Count what the cache holds in the default cache_max_bytes (closes #184)

The default limit was 75% of the space free at startup. The cache's
own files are not free space, so a fuller cache got a smaller limit
after a restart and eviction then deleted most of it. The default is
now 75% of the sum of the free space and what the cache already holds
by its own size accounting, at least 500 MiB. The cache works it out
when it opens, after the database is open, so the computation and its
tests moved from internal/config to internal/imgcache; the config only
records whether cache_max_bytes was set, and the handlers turn the
disk cache off only for an explicit 0.

Model: opus-5-5

README.md

@@ -34,6 +34,66 @@ else has a built-in default. A config file mounted at `/etc/pixa/config.yml`
 is optional: it is read when present, and an environment variable wins over
 the same setting in it.
 
+## Deployment
+
+pixa listens on plain HTTP and runs behind a reverse proxy that terminates TLS.
+[`configs/Caddyfile`](configs/Caddyfile) is an example for Caddy, chosen because
+it is the smallest correct one: Caddy gets the TLS certificate itself and does
+everything in this list without further settings. The reverse proxy must:
+
+- terminate TLS, as the login and generator pages work only over HTTPS (see
+  Routes);
+- pass the `Host`, `Origin` and `Referer` headers on unchanged, as pixa refuses
+  a form from those pages unless `Origin` or `Referer` names the host in `Host`,
+  and builds encrypted URLs from `Host`;
+- set `X-Forwarded-For` to the client's address, with `trusted_proxies` set to
+  the address pixa sees the proxy's requests come from, so the login limit
+  counts each client by its own address (see `trusted_proxies` under
+  Configuration);
+- wait for pixa's answer for at least `downstream_timeout` (default `60s`), the
+  longest pixa takes to fetch, convert and send an image.
+
+It may also refuse `/metrics`, as the example does, so that only a scraper that
+reaches pixa directly can read it; pixa itself asks for the metrics username and
+password there.
+
+pixa does the rest itself: it checks signatures and encrypted URLs, applies the
+allowlist, refuses upstream hosts with private or local addresses, limits login
+attempts, upstream response size and image dimensions, and sends the security
+headers, `Strict-Transport-Security` included, with every response.
+
+The state directory (`state_dir`, `/var/lib/pixa` in the container) holds the
+database and the disk cache:
+
+- It needs a persistent volume: without one, every restart starts with an empty
+  cache. In the container, the startup script gives the directory to the user
+  pixa runs as (uid 65532) and sets its mode to `750`; outside it, that user
+  must be able to write the directory.
+- `cache_max_bytes` limits the source and transformed images together. The
+  database, the metadata files, the `.meta` file beside each transformed image
+  and files still being written come on top, and eviction runs in the
+  background, so the cache can pass the limit for a while: leave room on the
+  volume beyond it.
+- Set `cache_max_bytes` for a lasting deployment. Its default, worked out each
+  time pixa starts, is 75% of the sum of the space free on the volume and the
+  space the cached images already take, so a restart keeps the limit the cache
+  had, but anything else that fills or frees space on the volume moves it.
+
+A load balancer's health check can request `/.well-known/healthcheck.json`,
+which answers 200 whenever pixa is running, in maintenance mode too (see
+`maintenance_mode`).
+
+On SIGTERM or SIGINT pixa stops accepting connections, gives the requests in
+progress and the images being processed 5 seconds to finish, and exits: with 0,
+or with 1 when images were still being processed after those 5 seconds or
+another part of pixa failed to stop. A request not finished by then is cut off.
+`docker stop` waits 10 seconds before it kills the container.
+
+Outside Docker, pixa needs libvips (the image has 8.15) and libheif to run, as
+it uses libvips through CGO; building it also needs their development files,
+`pkg-config` and a C compiler. `script/bootstrap` installs all of these with
+nix, apt, brew or apk.
+
 ## Running under upaas
 
 What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
@@ -49,7 +109,7 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
     - `PIXA_ALLOWLIST_HOSTS`: upstream hosts served without a signature,
       comma-separated
     - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it;
-      default 75% of free space
+      default 75% of (free space + what the cache holds)
     - the rest are in the table under Configuration below
 - **Health check:** the image's `HEALTHCHECK` requests
   `/.well-known/healthcheck.json`. upaas reads the container's health 60
@@ -71,13 +131,17 @@ prevent abuse, and allowlisted source hosts for open access.
 ### Storage
 
 - **Source content**:
-  `<statedir>/cache/src-content/<ab>/<cd>/<sha256 of source content>`
+  `<state_dir>/cache/sources/<ab>/<cd>/<sha256 of source content>`
 - **Source metadata**:
-  `<statedir>/cache/src-metadata/<hostname>/<sha256 of path>.json`
-  (fetch time, original headers, request, content hash)
-- **Database**: `<statedir>/state.sqlite3` (SQLite)
-- **Output documents**:
-  `<statedir>/cache/dst-content/<ab>/<cd>/<sha256 of output content>`
+  `<state_dir>/cache/metadata/<hostname>/<sha256 of path and query>.json`
+  (host, path and query, content hash, upstream status and headers, fetch time)
+- **Database**: `<state_dir>/state.sqlite3` (SQLite)
+- **Transformed images**:
+  `<state_dir>/cache/variants/<ab>/<cd>/<sha256 of host, path, query, size, format, quality and fit>`,
+  each with a `.meta` file beside it holding its content type
+
+`<ab>` and `<cd>` are the first and second pairs of characters of the file's
+name.
 
 Multiple source paths may reference the same content blob; the
 database tracks references rather than using filesystem refcounting.
@@ -88,17 +152,96 @@ the metadata file stored beside it.
 
 ### Routes
 
+pixa answers these routes; any other path answers 404. A path in this list asked
+with a method the list does not give answers 405, except `/static/<file>`, which
+answers any method as it answers `GET`. A browser's CORS preflight request
+(`OPTIONS` with `Origin` and `Access-Control-Request-Method` headers) to any
+path under `/v1/` answers 200, in maintenance mode too.
+
+- `GET /` — the login page, or the URL generator page with a login session
+  (see Encrypted URLs). Needs: nothing. Answers: 200.
+- `POST /` — log in with the signing key typed into the login page. Needs: the
+  login page's form (below). Answers: 303 to `/` with a login session cookie
+  that lasts 30 days for the right key; 200 with the login page and an error for
+  a wrong key; 429 over the login limit (below).
+- `POST /generate` — make an encrypted URL from the generator page's form.
+  Needs: a login session and the generator page's form (below); without a login
+  session it answers 303 to `/`. Answers: 200 with the page showing the URL; 400
+  with the page naming a field that is not valid; 500 when the URL cannot be
+  made.
+- `GET /logout` — end the login session. Needs: nothing. Answers: 303 to `/`.
+- `GET` or `HEAD` `/v1/image/<host>/<path>/<size>.<format>` — an image, fetched,
+  resized and converted (below). Needs: a signature, unless the host is
+  allowlisted (see Source Hosts). Answers: 200; 304 when `If-None-Match` matches
+  the image's `ETag`; 400 for a URL or parameter that is not valid; 401 for a
+  missing or wrong signature, a missing `exp` or an `exp` in the past; 403 when
+  the upstream host, or a host it redirects to, is `localhost`, ends in
+  `.localhost` or `.local`, or has an address in a blocked network (see
+  `blocked_networks`); 502 when the upstream answered with an error status, and
+  for 5 minutes after that for the same source URL; 503 when pixa is busy or in
+  maintenance mode; 500 for any other failure.
+- `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
+  (see Encrypted URLs). Needs: nothing but the URL. Answers: 200; 304 when
+  `If-None-Match` matches the image's `ETag`; 400 for a token that does not
+  decrypt, or that asks for a size or fit that is not valid; 410 once it has
+  expired; 504 when the upstream has not sent its response headers within
+  `upstream_fetch_timeout`, but 500 when that time runs out while the image
+  itself is still arriving; 403, 502, 503 and 500 as for `/v1/image/`.
+- `GET /robots.txt` — asks every crawler to stay away (`Disallow: /`). Needs:
+  nothing. Answers: 200.
+- `GET /.well-known/healthcheck.json` — JSON with `status` (`ok`), `now`,
+  `uptime_seconds`, `uptime_human`, `version`, `appname` and
+  `maintenance_mode`. Needs: nothing. Answers: 200, always.
+- `GET /static/<file>` — the script the login and generator pages load. Needs:
+  nothing. Answers: 200, or 404 for a file that does not exist.
+- `GET /metrics` — Prometheus metrics (see Architecture). Needs: HTTP basic
+  authentication with `metrics.username` and `metrics.password`. Answers: 200;
+  401 without them; 404 when they are not set, as the route then does not exist.
+
+Every response carries an `X-Request-ID` header holding the request's ID, which
+a client can quote when reporting a problem: the request's own `X-Request-ID`,
+as a reverse proxy in front of pixa may send, when it is at most 64 letters,
+digits, `-`, `_` or `.`; otherwise a random one pixa makes for the request,
+which tells nothing about the machine or the other requests. pixa's log line for
+the request carries the same ID as `request_id`, and so do the lines it logs
+when it fetches, converts and serves an image; the fetch sends it to the
+upstream host as `X-Request-ID`.
+
+Both `POST` routes accept only a form that pixa's own page served: the page puts
+a token in the form and sets a cookie to match, and a request without both is
+refused with 403, so another site cannot submit the form from a visitor's
+browser. The login and generator pages are meant to be opened over HTTPS: while
+`debug` is off, a form sent from a page opened over plain HTTP is refused with
+403, and while it is on, so is one sent from a page opened over HTTPS. Plain
+HTTP is for development on the browser's own machine: the login session cookie
+is always marked `Secure`, and over plain HTTP a browser keeps such a cookie
+only for its own machine (`localhost`), if at all. A form is also refused with
+403 when the page's host is not the `Host` header pixa receives, so a reverse
+proxy in front of pixa must pass that header on unchanged. A form body over
+1 MiB is refused with 413. The image routes answer the errors listed for them
+with JSON holding `error`, `status` and `timestamp`.
+
+An image URL has this form:
+
 ```
-/v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>
+/v1/image/<host>/<path>/<size>.<format>?sig=<signature>&exp=<expiration>&q=<quality>&fit=<fit>
 ```
 
-Images are only fetched from origins using TLS with valid certificates.
+Images are only fetched from origins using TLS with valid certificates, unless
+`allow_http` is set: then pixa fetches every image over plain HTTP, which is for
+testing only.
 
 A request whose query string cannot be decoded, or gives any parameter more
 than once, is refused with 400.
 
-- `<format>`: one of `orig`, `png`, `jpeg`, `webp`
+- `<format>`: one of `orig` (or `original`), `jpeg` (or `jpg`), `png`, `webp`,
+  `avif`, `gif`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
+- `sig` and `exp`: the signature and its expiry, needed unless the host is
+  allowlisted (see Signature Specification)
+- `q` and `fit`: the output quality and how the image is fitted to `<size>`,
+  both optional (values under Signature Specification). Both are part of what
+  is cached, so each value of either is a separate cached image.
 
 An image is served with `Cache-Control: public, max-age=<seconds>, immutable`.
 When the URL has an expiry (an `exp`, or the TTL of an encrypted URL),
@@ -113,7 +256,9 @@ source) and one transcode: the first request does the work, and the others wait
 for its image or its error, holding no upstream connection or processing slot
 of their own. A waiting request stops waiting when its own client goes away.
 The work goes on for the others even if the first request's client goes away,
-until that request's `downstream_timeout` ends.
+until that request's `downstream_timeout` ends. The shared fetch sends the first
+request's ID upstream, and the lines logged for the fetch and the transcode
+carry that ID.
 
 The login form (`POST /`) is limited to 5 attempts per minute per client
 address, counting an IPv6 client by its /64; an attempt over the limit is
@@ -132,6 +277,39 @@ its own `X-Forwarded-For`, whether it connects directly or through the proxy,
 because its own address is trusted too. Setting `trusted_proxies` to only the
 address pixa sees for requests that come through the proxy closes this.
 
+### Encrypted URLs
+
+An encrypted URL is an image URL made on pixa's own web page by someone who
+knows the signing key. It works for any upstream host, allowlisted or not,
+without a signature, and whoever gets it can neither read the source URL from it
+nor change what it asks for.
+
+1. Open `/` in a browser over HTTPS (or over plain HTTP while `debug` is on, see
+   Routes) and log in with the signing key (`signing_key`). The login session
+   lasts 30 days, or until `/logout`.
+2. On the generator page, give the source image's URL, the width and height, the
+   format, quality and fit, and how long the URL lasts, then submit the form
+   (`POST /generate`). Width and height both empty or `0` keep the original
+   size; if only one of them is empty or `0`, that side is scaled to keep the
+   image's proportions.
+3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
+   it expires. `<host>` is the host the page was opened on, and the URL starts
+   with `http` instead while `debug` is on. The name after the token is ignored
+   and only gives the URL a file extension, `jpg` for `orig`.
+
+The token holds the source's host, path and query and the size, format,
+quality, fit and expiry, encrypted with a key derived from `signing_key`. The
+source URL's scheme is not kept: the image is fetched like any other (see
+Routes), and the blocked networks still apply.
+
+How long the URL lasts is chosen on the page, from 1 minute to 1 year, or
+never. The expiry is fixed in the token when the URL is made and cannot be
+changed or revoked afterwards. Until then the image is served with a `max-age`
+that ends at the expiry (see Routes); after it the URL answers 410
+`URL has expired`. A URL made to last forever stops working only when
+`signing_key` changes: changing it makes every encrypted URL already handed out
+answer 400, and ends every login session.
+
 ### Image Metadata
 
 pixa decodes and re-encodes every image it serves, and removes all metadata from
@@ -173,7 +351,8 @@ Where:
 - `query` — source query string, empty string if none
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
-- `format` — output format (jpeg, png, webp, avif, gif, orig)
+- `format` — output format, one of those listed under Routes, with `original`
+  signed as `orig` and `jpg` as `jpeg`
 - `expiration` — the URL's `exp` query parameter, the Unix timestamp when
   the signature expires; a request whose `exp` is not a whole number, an
   empty `exp=` included, is refused with 400
@@ -229,13 +408,24 @@ 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 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, one it finds but cannot read or parse aborts startup; one it cannot
+find, for any reason, is passed over without a message, even when the file is
+there in a directory pixa may not enter. 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 space        |
+| `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_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 |
@@ -300,8 +490,10 @@ Key settings in more detail:
   each), so keep it longer than `upstream_fetch_timeout` plus 20 seconds
 - `signing_key` — HMAC secret for URL signatures
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
-  disk cache entirely; omitted defaults to 75% of the free space on
-  the filesystem containing `<state_dir>/cache/` (minimum 500 MiB)
+  disk cache entirely; omitted defaults to 75% of the sum of the free space on
+  the filesystem containing `<state_dir>/cache/` and the bytes of source and
+  transformed images the cache already holds, worked out at startup (minimum
+  500 MiB)
 - `upstream_connections` — the most connections to upstream hosts at once, all
   hosts together, on top of `upstream_connections_per_host`; default `64`. A
   fetch holds its connection until its image has been processed. A fetch that
@@ -314,12 +506,12 @@ Key settings in more detail:
   seconds for one to free up; if none does, and `downstream_timeout` has not
   ended first, it is answered 503 the same way
 - `maintenance_mode` — while `true`, the image routes (`/v1/image/` and
-  `/v1/e/`) answer every request with 503, a `Retry-After` header and a JSON
-  error body. The health check (`/.well-known/healthcheck.json`) still answers
-  200 and reports `"maintenance_mode": true`. It stays 200 because the image's
-  Docker `HEALTHCHECK` requests it: a 503 there would make the container
-  unhealthy, and upaas marks a deploy failed when its container is unhealthy.
-  The login and URL generator pages and `/metrics` keep working
+  `/v1/e/`) answer every request for an image with 503, a `Retry-After` header
+  and a JSON error body. The health check (`/.well-known/healthcheck.json`)
+  still answers 200 and reports `"maintenance_mode": true`. It stays 200
+  because the image's Docker `HEALTHCHECK` requests it: a 503 there would make
+  the container unhealthy, and upaas marks a deploy failed when its container
+  is unhealthy. The login and URL generator pages and `/metrics` keep working
 
 See `config.example.yml` for all options with defaults.
 
