same
+// directory: the temp file is synced to disk, renamed to path, and the
+// directory synced after, so that neither a concurrent reader nor a
+// crash finds a partial file under path. When the directory sync fails,
+// the file is removed from path, so that a failed write leaves no file
+// there.
+func writeFileSynced(path string, data []byte) error {
+	dir := filepath.Dir(path)
+
+	tmpFile, err := os.CreateTemp(dir, ".tmp-*")
+	if err != nil {
+		return fmt.Errorf("failed to create temp file: %w", err)
+	}
+
+	tmpPath := tmpFile.Name()
+
+	_, err = tmpFile.Write(data)
+	if err != nil {
+		_ = tmpFile.Close()
+		_ = os.Remove(tmpPath)
+
+		return fmt.Errorf("failed to write temp file: %w", err)
+	}
+
+	err = tmpFile.Sync()
+	if err != nil {
+		_ = tmpFile.Close()
+		_ = os.Remove(tmpPath)
+
+		return fmt.Errorf("failed to sync temp file: %w", err)
+	}
+
+	err = tmpFile.Close()
+	if err != nil {
+		_ = os.Remove(tmpPath)
+
+		return fmt.Errorf("failed to close temp file: %w", err)
+	}
+
+	err = os.Rename(filepath.Clean(tmpPath), filepath.Clean(path))
+	if err != nil {
+		_ = os.Remove(tmpPath)
+
+		return fmt.Errorf("failed to rename temp file: %w", err)
+	}
+
+	err = syncDirectory(dir)
+	if err != nil {
+		_ = os.Remove(path)
+
+		return err
+	}
+
+	return nil
+}
+
+// syncDirectory syncs the directory dir to disk, so that a file just
+// renamed into it is still there after a crash.
+func syncDirectory(dir string) error {
+	d, err := os.Open(filepath.Clean(dir))
+	if err != nil {
+		return fmt.Errorf("failed to open directory: %w", err)
+	}
+
+	err = d.Sync()
+	_ = d.Close()
+
+	if err != nil {
+		return fmt.Errorf("failed to sync directory: %w", err)
+	}
+
+	return nil
+}
+
+// keyToPath converts a key to a file path: <basedir>/<ab>/<cd>/<key>
+func (s *FileStore) keyToPath(key string) string {
+	if len(key) < MinHashLength {
+		return filepath.Join(s.baseDir, key)
+	}
+
+	return filepath.Join(s.baseDir, key[0:2], key[2:4], key)
+}

internal/imgcache/filestore_internal_test.go

@@ -0,0 +1,184 @@
+package imgcache
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+func TestFileStore_WriteAndOpen(t *testing.T) {
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+
+	storage, err := NewFileStore(tmpDir)
+	if err != nil {
+		t.Fatalf("NewFileStore() error = %v", err)
+	}
+
+	content := []byte("hello world")
+	key := string(imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key())
+
+	err = storage.Write(key, content)
+	if err != nil {
+		t.Fatalf("Write() error = %v", err)
+	}
+
+	// Verify file exists at expected path
+	expectedPath := filepath.Join(tmpDir, key[0:2], key[2:4], key)
+
+	_, err = os.Stat(expectedPath)
+	if err != nil {
+		t.Errorf("File not at expected path %s: %v", expectedPath, err)
+	}
+
+	// Open and verify size and content
+	r, size, err := storage.Open(key)
+	if err != nil {
+		t.Fatalf("Open() error = %v", err)
+	}
+
+	defer func() { _ = r.Close() }()
+
+	if size != int64(len(content)) {
+		t.Errorf("Open() size = %d, want %d", size, len(content))
+	}
+
+	loaded, err := io.ReadAll(r)
+	if err != nil {
+		t.Fatalf("ReadAll() error = %v", err)
+	}
+
+	if !bytes.Equal(loaded, content) {
+		t.Errorf("Open() content = %q, want %q", loaded, content)
+	}
+}
+
+func TestFileStore_OpenNotFound(t *testing.T) {
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+
+	storage, err := NewFileStore(tmpDir)
+	if err != nil {
+		t.Fatalf("NewFileStore() error = %v", err)
+	}
+
+	_, _, err = storage.Open("nonexistent")
+	if !errors.Is(err, ErrNotFound) {
+		t.Errorf("Open() error = %v, want ErrNotFound", err)
+	}
+}
+
+// TestFileStore_Size checks that Size returns the size of a written file,
+// and ErrNotFound for a key with no file.
+func TestFileStore_Size(t *testing.T) {
+	t.Parallel()
+
+	storage, err := NewFileStore(t.TempDir())
+	if err != nil {
+		t.Fatalf("NewFileStore() error = %v", err)
+	}
+
+	content := []byte("the file's content")
+	key := string(imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key())
+
+	err = storage.Write(key, content)
+	if err != nil {
+		t.Fatalf("Write() error = %v", err)
+	}
+
+	size, err := storage.Size(key)
+	if err != nil || size != int64(len(content)) {
+		t.Errorf("Size() = %d, error %v; want %d", size, err, len(content))
+	}
+
+	_, err = storage.Size("nonexistent")
+	if !errors.Is(err, ErrNotFound) {
+		t.Errorf("Size() of a missing file error = %v, want ErrNotFound", err)
+	}
+}
+
+func TestFileStore_Delete(t *testing.T) {
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+
+	storage, err := NewFileStore(tmpDir)
+	if err != nil {
+		t.Fatalf("NewFileStore() error = %v", err)
+	}
+
+	content := []byte("to be deleted")
+	key := string(imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key())
+	path := storage.keyToPath(key)
+
+	err = storage.Write(key, content)
+	if err != nil {
+		t.Fatalf("Write() error = %v", err)
+	}
+
+	_, err = os.Stat(path)
+	if err != nil {
+		t.Errorf("file missing before Delete(): %v", err)
+	}
+
+	err = storage.Delete(key)
+	if err != nil {
+		t.Fatalf("Delete() error = %v", err)
+	}
+
+	_, err = os.Stat(path)
+	if err == nil {
+		t.Error("file still on disk after Delete()")
+	}
+}
+
+func TestFileStore_DeleteNonexistent(t *testing.T) {
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+
+	storage, err := NewFileStore(tmpDir)
+	if err != nil {
+		t.Fatalf("NewFileStore() error = %v", err)
+	}
+
+	// Should not error
+	err = storage.Delete("nonexistent")
+	if err != nil {
+		t.Errorf("Delete() error = %v, want nil", err)
+	}
+}
+
+func TestFileStore_KeyToPath(t *testing.T) {
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+
+	storage, err := NewFileStore(tmpDir)
+	if err != nil {
+		t.Fatalf("NewFileStore() error = %v", err)
+	}
+
+	// Test by writing and verifying the resulting path structure
+	content := []byte("test content for path verification")
+	key := string(imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key())
+
+	err = storage.Write(key, content)
+	if err != nil {
+		t.Fatalf("Write() error = %v", err)
+	}
+
+	expectedPath := filepath.Join(tmpDir, key[0:2], key[2:4], key)
+
+	_, err = os.Stat(expectedPath)
+	if err != nil {
+		t.Errorf("File not at expected path %s: %v", expectedPath, err)
+	}
+}