@@ -330,7 +522,10 @@ See `config.example.yml` for all options with defaults.
 - **Image processing**: govips (CGO wrapper for libvips)
 - **Database**: SQLite via modernc.org/sqlite
 - **Static assets**: embedded via `//go:embed`
-- **Metrics**: Prometheus
+- **Metrics**: Prometheus, at `/metrics`: generic HTTP request metrics
+  (duration, response size, requests in flight) and the Go runtime and process
+  metrics; requests are measured and `/metrics` is served only when
+  `metrics.username` and `metrics.password` are set
 - **Logging**: stdlib slog
 
 ## Entrypoints

TODO.md

@@ -29,6 +29,107 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 the default `cache_max_bytes` no longer shrinks as the cache fills
+  (closes #184): for an omitted key, the cache works out the limit when it
+  opens, after the database is open, as 75% of the sum of the free space on the
+  filesystem containing `<state_dir>/cache/` and what the cache already holds by
+  its own size accounting, at least 500 MiB, so a cache filled to its limit
+  keeps that limit across a restart. The computation and its tests moved from
+  `internal/config` to `internal/imgcache`; the config only records whether the
+  key was set.
+- 2026-10-04 `TestEvictionRunsOnPeriodicSchedule` no longer races the evictor
+  (closes #183): it wrote each variant file and then inserted its accounting row
+  by hand, and a reconciliation pass between the two adopted the file first, so
+  the insert failed. It now writes the files only, while holding the test
+  database's only connection so the evictor's startup pass waits after walking
+  the empty variant directory; a periodic reconciliation pass then adopts the
+  files and the eviction pass after it evicts them. No other test in
+  `internal/imgcache` inserts a row by hand after starting the evictor. Test
+  only.
+- 2026-10-04 deployment guide and example Caddy config (closes #89):
+  "Deployment" in `README.md` says what the reverse proxy in front of pixa must
+  do (terminate TLS; pass `Host`, `Origin` and `Referer` on unchanged; set
+  `X-Forwarded-For`, with `trusted_proxies` to match; wait at least
+  `downstream_timeout`; optionally refuse `/metrics`) and what pixa does itself,
+  that the state directory needs a persistent volume and what `cache_max_bytes`
+  counts, the health check for a load balancer, what a stop does and its exit
+  codes, and what running outside Docker needs; `configs/Caddyfile` is the
+  example, checked with `caddy validate`.
+- 2026-10-04 the metrics basic auth, CORS preflight, request logging and
+  metrics recording have tests (closes #79): `MetricsAuth` on its own answers
+  401 with a challenge without credentials or with a wrong username or password
+  and lets the configured ones through; a preflight request gets `*` for any
+  origin when `access_control_allow_origin` is `*` and no
+  `Access-Control-Allow-Origin` from another origin than the configured one; a
+  `POST /` carrying the signing key leaves no trace of it in the request log
+  line, and the login handler's own log lines leave out the submitted key; the
+  metrics middleware on its own records a request it served, and the router
+  records nothing while no metrics username is set. Not tested: that the router
+  puts the basic auth in front of `/metrics` and records requests when a
+  metrics username is set. Only one test per package can set up `/metrics`, and
+  in `internal/server` that is `TestMaintenanceModeKeepsOtherRoutes`, which
+  needs the owner's approval to change; #180 holds it. Tests only; the basic
+  auth library already compares the password in constant time.
+- 2026-10-04 the image route's signature check and error answers are tested
+  (closes #76): new tests in `internal/handlers`, with no network, check the
+  status and JSON error body for a missing, wrong, unpadded, upper-case or
+  expired signature on a host not on the allowlist, or a valid one sent for
+  its parent domain, a sibling host, a subdomain or the host with another
+  domain appended (401), an unparseable path (400), `localhost` as the
+  upstream host (403) and an upstream error (502); that an allowlisted host is
+  served without a signature, another host only with a valid one; and the
+  answers of `/robots.txt` and the health check. No code changes.
+- 2026-10-04 request IDs returned and passed on, and `/v1/e/` revalidates
+  (closes #84): pixa's own `RequestID` middleware, in place of chi's, gives each
+  request an ID, its own `X-Request-ID` when that is at most 64 letters, digits,
+  `-`, `_` or `.` and a random one otherwise, stores it where chi's did and
+  sends it back as `X-Request-ID` on every response; the upstream fetch sends
+  that ID, and the "upstream fetched", "image converted" and "image served" log
+  lines carry it as `request_id`, a fetch shared by several requests carrying
+  the first request's; `/v1/e/` sets `ETag`, answers a matching `If-None-Match`
+  with 304 and is routed for `HEAD`, the `ETag` and 304 code being
+  `notModified`, which `/v1/image/` calls too; its token checks moved unchanged
+  into `parseImageEncRequest` to keep `HandleImageEnc` within the line limit; no
+  `Vary` is added, as no response depends on a request header except the image
+  routes' CORS headers, for which `go-chi/cors` already sends `Vary: Origin`;
+  `Vary: Accept` is left to #88.
+- 2026-10-04 routes, encrypted URLs and config file documented (closes #75):
+  "Routes" in `README.md` lists every route with its method, purpose, what it
+  needs and the status codes it answers with, and says `q` and `fit` are part
+  of what is cached; "Encrypted URLs" covers logging in, making one on the
+  generator page, how long it lasts and the 410 once it has expired;
+  "Configuration" gives the order in which pixa looks for its config file;
+  `config.example.yml` lists `db_url` and `env` and gives every key's default;
+  `scripts/manual-test.sh` is left to #97.
+- 2026-10-04 shutdown stops cache eviction in progress (closes #102):
+  `StartEviction` runs the eviction goroutine with its own context, which
+  `StopEviction` cancels, so a pass in progress stops at its next database
+  call, file, row or eviction candidate instead of running to completion, and
+  no pass starts after it, so a stop logs at most one warning;
+  `StopEviction` takes a context and, when that context ends before the
+  goroutine exits, stops waiting and returns its error; the handlers' stop hook
+  passes fx's stop context, so an eviction still running when fx's stop
+  deadline ends fails the stop and makes the exit code 1.
+- 2026-10-04 dead code in `internal/imgcache` is gone (closes #73): `Purge`,
+  which only returned an error and which nothing called, is no longer part of
+  the `ImageCache` interface or `Service`; the `SignatureValidator`,
+  `Allowlist` and `Storage` interfaces, which nothing implemented or used, are
+  deleted. Nothing else changes.
+- 2026-10-04 upstream host semaphores and variant `.meta` files no longer
+  outlive their use (closes #87): the fetcher counts the fetches holding or
+  waiting for a slot of each upstream host's semaphore and removes the host's
+  semaphore once none is left, so fetches from many hosts no longer leave one
+  semaphore each until restart; `VariantStorage.Delete` removes the variant's
+  `.meta` file along with it, a missing `.meta` file not being an error, and
+  `DeleteWithMeta`, which eviction called for that, is gone.
+- 2026-10-04 `README.md` matches the code (closes #74): "Storage" names the
+  cache directories pixa uses (`cache/sources`, `cache/metadata`,
+  `cache/variants`) and how files are named in each, and the comments in
+  `001_schema.sql` name the same paths; the routes and the signature section
+  list the same output formats, `jpg` and `original` included; the TLS
+  sentence names `allow_http` as its exception; "Metrics" says only generic
+  HTTP and Go runtime metrics exist, measured and served only when the metrics
+  username and password are set.
 - 2026-10-03 shutdown sets the exit code and waits for image processing
   (closes #86): fx alone handles SIGINT and SIGTERM, and the server's own
   signal handler is gone; fx's `Run` in `cmd/pixad` exits with the shutdown's
@@ -401,7 +502,6 @@ P2: security: referer blacklist
 - P2: HTTP response handling
     - Last-Modified headers
     - Vary header for content negotiation
-    - X-Request-ID propagation
 - P2: auto format selection (format=auto based on Accept header)
 - P2: configuration
     - YAML config file support
@@ -411,8 +511,3 @@ P2: security: referer blacklist
     - Prometheus performance metrics
     - integration tests for the image proxy flow
     - load tests to verify the 1k to 5k req/s target
-- P2: documentation
-    - configuration options
-    - API endpoints
-    - deployment guide
-    - example nginx or caddy reverse proxy config

config.example.yml

@@ -12,27 +12,38 @@
 # Durations are Go duration strings such as 30s or 2m and must be
 # positive; a bare number has no unit and aborts startup. Sizes are a
 # whole number of bytes.
+#
+# A key left out takes the default its comment gives.
 
-# Server settings
+# Port to listen on (default: 8080)
 port: 8080
+
+# Debug logging and plain-HTTP local development (default: false)
 debug: false
 
 # While true, the image routes (/v1/image/ and /v1/e/) answer every request
-# with 503 and a Retry-After header. The health check keeps answering 200 and
-# reports maintenance_mode as true. It stays 200 because the image's Docker
-# HEALTHCHECK requests it: a 503 there would make the container unhealthy, and
-# upaas marks a deploy failed when its container is unhealthy.
+# for an image with 503 and a Retry-After header. The health check keeps
+# answering 200 and reports maintenance_mode as true. It stays 200 because
+# the image's Docker HEALTHCHECK requests it: a 503 there would make the
+# container unhealthy, and upaas marks a deploy failed when its container is
+# unhealthy. (default: false)
 maintenance_mode: false
 
 # Data directory for SQLite database and cache files
+# (default: /var/lib/pixa)
 state_dir: ./data
 
+# SQLite database URL (default:
+# file:<state_dir>/state.sqlite3?_journal_mode=WAL). An empty value aborts
+# startup; leave the key out to use the default.
+# db_url: "file:./data/state.sqlite3?_journal_mode=WAL"
+
 # Image proxy settings
 # HMAC signing key for URL signatures (required, at least 32 characters)
 # Generate with: openssl rand -base64 32
 signing_key: "CHANGE_ME_generate_with_openssl_rand_base64_32"
 
-# Hosts that don't require signatures
+# Hosts that don't require signatures (default: none)
 # Use "." prefix for wildcard subdomain matching (e.g., ".example.com" matches "cdn.example.com")
 allowlist_hosts:
   - s3.sneak.cloud
@@ -45,7 +56,7 @@ allowlist_hosts:
 # SSRF protection. These are added to the always-enforced built-in ranges
 # (loopback, RFC 1918 private, link-local, CGNAT, benchmark, NAT64, and
 # similar), never replacing them. Each entry must be a valid CIDR in IPv4
-# or IPv6 form; an invalid entry aborts startup.
+# or IPv6 form; an invalid entry aborts startup. (default: none)
 # blocked_networks:
 #   - 100.64.0.0/10
 #   - 2001:db8::/32
@@ -72,6 +83,7 @@ allowlist_hosts:
 #   - 2001:db8::/32
 
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
+# (default: false)
 allow_http: false
 
 # Maximum concurrent connections per upstream host (default: 20)
@@ -116,15 +128,22 @@ access_control_allow_origin: "*"
 
 # Maximum disk cache size in bytes. Explicit values are used exactly as
 # given; 0 disables the disk cache entirely (every request fetches and
-# processes uncached). When omitted, the default is 75% of the free
-# space on the filesystem containing <state_dir>/cache/ at startup,
+# processes uncached). When omitted, the default is 75% of the sum of
+# the free space on the filesystem containing <state_dir>/cache/ and
+# the bytes of images the cache already holds, worked out at startup,
 # with a minimum of 500 MiB.
 # cache_max_bytes: 10737418240
 
-# Sentry error reporting (optional)
+# Sentry DSN for error reporting (default: empty, which turns it off)
 sentry_dsn: ""
 
-# Metrics endpoint authentication (optional)
+# Username and password for /metrics, set together (default: unset). Metrics
+# are measured and /metrics is served only when both are set.
 # metrics:
 #   username: "admin"
 #   password: "secret"
+
+# Environment variables set while this file loads, as described at the top
+# (default: none)
+# env:
+#   PIXA_DEBUG: "true"

configs/Caddyfile

@@ -0,0 +1,17 @@
+# Example Caddy config for running pixa behind Caddy; see "Deployment" in
+# README.md. Replace images.example.com with pixa's public host name, and
+# 127.0.0.1:8080 with the address Caddy reaches pixa on.
+#
+# Caddy gets and renews the TLS certificate for the host name, passes the
+# Host, Origin and Referer headers on unchanged, sets X-Forwarded-For to the
+# client's address, and waits for pixa's answer with no time limit of its
+# own, so pixa's downstream_timeout is what ends a slow request.
+
+images.example.com
+
+# pixa asks for metrics.username and metrics.password on /metrics. This
+# line also keeps it off the public address, for a scraper that reaches
+# pixa directly; remove it to read /metrics through Caddy.
+respond /metrics 404
+
+reverse_proxy 127.0.0.1:8080

internal/config/cache_max_bytes_internal_test.go

@@ -1,26 +1,10 @@
 package config
 
 import (
-	"errors"
-	"log/slog"
-	"os"
-	"path/filepath"
 	"strings"
 	"testing"
 )
 
-// Static errors returned by the stub free-space probes below.
-var (
-	errTestStatfsFailed     = errors.New("statfs failed")
-	errTestProbeNotExpected = errors.New("probe must not be called")
-)
-
-// discardLogger returns a logger that swallows all output, for tests
-// that exercise code paths which log.
-func discardLogger() *slog.Logger {
-	return slog.New(slog.DiscardHandler)
-}
-
 // TestCacheMaxBytesExplicitValueUsedWithoutFloor verifies that an
 // explicitly configured cache_max_bytes value is used exactly as
 // given: the 500 MiB floor applies only to the computed default, never
@@ -151,155 +135,30 @@ func TestCacheMaxBytesInvalidValuesAbortStartup(t *testing.T) {
 	}
 }
 
-// TestComputeDefaultCacheMaxBytesUses75PercentOfFreeSpace verifies the
-// computed default is 75% of the probed free space when that exceeds
-// the floor.
-func TestComputeDefaultCacheMaxBytesUses75PercentOfFreeSpace(t *testing.T) {
+// TestCacheMaxBytesExplicitIsRecorded verifies that an omitted
+// cache_max_bytes is recorded as not explicit, so the cache works out
+// the default when it opens, and that an explicit zero is recorded as
+// explicit, so it disables the disk cache instead.
+func TestCacheMaxBytesExplicitIsRecorded(t *testing.T) {
 	t.Parallel()
 
-	// 4 GiB free -> 3 GiB default.
-	probe := func(string) (uint64, error) { return 4294967296, nil }
+	signingKeyLine := "signing_key: " + validTestSigningKey + "\n"
 
-	got, err := ComputeDefaultCacheMaxBytes(t.TempDir(), probe)
-	if err != nil {
-		t.Fatalf("ComputeDefaultCacheMaxBytes returned error: %v", err)
-	}
-
-	if got != 3221225472 {
-		t.Errorf("ComputeDefaultCacheMaxBytes = %d, want 3221225472 (75%% of 4 GiB)",
-			got)
-	}
-}
-
-// TestComputeDefaultCacheMaxBytesAppliesFloorToComputedDefault
-// verifies that when 75% of free space is below 500 MiB, the computed
-// default is floored at DefaultCacheMaxBytesFloor.
-func TestComputeDefaultCacheMaxBytesAppliesFloorToComputedDefault(t *testing.T) {
-	t.Parallel()
-
-	cases := []struct {
-		name      string
-		freeBytes uint64
-	}{
-		{name: "100 MiB free", freeBytes: 104857600},
-		{name: "zero free", freeBytes: 0},
-		{name: "just below floor threshold", freeBytes: 699050665},
-	}
-
-	for _, tc := range cases {
-		t.Run(tc.name, func(t *testing.T) {
-			t.Parallel()
-
-			probe := func(string) (uint64, error) { return tc.freeBytes, nil }
-
-			got, err := ComputeDefaultCacheMaxBytes(t.TempDir(), probe)
-			if err != nil {
-				t.Fatalf("ComputeDefaultCacheMaxBytes returned error: %v", err)
-			}
-
-			if got != DefaultCacheMaxBytesFloor {
-				t.Errorf("ComputeDefaultCacheMaxBytes = %d, want floor %d",
-					got, DefaultCacheMaxBytesFloor)
-			}
-		})
-	}
-}
-
-// TestComputeDefaultCacheMaxBytesPropagatesProbeError verifies that a
-// failing free-space probe produces an error naming the config key,
-// instead of a silently wrong default.
-func TestComputeDefaultCacheMaxBytesPropagatesProbeError(t *testing.T) {
-	t.Parallel()
-
-	probe := func(string) (uint64, error) { return 0, errTestStatfsFailed }
-
-	_, err := ComputeDefaultCacheMaxBytes(t.TempDir(), probe)
-	if err == nil {
-		t.Fatal("probe failure must produce an error, got nil")
-	}
-
-	t.Logf("got expected error: %v", err)
-
-	if !strings.Contains(err.Error(), keyCacheMaxBytes) {
-		t.Errorf("error %q does not name the config key cache_max_bytes", err.Error())
-	}
-}
-
-// TestResolveCacheMaxBytesComputesDefaultWhenOmitted verifies that an
-// omitted cache_max_bytes key resolves to the computed default, that
-// the probe is pointed at <state_dir>/cache/ (which must be created
-// first so statfs measures the right filesystem), and that the result
-// lands on the Config.
-func TestResolveCacheMaxBytesComputesDefaultWhenOmitted(t *testing.T) {
-	t.Parallel()
-
-	c, err := configFromYAML(t, "signing_key: "+validTestSigningKey+"\n")
+	omitted, err := configFromYAML(t, signingKeyLine)
 	if err != nil {
 		t.Fatalf("minimal config should be valid, got error: %v", err)
 	}
 
-	c.StateDir = t.TempDir()
-	wantCacheDir := filepath.Join(c.StateDir, "cache")
-
-	var probedPath string
-
-	// 4 GiB free -> 3 GiB default.
-	probe := func(path string) (uint64, error) {
-		probedPath = path
-
-		return 4294967296, nil
+	if omitted.CacheMaxBytesExplicit {
+		t.Error("omitted cache_max_bytes recorded as explicit")
 	}
 
-	err = c.resolveCacheMaxBytes(discardLogger(), probe)
+	zero, err := configFromYAML(t, signingKeyLine+"cache_max_bytes: 0\n")
 	if err != nil {
-		t.Fatalf("resolveCacheMaxBytes returned error: %v", err)
+		t.Fatalf("cache_max_bytes: 0 must be accepted, got error: %v", err)
 	}
 
-	if c.CacheMaxBytes != 3221225472 {
-		t.Errorf("CacheMaxBytes = %d, want computed default 3221225472",
-			c.CacheMaxBytes)
-	}
-
-	if probedPath != wantCacheDir {
-		t.Errorf("free space probed at %q, want cache directory %q",
-			probedPath, wantCacheDir)
-	}
-
-	info, err := os.Stat(wantCacheDir)
-	if err != nil || !info.IsDir() {
-		t.Errorf("cache directory %q was not created before probing: info=%v err=%v",
-			wantCacheDir, info, err)
-	}
-}
-
-// TestResolveCacheMaxBytesDoesNotOverrideExplicitValue verifies that
-// an explicitly configured value survives resolution untouched and
-// that the free-space probe is never consulted for it.
-func TestResolveCacheMaxBytesDoesNotOverrideExplicitValue(t *testing.T) {
-	t.Parallel()
-
-	yamlContent := "signing_key: " + validTestSigningKey + "\ncache_max_bytes: 1024\n"
-
-	c, err := configFromYAML(t, yamlContent)
-	if err != nil {
-		t.Fatalf("explicit cache_max_bytes must be accepted, got error: %v", err)
-	}
-
-	c.StateDir = t.TempDir()
-
-	probe := func(string) (uint64, error) {
-		t.Error("free-space probe must not be consulted for explicit values")
-
-		return 0, errTestProbeNotExpected
-	}
-
-	err = c.resolveCacheMaxBytes(discardLogger(), probe)
-	if err != nil {
-		t.Fatalf("resolveCacheMaxBytes returned error: %v", err)
-	}
-
-	if c.CacheMaxBytes != 1024 {
-		t.Errorf("CacheMaxBytes = %d, want explicit 1024 (no floor, no recompute)",
-			c.CacheMaxBytes)
+	if !zero.CacheMaxBytesExplicit {
+		t.Error("cache_max_bytes: 0 not recorded as explicit")
 	}
 }

internal/config/cachesize.go

@@ -1,116 +0,0 @@
-package config
-
-import (
-	"fmt"
-	"log/slog"
-	"math"
-	"os"
-	"path/filepath"
-	"syscall"
-)
-
-// DefaultCacheMaxBytesFloor is the minimum computed default for the
-// cache_max_bytes setting: 500 MiB. The floor applies only to the
-// computed default (when the key is omitted from the configuration),
-// never to explicitly configured values.
-const DefaultCacheMaxBytesFloor int64 = 524288000
-
-// cacheDirPerms is the permission mode for the cache directory created
-// before probing free space, matching the state directory permissions.
-const cacheDirPerms = 0o750
-
-// freeSpaceFractionNumerator and freeSpaceFractionDenominator express
-// the 75% share of free space used for the computed default limit as
-// integer arithmetic (dividing before multiplying avoids overflow).
-const (
-	freeSpaceFractionNumerator   uint64 = 3
-	freeSpaceFractionDenominator uint64 = 4
-)
-
-// FreeSpaceProbeFunc reports the number of free bytes available on the
-// filesystem containing path. It is a function type so tests can
-// inject a fake probe instead of depending on the host disk.
-type FreeSpaceProbeFunc func(path string) (uint64, error)
-
-// defaultFreeSpaceProbe reports free filesystem bytes via statfs on
-// the given path, as available to unprivileged processes.
-func defaultFreeSpaceProbe(path string) (uint64, error) {
-	var stat syscall.Statfs_t
-
-	err := syscall.Statfs(path, &stat)
-	if err != nil {
-		return 0, err
-	}
-
-	if stat.Bsize < 0 {
-		return 0, fmt.Errorf("%w %d for %q", errNegativeBlockSize, stat.Bsize, path)
-	}
-
-	blockSize := uint64(stat.Bsize)
-
-	return stat.Bavail * blockSize, nil
-}
-
-// ComputeDefaultCacheMaxBytes returns the default cache size limit for
-// the filesystem containing cacheDir: 75% of the free bytes reported
-// by probe, with a floor of DefaultCacheMaxBytesFloor.
-func ComputeDefaultCacheMaxBytes(
-	cacheDir string, probe FreeSpaceProbeFunc,
-) (int64, error) {
-	freeBytes, err := probe(cacheDir)
-	if err != nil {
-		return 0, fmt.Errorf("config key %q: cannot determine free space for %q: %w",
-			"cache_max_bytes", cacheDir, err)
-	}
-
-	computed := freeBytes / freeSpaceFractionDenominator * freeSpaceFractionNumerator
-	computed = min(computed, math.MaxInt64)
-
-	// gosec cannot see that min() above bounds computed, so it reads
-	// this conversion as potentially overflowing. It cannot: computed is
-	// at most math.MaxInt64 on every path here.
-	//nolint:gosec // G115: clamped to MaxInt64 by min above
-	limit := int64(computed)
-	limit = max(limit, DefaultCacheMaxBytesFloor)
-
-	return limit, nil
-}
-
-// resolveCacheMaxBytes finalizes CacheMaxBytes after state_dir
-// validation: an explicitly configured value is kept as-is (no floor
-// applies), while an omitted key receives the computed default based
-// on free space in <state_dir>/cache/. The cache directory is created
-// first so statfs measures the filesystem that will actually hold the
-// cache. The effective limit is logged either way.
-func (c *Config) resolveCacheMaxBytes(
-	log *slog.Logger, probe FreeSpaceProbeFunc,
-) error {
-	if !c.cacheMaxBytesExplicit {
-		cacheDir := filepath.Join(c.StateDir, "cache")
-
-		err := os.MkdirAll(cacheDir, cacheDirPerms)
-		if err != nil {
-			return fmt.Errorf("config key %q: cannot create cache directory %q: %w",
-				keyCacheMaxBytes, cacheDir, err)
-		}
-
-		limit, err := ComputeDefaultCacheMaxBytes(cacheDir, probe)
-		if err != nil {
-			return err
-		}
-
-		c.CacheMaxBytes = limit
-
-		log.Info("computed default cache size limit from free space",
-			"cache_max_bytes", limit,
-			"cache_dir", cacheDir,
-		)
-	}
-
-	log.Info("effective cache size limit",
-		"cache_max_bytes", c.CacheMaxBytes,
-		"cache_disabled", c.CacheMaxBytes == 0,
-	)
-
-	return nil
-}

internal/config/config.go

@@ -91,9 +91,7 @@ var (
 	errMustBeSetTogether = errors.New("must be set together")
 	errMustNotBeNegative = errors.New("must not be negative")
 	errOverflowsInt64    = errors.New("overflows a 64-bit integer")
-	errNegativeBlockSize = errors.New(
-		"statfs reported negative block size")
-	errValueNull = errors.New(
+	errValueNull         = errors.New(
 		"value is null; omit the key entirely to use the default")
 	errValuesNull = errors.New(
 		"value is null; omit a key entirely to use its default")
@@ -169,18 +167,19 @@ type Config struct {
 	// address, and an explicit list replaces the default.
 	TrustedProxies []netip.Prefix
 
-	// CacheMaxBytes is the disk cache size limit in bytes. Zero
-	// disables the disk cache entirely. When cache_max_bytes is
-	// omitted from the configuration, this holds the computed default
-	// (75% of free space on the filesystem containing
-	// <state_dir>/cache/, floored at DefaultCacheMaxBytesFloor).
+	// CacheMaxBytes is the disk cache size limit in bytes. Only an
+	// explicit zero (CacheMaxBytesExplicit true) disables the disk
+	// cache. Zero with CacheMaxBytesExplicit false means
+	// cache_max_bytes was omitted, and the cache works out the default
+	// limit when it opens.
 	CacheMaxBytes int64
 
-	// cacheMaxBytesExplicit records whether cache_max_bytes was
+	// CacheMaxBytesExplicit records whether cache_max_bytes was
 	// explicitly set, in the environment or the configuration file.
-	// Explicit values are used exactly as given; only an omitted key
-	// gets the computed default (and its floor) in resolveCacheMaxBytes.
-	cacheMaxBytesExplicit bool
+	// Explicit values are used exactly as given; for an omitted key the
+	// cache works out the default limit when it opens (see
+	// imgcache.CacheConfig.UseDefaultMaxBytes).
+	CacheMaxBytesExplicit bool
 }
 
 // New creates a new Config instance from the environment and the
@@ -217,9 +216,13 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		return nil, err
 	}
 
-	err = c.resolveCacheMaxBytes(log, defaultFreeSpaceProbe)
-	if err != nil {
-		return nil, err
+	// An omitted cache_max_bytes is worked out and logged when the
+	// cache opens.
+	if c.CacheMaxBytesExplicit {
+		log.Info("effective cache size limit",
+			"cache_max_bytes", c.CacheMaxBytes,
+			"cache_disabled", c.CacheMaxBytes == 0,
+		)
 	}
 
 	if c.Debug {
@@ -298,11 +301,11 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		TrustedProxies:  trustedProxies,
 	}
 
-	// The computed default for cache_max_bytes needs a validated
-	// state_dir, so it is resolved later (resolveCacheMaxBytes); here
-	// we only record whether the operator set the key explicitly.
+	// The default for an omitted cache_max_bytes is worked out when
+	// the cache opens; here we only record whether the operator set
+	// the key explicitly.
 	if _, present := lookupValue(sc, keyCacheMaxBytes); present {
-		c.cacheMaxBytesExplicit = true
+		c.CacheMaxBytesExplicit = true
 	}
 
 	// Build DBURL from StateDir if not explicitly set. The derived URL

internal/config/env_internal_test.go

@@ -98,7 +98,7 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 		UpstreamConnections:        10,
 		MaxConcurrentProcessing:    3,
 		CacheMaxBytes:              1024,
-		cacheMaxBytesExplicit:      true,
+		CacheMaxBytesExplicit:      true,
 		BlockedNetworks:            []netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")},
 		TrustedProxies:             []netip.Prefix{netip.MustParsePrefix("192.0.2.0/24")},
 		AccessControlAllowOrigin:   "https://app.example.com",

internal/db/migrations/001_schema.sql

@@ -2,7 +2,7 @@
 -- Creates all tables for the pixa caching image proxy
 
 -- Source content blobs
--- Files stored at: cache/src-content/<ab>/<cd>/<sha256>
+-- Files stored at: cache/sources/<ab>/<cd>/<sha256>
 -- last_accessed_at is NULL until the first LRU touch; eviction falls
 -- back to fetched_at for rows that have never been touched.
 CREATE TABLE IF NOT EXISTS source_content (
@@ -16,7 +16,7 @@ CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
     ON source_content(last_accessed_at);
 
 -- Source URL metadata - maps URLs to content hashes
--- JSON stored at: cache/src-metadata/<hostname>/<path_hash>.json
+-- JSON stored at: cache/metadata/<hostname>/<path_hash>.json
 CREATE TABLE IF NOT EXISTS source_metadata (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     source_host TEXT NOT NULL,
@@ -56,7 +56,8 @@ CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
     ON variant_content(last_accessed_at);
 