internal/imgcache/partial_cache_files_internal_test.go

@@ -67,8 +67,8 @@ func TestReconciliationRemovesDamagedFiles(t *testing.T) {
 			sourceKey := storeEvictionTestSource(t, cache, testHostCDN, testPathCat, content)
 			storeEvictionTestVariant(t, cache, testVariantKeyOne, content)
 
-			sourcePath := cache.srcContent.keyToPath(sourceKey)
-			variantPath := cache.variants.keyToPath(testVariantKeyOne)
+			sourcePath := cache.srcContent.keyToPath(string(sourceKey))
+			variantPath := cache.variants.keyToPath(string(testVariantKeyOne))
 
 			for _, path := range []string{sourcePath, variantPath} {
 				err := os.Truncate(path, size)
@@ -82,11 +82,13 @@ func TestReconciliationRemovesDamagedFiles(t *testing.T) {
 			assertWarningNames(t, logs, sourcePath)
 			assertWarningNames(t, logs, variantPath)
 
-			if cache.srcContent.Exists(sourceKey) {
+			_, err := os.Stat(sourcePath)
+			if err == nil {
 				t.Error("the damaged source file is still on disk")
 			}
 
-			if cache.variants.Exists(testVariantKeyOne) {
+			_, err = os.Stat(variantPath)
+			if err == nil {
 				t.Error("the damaged variant file is still on disk")
 			}
 
@@ -116,16 +118,19 @@ func TestReconciliationRemovesEmptyVariantFileWithoutRow(t *testing.T) {
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
 
-	_, err := cache.variants.Store(testVariantKeyOne, bytes.NewReader(nil))
+	err := cache.variants.Write(string(testVariantKeyOne), nil)
 	if err != nil {
 		t.Fatalf("failed to store an empty variant file: %v", err)
 	}
 
+	variantPath := cache.variants.keyToPath(string(testVariantKeyOne))
+
 	logs := reconcileAsAtStartup(t, cache)
 
-	assertWarningNames(t, logs, cache.variants.keyToPath(testVariantKeyOne))
+	assertWarningNames(t, logs, variantPath)
 
-	if cache.variants.Exists(testVariantKeyOne) {
+	_, err = os.Stat(variantPath)
+	if err == nil {
 		t.Error("the empty variant file is still on disk")
 	}
 
@@ -137,19 +142,19 @@ func TestReconciliationRemovesEmptyVariantFileWithoutRow(t *testing.T) {
 	}
 }
 
-// TestContentStorage_StoreReplacesFileOfAnotherSize checks that a
-// store over a file of another size at the source's path, such as a
-// partial file a crash left, replaces it with the content.
-func TestContentStorage_StoreReplacesFileOfAnotherSize(t *testing.T) {
+// TestFileStore_WriteReplacesFileOfAnotherSize checks that a write over a
+// file of another size at the source's path, such as a partial file a crash
+// left, replaces it with the content.
+func TestFileStore_WriteReplacesFileOfAnotherSize(t *testing.T) {
 	t.Parallel()
 
-	storage, err := NewContentStorage(t.TempDir())
+	storage, err := NewFileStore(t.TempDir())
 	if err != nil {
-		t.Fatalf("NewContentStorage() error = %v", err)
+		t.Fatalf("NewFileStore() error = %v", err)
 	}
 
 	content := []byte("the whole source image")
-	key := imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key()
+	key := string(imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key())
 	path := storage.keyToPath(key)
 
 	err = os.MkdirAll(filepath.Dir(path), StorageDirPerm)
@@ -162,14 +167,14 @@ func TestContentStorage_StoreReplacesFileOfAnotherSize(t *testing.T) {
 		t.Fatalf("writing a partial file: %v", err)
 	}
 
-	_, err = storage.Store(key, content)
+	err = storage.Write(key, content)
 	if err != nil {
-		t.Fatalf("Store() error = %v", err)
+		t.Fatalf("Write() error = %v", err)
 	}
 
-	stored, _, err := storage.LoadWithSize(key)
+	stored, _, err := storage.Open(key)
 	if err != nil {
-		t.Fatalf("LoadWithSize() error = %v", err)
+		t.Fatalf("Open() error = %v", err)
 	}
 
 	defer func() { _ = stored.Close() }()

internal/imgcache/pendingaccesstimes_internal_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"database/sql"
 	"errors"
+	"image/color"
 	"io"
 	"log/slog"
 	"strings"
@@ -204,8 +205,8 @@ func TestSource_HandsAccessTimeToTheBackgroundWriter(t *testing.T) {
 	t.Parallel()
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
-	key := storeEvictionTestSource(t, cache, "example.com", "/photo.jpg",
-		[]byte("source bytes"))
+	content := generateTestJPEG(t, 10, 10, color.RGBA{0, 0, 255, 255})
+	key := storeEvictionTestSource(t, cache, "example.com", "/photo.jpg", content)
 	anHourAgo := time.Now().Add(-time.Hour)
 	setSourceLastAccessed(t, cache, key, anHourAgo)
 
@@ -215,7 +216,7 @@ func TestSource_HandsAccessTimeToTheBackgroundWriter(t *testing.T) {
 	`
 	anHourAgoInDatabase := anHourAgo.UTC().Format(sqliteTimestampFormat)
 
-	reader, _, err := cache.Source(t.Context(),
+	reader, _, _, err := cache.Source(t.Context(),
 		imagerequest.Source{Host: "example.com", Path: "/photo.jpg"})
 	if err != nil {
 		t.Fatalf("Source() error = %v; want the source of %q", err, key)
@@ -224,7 +225,7 @@ func TestSource_HandsAccessTimeToTheBackgroundWriter(t *testing.T) {
 	found, err := io.ReadAll(reader)
 	_ = reader.Close()
 
-	if err != nil || string(found) != "source bytes" {
+	if err != nil || !bytes.Equal(found, content) {
 		t.Fatalf("Source() reads %q, %v; want the source of %q", found, err, key)
 	}
 

internal/imgcache/source_content_internal_test.go

@@ -2,12 +2,17 @@ package imgcache
 
 import (
 	"bytes"
+	"errors"
+	"fmt"
+	"image/color"
 	"io"
 	"os"
 	"path/filepath"
 	"testing"
 
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/imagerequest"
+	"sneak.berlin/go/pixa/internal/magic"
 )
 
 // TestPutSource_StoresIdenticalBytesOfTwoURLsTwice stores the same bytes
@@ -64,7 +69,7 @@ func TestEvictSourceBlob_LeavesSourceWithIdenticalBytesServed(t *testing.T) {
 	t.Parallel()
 
 	cache, _ := newEvictionTestCache(t, 1<<30)
-	content := bytes.Repeat([]byte{0x5B}, 400)
+	content := generateTestJPEG(t, 20, 20, color.RGBA{0x5B, 0x5B, 0x5B, 255})
 
 	evicted := storeEvictionTestSource(t, cache, testHostCDN, "/evicted.jpg", content)
 	storeEvictionTestSource(t, cache, testHostCDN, "/kept.jpg", content)
@@ -74,7 +79,8 @@ func TestEvictSourceBlob_LeavesSourceWithIdenticalBytesServed(t *testing.T) {
 		t.Fatalf("evictSourceBlob() error = %v", err)
 	}
 
-	if cache.srcContent.Exists(evicted) {
+	_, err = os.Stat(cache.srcContent.keyToPath(string(evicted)))
+	if err == nil {
 		t.Error("the evicted source's file is still on disk")
 	}
 
@@ -84,7 +90,7 @@ func TestEvictSourceBlob_LeavesSourceWithIdenticalBytesServed(t *testing.T) {
 		t.Errorf("the evicted source still has %d source_content rows", n)
 	}
 
-	reader, _, err := cache.Source(t.Context(),
+	reader, _, _, err := cache.Source(t.Context(),
 		imagerequest.Source{Host: testHostCDN, Path: "/kept.jpg"})
 	t.Logf("Source() of the kept source: error %v", err)
 
@@ -129,7 +135,104 @@ func TestPutSource_FailedRowWriteLeavesNoFile(t *testing.T) {
 		t.Error("PutSource() error = nil, want the failed row write")
 	}
 
-	if cache.srcContent.Exists(req.Source.Key()) {
+	_, err = os.Stat(cache.srcContent.keyToPath(string(req.Source.Key())))
+	if err == nil {
 		t.Error("the source's file is on disk although its row was not written")
 	}
 }
+
+// TestSource_FormatFromFirstBytes stores an image in each source format
+// under one source URL, in turn. After each store, Source must return that
+// image whole, with its format read from the file.
+func TestSource_FormatFromFirstBytes(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	processor := imageprocessor.New(imageprocessor.Params{
+		MaxInputBytes:           50 << 20,
+		MaxConcurrentProcessing: 1,
+	})
+	jpegImage := generateTestJPEG(t, 40, 30, color.RGBA{200, 40, 40, 255})
+	src := imagerequest.Source{Host: testHostCDN, Path: testPathCat}
+
+	for _, format := range []imagerequest.Format{
+		imagerequest.FormatJPEG, imagerequest.FormatPNG, imagerequest.FormatGIF,
+		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
+	} {
+		result, err := processor.Process(t.Context(), bytes.NewReader(jpegImage),
+			imagerequest.FormatJPEG, &imagerequest.Transform{Format: format})
+		if err != nil {
+			t.Fatalf("making a %s image: %v", format, err)
+		}
+
+		image, err := io.ReadAll(result.Content)
+		if err != nil {
+			t.Fatalf("reading the %s image: %v", format, err)
+		}
+
+		_, err = cache.PutSource(t.Context(), src, bytes.NewReader(image))
+		if err != nil {
+			t.Fatalf("PutSource(%s) error = %v", format, err)
+		}
+
+		reader, size, got, err := cache.Source(t.Context(), src)
+		if err != nil {
+			t.Fatalf("Source() of the %s image error = %v", format, err)
+		}
+
+		served, err := io.ReadAll(reader)
+		_ = reader.Close()
+
+		if err != nil {
+			t.Fatalf("reading the %s source: %v", format, err)
+		}
+
+		t.Logf("%s: %d bytes read as %s", format, size, got)
+
+		if got != format {
+			t.Errorf("Source() of the %s image format = %q, want %q",
+				format, got, format)
+		}
+
+		if size != int64(len(image)) || !bytes.Equal(served, image) {
+			t.Errorf("%s source read as %d bytes, size %d, want the %d bytes stored",
+				format, len(served), size, len(image))
+		}
+	}
+}
+
+// TestSource_RefusesFileOfNoSourceFormat checks that Source refuses a
+// cached source whose file's first bytes are not those of a source format,
+// with an error other than ErrNotFound.
+func TestSource_RefusesFileOfNoSourceFormat(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+	jpegImage := generateTestJPEG(t, 10, 10, color.RGBA{0, 0, 255, 255})
+
+	tests := []struct {
+		name    string
+		content []byte
+		wantErr error
+	}{
+		{"text", []byte("this is text, not an image"), magic.ErrUnknownFormat},
+		{"SVG", []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
+			magic.ErrUnknownFormat},
+		{"JPEG cut to 10 bytes", jpegImage[:10], io.EOF},
+		{"empty file", nil, io.EOF},
+	}
+
+	for i, tt := range tests {
+		path := fmt.Sprintf("/refused%d.jpg", i)
+		storeEvictionTestSource(t, cache, testHostCDN, path, tt.content)
+
+		reader, _, format, err := cache.Source(t.Context(),
+			imagerequest.Source{Host: testHostCDN, Path: path})
+		t.Logf("%s: Source() error = %v", tt.name, err)
+
+		if reader != nil || format != "" || !errors.Is(err, tt.wantErr) {
+			t.Errorf("%s: Source() = reader %v, format %q, error %v; "+
+				"want no reader and %v", tt.name, reader, format, err, tt.wantErr)
+		}
+	}
+}