 -- Output/transformed content blobs
--- Files stored at: cache/dst-content/<ab>/<cd>/<sha256>
+-- Not written: transformed images are stored in cache/variants and
+-- tracked in variant_content above.
 CREATE TABLE IF NOT EXISTS output_content (
     content_hash TEXT PRIMARY KEY,
     content_type TEXT NOT NULL,

internal/handlers/auth_login_log_internal_test.go

@@ -0,0 +1,59 @@
+package handlers
+
+import (
+	"bytes"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/session"
+)
+
+// TestLoginLogLeavesOutSubmittedKey verifies that the log lines for a
+// failed and for a successful login do not contain the submitted key.
+func TestLoginLogLeavesOutSubmittedKey(t *testing.T) {
+	t.Parallel()
+
+	const wrongKey = "wrong-signing-key-fedcba9876543210"
+
+	var buf bytes.Buffer
+
+	sessMgr, err := session.NewManager(testSigningKey)
+	if err != nil {
+		t.Fatalf("session.NewManager() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:     slog.New(slog.NewJSONHandler(&buf, nil)),
+		config:  &config.Config{SigningKey: testSigningKey},
+		sessMgr: sessMgr,
+	}
+
+	submittedKeys := []string{wrongKey, testSigningKey}
+
+	for _, key := range submittedKeys {
+		form := url.Values{loginKeyField: {key}}
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodPost, "/",
+			strings.NewReader(form.Encode()))
+		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+		h.handleLoginPost(httptest.NewRecorder(), req)
+	}
+
+	for _, msg := range []string{"failed login attempt", "successful login"} {
+		if !strings.Contains(buf.String(), msg) {
+			t.Fatalf("log missing %q; got %q", msg, buf.String())
+		}
+	}
+
+	for _, key := range submittedKeys {
+		if strings.Contains(buf.String(), key) {
+			t.Errorf("log contains submitted key %q; got %q", key, buf.String())
+		}
+	}
+}

internal/handlers/cache_max_bytes_internal_test.go

@@ -0,0 +1,74 @@
+package handlers
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"go.uber.org/fx/fxtest"
+
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/database"
+	"sneak.berlin/go/pixa/internal/globals"
+	"sneak.berlin/go/pixa/internal/logger"
+)
+
+// TestDiskCacheOffOnlyForExplicitZeroCacheMaxBytes starts the handlers
+// once with cache_max_bytes omitted and once with cache_max_bytes: 0,
+// and checks by whether the cache directories were created that the
+// disk cache is on in the first case and off in the second.
+func TestDiskCacheOffOnlyForExplicitZeroCacheMaxBytes(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name                  string
+		cacheMaxBytesExplicit bool
+		wantDiskCache         bool
+	}{
+		{name: "cache_max_bytes omitted", cacheMaxBytesExplicit: false, wantDiskCache: true},
+		{name: "cache_max_bytes: 0", cacheMaxBytesExplicit: true, wantDiskCache: false},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			stateDir := t.TempDir()
+			cfg := &config.Config{
+				SigningKey:            testSigningKey,
+				StateDir:              stateDir,
+				DBURL:                 "file:" + filepath.Join(stateDir, "state.sqlite3"),
+				CacheMaxBytes:         0,
+				CacheMaxBytesExplicit: tc.cacheMaxBytesExplicit,
+			}
+
+			lc := fxtest.NewLifecycle(t)
+
+			log, err := logger.New(lc, logger.Params{Globals: &globals.Globals{}})
+			if err != nil {
+				t.Fatalf("logger.New() error = %v", err)
+			}
+
+			db, err := database.New(lc, database.Params{Logger: log, Config: cfg})
+			if err != nil {
+				t.Fatalf("database.New() error = %v", err)
+			}
+
+			_, err = New(lc, Params{Logger: log, Database: db, Config: cfg})
+			if err != nil {
+				t.Fatalf("New() error = %v", err)
+			}
+
+			lc.RequireStart()
+			t.Cleanup(lc.RequireStop)
+
+			_, err = os.Stat(filepath.Join(stateDir, "cache", "variants"))
+			gotDiskCache := err == nil
+
+			if gotDiskCache != tc.wantDiskCache {
+				t.Errorf("cache directories created = %v, want %v",
+					gotDiskCache, tc.wantDiskCache)
+			}
+		})
+	}
+}

internal/handlers/handlers.go

@@ -58,23 +58,16 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 	}
 
 	lc.Append(fx.Hook{
-		// The eviction goroutine must outlive OnStart, so it cannot
-		// inherit this hook's context. It makes its own instead, which
-		// leaves it uncancellable: an in-flight pass runs to completion
-		// during OnStop regardless of the shutdown deadline. Making the
-		// loop cancellable changes shutdown semantics and is tracked
-		// separately in issue #102, rather than being folded into the
-		// lint-conformance change that surfaced it.
-		//nolint:contextcheck // see issue #102
+		//nolint:contextcheck // the eviction loop outlives OnStart; OnStop cancels it
 		OnStart: func(_ context.Context) error {
 			return s.initImageService()
 		},
-		OnStop: func(_ context.Context) error {
-			if s.imgCache != nil {
-				s.imgCache.StopEviction()
+		OnStop: func(ctx context.Context) error {
+			if s.imgCache == nil {
+				return nil
 			}
 
-			return nil
+			return s.imgCache.StopEviction(ctx)
 		},
 	})
 
@@ -90,14 +83,16 @@ func (s *Handlers) WaitForProcessing(ctx context.Context) int {
 // initImageService initializes the image cache and service.
 func (s *Handlers) initImageService() error {
 	// Create the cache. cache_max_bytes: 0 disables the disk cache
-	// entirely; any other value is the eviction limit in bytes.
+	// entirely; any other value is the eviction limit in bytes; when
+	// it is omitted, the cache works out the default limit itself.
 	cache, err := imgcache.NewCache(s.db.DB(), imgcache.CacheConfig{
-		StateDir:         s.config.StateDir,
-		CacheTTL:         imgcache.DefaultCacheTTL,
-		NegativeTTL:      imgcache.DefaultNegativeTTL,
-		MaxBytes:         s.config.CacheMaxBytes,
-		DisableDiskCache: s.config.CacheMaxBytes == 0,
-		Logger:           s.log,
+		StateDir:           s.config.StateDir,
+		CacheTTL:           imgcache.DefaultCacheTTL,
+		NegativeTTL:        imgcache.DefaultNegativeTTL,
+		MaxBytes:           s.config.CacheMaxBytes,
+		UseDefaultMaxBytes: !s.config.CacheMaxBytesExplicit,
+		DisableDiskCache:   s.config.CacheMaxBytesExplicit && s.config.CacheMaxBytes == 0,
+		Logger:             s.log,
 	})
 	if err != nil {
 		return err

internal/handlers/image.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
@@ -247,6 +248,25 @@ func cacheControl(expires time.Time) string {
 	return fmt.Sprintf("public, max-age=%d, immutable", int64(maxAge/time.Second))
 }
 
+// notModified sets the ETag header to etag and, when the request's
+// If-None-Match is that ETag, answers 304 Not Modified. It reports whether it
+// answered. An empty etag sets no header and never answers.
+func notModified(w http.ResponseWriter, r *http.Request, etag string) bool {
+	if etag == "" {
+		return false
+	}
+
+	w.Header().Set("ETag", etag)
+
+	if r.Header.Get("If-None-Match") != etag {
+		return false
+	}
+
+	w.WriteHeader(http.StatusNotModified)
+
+	return true
+}
+
 // writeImageResponse writes headers and streams the image content,
 // handling conditional and HEAD requests.
 func (s *Handlers) writeImageResponse(
@@ -265,17 +285,8 @@ func (s *Handlers) writeImageResponse(
 	w.Header().Set("Cache-Control", cacheControl(req.Expires))
 	w.Header().Set("X-Pixa-Cache", string(resp.CacheStatus))
 
-	if resp.ETag != "" {
-		w.Header().Set("ETag", resp.ETag)
-
-		// Check for conditional request (If-None-Match)
-		if ifNoneMatch := r.Header.Get("If-None-Match"); ifNoneMatch != "" {
-			if ifNoneMatch == resp.ETag {
-				w.WriteHeader(http.StatusNotModified)
-
-				return
-			}
-		}
+	if notModified(w, r, resp.ETag) {
+		return
 	}
 
 	// Handle HEAD request - return headers only
@@ -298,6 +309,7 @@ func (s *Handlers) writeImageResponse(
 	// Log cache status and timing after serving
 	duration := time.Since(startTime)
 	s.log.Info("image served",
+		"request_id", middleware.GetReqID(r.Context()),
 		"cache_key", cacheKey,
 		"cache_status", resp.CacheStatus,
 		"duration_ms", duration.Milliseconds(),

internal/handlers/image_cache_control_internal_test.go

@@ -23,8 +23,10 @@ const photoPath = "/images/photo.jpg"
 // newSignedHostServer returns a router for both image routes, and the Handlers
 // behind it, whose fetcher serves a JPEG at photoPath on signedHost. signedHost
 // is not on the allowlist, so a /v1/image/ URL for it is served only with a
-// valid signature.
-func newSignedHostServer(t *testing.T) (*Handlers, http.Handler) {
+// valid signature. The handlers and the image service log to log.
+func newSignedHostServer(
+	t *testing.T, log *slog.Logger,
+) (*Handlers, http.Handler) {
 	t.Helper()
 
 	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
@@ -44,6 +46,7 @@ func newSignedHostServer(t *testing.T) (*Handlers, http.Handler) {
 			signedHost + photoPath: &fstest.MapFile{Data: jpegData},
 		}),
 		SigningKey: testSigningKey,
+		Logger:     log,
 	})
 	if err != nil {
 		t.Fatalf("imgcache.NewService() error = %v", err)
@@ -55,7 +58,7 @@ func newSignedHostServer(t *testing.T) (*Handlers, http.Handler) {
 	}
 
 	h := &Handlers{
-		log:    slog.New(slog.DiscardHandler),
+		log:    log,
 		imgSvc: svc,
 		encGen: encGen,
 	}
@@ -103,7 +106,7 @@ func getMaxAge(t *testing.T, srv http.Handler, target string) int {
 func TestHandleImage_SignedURL_MaxAgeEndsAtExp(t *testing.T) {
 	t.Parallel()
 
-	h, srv := newSignedHostServer(t)
+	h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
 
 	signedURL, err := h.imgSvc.GenerateSignedURL("", &imgcache.ImageRequest{
 		SourceHost: signedHost,
@@ -179,7 +182,7 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			h, srv := newSignedHostServer(t)
+			h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
 
 			token, err := h.encGen.Generate(&encurl.Payload{
 				SourceHost: signedHost,

internal/handlers/image_errors_internal_test.go

@@ -0,0 +1,267 @@
+package handlers
+
+import (
+	"encoding/json"
+	"fmt"
+	"image/color"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/signature"
+)
+
+// allowlistedHost is the only host on the allowlist of the image route
+// newImageRoute builds.
+const allowlistedHost = "allowed.example.com"
+
+// newImageRoute returns the image route of a Handlers whose service fetches
+// with fetcher and checks signatures with testSigningKey.
+func newImageRoute(t *testing.T, fetcher httpfetcher.Fetcher) http.Handler {
+	t.Helper()
+
+	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
+		StateDir:    t.TempDir(),
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+	})
+	if err != nil {
+		t.Fatalf("failed to create cache: %v", err)
+	}
+
+	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
+		Cache:      cache,
+		Fetcher:    fetcher,
+		SigningKey: testSigningKey,
+		Allowlist:  []string{allowlistedHost},
+	})
+	if err != nil {
+		t.Fatalf("failed to create service: %v", err)
+	}
+
+	h := &Handlers{imgSvc: svc, log: slog.New(slog.DiscardHandler)}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+
+	return r
+}
+
+// newPhotoFetcher returns a mock fetcher that serves a JPEG at photoPath on
+// each of hosts, and answers any other URL with an upstream error.
+func newPhotoFetcher(t *testing.T, hosts ...string) *httpfetcher.MockFetcher {
+	t.Helper()
+
+	photo := &fstest.MapFile{
+		Data: generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255}),
+	}
+
+	files := fstest.MapFS{}
+	for _, host := range hosts {
+		files[host+photoPath] = photo
+	}
+
+	return httpfetcher.NewMock(files)
+}
+
+// photoURL returns the image route URL of photoPath on host, as a 50x50 JPEG.
+func photoURL(host string) string {
+	return "/v1/image/" + host + photoPath + "/50x50.jpeg"
+}
+
+// photoURLWithSig returns photoURL(host) with sig and expires as its sig and
+// exp.
+func photoURLWithSig(host, sig string, expires time.Time) string {
+	return fmt.Sprintf("%s?sig=%s&exp=%d", photoURL(host), sig, expires.Unix())
+}
+
+// photoSignature returns the signature of photoURL(host) at the default
+// quality and fit, made with key and expiring at expires.
+func photoSignature(key, host string, expires time.Time) string {
+	return signature.New(key).Sign(&signature.Request{
+		SourceHost: host,
+		SourcePath: photoPath,
+		Width:      50,
+		Height:     50,
+		Format:     string(imgcache.FormatJPEG),
+		Quality:    85,
+		FitMode:    string(imgcache.FitCover),
+		Expires:    expires,
+	})
+}
+
+// sendGet sends a GET for target to route and returns the response.
+func sendGet(
+	t *testing.T, route http.Handler, target string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	rec := httptest.NewRecorder()
+
+	route.ServeHTTP(rec, req)
+	t.Logf("GET %s: %d", target, rec.Code)
+
+	return rec
+}
+
+// checkErrorBody checks that rec has status wantStatus and the JSON error body
+// the image route sends: wantError, wantStatus and the time in RFC 3339.
+func checkErrorBody(
+	t *testing.T, rec *httptest.ResponseRecorder, wantStatus int, wantError string,
+) {
+	t.Helper()
+
+	if rec.Code != wantStatus {
+		t.Errorf("status = %d, want %d", rec.Code, wantStatus)
+	}
+
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", ct)
+	}
+
+	var body struct {
+		Error     string `json:"error"`
+		Status    int    `json:"status"`
+		Timestamp string `json:"timestamp"`
+	}
+
+	err := json.NewDecoder(rec.Body).Decode(&body)
+	if err != nil {
+		t.Fatalf("decoding response body: %v", err)
+	}
+
+	if body.Error != wantError || body.Status != wantStatus {
+		t.Errorf("body error and status = %q %d, want %q %d",
+			body.Error, body.Status, wantError, wantStatus)
+	}
+
+	_, err = time.Parse(time.RFC3339, body.Timestamp)
+	if err != nil {
+		t.Errorf("body timestamp: %v", err)
+	}
+}
+
+// TestHandleImage_ErrorAnswers checks the status and the JSON error body the
+// image route answers each request below with. The JPEG at photoPath exists on
+// signedHost and on each host below that differs from it, so a request refused
+// with 401 would otherwise be served.
+func TestHandleImage_ErrorAnswers(t *testing.T) {
+	t.Parallel()
+
+	// A signature for signedHost must not verify for any of these.
+	parentHost := "example.com"
+	siblingHost := "other.example.com"
+	subdomainHost := "img." + signedHost
+	appendedHost := signedHost + ".example.net"
+
+	photos := newPhotoFetcher(t,
+		signedHost, parentHost, siblingHost, subdomainHost, appendedHost)
+	// The real fetcher refuses localhost before any lookup or connection.
+	realFetcher := httpfetcher.New(httpfetcher.DefaultConfig())
+
+	exp := time.Now().Add(time.Hour)
+	expired := time.Now().Add(-time.Hour)
+	sig := photoSignature(testSigningKey, signedHost, exp)
+	otherKeySig := photoSignature("another-signing-key", signedHost, exp)
+	expiredSig := photoSignature(testSigningKey, signedHost, expired)
+	localhostSig := photoSignature(testSigningKey, "localhost", exp)
+
+	// The error every request refused for its signature gets.
+	const unauthorized = "unauthorized"
+
+	tests := []struct {
+		name       string
+		fetcher    httpfetcher.Fetcher
+		target     string
+		wantStatus int
+		wantError  string
+	}{
+		{"no sig or exp", photos, photoURL(signedHost),
+			http.StatusUnauthorized, unauthorized},
+		{"exp but no sig", photos,
+			fmt.Sprintf("%s?exp=%d", photoURL(signedHost), exp.Unix()),
+			http.StatusUnauthorized, unauthorized},
+		{"sig made with another key", photos,
+			photoURLWithSig(signedHost, otherKeySig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig without its = padding", photos,
+			photoURLWithSig(signedHost, strings.TrimRight(sig, "="), exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig in upper case", photos,
+			photoURLWithSig(signedHost, strings.ToUpper(sig), exp),
+			http.StatusUnauthorized, unauthorized},
+		{"expired sig", photos, photoURLWithSig(signedHost, expiredSig, expired),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent for the parent domain", photos,
+			photoURLWithSig(parentHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent for a sibling host", photos,
+			photoURLWithSig(siblingHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent for a subdomain", photos,
+			photoURLWithSig(subdomainHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"sig sent with another domain appended", photos,
+			photoURLWithSig(appendedHost, sig, exp),
+			http.StatusUnauthorized, unauthorized},
+		{"unparseable path", photos,
+			"/v1/image/" + allowlistedHost + photoPath + "/big.jpeg",
+			http.StatusBadRequest, "invalid image URL: invalid size format"},
+		{"blocked upstream address", realFetcher,
+			photoURLWithSig("localhost", localhostSig, exp),
+			http.StatusForbidden, "forbidden"},
+		{"upstream error", photos,
+			"/v1/image/" + allowlistedHost + "/images/missing.jpg/50x50.jpeg",
+			http.StatusBadGateway, "upstream error"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := sendGet(t, newImageRoute(t, tt.fetcher), tt.target)
+			checkErrorBody(t, rec, tt.wantStatus, tt.wantError)
+		})
+	}
+}
+
+// TestHandleImage_AllowlistOrSignature checks that the image route serves an
+// image without a signature for a host on the allowlist only, and for another
+// host only with a valid signature.
+func TestHandleImage_AllowlistOrSignature(t *testing.T) {
+	t.Parallel()
+
+	photos := newPhotoFetcher(t, allowlistedHost, signedHost)
+	exp := time.Now().Add(time.Hour)
+	sig := photoSignature(testSigningKey, signedHost, exp)
+
+	tests := []struct {
+		name       string
+		target     string
+		wantStatus int
+	}{
+		{"allowlisted host, no sig", photoURL(allowlistedHost), http.StatusOK},
+		{"other host, no sig", photoURL(signedHost), http.StatusUnauthorized},
+		{"other host, valid sig", photoURLWithSig(signedHost, sig, exp),
+			http.StatusOK},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := sendGet(t, newImageRoute(t, photos), tt.target)
+			if rec.Code != tt.wantStatus {
+				t.Errorf("status = %d, want %d", rec.Code, tt.wantStatus)
+			}
+		})
+	}
+}