internal/imgcache/storage.go

@@ -1,326 +0,0 @@
-package imgcache
-
-import (
-	"errors"
-	"fmt"
-	"io"
-	"os"
-	"path/filepath"
-
-	"sneak.berlin/go/pixa/internal/imagerequest"
-	"sneak.berlin/go/pixa/internal/magic"
-)
-
-// Storage constants.
-const (
-	// StorageDirPerm is the permission mode for storage directories.
-	StorageDirPerm = 0750
-	// StorageFilePerm is the permission mode for storage files.
-	StorageFilePerm = 0600
-	// MinHashLength is the minimum hash length for path splitting.
-	MinHashLength = 4
-)
-
-// Storage errors.
-var (
-	ErrNotFound = errors.New("content not found")
-)
-
-// ContentStorage handles source image file storage, one file per source
-// URL. Files are stored at: <basedir>/<ab>/<cd>/<source key>
-type ContentStorage struct {
-	baseDir string
-}
-
-// NewContentStorage creates a new content storage at the given base directory.
-func NewContentStorage(baseDir string) (*ContentStorage, error) {
-	err := os.MkdirAll(baseDir, StorageDirPerm)
-	if err != nil {
-		return nil, fmt.Errorf("failed to create storage directory: %w", err)
-	}
-
-	return &ContentStorage{baseDir: baseDir}, nil
-}
-
-// Store writes data with writeFileSynced to the path derived from key,
-// replacing any file already there, and returns its size.
-func (s *ContentStorage) Store(
-	key imagerequest.SourceKey, data []byte,
-) (int64, error) {
-	path := s.keyToPath(key)
-
-	err := os.MkdirAll(filepath.Dir(path), StorageDirPerm)
-	if err != nil {
-		return 0, fmt.Errorf("failed to create directory: %w", err)
-	}
-
-	err = writeFileSynced(path, data)
-	if err != nil {
-		return 0, err
-	}
-
-	return int64(len(data)), nil
-}
-
-// LoadWithSize returns a reader and file size for the content with the given key.
-func (s *ContentStorage) LoadWithSize(
-	key imagerequest.SourceKey,
-) (io.ReadCloser, int64, error) {
-	path := s.keyToPath(key)
-
-	f, err := os.Open(path) //nolint:gosec // path derived from source key
-	if err != nil {
-		if os.IsNotExist(err) {
-			return nil, 0, ErrNotFound
-		}
-
-		return nil, 0, fmt.Errorf("failed to open content: %w", err)
-	}
-
-	stat, err := f.Stat()
-	if err != nil {
-		_ = f.Close()
-
-		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
-	}
-
-	return f, stat.Size(), nil
-}
-
-// Delete removes content with the given key.
-func (s *ContentStorage) Delete(key imagerequest.SourceKey) error {
-	path := s.keyToPath(key)
-
-	err := os.Remove(path)
-	if err != nil && !os.IsNotExist(err) {
-		return fmt.Errorf("failed to delete content: %w", err)
-	}
-
-	return nil
-}
-
-// Exists checks if content with the given key exists.
-func (s *ContentStorage) Exists(key imagerequest.SourceKey) bool {
-	path := s.keyToPath(key)
-	_, err := os.Stat(path)
-
-	return err == nil
-}
-
-// writeFileSynced writes data to path through a temp file in the same
-// directory: the temp file is synced to disk, renamed to path, and the
-// directory synced after, so that neither a concurrent reader nor a
-// crash finds a partial file under path. When the directory sync fails,
-// the file is removed from path, so that a failed write leaves no file
-// there.
-func writeFileSynced(path string, data []byte) error {
-	dir := filepath.Dir(path)
-
-	tmpFile, err := os.CreateTemp(dir, ".tmp-*")
-	if err != nil {
-		return fmt.Errorf("failed to create temp file: %w", err)
-	}
-
-	tmpPath := tmpFile.Name()
-
-	_, err = tmpFile.Write(data)
-	if err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(tmpPath)
-
-		return fmt.Errorf("failed to write temp file: %w", err)
-	}
-
-	err = tmpFile.Sync()
-	if err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(tmpPath)
-
-		return fmt.Errorf("failed to sync temp file: %w", err)
-	}
-
-	err = tmpFile.Close()
-	if err != nil {
-		_ = os.Remove(tmpPath)
-
-		return fmt.Errorf("failed to close temp file: %w", err)
-	}
-
-	err = os.Rename(filepath.Clean(tmpPath), filepath.Clean(path))
-	if err != nil {
-		_ = os.Remove(tmpPath)
-
-		return fmt.Errorf("failed to rename temp file: %w", err)
-	}
-
-	err = syncDirectory(dir)
-	if err != nil {
-		_ = os.Remove(path)
-
-		return err
-	}
-
-	return nil
-}
-
-// syncDirectory syncs the directory dir to disk, so that a file just
-// renamed into it is still there after a crash.
-func syncDirectory(dir string) error {
-	d, err := os.Open(filepath.Clean(dir))
-	if err != nil {
-		return fmt.Errorf("failed to open directory: %w", err)
-	}
-
-	err = d.Sync()
-	_ = d.Close()
-
-	if err != nil {
-		return fmt.Errorf("failed to sync directory: %w", err)
-	}
-
-	return nil
-}
-
-// keyToPath converts a key to a file path: <basedir>/<ab>/<cd>/<key>
-func (s *ContentStorage) keyToPath(key imagerequest.SourceKey) string {
-	k := string(key)
-	if len(k) < MinHashLength {
-		return filepath.Clean(filepath.Join(s.baseDir, k))
-	}
-
-	return filepath.Clean(filepath.Join(s.baseDir, k[0:2], k[2:4], k))
-}
-
-// VariantStorage handles key-based file storage for processed image variants.
-// Files are stored at: <basedir>/<ab>/<cd>/<cache_key>
-type VariantStorage struct {
-	baseDir string
-}
-
-// errNotAnOutputFormat is returned for a variant file whose first bytes are
-// those of a format pixa reads but never writes, such as SVG.
-var errNotAnOutputFormat = errors.New("variant file is not in an output format")
-
-// NewVariantStorage creates a new variant storage at the given base directory.
-func NewVariantStorage(baseDir string) (*VariantStorage, error) {
-	err := os.MkdirAll(baseDir, StorageDirPerm)
-	if err != nil {
-		return nil, fmt.Errorf("failed to create variant storage directory: %w", err)
-	}
-
-	return &VariantStorage{baseDir: baseDir}, nil
-}
-
-// Store writes content to storage at the given key with writeFileSynced.
-func (s *VariantStorage) Store(
-	key imagerequest.VariantKey, r io.Reader,
-) (int64, error) {
-	data, err := io.ReadAll(r)
-	if err != nil {
-		return 0, fmt.Errorf("failed to read content: %w", err)
-	}
-
-	path := s.keyToPath(key)
-
-	err = os.MkdirAll(filepath.Dir(path), StorageDirPerm)
-	if err != nil {
-		return 0, fmt.Errorf("failed to create directory: %w", err)
-	}
-
-	err = writeFileSynced(path, data)
-	if err != nil {
-		return 0, err
-	}
-
-	return int64(len(data)), nil
-}
-
-// LoadWithContentType opens the content at the given key and returns a
-// reader, the file size and the content type, which is read from the
-// file's first bytes. A missing file is ErrNotFound. A file whose first
-// bytes are not those of an output format, an empty one included, is
-// closed again and the reason returned.
-func (s *VariantStorage) LoadWithContentType(
-	key imagerequest.VariantKey,
-) (io.ReadCloser, int64, string, error) {
-	path := s.keyToPath(key)
-
-	f, err := os.Open(path) //nolint:gosec // path derived from cache key
-	if err != nil {
-		if os.IsNotExist(err) {
-			return nil, 0, "", ErrNotFound
-		}
-
-		return nil, 0, "", fmt.Errorf("failed to open content: %w", err)
-	}
-
-	stat, err := f.Stat()
-	if err != nil {
-		_ = f.Close()
-
-		return nil, 0, "", fmt.Errorf("failed to stat content: %w", err)
-	}
-
-	contentType, err := outputContentType(f)
-	if err != nil {
-		_ = f.Close()
-
-		return nil, 0, "", err
-	}
-
-	return f, stat.Size(), contentType, nil
-}
-
-// outputContentType reads the first bytes of content, without moving its
-// offset, and returns the content type of the output format they show.
-func outputContentType(content io.ReaderAt) (string, error) {
-	firstBytes := make([]byte, magic.MinMagicBytes)
-
-	_, err := content.ReadAt(firstBytes, 0)
-	if err != nil {
-		return "", fmt.Errorf("failed to read the first bytes: %w", err)
-	}
-
-	detected, err := magic.DetectFormat(firstBytes)
-	if err != nil {
-		return "", err
-	}
-
-	_, isOutputFormat := magic.MIMEToImageFormat(string(detected))
-	if !isOutputFormat {
-		return "", fmt.Errorf("%w: %s", errNotAnOutputFormat, detected)
-	}
-
-	return string(detected), nil
-}
-
-// Exists checks if content exists at the given key.
-func (s *VariantStorage) Exists(key imagerequest.VariantKey) bool {
-	path := s.keyToPath(key)
-	_, err := os.Stat(path)
-
-	return err == nil
-}
-
-// Delete removes the content at the given key. A missing file is not an
-// error.
-func (s *VariantStorage) Delete(key imagerequest.VariantKey) error {
-	path := s.keyToPath(key)
-
-	err := os.Remove(path)
-	if err != nil && !os.IsNotExist(err) {
-		return fmt.Errorf("failed to delete content: %w", err)
-	}
-
-	return nil
-}
-
-// keyToPath converts a key to a file path: <basedir>/<ab>/<cd>/<key>
-func (s *VariantStorage) keyToPath(key imagerequest.VariantKey) string {
-	k := string(key)
-	if len(k) < MinHashLength {
-		return filepath.Join(s.baseDir, k)
-	}
-
-	return filepath.Join(s.baseDir, k[0:2], k[2:4], k)
-}