internal/handlers/imageenc.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
 
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
@@ -24,43 +25,8 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		ctx := r.Context()
 		start := time.Now()
 
-		// Extract token from URL
-		token := chi.URLParam(r, "token")
-		if token == "" {
-			s.respondError(w, "missing token", http.StatusBadRequest)
-
-			return
-		}
-
-		// Decrypt and validate the payload
-		payload, err := s.encGen.Parse(token)
-		if err != nil {
-			if errors.Is(err, encurl.ErrExpired) {
-				s.log.Debug("encrypted URL expired", "error", err)
-				s.respondError(w, "URL has expired", http.StatusGone)
-
-				return
-			}
-
-			s.log.Debug("failed to decrypt URL", "error", err)
-			s.respondError(w, "invalid encrypted URL", http.StatusBadRequest)
-
-			return
-		}
-
-		// Convert payload to ImageRequest
-		req := payload.ToImageRequest()
-
-		// Apply the same dimension and fit-mode bounds as the plain image
-		// route: a sealed payload is trusted for its origin, not for staying
-		// within limits, so an over-limit size or unknown fit mode is a 400
-		// here rather than an out-of-memory or a 500 from the processor.
-		err = imgcache.ValidateImageRequest(req)
-		if err != nil {
-			s.log.Debug("encrypted URL failed validation", "error", err)
-			s.respondError(w, "invalid encrypted URL: "+err.Error(),
-				http.StatusBadRequest)
-
+		req, ok := s.parseImageEncRequest(w, r)
+		if !ok {
 			return
 		}
 
@@ -94,6 +60,17 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		w.Header().Set("Cache-Control", cacheControl(req.Expires))
 		w.Header().Set("X-Pixa-Cache", string(resp.CacheStatus))
 
+		if notModified(w, r, resp.ETag) {
+			return
+		}
+
+		// A HEAD request gets the headers only
+		if r.Method == http.MethodHead {
+			w.WriteHeader(http.StatusOK)
+
+			return
+		}
+
 		// Stream the response
 		written, err := io.Copy(w, resp.Content)
 		if err != nil {
@@ -105,6 +82,7 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		// Log completion
 		duration := time.Since(start)
 		s.log.Info("image served",
+			"request_id", middleware.GetReqID(ctx),
 			"cache_key", imgcache.CacheKey(req),
 			"host", req.SourceHost,
 			"path", req.SourcePath,
@@ -116,6 +94,56 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 	}
 }
 
+// parseImageEncRequest decrypts the token of an encrypted image URL into an
+// ImageRequest and checks it. On a token that is missing, does not decrypt,
+// has expired or asks for something not valid, it writes an error response
+// and returns false.
+func (s *Handlers) parseImageEncRequest(
+	w http.ResponseWriter, r *http.Request,
+) (*imgcache.ImageRequest, bool) {
+	// Extract token from URL
+	token := chi.URLParam(r, "token")
+	if token == "" {
+		s.respondError(w, "missing token", http.StatusBadRequest)
+
+		return nil, false
+	}
+
+	// Decrypt and validate the payload
+	payload, err := s.encGen.Parse(token)
+	if err != nil {
+		if errors.Is(err, encurl.ErrExpired) {
+			s.log.Debug("encrypted URL expired", "error", err)
+			s.respondError(w, "URL has expired", http.StatusGone)
+
+			return nil, false
+		}
+
+		s.log.Debug("failed to decrypt URL", "error", err)
+		s.respondError(w, "invalid encrypted URL", http.StatusBadRequest)
+
+		return nil, false
+	}
+
+	// Convert payload to ImageRequest
+	req := payload.ToImageRequest()
+
+	// Apply the same dimension and fit-mode bounds as the plain image
+	// route: a sealed payload is trusted for its origin, not for staying
+	// within limits, so an over-limit size or unknown fit mode is a 400
+	// here rather than an out-of-memory or a 500 from the processor.
+	err = imgcache.ValidateImageRequest(req)
+	if err != nil {
+		s.log.Debug("encrypted URL failed validation", "error", err)
+		s.respondError(w, "invalid encrypted URL: "+err.Error(),
+			http.StatusBadRequest)
+
+		return nil, false
+	}
+
+	return req, true
+}
+
 // handleImageError converts image service errors to HTTP responses.
 func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 	switch {

internal/handlers/imageenc_internal_test.go

@@ -96,3 +96,70 @@ func TestHandleImageEnc_InvalidFitMode_Returns400(t *testing.T) {
 		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
 	}
 }
+
+// TestHandleImageEnc_IfNoneMatch_Returns304 verifies that an image served
+// through an encrypted URL carries an ETag, and that a request whose
+// If-None-Match is that ETag is answered 304 Not Modified with no body.
+func TestHandleImageEnc_IfNoneMatch_Returns304(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
+	target := encPhotoURL(t, h)
+
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, target, nil))
+
+	etag := rec.Header().Get("ETag")
+	t.Logf("GET: %d, ETag %q", rec.Code, etag)
+
+	if rec.Code != http.StatusOK || etag == "" {
+		t.Fatalf("GET: status = %d, ETag = %q, want %d and an ETag",
+			rec.Code, etag, http.StatusOK)
+	}
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	req.Header.Set("If-None-Match", etag)
+
+	rec = httptest.NewRecorder()
+	srv.ServeHTTP(rec, req)
+	t.Logf("GET with If-None-Match: %d, %d body bytes", rec.Code, rec.Body.Len())
+
+	if rec.Code != http.StatusNotModified || rec.Body.Len() != 0 {
+		t.Errorf("status = %d with %d body bytes, want %d with none",
+			rec.Code, rec.Body.Len(), http.StatusNotModified)
+	}
+}
+
+// TestHandleImageEnc_HEAD_ReturnsHeadersOnly verifies that HEAD on an
+// encrypted URL is answered 200 with the headers GET sends and no body.
+func TestHandleImageEnc_HEAD_ReturnsHeadersOnly(t *testing.T) {
+	t.Parallel()
+
+	h, _ := newSignedHostServer(t, slog.New(slog.DiscardHandler))
+
+	r := chi.NewRouter()
+	r.Head("/v1/e/{token}/*", h.HandleImageEnc())
+
+	rec := httptest.NewRecorder()
+	r.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodHead, encPhotoURL(t, h), nil))
+	t.Logf("HEAD: %d, headers %v, %d body bytes",
+		rec.Code, rec.Header(), rec.Body.Len())
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	for _, name := range []string{
+		"Content-Type", "Content-Length", "Cache-Control", "ETag",
+	} {
+		if rec.Header().Get(name) == "" {
+			t.Errorf("HEAD response has no %s", name)
+		}
+	}
+
+	if rec.Body.Len() != 0 {
+		t.Errorf("HEAD response body has %d bytes, want none", rec.Body.Len())
+	}
+}

internal/handlers/request_id_internal_test.go

@@ -0,0 +1,131 @@
+package handlers
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/go-chi/chi/v5/middleware"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// signedPhotoURL returns a signed /v1/image/ URL, valid for a minute, for the
+// JPEG at photoPath on signedHost at 50x50, made with h's image service.
+func signedPhotoURL(t *testing.T, h *Handlers) string {
+	t.Helper()
+
+	signedURL, err := h.imgSvc.GenerateSignedURL("", &imgcache.ImageRequest{
+		SourceHost: signedHost,
+		SourcePath: photoPath,
+		Size:       imgcache.Size{Width: 50, Height: 50},
+		Format:     imgcache.FormatJPEG,
+	}, time.Minute)
+	if err != nil {
+		t.Fatalf("GenerateSignedURL() error = %v", err)
+	}
+
+	return signedURL
+}
+
+// encPhotoURL returns an encrypted /v1/e/ URL, which never expires, for the
+// JPEG at photoPath on signedHost at 50x50, made with h's generator.
+func encPhotoURL(t *testing.T, h *Handlers) string {
+	t.Helper()
+
+	token, err := h.encGen.Generate(&encurl.Payload{
+		SourceHost: signedHost,
+		SourcePath: photoPath,
+		Width:      50,
+		Height:     50,
+		Format:     imgcache.FormatJPEG,
+	})
+	if err != nil {
+		t.Fatalf("Generate() error = %v", err)
+	}
+
+	return "/v1/e/" + token + "/img.jpg"
+}
+
+// requestIDByMessage reads the JSON log lines in logs and returns the
+// request_id of each line, by its message.
+func requestIDByMessage(t *testing.T, logs io.Reader) map[string]string {
+	t.Helper()
+
+	logged := make(map[string]string)
+
+	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)
+		}
+
+		msg, _ := line["msg"].(string)
+		requestID, _ := line["request_id"].(string)
+		logged[msg] = requestID
+	}
+
+	return logged
+}
+
+// TestImageLogLinesCarryRequestID verifies that the lines logged when an image
+// is fetched, converted and served through either image route carry the
+// request's ID as request_id, as the request log line does, so they can be
+// found from it.
+func TestImageLogLinesCarryRequestID(t *testing.T) {
+	t.Parallel()
+
+	const requestID = "test-request-id"
+
+	imageURLs := map[string]func(*testing.T, *Handlers) string{
+		"/v1/image/": signedPhotoURL,
+		"/v1/e/":     encPhotoURL,
+	}
+
+	for route, imageURL := range imageURLs {
+		t.Run(route, func(t *testing.T) {
+			t.Parallel()
+
+			var logs bytes.Buffer
+
+			h, srv := newSignedHostServer(t,
+				slog.New(slog.NewJSONHandler(&logs, nil)))
+
+			ctx := context.WithValue(t.Context(),
+				middleware.RequestIDKey, requestID)
+			rec := httptest.NewRecorder()
+			srv.ServeHTTP(rec, httptest.NewRequestWithContext(
+				ctx, http.MethodGet, imageURL(t, h), nil))
+
+			if rec.Code != http.StatusOK {
+				t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+			}
+
+			t.Logf("logged:\n%s", logs.String())
+
+			logged := requestIDByMessage(t, &logs)
+
+			for _, msg := range []string{
+				"upstream fetched", "image converted", "image served",
+			} {
+				got, ok := logged[msg]
+				if !ok {
+					t.Errorf("no %q line logged", msg)
+				} else if got != requestID {
+					t.Errorf("%q line has request_id %q, want %q",
+						msg, got, requestID)
+				}
+			}
+		})
+	}
+}

internal/handlers/robots_healthcheck_internal_test.go

@@ -0,0 +1,90 @@
+package handlers
+
+import (
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"testing"
+
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/globals"
+	"sneak.berlin/go/pixa/internal/healthcheck"
+	"sneak.berlin/go/pixa/internal/logger"
+)
+
+// TestHandleRobotsTxt checks that /robots.txt asks every crawler to stay off
+// the whole site.
+func TestHandleRobotsTxt(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	rec := sendGet(t, h.HandleRobotsTxt(), "/robots.txt")
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if ct := rec.Header().Get("Content-Type"); ct != "text/plain" {
+		t.Errorf("Content-Type = %q, want text/plain", ct)
+	}
+
+	want := "User-agent: *\nDisallow: /\n"
+	if rec.Body.String() != want {
+		t.Errorf("body = %q, want %q", rec.Body.String(), want)
+	}
+}
+
+// TestHandleHealthCheck checks that the health check answers 200 with status
+// ok, the app's name and version, now, uptime_seconds, uptime_human and
+// maintenance_mode, which is true here: the health check stays 200 while
+// maintenance mode is on.
+func TestHandleHealthCheck(t *testing.T) {
+	t.Parallel()
+
+	lc := fxtest.NewLifecycle(t)
+
+	log, err := logger.New(lc, logger.Params{Globals: &globals.Globals{}})
+	if err != nil {
+		t.Fatalf("logger.New() error = %v", err)
+	}
+
+	hc, err := healthcheck.New(lc, healthcheck.Params{
+		Globals: &globals.Globals{Appname: "pixad", Version: "v1.2.3"},
+		Config:  &config.Config{MaintenanceMode: true},
+		Logger:  log,
+	})
+	if err != nil {
+		t.Fatalf("healthcheck.New() error = %v", err)
+	}
+
+	h := &Handlers{hc: hc, log: slog.New(slog.DiscardHandler)}
+	rec := sendGet(t, h.HandleHealthCheck(), "/.well-known/healthcheck.json")
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if ct := rec.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", ct)
+	}
+
+	var body map[string]any
+
+	err = json.NewDecoder(rec.Body).Decode(&body)
+	if err != nil {
+		t.Fatalf("decoding response body: %v", err)
+	}
+
+	if body["status"] != "ok" || body["appname"] != "pixad" ||
+		body["version"] != "v1.2.3" || body["maintenance_mode"] != true {
+		t.Errorf("body = %v, want status ok, appname pixad, version v1.2.3 "+
+			"and maintenance_mode true", body)
+	}
+
+	for _, key := range []string{"now", "uptime_seconds", "uptime_human"} {
+		if _, ok := body[key]; !ok {
+			t.Errorf("body = %v, has no %s", body, key)
+		}
+	}
+}

internal/httpfetcher/fetch_internal_test.go

@@ -191,7 +191,23 @@ func fetchBody(t *testing.T, f *HTTPFetcher, path string) string {
 
 // semLen reports how many per-host semaphore slots are currently held.
 func semLen(f *HTTPFetcher, host string) int {
-	return len(f.getHostSemaphore(host))
+	f.hostSemMu.Lock()
+	defer f.hostSemMu.Unlock()
+
+	sem, ok := f.hostSems[host]
+	if !ok {
+		return 0
+	}
+
+	return len(sem.slots)
+}
+
+// hostSemCount reports how many hosts have a semaphore in hostSems.
+func hostSemCount(f *HTTPFetcher) int {
+	f.hostSemMu.Lock()
+	defer f.hostSemMu.Unlock()
+
+	return len(f.hostSems)
 }
 
 func TestFetchRedirectToPrivateIPBlocked(t *testing.T) {

internal/httpfetcher/httpfetcher.go

@@ -18,6 +18,8 @@ import (
 	"strings"
 	"sync"
 	"time"
+
+	"github.com/go-chi/chi/v5/middleware"
 )
 
 // Fetcher configuration constants.
@@ -160,10 +162,13 @@ func DefaultConfig() *Config {
 // HTTPFetcher implements Fetcher with SSRF protection and connection limits
 // per host and for all hosts together.
 type HTTPFetcher struct {
-	client    *http.Client
-	config    *Config
-	hostSems  map[string]chan struct{} // per-host semaphores
-	hostSemMu sync.Mutex               // protects hostSems map
+	client *http.Client
+	config *Config
+	// hostSems holds the semaphore of each host with a fetch holding or
+	// waiting for one of its slots; the entry is removed when the host's
+	// last such fetch gives its slot back or stops waiting.
+	hostSems  map[string]*hostSemaphore
+	hostSemMu sync.Mutex // protects hostSems and each entry's count
 	// allHostsSemaphore has one slot per connection allowed to all hosts
 	// together (config.MaxConnections).
 	allHostsSemaphore chan struct{}
@@ -171,6 +176,14 @@ type HTTPFetcher struct {
 	connectionWaitTimeout time.Duration
 }
 
+// hostSemaphore is one host's connection slots
+// (config.MaxConnectionsPerHost) and the number of fetches holding or
+// waiting for one of them.
+type hostSemaphore struct {
+	slots chan struct{}
+	count int
+}
+
 // New creates a new HTTPFetcher with SSRF protection.
 func New(config *Config) *HTTPFetcher {
 	if config == nil {
@@ -211,7 +224,7 @@ func New(config *Config) *HTTPFetcher {
 	return &HTTPFetcher{
 		client:                client,
 		config:                config,
-		hostSems:              make(map[string]chan struct{}),
+		hostSems:              make(map[string]*hostSemaphore),
 		allHostsSemaphore:     make(chan struct{}, config.MaxConnections),
 		connectionWaitTimeout: ConnectionWaitTimeout,
 	}
@@ -256,6 +269,13 @@ func (f *HTTPFetcher) Fetch(ctx context.Context, url string) (*FetchResult, erro
 	req.Header.Set("User-Agent", f.config.UserAgent)
 	req.Header.Set("Accept", strings.Join(f.config.AllowedContentTypes, ", "))
 
+	// The ID of the request this fetch serves, so the fetch can be found in
+	// the upstream host's logs
+	requestID := middleware.GetReqID(ctx)
+	if requestID != "" {
+		req.Header.Set(middleware.RequestIDHeader, requestID)
+	}
+
 	// Use httptrace to capture connection details
 	var remoteAddr string
 
@@ -307,6 +327,8 @@ func (f *HTTPFetcher) acquireConnection(
 	select {
 	case hostSem <- struct{}{}:
 	case <-ctx.Done():
+		f.putHostSemaphore(host)
+
 		return nil, ctx.Err()
 	}
 
@@ -314,32 +336,55 @@ func (f *HTTPFetcher) acquireConnection(
 	case f.allHostsSemaphore <- struct{}{}:
 	case <-time.After(f.connectionWaitTimeout):
 		<-hostSem
+		f.putHostSemaphore(host)
 
 		return nil, ErrTooManyConnections
 	case <-ctx.Done():
 		<-hostSem
+		f.putHostSemaphore(host)
 
 		return nil, ctx.Err()
 	}
 
 	return func() {
 		<-hostSem
+		f.putHostSemaphore(host)
 		<-f.allHostsSemaphore
 	}, nil
 }
 
-// getHostSemaphore returns the semaphore for a host, creating it if necessary.
+// getHostSemaphore returns the semaphore for a host, creating it if
+// necessary, and counts the caller among the fetches using it. The caller
+// calls putHostSemaphore once it holds no slot and waits for none.
 func (f *HTTPFetcher) getHostSemaphore(host string) chan struct{} {
 	f.hostSemMu.Lock()
 	defer f.hostSemMu.Unlock()
 
 	sem, ok := f.hostSems[host]
 	if !ok {
-		sem = make(chan struct{}, f.config.MaxConnectionsPerHost)
+		sem = &hostSemaphore{
+			slots: make(chan struct{}, f.config.MaxConnectionsPerHost),
+		}
 		f.hostSems[host] = sem
 	}
 
-	return sem
+	sem.count++
+
+	return sem.slots
+}
+
+// putHostSemaphore stops counting the caller among the fetches using the
+// host's semaphore, and removes the semaphore when no fetch uses it.
+func (f *HTTPFetcher) putHostSemaphore(host string) {
+	f.hostSemMu.Lock()
+	defer f.hostSemMu.Unlock()
+
+	sem := f.hostSems[host]
+
+	sem.count--
+	if sem.count == 0 {
+		delete(f.hostSems, host)
+	}
 }
 
 // buildResult validates the upstream response and assembles a FetchResult

internal/httpfetcher/max_connections_internal_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net"
 	"strconv"
+	"sync"
 	"testing"
 	"time"
 )
@@ -119,6 +120,98 @@ func TestFetchFreesHostSlotWhenContextEndsWaitingForConnection(t *testing.T) {
 	}
 }
 
+// TestFetchRemovesIdleHostSemaphores checks that a host's semaphore is
+// removed once no fetch holds or waits for one of its slots: after 100
+// concurrent fetches from 50 hosts have all finished, no semaphore is left.
+func TestFetchRemovesIdleHostSemaphores(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+	f, _ := newServerFetcher(t, srv, nil)
+	ctx := testContext(t)
+
+	var wg sync.WaitGroup
+
+	for i := range 100 {
+		wg.Go(func() {
+			res, err := f.Fetch(ctx, imageURLOnPort(1+i%50))
+			if err != nil {
+				t.Errorf("Fetch() error = %v", err)
+
+				return
+			}
+
+			_ = res.Content.Close()
+		})
+	}
+
+	wg.Wait()
+
+	if n := hostSemCount(f); n != 0 {
+		t.Errorf("%d host semaphores left after every fetch finished, want 0", n)
+	}
+}
+
+// TestFetchRemovesHostSemaphoreWhenNoConnection checks that a fetch that
+// ends without a connection leaves no semaphore behind: when it is refused
+// after waiting for a connection shared by all hosts, when its context ends
+// while it waits for its host's slot, and when its context ends while it
+// waits for a connection shared by all hosts, long before the 10 second
+// wait timeout.
+func TestFetchRemovesHostSemaphoreWhenNoConnection(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxConnections = 1
+	cfg.MaxConnectionsPerHost = 1
+
+	f, _ := newServerFetcher(t, srv, cfg)
+	f.connectionWaitTimeout = 100 * time.Millisecond
+
+	open, err := f.Fetch(testContext(t), imageURLOnPort(81))
+	if err != nil {
+		t.Fatalf("first Fetch() error = %v", err)
+	}
+
+	_, err = f.Fetch(testContext(t), imageURLOnPort(82))
+	if !errors.Is(err, ErrTooManyConnections) {
+		t.Fatalf("Fetch() from another host: error = %v, "+
+			"want ErrTooManyConnections", err)
+	}
+
+	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
+	defer cancel()
+
+	_, err = f.Fetch(ctx, imageURLOnPort(81))
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("Fetch() from the busy host: error = %v, "+
+			"want context.DeadlineExceeded", err)
+	}
+
+	// Back to the 10 second wait, so the next fetch's context ends first.
+	f.connectionWaitTimeout = ConnectionWaitTimeout
+
+	ctx, cancel = context.WithTimeout(t.Context(), 100*time.Millisecond)
+	defer cancel()
+
+	_, err = f.Fetch(ctx, imageURLOnPort(83))
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("Fetch() from a host with nothing open: error = %v, "+
+			"want context.DeadlineExceeded", err)
+	}
+
+	err = open.Content.Close()
+	if err != nil {
+		t.Fatalf("close first body: %v", err)
+	}
+
+	if n := hostSemCount(f); n != 0 {
+		t.Errorf("%d host semaphores left after every fetch finished, want 0", n)
+	}
+}
+
 // TestFetchReleasesConnectionOnError checks that a fetch that fails after
 // taking its connection gives it back: with MaxConnections at 1, the slot
 // must be free after the failure and the next fetch must succeed.

internal/httpfetcher/request_id_internal_test.go

@@ -0,0 +1,50 @@
+package httpfetcher
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/go-chi/chi/v5/middleware"
+)
+
+// TestFetchSendsRequestID verifies that a fetch sends the ID of the request
+// it serves, which the RequestID middleware stores in the request context,
+// to the upstream host as X-Request-Id, so the fetch can be found in that
+// host's logs.
+func TestFetchSendsRequestID(t *testing.T) {
+	t.Parallel()
+
+	const requestID = "test-request-id"
+
+	received := make(chan string, 1)
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			received <- r.Header.Get("X-Request-Id")
+
+			w.Header().Set("Content-Type", contentTypeJPEG)
+			_, _ = io.WriteString(w, imagePayload)
+		}))
+	t.Cleanup(srv.Close)
+
+	f, _ := newServerFetcher(t, srv, nil)
+
+	ctx := context.WithValue(testContext(t), middleware.RequestIDKey, requestID)
+
+	res, err := f.Fetch(ctx, upstreamURL("/image"))
+	if err != nil {
+		t.Fatalf("Fetch() error = %v", err)
+	}
+
+	_ = res.Content.Close()
+
+	got := <-received
+	t.Logf("upstream received X-Request-Id %q", got)
+
+	if got != requestID {
+		t.Errorf("upstream X-Request-Id = %q, want %q", got, requestID)
+	}
+}

internal/imgcache/cache.go

@@ -11,7 +11,6 @@ import (
 	"io"
 	"log/slog"
 	"path/filepath"
-	"sync"
 	"time"
 
 	lru "github.com/hashicorp/golang-lru/v2"
@@ -38,11 +37,16 @@ type CacheConfig struct {
 	NegativeTTL time.Duration
 
 	// MaxBytes is the disk cache size limit in bytes that eviction
-	// enforces. Zero means no limit is enforced (no eviction). The
-	// config layer supplies the computed default when the operator
-	// omits cache_max_bytes.
+	// enforces. Zero means no limit is enforced (no eviction).
 	MaxBytes int64
 
+	// UseDefaultMaxBytes makes NewCache replace MaxBytes with the
+	// default limit: 75% of the sum of the space free on the filesystem
+	// holding the cache and the bytes the cache already holds, at least
+	// DefaultCacheMaxBytesFloor. The config layer sets this when the
+	// operator omits cache_max_bytes.
+	UseDefaultMaxBytes bool
+
 	// DisableDiskCache turns the disk cache off entirely: no cache
 	// directories are created, lookups always miss, stores are
 	// no-ops, and no eviction machinery runs. The config layer sets
@@ -69,11 +73,11 @@ type Cache struct {
 
 	// Eviction machinery. The channels are created in NewCache so
 	// stores can signal write pressure without racing StartEviction.
+	// evictionCancel, set by StartEviction, cancels the eviction
+	// goroutine's context.
 	evictionPressure chan struct{}
-	evictionStop     chan struct{}
 	evictionDone     chan struct{}
-	evictionStarted  bool
-	evictionStopOnce sync.Once
+	evictionCancel   context.CancelFunc
 
 	// metaCache holds the content types of the variants most recently
 	// stored or served, so a hit does not read the variant's .meta file.
@@ -96,6 +100,14 @@ type Cache struct {
 
 // NewCache creates a new cache instance.
 func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
+	return newCache(db, config, defaultFreeSpaceProbe)
+}
+
+// newCache is NewCache with the free-space probe passed in, so tests
+// can fake the free space the default limit is worked out from.
+func newCache(
+	db *sql.DB, config CacheConfig, probe FreeSpaceProbeFunc,
+) (*Cache, error) {
 	log := config.Logger
 	if log == nil {
 		log = slog.Default()
@@ -112,7 +124,6 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 		log:              log,
 		disabled:         config.DisableDiskCache,
 		evictionPressure: make(chan struct{}, 1),
-		evictionStop:     make(chan struct{}),
 		evictionDone:     make(chan struct{}),
 		metaCache:        metaCache,
 		contentLocks:     newContentLock(),
@@ -147,6 +158,20 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 	c.variants = variants
 	c.srcMetadata = srcMetadata
 