internal/imgcache/storage_internal_test.go

@@ -1,156 +0,0 @@
-package imgcache
-
-import (
-	"bytes"
-	"errors"
-	"io"
-	"os"
-	"path/filepath"
-	"testing"
-
-	"sneak.berlin/go/pixa/internal/imagerequest"
-)
-
-func TestContentStorage_StoreAndLoadWithSize(t *testing.T) {
-	t.Parallel()
-
-	tmpDir := t.TempDir()
-
-	storage, err := NewContentStorage(tmpDir)
-	if err != nil {
-		t.Fatalf("NewContentStorage() error = %v", err)
-	}
-
-	content := []byte("hello world")
-	key := imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key()
-
-	size, err := storage.Store(key, content)
-	if err != nil {
-		t.Fatalf("Store() error = %v", err)
-	}
-
-	if size != int64(len(content)) {
-		t.Errorf("Store() size = %d, want %d", size, len(content))
-	}
-
-	// Verify file exists at expected path
-	keyStr := string(key)
-
-	expectedPath := filepath.Join(tmpDir, keyStr[0:2], keyStr[2:4], keyStr)
-
-	_, err = os.Stat(expectedPath)
-	if err != nil {
-		t.Errorf("File not at expected path %s: %v", expectedPath, err)
-	}
-
-	// Load and verify content
-	r, _, err := storage.LoadWithSize(key)
-	if err != nil {
-		t.Fatalf("LoadWithSize() error = %v", err)
-	}
-
-	defer func() { _ = r.Close() }()
-
-	loaded, err := io.ReadAll(r)
-	if err != nil {
-		t.Fatalf("ReadAll() error = %v", err)
-	}
-
-	if !bytes.Equal(loaded, content) {
-		t.Errorf("LoadWithSize() content = %q, want %q", loaded, content)
-	}
-}
-
-func TestContentStorage_LoadWithSizeNotFound(t *testing.T) {
-	t.Parallel()
-
-	tmpDir := t.TempDir()
-
-	storage, err := NewContentStorage(tmpDir)
-	if err != nil {
-		t.Fatalf("NewContentStorage() error = %v", err)
-	}
-
-	_, _, err = storage.LoadWithSize(imagerequest.SourceKey("nonexistent"))
-	if !errors.Is(err, ErrNotFound) {
-		t.Errorf("LoadWithSize() error = %v, want ErrNotFound", err)
-	}
-}
-
-func TestContentStorage_Delete(t *testing.T) {
-	t.Parallel()
-
-	tmpDir := t.TempDir()
-
-	storage, err := NewContentStorage(tmpDir)
-	if err != nil {
-		t.Fatalf("NewContentStorage() error = %v", err)
-	}
-
-	content := []byte("to be deleted")
-	key := imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key()
-
-	_, err = storage.Store(key, content)
-	if err != nil {
-		t.Fatalf("Store() error = %v", err)
-	}
-
-	if !storage.Exists(key) {
-		t.Error("Exists() = false, want true")
-	}
-
-	err = storage.Delete(key)
-	if err != nil {
-		t.Fatalf("Delete() error = %v", err)
-	}
-
-	if storage.Exists(key) {
-		t.Error("Exists() = true after delete, want false")
-	}
-}
-
-func TestContentStorage_DeleteNonexistent(t *testing.T) {
-	t.Parallel()
-
-	tmpDir := t.TempDir()
-
-	storage, err := NewContentStorage(tmpDir)
-	if err != nil {
-		t.Fatalf("NewContentStorage() error = %v", err)
-	}
-
-	// Should not error
-	err = storage.Delete(imagerequest.SourceKey("nonexistent"))
-	if err != nil {
-		t.Errorf("Delete() error = %v, want nil", err)
-	}
-}
-
-func TestContentStorage_KeyToPath(t *testing.T) {
-	t.Parallel()
-
-	tmpDir := t.TempDir()
-
-	storage, err := NewContentStorage(tmpDir)
-	if err != nil {
-		t.Fatalf("NewContentStorage() error = %v", err)
-	}
-
-	// Test by storing and verifying the resulting path structure
-	content := []byte("test content for path verification")
-	key := imagerequest.Source{Host: testHostCDN, Path: testPathCat}.Key()
-
-	_, err = storage.Store(key, content)
-	if err != nil {
-		t.Fatalf("Store() error = %v", err)
-	}
-
-	keyStr := string(key)
-
-	expectedPath := filepath.Join(tmpDir, keyStr[0:2], keyStr[2:4], keyStr)
-
-	_, err = os.Stat(expectedPath)
-	if err != nil {
-		t.Errorf("File not at expected path %s: %v", expectedPath, err)
-	}
-}