+	if config.UseDefaultMaxBytes {
+		limit, err := c.computeDefaultMaxBytes(context.Background(), probe)
+		if err != nil {
+			return nil, err
+		}
+
+		c.config.MaxBytes = limit
+
+		log.Info("computed default cache size limit from free space and cache contents",
+			"cache_max_bytes", limit,
+			"cache_dir", filepath.Join(config.StateDir, "cache"),
+		)
+	}
+
 	return c, nil
 }
 

internal/imgcache/cachesize.go

@@ -0,0 +1,89 @@
+package imgcache
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math"
+	"path/filepath"
+	"syscall"
+)
+
+// DefaultCacheMaxBytesFloor is the minimum computed default for the
+// cache_max_bytes setting: 500 MiB. The floor applies only to the
+// computed default (when the key is omitted from the configuration),
+// never to explicitly configured values.
+const DefaultCacheMaxBytesFloor int64 = 524288000
+
+// freeSpaceFractionNumerator and freeSpaceFractionDenominator express
+// the 75% share used for the computed default limit as integer
+// arithmetic (dividing before multiplying avoids overflow).
+const (
+	freeSpaceFractionNumerator   uint64 = 3
+	freeSpaceFractionDenominator uint64 = 4
+)
+
+var errNegativeBlockSize = errors.New("statfs reported negative block size")
+
+// FreeSpaceProbeFunc reports the number of free bytes available on the
+// filesystem containing path. It is a function type so tests can
+// inject a fake probe instead of depending on the host disk.
+type FreeSpaceProbeFunc func(path string) (uint64, error)
+
+// defaultFreeSpaceProbe reports free filesystem bytes via statfs on
+// the given path, as available to unprivileged processes.
+func defaultFreeSpaceProbe(path string) (uint64, error) {
+	var stat syscall.Statfs_t
+
+	err := syscall.Statfs(path, &stat)
+	if err != nil {
+		return 0, err
+	}
+
+	if stat.Bsize < 0 {
+		return 0, fmt.Errorf("%w %d for %q", errNegativeBlockSize, stat.Bsize, path)
+	}
+
+	blockSize := uint64(stat.Bsize)
+
+	return stat.Bavail * blockSize, nil
+}
+
+// computeDefaultMaxBytes returns the default cache size limit: 75% of
+// the sum of the free bytes probe reports for <state_dir>/cache/ and
+// the bytes the cache already holds, with a floor of
+// DefaultCacheMaxBytesFloor. Counting what the cache holds keeps the
+// limit from shrinking as the cache fills.
+func (c *Cache) computeDefaultMaxBytes(
+	ctx context.Context, probe FreeSpaceProbeFunc,
+) (int64, error) {
+	cacheDir := filepath.Join(c.config.StateDir, "cache")
+
+	freeBytes, err := probe(cacheDir)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"default cache_max_bytes: cannot determine free space for %q: %w",
+			cacheDir, err)
+	}
+
+	usedBytes, err := c.UsageBytes(ctx)
+	if err != nil {
+		return 0, err
+	}
+
+	// Both terms are at most math.MaxInt64, so the sum cannot overflow.
+	//nolint:gosec // G115: UsageBytes sums file sizes, never negative
+	spaceBytes := min(freeBytes, math.MaxInt64) + uint64(usedBytes)
+
+	computed := spaceBytes / freeSpaceFractionDenominator * freeSpaceFractionNumerator
+	computed = min(computed, math.MaxInt64)
+
+	// gosec cannot see that min() above bounds computed, so it reads
+	// this conversion as potentially overflowing. It cannot: computed is
+	// at most math.MaxInt64 on every path here.
+	//nolint:gosec // G115: clamped to MaxInt64 by min above
+	limit := int64(computed)
+	limit = max(limit, DefaultCacheMaxBytesFloor)
+
+	return limit, nil
+}

internal/imgcache/cachesize_internal_test.go

@@ -0,0 +1,188 @@
+package imgcache
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// Static errors returned by the stub free-space probes below.
+var (
+	errTestStatfsFailed     = errors.New("statfs failed")
+	errTestProbeNotExpected = errors.New("probe must not be called")
+)
+
+// TestComputeDefaultMaxBytesCountsWhatTheCacheHolds verifies that the
+// default limit is 75% of the free space plus what the cache already
+// holds, so a cache filled to its limit keeps that limit across a
+// restart instead of shrinking to 75% of the space left free.
+func TestComputeDefaultMaxBytesCountsWhatTheCacheHolds(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	// Empty cache, 4 GiB free -> 3 GiB default.
+	got, err := cache.computeDefaultMaxBytes(t.Context(),
+		func(string) (uint64, error) { return 4294967296, nil })
+	if err != nil {
+		t.Fatalf("computeDefaultMaxBytes returned error: %v", err)
+	}
+
+	t.Logf("default for an empty cache with 4 GiB free: %d", got)
+
+	if got != 3221225472 {
+		t.Errorf("default for an empty cache = %d, want 3221225472 (75%% of 4 GiB)",
+			got)
+	}
+
+	// The cache now holds those 3 GiB, which leaves 1 GiB free.
+	_, err = cache.db.ExecContext(t.Context(),
+		`INSERT INTO variant_content (cache_key, size_bytes, content_type)
+		 VALUES (?, ?, ?)`,
+		string(testVariantKeyOne), 3221225472, testContentTypeWebP,
+	)
+	if err != nil {
+		t.Fatalf("failed to insert variant accounting row: %v", err)
+	}
+
+	got, err = cache.computeDefaultMaxBytes(t.Context(),
+		func(string) (uint64, error) { return 1073741824, nil })
+	if err != nil {
+		t.Fatalf("computeDefaultMaxBytes returned error: %v", err)
+	}
+
+	t.Logf("default for a cache holding 3 GiB with 1 GiB free: %d", got)
+
+	if got != 3221225472 {
+		t.Errorf("default for a cache holding 3 GiB with 1 GiB free = %d, "+
+			"want 3221225472 (75%% of 1 GiB + 3 GiB)", got)
+	}
+}
+
+// TestComputeDefaultMaxBytesAppliesFloor verifies that when 75% of the
+// free space plus what the cache holds is below 500 MiB, the default
+// is floored at DefaultCacheMaxBytesFloor.
+func TestComputeDefaultMaxBytesAppliesFloor(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name      string
+		freeBytes uint64
+	}{
+		{name: "100 MiB free", freeBytes: 104857600},
+		{name: "zero free", freeBytes: 0},
+		{name: "just below floor threshold", freeBytes: 699050665},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			cache, _ := newEvictionTestCache(t, 1<<30)
+
+			got, err := cache.computeDefaultMaxBytes(t.Context(),
+				func(string) (uint64, error) { return tc.freeBytes, nil })
+			if err != nil {
+				t.Fatalf("computeDefaultMaxBytes returned error: %v", err)
+			}
+
+			if got != DefaultCacheMaxBytesFloor {
+				t.Errorf("computeDefaultMaxBytes = %d, want floor %d",
+					got, DefaultCacheMaxBytesFloor)
+			}
+		})
+	}
+}
+
+// TestComputeDefaultMaxBytesPropagatesProbeError verifies that a
+// failing free-space probe produces an error naming cache_max_bytes,
+// instead of a silently wrong default.
+func TestComputeDefaultMaxBytesPropagatesProbeError(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	_, err := cache.computeDefaultMaxBytes(t.Context(),
+		func(string) (uint64, error) { return 0, errTestStatfsFailed })
+	if err == nil {
+		t.Fatal("probe failure must produce an error, got nil")
+	}
+
+	t.Logf("got expected error: %v", err)
+
+	if !strings.Contains(err.Error(), "cache_max_bytes") {
+		t.Errorf("error %q does not name cache_max_bytes", err.Error())
+	}
+}
+
+// TestNewCacheComputesDefaultMaxBytesWhenAsked verifies that with
+// UseDefaultMaxBytes set, the cache's limit becomes the computed
+// default, and that the probe is pointed at <state_dir>/cache/, which
+// must be created first so statfs measures the right filesystem.
+func TestNewCacheComputesDefaultMaxBytesWhenAsked(t *testing.T) {
+	t.Parallel()
+
+	stateDir := t.TempDir()
+	wantCacheDir := filepath.Join(stateDir, "cache")
+
+	var probedPath string
+
+	// 4 GiB free -> 3 GiB default.
+	probe := func(path string) (uint64, error) {
+		probedPath = path
+
+		info, err := os.Stat(path)
+		if err != nil || !info.IsDir() {
+			t.Errorf("cache directory %q was not created before probing: info=%v err=%v",
+				path, info, err)
+		}
+
+		return 4294967296, nil
+	}
+
+	cache, err := newCache(evictionTestDB(t), CacheConfig{
+		StateDir:           stateDir,
+		UseDefaultMaxBytes: true,
+	}, probe)
+	if err != nil {
+		t.Fatalf("newCache returned error: %v", err)
+	}
+
+	if cache.config.MaxBytes != 3221225472 {
+		t.Errorf("MaxBytes = %d, want computed default 3221225472",
+			cache.config.MaxBytes)
+	}
+
+	if probedPath != wantCacheDir {
+		t.Errorf("free space probed at %q, want cache directory %q",
+			probedPath, wantCacheDir)
+	}
+}
+
+// TestNewCacheKeepsExplicitMaxBytes verifies that without
+// UseDefaultMaxBytes the cache keeps MaxBytes exactly as given and
+// never consults the free-space probe.
+func TestNewCacheKeepsExplicitMaxBytes(t *testing.T) {
+	t.Parallel()
+
+	probe := func(string) (uint64, error) {
+		t.Error("free-space probe must not be consulted for explicit values")
+
+		return 0, errTestProbeNotExpected
+	}
+
+	cache, err := newCache(evictionTestDB(t), CacheConfig{
+		StateDir: t.TempDir(),
+		MaxBytes: 1024,
+	}, probe)
+	if err != nil {
+		t.Fatalf("newCache returned error: %v", err)
+	}
+
+	if cache.config.MaxBytes != 1024 {
+		t.Errorf("MaxBytes = %d, want explicit 1024 (no floor, no recompute)",
+			cache.config.MaxBytes)
+	}
+}

internal/imgcache/eviction.go

@@ -117,7 +117,10 @@ func (c *Cache) EvictToLimit(ctx context.Context) error {
 
 // evictBatch fetches one batch of LRU candidates across variants and
 // source blobs and evicts them oldest-first until excessBytes are
-// freed or the batch is exhausted. It returns the bytes freed.
+// freed or the batch is exhausted. It returns the bytes freed. A
+// candidate that fails once ctx is cancelled (every one started after
+// that fails at its first database call) ends the batch with ctx's
+// error, without a warning.
 func (c *Cache) evictBatch(ctx context.Context, excessBytes int64) (int64, error) {
 	candidates, err := c.evictionCandidates(ctx)
 	if err != nil {
@@ -133,6 +136,10 @@ func (c *Cache) evictBatch(ctx context.Context, excessBytes int64) (int64, error
 
 		err := c.evictCandidate(ctx, candidate)
 		if err != nil {
+			if ctx.Err() != nil {
+				return freed, ctx.Err()
+			}
+
 			c.log.Warn("failed to evict cache entry",
 				"cache_key", candidate.cacheKey,
 				"content_hash", candidate.contentHash,
@@ -283,7 +290,7 @@ func (c *Cache) evictVariant(ctx context.Context, cacheKey VariantKey) error {
 
 	c.metaCache.Remove(cacheKey)
 
-	err = c.variants.DeleteWithMeta(cacheKey)
+	err = c.variants.Delete(cacheKey)
 	if err != nil {
 		return err
 	}
@@ -424,37 +431,44 @@ func (c *Cache) notifyWritePressure() {
 // startup and again on every periodic tick thereafter, and evicts to
 // the configured limit on the given periodic interval and on
 // write-pressure notifications. It is a no-op on a disabled cache or
-// when already started.
+// when already started. The goroutine outlives the caller, so it runs
+// with its own context, which StopEviction cancels.
 func (c *Cache) StartEviction(interval time.Duration) {
-	if c.disabled || c.evictionStarted {
+	if c.disabled || c.evictionCancel != nil {
 		return
 	}
 
-	c.evictionStarted = true
+	ctx, cancel := context.WithCancel(context.Background())
+	c.evictionCancel = cancel
 
-	go c.evictionLoop(interval)
+	go c.evictionLoop(ctx, interval)
 }
 
-// StopEviction stops the background eviction goroutine and waits for
-// it to exit. It is safe to call when eviction was never started, and
-// safe to call more than once.
-func (c *Cache) StopEviction() {
-	if !c.evictionStarted {
-		return
+// StopEviction cancels the background eviction goroutine, which
+// interrupts a pass in progress, and waits for it to exit or for ctx to
+// end, whichever comes first. In the second case it returns an error
+// wrapping ctx's error. It is safe to call when eviction was never
+// started, and safe to call more than once.
+func (c *Cache) StopEviction(ctx context.Context) error {
+	if c.evictionCancel == nil {
+		return nil
 	}
 
-	c.evictionStopOnce.Do(func() {
-		close(c.evictionStop)
-		<-c.evictionDone
-	})
+	c.evictionCancel()
+
+	select {
+	case <-c.evictionDone:
+		return nil
+	case <-ctx.Done():
+		return fmt.Errorf("cache eviction still running: %w", ctx.Err())
+	}
 }
 
-// evictionLoop is the body of the background eviction goroutine.
-func (c *Cache) evictionLoop(interval time.Duration) {
+// evictionLoop is the body of the background eviction goroutine. It
+// returns when ctx is cancelled, and starts no pass after that.
+func (c *Cache) evictionLoop(ctx context.Context, interval time.Duration) {
 	defer close(c.evictionDone)
 
-	ctx := context.Background()
-
 	c.runReconciliationPass(ctx)
 	c.runEvictionPass(ctx)
 
@@ -463,7 +477,7 @@ func (c *Cache) evictionLoop(interval time.Duration) {
 
 	for {
 		select {
-		case <-c.evictionStop:
+		case <-ctx.Done():
 			return
 		case <-ticker.C:
 			// Reconciliation walks the cache directories, so it only
@@ -484,8 +498,13 @@ func (c *Cache) evictionLoop(interval time.Duration) {
 }
 
 // runEvictionPass runs one eviction pass, logging failures instead of
-// propagating them (the loop must keep running).
+// propagating them (the loop must keep running). It does nothing once
+// ctx is cancelled.
 func (c *Cache) runEvictionPass(ctx context.Context) {
+	if ctx.Err() != nil {
+		return
+	}
+
 	err := c.EvictToLimit(ctx)
 	if err != nil {
 		c.log.Warn("cache eviction pass failed", "error", err)
@@ -493,8 +512,13 @@ func (c *Cache) runEvictionPass(ctx context.Context) {
 }
 
 // runReconciliationPass runs one reconciliation pass, logging failures
-// instead of propagating them (the loop must keep running).
+// instead of propagating them (the loop must keep running). It does
+// nothing once ctx is cancelled.
 func (c *Cache) runReconciliationPass(ctx context.Context) {
+	if ctx.Err() != nil {
+		return
+	}
+
 	err := c.reconcileAccounting(ctx)
 	if err != nil {
 		c.log.Warn("cache accounting reconciliation failed", "error", err)
@@ -511,7 +535,8 @@ func (c *Cache) runReconciliationPass(ctx context.Context) {
 // know (and rows whose files are gone), and sweeps stale temp files
 // left behind by crashed writes. Running it periodically, not just
 // once, bounds how long such drift can accumulate unaccounted for on a
-// long-running process to one eviction interval.
+// long-running process to one eviction interval. Once ctx is cancelled,
+// it stops at the next file or row and returns ctx's error.
 func (c *Cache) reconcileAccounting(ctx context.Context) error {
 	if c.disabled {
 		return nil
@@ -546,6 +571,10 @@ func (c *Cache) reconcileVariantFiles(ctx context.Context) error {
 	return filepath.WalkDir(
 		c.variants.baseDir,
 		func(path string, entry fs.DirEntry, err error) error {
+			if ctx.Err() != nil {
+				return ctx.Err()
+			}
+
 			if err != nil || entry.IsDir() {
 				return err
 			}
@@ -634,6 +663,10 @@ func (c *Cache) reconcileVariantRows(ctx context.Context) error {
 	}
 
 	for _, key := range keys {
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+
 		if c.variants.Exists(key) {
 			continue
 		}
@@ -699,6 +732,10 @@ func (c *Cache) reconcileSourceFiles(ctx context.Context) error {
 	return filepath.WalkDir(
 		c.srcContent.baseDir,
 		func(path string, entry fs.DirEntry, err error) error {
+			if ctx.Err() != nil {
+				return ctx.Err()
+			}
+
 			if err != nil || entry.IsDir() {
 				return err
 			}
@@ -760,6 +797,10 @@ func (c *Cache) reconcileSourceRows(ctx context.Context) error {
 	}
 
 	for _, hash := range hashes {
+		if ctx.Err() != nil {
+			return ctx.Err()
+		}
+
 		if c.srcContent.Exists(hash) {
 			continue
 		}

internal/imgcache/eviction_internal_test.go

@@ -4,9 +4,12 @@ import (
 	"bytes"
 	"context"
 	"database/sql"
+	"errors"
 	"io/fs"
+	"log/slog"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -657,7 +660,7 @@ func TestEvictionRunsUnderWritePressure(t *testing.T) {
 	// An interval far longer than the test ensures only write
 	// pressure can trigger eviction here.
 	cache.StartEviction(time.Hour)
-	defer cache.StopEviction()
+	defer func() { _ = cache.StopEviction(t.Context()) }()
 
 	keys := []VariantKey{
 		testVariantKeyOne, testVariantKeyTwo, testVariantKeyThree,
@@ -678,6 +681,11 @@ func TestEvictionRunsUnderWritePressure(t *testing.T) {
 	assertNoDanglingReferences(t, cache)
 }
 