internal/magic/jpegxl_internal_test.go

@@ -1,12 +1,17 @@
 package magic
 
-import "testing"
+import (
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
 
 // testMIMEJXL is the MIME type of JPEG XL.
 const testMIMEJXL = "image/jxl"
 
 // TestDetectFormatJPEGXL verifies that both JPEG XL signatures, that of a
-// bare codestream and that of the container, are detected as image/jxl.
+// bare codestream and that of the container, are detected as JPEG XL and
+// match the MIME type image/jxl.
 func TestDetectFormatJPEGXL(t *testing.T) {
 	t.Parallel()
 
@@ -25,25 +30,15 @@ func TestDetectFormatJPEGXL(t *testing.T) {
 			t.Parallel()
 
 			got, err := DetectFormat(tt.data)
-			if err != nil || got != MIMETypeJXL {
-				t.Errorf("DetectFormat() = %q, %v, want %q", got, err, testMIMEJXL)
+			if err != nil || got != imagerequest.FormatJXL {
+				t.Errorf("DetectFormat() = %q, %v, want %q",
+					got, err, imagerequest.FormatJXL)
 			}
 
-			err = ValidateMagicBytes(tt.data, testMIMEJXL)
+			_, err = ValidateMagicBytes(tt.data, testMIMEJXL)
 			if err != nil {
 				t.Errorf("ValidateMagicBytes(%q) error = %v", testMIMEJXL, err)
 			}
 		})
 	}
 }
-
-// TestJPEGXLMIMEType verifies that image/jxl maps to the format jxl.
-func TestJPEGXLMIMEType(t *testing.T) {
-	t.Parallel()
-
-	format, ok := MIMEToImageFormat(testMIMEJXL)
-	if format != FormatJXL || !ok {
-		t.Errorf("MIMEToImageFormat(%q) = %q, %v, want %q, true",
-			testMIMEJXL, format, ok, FormatJXL)
-	}
-}

internal/magic/magic.go

@@ -6,6 +6,8 @@ import (
 	"bytes"
 	"errors"
 	"strings"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // Magic byte errors.
@@ -15,34 +17,6 @@ var (
 	ErrNotEnoughData     = errors.New("not enough data to detect format")
 )
 
-// MIMEType represents a supported MIME type for input images.
-type MIMEType string
-
-// Supported input MIME types.
-const (
-	MIMETypeJPEG = MIMEType("image/jpeg")
-	MIMETypePNG  = MIMEType("image/png")
-	MIMETypeWebP = MIMEType("image/webp")
-	MIMETypeGIF  = MIMEType("image/gif")
-	MIMETypeAVIF = MIMEType("image/avif")
-	MIMETypeJXL  = MIMEType("image/jxl")
-	MIMETypeSVG  = MIMEType("image/svg+xml")
-)
-
-// ImageFormat represents supported output image formats.
-// This mirrors the type in imgcache to avoid circular imports.
-type ImageFormat string
-
-// Supported image output formats.
-const (
-	FormatJPEG ImageFormat = "jpeg"
-	FormatPNG  ImageFormat = "png"
-	FormatWebP ImageFormat = "webp"
-	FormatAVIF ImageFormat = "avif"
-	FormatJXL  ImageFormat = "jxl"
-	FormatGIF  ImageFormat = "gif"
-)
-
 // MinMagicBytes is the minimum number of bytes needed to detect format.
 const MinMagicBytes = 12
 
@@ -78,32 +52,32 @@ var (
 	avisBrand = []byte{0x61, 0x76, 0x69, 0x73} // "avis" (AVIF sequence)
 )
 
-// DetectFormat detects the image format from magic bytes.
-// Returns the MIME type and nil error on success.
-func DetectFormat(data []byte) (MIMEType, error) {
+// DetectFormat detects the image format from magic bytes. Data in none of
+// the formats pixa reads, SVG included, is ErrUnknownFormat.
+func DetectFormat(data []byte) (imagerequest.Format, error) {
 	if len(data) < MinMagicBytes {
 		return "", ErrNotEnoughData
 	}
 
 	// Check JPEG (FFD8FF)
 	if bytes.HasPrefix(data, magicJPEG) {
-		return MIMETypeJPEG, nil
+		return imagerequest.FormatJPEG, nil
 	}
 
 	// Check PNG (89504E47 0D0A1A0A)
 	if bytes.HasPrefix(data, magicPNG) {
-		return MIMETypePNG, nil
+		return imagerequest.FormatPNG, nil
 	}
 
 	// Check GIF (GIF87a or GIF89a)
 	if bytes.HasPrefix(data, magicGIF) {
-		return MIMETypeGIF, nil
+		return imagerequest.FormatGIF, nil
 	}
 
 	// Check WebP (RIFF....WEBP)
 	if bytes.HasPrefix(data, magicWebP) && len(data) >= 12 {
 		if bytes.Equal(data[8:12], webpIdent) {
-			return MIMETypeWebP, nil
+			return imagerequest.FormatWebP, nil
 		}
 	}
 
@@ -112,66 +86,39 @@ func DetectFormat(data []byte) (MIMEType, error) {
 	if len(data) >= 12 && bytes.Equal(data[4:8], magicFtyp) {
 		brand := data[8:12]
 		if bytes.Equal(brand, avifBrand) || bytes.Equal(brand, avisBrand) {
-			return MIMETypeAVIF, nil
+			return imagerequest.FormatAVIF, nil
 		}
 	}
 
 	// Check JPEG XL (FF0A, or the 12-byte container signature)
 	if bytes.HasPrefix(data, magicJXLCodestream) ||
 		bytes.HasPrefix(data, magicJXLContainer) {
-		return MIMETypeJXL, nil
-	}
-
-	// Check SVG - look for XML declaration or SVG tag
-	if detectSVG(data) {
-		return MIMETypeSVG, nil
+		return imagerequest.FormatJXL, nil
 	}
 
 	return "", ErrUnknownFormat
 }
 
-// detectSVG checks if data appears to be SVG content.
-func detectSVG(data []byte) bool {
-	// Skip BOM if present
-	content := skipBOM(data)
-
-	// Convert to string for easier pattern matching
-	s := strings.ToLower(string(content))
-
-	// Skip leading whitespace
-	s = strings.TrimSpace(s)
-
-	// Check for XML declaration or SVG element
-	return strings.HasPrefix(s, "<?xml") ||
-		strings.HasPrefix(s, "<svg") ||
-		strings.HasPrefix(s, "<!doctype svg")
-}
-
-// skipBOM removes UTF-8 BOM if present.
-func skipBOM(data []byte) []byte {
-	if len(data) >= 3 && data[0] == 0xEF && data[1] == 0xBB && data[2] == 0xBF {
-		return data[3:]
-	}
-
-	return data
-}
-
-// ValidateMagicBytes validates that the content matches the declared MIME type.
-func ValidateMagicBytes(data []byte, declaredType string) error {
+// ValidateMagicBytes detects the format of data from its magic bytes, and
+// returns it when it is the format the declared MIME type names. Data in
+// another format is ErrMagicByteMismatch.
+func ValidateMagicBytes(
+	data []byte, declaredType string,
+) (imagerequest.Format, error) {
 	detected, err := DetectFormat(data)
 	if err != nil {
-		return err
+		return "", err
 	}
 
 	// Normalize the declared type (remove parameters like charset)
 	normalizedDeclared := normalizeMIMEType(declaredType)
 
 	// Check if they match
-	if string(detected) != normalizedDeclared {
-		return ErrMagicByteMismatch
+	if detected.MIMEType() != normalizedDeclared {
+		return "", ErrMagicByteMismatch
 	}
 
-	return nil
+	return detected, nil
 }
 
 // normalizeMIMEType extracts just the media type, removing parameters.
@@ -183,27 +130,3 @@ func normalizeMIMEType(mimeType string) string {
 
 	return strings.TrimSpace(strings.ToLower(mimeType))
 }
-
-// MIMEToImageFormat converts a MIME type to an ImageFormat.
-func MIMEToImageFormat(mimeType string) (ImageFormat, bool) {
-	normalized := normalizeMIMEType(mimeType)
-	switch MIMEType(normalized) {
-	case MIMETypeJPEG:
-		return FormatJPEG, true
-	case MIMETypePNG:
-		return FormatPNG, true
-	case MIMETypeWebP:
-		return FormatWebP, true
-	case MIMETypeGIF:
-		return FormatGIF, true
-	case MIMETypeAVIF:
-		return FormatAVIF, true
-	case MIMETypeJXL:
-		return FormatJXL, true
-	case MIMETypeSVG:
-		// SVG has no corresponding output format.
-		return "", false
-	default:
-		return "", false
-	}
-}

internal/magic/magic_internal_test.go

@@ -1,9 +1,10 @@
 package magic
 
 import (
-	"bytes"
 	"errors"
 	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
 )
 
 // Shared test fixture strings.
@@ -12,9 +13,6 @@ const (
 	testMIMEJPEG       = "image/jpeg"
 	testMIMEJPEGParams = "image/jpeg; charset=utf-8"
 	testMIMEPNG        = "image/png"
-	testMIMEWebP       = "image/webp"
-	testMIMEGIF        = "image/gif"
-	testMIMEAVIF       = "image/avif"
 )
 
 // pad appends zero bytes so data is comfortably above MinMagicBytes.
@@ -36,37 +34,22 @@ func TestDetectFormat(t *testing.T) {
 	avis := pad(0x00, 0x00, 0x00, 0x1C, 0x66, 0x74, 0x79, 0x70, 0x61, 0x76, 0x69, 0x73)
 
 	tests := []struct {
-		name     string
-		data     []byte
-		wantMIME MIMEType
-		wantErr  error
+		name       string
+		data       []byte
+		wantFormat imagerequest.Format
+		wantErr    error
 	}{
-		{name: "JPEG", data: jpeg, wantMIME: MIMETypeJPEG},
-		{name: "PNG", data: png, wantMIME: MIMETypePNG},
-		{name: "GIF87a", data: gif87a, wantMIME: MIMETypeGIF},
-		{name: "GIF89a", data: gif89a, wantMIME: MIMETypeGIF},
-		{name: "WebP", data: webp, wantMIME: MIMETypeWebP},
-		{name: "AVIF", data: avif, wantMIME: MIMETypeAVIF},
-		{name: "AVIF sequence", data: avis, wantMIME: MIMETypeAVIF},
+		{name: "JPEG", data: jpeg, wantFormat: imagerequest.FormatJPEG},
+		{name: "PNG", data: png, wantFormat: imagerequest.FormatPNG},
+		{name: "GIF87a", data: gif87a, wantFormat: imagerequest.FormatGIF},
+		{name: "GIF89a", data: gif89a, wantFormat: imagerequest.FormatGIF},
+		{name: "WebP", data: webp, wantFormat: imagerequest.FormatWebP},
+		{name: "AVIF", data: avif, wantFormat: imagerequest.FormatAVIF},
+		{name: "AVIF sequence", data: avis, wantFormat: imagerequest.FormatAVIF},
 		{
-			name:     "SVG with XML declaration",
-			data:     []byte(`<?xml version="1.0"?><svg></svg>`),
-			wantMIME: MIMETypeSVG,
-		},
-		{
-			name:     "SVG without declaration",
-			data:     []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			wantMIME: MIMETypeSVG,
-		},
-		{
-			name:     "SVG with whitespace",
-			data:     []byte(`   <?xml version="1.0"?><svg></svg>`),
-			wantMIME: MIMETypeSVG,
-		},
-		{
-			name:     "SVG with BOM",
-			data:     append([]byte{0xEF, 0xBB, 0xBF}, []byte(`<svg></svg>`)...),
-			wantMIME: MIMETypeSVG,
+			name:    "SVG",
+			data:    []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
+			wantErr: ErrUnknownFormat,
 		},
 		{
 			name:    "unknown format",
@@ -88,8 +71,8 @@ func TestDetectFormat(t *testing.T) {
 				return
 			}
 
-			if got != tt.wantMIME {
-				t.Errorf("DetectFormat() = %v, want %v", got, tt.wantMIME)
+			if got != tt.wantFormat {
+				t.Errorf("DetectFormat() = %v, want %v", got, tt.wantFormat)
 			}
 		})
 	}
@@ -105,24 +88,28 @@ func TestValidateMagicBytes(t *testing.T) {
 		name         string
 		data         []byte
 		declaredType string
+		wantFormat   imagerequest.Format
 		wantErr      error
 	}{
 		{
 			name:         "matching JPEG",
 			data:         jpegData,
 			declaredType: testMIMEJPEG,
+			wantFormat:   imagerequest.FormatJPEG,
 			wantErr:      nil,
 		},
 		{
 			name:         "matching JPEG with params",
 			data:         jpegData,
 			declaredType: testMIMEJPEGParams,
+			wantFormat:   imagerequest.FormatJPEG,
 			wantErr:      nil,
 		},
 		{
 			name:         "matching PNG",
 			data:         pngData,
 			declaredType: testMIMEPNG,
+			wantFormat:   imagerequest.FormatPNG,
 			wantErr:      nil,
 		},
 		{
@@ -143,45 +130,14 @@ func TestValidateMagicBytes(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			err := ValidateMagicBytes(tt.data, tt.declaredType)
+			got, err := ValidateMagicBytes(tt.data, tt.declaredType)
 
 			if !errors.Is(err, tt.wantErr) {
 				t.Errorf("ValidateMagicBytes() error = %v, wantErr %v", err, tt.wantErr)
 			}
-		})
-	}
-}
-
-func TestMIMEToImageFormat(t *testing.T) {
-	t.Parallel()
-
-	tests := []struct {
-		mimeType   string
-		wantFormat ImageFormat
-		wantOk     bool
-	}{
-		{testMIMEJPEG, FormatJPEG, true},
-		{testMIMEPNG, FormatPNG, true},
-		{testMIMEWebP, FormatWebP, true},
-		{testMIMEGIF, FormatGIF, true},
-		{testMIMEAVIF, FormatAVIF, true},
-		{"image/svg+xml", "", false}, // SVG doesn't convert to ImageFormat
-		{"image/tiff", "", false},
-		{"text/plain", "", false},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.mimeType, func(t *testing.T) {
-			t.Parallel()
-
-			got, ok := MIMEToImageFormat(tt.mimeType)
-
-			if ok != tt.wantOk {
-				t.Errorf("MIMEToImageFormat(%q) ok = %v, want %v", tt.mimeType, ok, tt.wantOk)
-			}
 
 			if got != tt.wantFormat {
-				t.Errorf("MIMEToImageFormat(%q) = %v, want %v", tt.mimeType, got, tt.wantFormat)
+				t.Errorf("ValidateMagicBytes() = %q, want %q", got, tt.wantFormat)
 			}
 		})
 	}
@@ -214,109 +170,6 @@ func TestNormalizeMIMEType(t *testing.T) {
 	}
 }
 
-func TestDetectSVG(t *testing.T) {
-	t.Parallel()
-
-	tests := []struct {
-		name string
-		data string
-		want bool
-	}{
-		{"xml declaration", `<?xml version="1.0"?><svg></svg>`, true},
-		{"svg element", `<svg xmlns="http://www.w3.org/2000/svg"></svg>`, true},
-		{
-			"doctype",
-			`<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" ` +
-				`"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">`,
-			true,
-		},
-		{"with whitespace", `
-  <?xml version="1.0"?><svg></svg>`, true},
-		{"uppercase", `<SVG></SVG>`, true},
-		{"not svg", `<html></html>`, false},
-		{"random text", `hello world`, false},
-		{testNameEmpty, ``, false},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			got := detectSVG([]byte(tt.data))
-
-			if got != tt.want {
-				t.Errorf("detectSVG(%q) = %v, want %v", tt.data, got, tt.want)
-			}
-		})
-	}
-}
-
-func TestSkipBOM(t *testing.T) {
-	t.Parallel()
-
-	tests := []struct {
-		name string
-		data []byte
-		want []byte
-	}{
-		{"with BOM", []byte{0xEF, 0xBB, 0xBF, 'h', 'e', 'l', 'l', 'o'}, []byte("hello")},
-		{"without BOM", []byte("hello"), []byte("hello")},
-		{testNameEmpty, []byte{}, []byte{}},
-		{"only BOM", []byte{0xEF, 0xBB, 0xBF}, []byte{}},
-		{"partial BOM", []byte{0xEF, 0xBB}, []byte{0xEF, 0xBB}},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			got := skipBOM(tt.data)
-
-			if !bytes.Equal(got, tt.want) {
-				t.Errorf("skipBOM() = %v, want %v", got, tt.want)
-			}
-		})
-	}
-}
-
-func TestRealWorldSVGPatterns(t *testing.T) {
-	t.Parallel()
-
-	// Test various real-world SVG patterns
-	svgPatterns := []string{
-		`<?xml version="1.0" encoding="UTF-8"?>
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
-  <circle cx="50" cy="50" r="40"/>
-</svg>`,
-		`<svg viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
-  <path d="M12 2L2 7l10 5 10-5-10-5z"/>
-</svg>`,
-		`<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" ` +
-			`"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">` + `
-<svg xmlns="http://www.w3.org/2000/svg">
-</svg>`,
-	}
-
-	for i, pattern := range svgPatterns {
-		data := []byte(pattern)
-		if len(data) < MinMagicBytes {
-			// Pad short SVGs for detection
-			data = append(data, make([]byte, MinMagicBytes-len(data))...)
-		}
-
-		got, err := DetectFormat(data)
-		if err != nil {
-			t.Errorf("Pattern %d: DetectFormat() error = %v", i, err)
-
-			continue
-		}
-
-		if got != MIMETypeSVG {
-			t.Errorf("Pattern %d: DetectFormat() = %v, want %v", i, got, MIMETypeSVG)
-		}
-	}
-}
-
 func TestDetectFormatRIFFNotWebP(t *testing.T) {
 	t.Parallel()
 

internal/providers/providers.go

@@ -227,7 +227,7 @@ type imageServiceParams struct {
 func newImageService(
 	_ fx.Lifecycle, params imageServiceParams,
 ) (*imageproxy.Proxy, error) {
-	return imageproxy.NewProxy(&imageproxy.Config{
+	return imageproxy.New(&imageproxy.Config{
 		Cache:           params.Cache,
 		Fetcher:         params.Fetcher,
 		Processor:       params.Processor,