+// TestEvictionRunsOnPeriodicSchedule writes three variant files straight
+// to disk, bypassing StoreVariant, so they have no accounting rows and no
+// write-pressure notification fires. Only a periodic reconciliation pass
+// can then adopt them, and only the eviction pass that follows it can
+// evict them.
 func TestEvictionRunsOnPeriodicSchedule(t *testing.T) {
 	t.Parallel()
 
@@ -685,12 +693,28 @@ func TestEvictionRunsOnPeriodicSchedule(t *testing.T) {
 
 	cache, _ := newEvictionTestCache(t, limit)
 
-	// Start the evictor while the cache is empty, then create tracked
-	// over-limit state WITHOUT going through the store methods, so no
-	// write-pressure notification fires and only the periodic ticker
-	// can trigger eviction.
+	// Hold the test database's only connection, so the startup pass
+	// waits for it after walking the still empty variant directory: the
+	// files written while it waits are first seen by a periodic pass.
+	conn, err := cache.db.Conn(t.Context())
+	if err != nil {
+		t.Fatalf("failed to take the database connection: %v", err)
+	}
+
+	defer func() { _ = conn.Close() }()
+
 	cache.StartEviction(100 * time.Millisecond)
-	defer cache.StopEviction()
+	defer func() { _ = cache.StopEviction(t.Context()) }()
+
+	deadline := time.Now().Add(5 * time.Second)
+
+	for cache.db.Stats().WaitCount == 0 {
+		if time.Now().After(deadline) {
+			t.Fatal("the startup pass never waited for the database")
+		}
+
+		time.Sleep(10 * time.Millisecond)
+	}
 
 	keys := []VariantKey{
 		testVariantKeyOne, testVariantKeyTwo, testVariantKeyThree,
@@ -700,25 +724,43 @@ 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), "image/webp")
+		_, err = cache.variants.Store(key, bytes.NewReader(content), "image/webp")
 		if err != nil {
 			t.Fatalf("failed to store variant file: %v", err)
 		}
+	}
 
-		_, err = cache.db.ExecContext(t.Context(),
-			`INSERT INTO variant_content (cache_key, size_bytes, content_type)
-			 VALUES (?, ?, ?)`,
-			string(key), len(content), "image/webp",
-		)
-		if err != nil {
-			t.Fatalf("failed to insert variant accounting row: %v", err)
+	_ = conn.Close()
+
+	// Only one of the 1000-byte files fits under the limit: wait until
+	// the evictor has removed the other two.
+	stored := len(keys)
+	deadline = time.Now().Add(5 * time.Second)
+
+	for stored > 1 && time.Now().Before(deadline) {
+		time.Sleep(25 * time.Millisecond)
+
+		stored = 0
+
+		for _, key := range keys {
+			if cache.variants.Exists(key) {
+				stored++
+			}
 		}
 	}
 
-	usage := waitForUsageAtOrBelow(t, cache, limit, 5*time.Second)
+	if stored > 1 {
+		t.Fatalf("periodic schedule did not trigger eviction: %d of %d "+
+			"variant files still on disk, want at most 1", stored, len(keys))
+	}
+
+	usage, err := cache.UsageBytes(t.Context())
+	if err != nil {
+		t.Fatalf("UsageBytes failed: %v", err)
+	}
+
 	if usage > limit {
-		t.Errorf("periodic schedule did not trigger eviction: usage = %d, want <= %d",
-			usage, limit)
+		t.Errorf("usage after eviction = %d, want <= %d", usage, limit)
 	}
 
 	assertNoDanglingReferences(t, cache)
@@ -751,7 +793,7 @@ func TestStartEvictionReconcilesAccountingWithDisk(t *testing.T) {
 	}
 
 	cache.StartEviction(time.Hour)
-	defer cache.StopEviction()
+	defer func() { _ = cache.StopEviction(t.Context()) }()
 
 	deadline := time.Now().Add(5 * time.Second)
 
@@ -808,7 +850,7 @@ func TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup(t *testing.T) {
 	const interval = 100 * time.Millisecond
 
 	cache.StartEviction(interval)
-	defer cache.StopEviction()
+	defer func() { _ = cache.StopEviction(t.Context()) }()
 
 	// Let startup reconciliation run and settle on an empty cache
 	// before introducing the untracked file, so the adoption we assert
@@ -862,6 +904,242 @@ func TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup(t *testing.T) {
 	}
 }
 
+// TestStopEvictionInterruptsPassInProgress holds the test database's
+// only connection, so the startup reconciliation pass waits for it, and
+// checks that StopEviction stops that pass instead of waiting for the
+// connection to come free, and that the stop logs one warning: the
+// interrupted reconciliation's, with no eviction pass started after it.
+func TestStopEvictionInterruptsPassInProgress(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	var logBuf bytes.Buffer
+
+	cache.log = slog.New(slog.NewJSONHandler(&logBuf, nil))
+
+	conn, err := cache.db.Conn(t.Context())
+	if err != nil {
+		t.Fatalf("failed to take the database connection: %v", err)
+	}
+
+	defer func() { _ = conn.Close() }()
+
+	cache.StartEviction(time.Hour)
+
+	// The pass is in progress once it waits for the connection.
+	deadline := time.Now().Add(5 * time.Second)
+
+	for cache.db.Stats().WaitCount == 0 {
+		if time.Now().After(deadline) {
+			t.Fatal("the reconciliation pass never waited for the database")
+		}
+
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
+	defer cancel()
+
+	err = cache.StopEviction(ctx)
+	t.Logf("StopEviction() error = %v", err)
+
+	if err != nil {
+		t.Fatalf("StopEviction() error = %v, want nil: the pass waiting for "+
+			"the database did not stop", err)
+	}
+
+	t.Logf("log output: %s", logBuf.String())
+
+	warnings := strings.Count(logBuf.String(), `"level":"WARN"`)
+	if warnings != 1 {
+		t.Errorf("the stop logged %d warnings, want 1", warnings)
+	}
+}
+
+// TestEvictToLimitStopsAtNextCandidateOnceCancelled cancels the context
+// while the oldest of three source blobs is being evicted, and checks that
+// EvictToLimit then returns context.Canceled without evicting the other
+// two or logging a warning for either of them.
+func TestEvictToLimitStopsAtNextCandidateOnceCancelled(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1)
+
+	var logBuf bytes.Buffer
+
+	cache.log = slog.New(slog.NewJSONHandler(&logBuf, nil))
+
+	hashes := []ContentHash{
+		storeEvictionTestSource(t, cache, "cancel.example.com", "/a.jpg",
+			bytes.Repeat([]byte{0x61}, 1000)),
+		storeEvictionTestSource(t, cache, "cancel.example.com", "/b.jpg",
+			bytes.Repeat([]byte{0x62}, 1000)),
+		storeEvictionTestSource(t, cache, "cancel.example.com", "/c.jpg",
+			bytes.Repeat([]byte{0x63}, 1000)),
+	}
+
+	base := time.Now().Add(-time.Hour)
+
+	for i, hash := range hashes {
+		setSourceLastAccessed(t, cache, hash, base.Add(time.Duration(i)*time.Minute))
+	}
+
+	ctx, cancel := context.WithCancel(t.Context())
+	defer cancel()
+
+	cache.evictSourceBlobTestHook = func(ContentHash) { cancel() }
+
+	err := cache.EvictToLimit(ctx)
+	t.Logf("EvictToLimit() error = %v", err)
+	t.Logf("log output: %s", logBuf.String())
+
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("EvictToLimit() error = %v, want context.Canceled", err)
+	}
+
+	if cache.srcContent.Exists(hashes[0]) {
+		t.Errorf("source blob %s, evicted when the context was cancelled, "+
+			"is still on disk", hashes[0])
+	}
+
+	for _, hash := range hashes[1:] {
+		if !cache.srcContent.Exists(hash) {
+			t.Errorf("source blob %s was evicted after the context was cancelled", hash)
+		}
+	}
+
+	if strings.Contains(logBuf.String(), `"level":"WARN"`) {
+		t.Errorf("EvictToLimit logged a warning after the context was cancelled")
+	}
+
+	assertNoDanglingReferences(t, cache)
+}
+
+// TestStopEvictionReturnsWhenItsContextEnds pauses an eviction pass where
+// cancellation cannot reach it, after a source blob's rows are deleted and
+// before its file is removed, and checks that StopEviction returns its
+// context's error when that context ends instead of waiting for the pass.
+// Once the pass goes on, the goroutine exits and no row points at a
+// missing file.
+func TestStopEvictionReturnsWhenItsContextEnds(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1)
+
+	paused := make(chan struct{})
+	resume := make(chan struct{})
+
+	cache.evictSourceBlobTestHook = func(ContentHash) {
+		close(paused)
+		<-resume
+	}
+
+	hash := storeEvictionTestSource(t, cache, "stop.example.com", "/a.jpg",
+		bytes.Repeat([]byte{0x61}, 1000))
+
+	cache.StartEviction(time.Hour)
+
+	select {
+	case <-paused:
+	case <-time.After(5 * time.Second):
+		t.Fatal("the eviction pass never reached the source blob")
+	}
+
+	ctx, cancel := context.WithTimeout(t.Context(), 50*time.Millisecond)
+	defer cancel()
+
+	err := cache.StopEviction(ctx)
+	t.Logf("StopEviction() error = %v", err)
+
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Errorf("StopEviction() error = %v, want context.DeadlineExceeded", err)
+	}
+
+	close(resume)
+
+	err = cache.StopEviction(t.Context())
+	if err != nil {
+		t.Fatalf("second StopEviction() error = %v, want nil", err)
+	}
+
+	assertNoDanglingReferences(t, cache)
+
+	if cache.srcContent.Exists(hash) {
+		t.Errorf("source blob %s is still on disk after its rows were deleted", hash)
+	}
+}
+
+// TestReconciliationWalksStopOnceCancelled checks that both directory
+// walks of a reconciliation pass return the context's error once it is
+// cancelled, leaving in place a stale temp file they would otherwise
+// remove.
+func TestReconciliationWalksStopOnceCancelled(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	staleTime := time.Now().Add(-2 * staleTempFileAge)
+
+	walks := map[string]func(context.Context) error{
+		cache.variants.baseDir:   cache.reconcileVariantFiles,
+		cache.srcContent.baseDir: cache.reconcileSourceFiles,
+	}
+
+	for dir, walk := range walks {
+		tempFile := filepath.Join(dir, tempFilePrefix+"stale")
+
+		err := os.WriteFile(tempFile, []byte("partial"), 0o600)
+		if err != nil {
+			t.Fatalf("failed to write temp file: %v", err)
+		}
+
+		err = os.Chtimes(tempFile, staleTime, staleTime)
+		if err != nil {
+			t.Fatalf("failed to backdate temp file: %v", err)
+		}
+
+		err = walk(ctx)
+		t.Logf("walk of %s: error = %v", dir, err)
+
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("walk of %s: error = %v, want context.Canceled", dir, err)
+		}
+
+		_, err = os.Stat(tempFile)
+		if err != nil {
+			t.Errorf("walk of %s went on after cancellation: %v", dir, err)
+		}
+	}
+}
+
+// TestReconciliationPassLogsNoWarningOnceCancelled checks that a
+// reconciliation pass run with an already cancelled context logs no
+// warning, so a periodic tick the loop takes after a stop adds no
+// warning to the one from the pass the stop interrupted.
+func TestReconciliationPassLogsNoWarningOnceCancelled(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	var logBuf bytes.Buffer
+
+	cache.log = slog.New(slog.NewJSONHandler(&logBuf, nil))
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	cache.runReconciliationPass(ctx)
+	t.Logf("log output: %s", logBuf.String())
+
+	if strings.Contains(logBuf.String(), `"level":"WARN"`) {
+		t.Errorf("runReconciliationPass logged a warning with a cancelled context")
+	}
+}
+
 // TestEvictSourceBlobExcludesConcurrentStoreOfIdenticalContent exercises
 // the exact TOCTOU window between evictSourceBlob's row-deletion
 // transaction commit and its content file unlink: a concurrent

internal/imgcache/imgcache.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"errors"
 	"io"
-	"net/url"
 	"time"
 )
 
@@ -154,9 +153,6 @@ type ImageCache interface {
 	// Warm pre-fetches and caches an image without returning it
 	Warm(ctx context.Context, req *ImageRequest) error
 
-	// Purge removes a cached image
-	Purge(ctx context.Context, req *ImageRequest) error
-
 	// Stats returns cache statistics
 	Stats(ctx context.Context) (*CacheStats, error)
 }
@@ -176,29 +172,3 @@ type CacheStats struct {
 	// HitRate is HitCount / (HitCount + MissCount)
 	HitRate float64
 }
-
-// SignatureValidator validates request signatures
-type SignatureValidator interface {
-	// Validate checks if the signature is valid for the request
-	Validate(req *ImageRequest) error
-	// Generate creates a signature for a request
-	Generate(req *ImageRequest) string
-}
-
-// Allowlist checks if a URL is allowlisted (no signature required)
-type Allowlist interface {
-	// IsAllowlisted returns true if the URL doesn't require a signature
-	IsAllowlisted(u *url.URL) bool
-}
-
-// Storage handles persistent storage of cached content
-type Storage interface {
-	// Store saves content and returns its hash
-	Store(ctx context.Context, content io.Reader) (hash string, err error)
-	// Load retrieves content by hash
-	Load(ctx context.Context, hash string) (io.ReadCloser, error)
-	// Delete removes content by hash
-	Delete(ctx context.Context, hash string) error
-	// Exists checks if content exists
-	Exists(ctx context.Context, hash string) (bool, error)
-}

internal/imgcache/service.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/dustin/go-humanize"
 	"github.com/getsentry/sentry-go"
+	"github.com/go-chi/chi/v5/middleware"
 	"golang.org/x/sync/singleflight"
 	"sneak.berlin/go/pixa/internal/allowlist"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
@@ -56,11 +57,10 @@ type ServiceConfig struct {
 	Logger *slog.Logger
 }
 
-// Static errors for service construction and unimplemented operations.
+// Static errors for service construction.
 var (
-	errCacheRequired       = errors.New("cache is required")
-	errSigningKeyRequired  = errors.New("signing key is required")
-	errPurgeNotImplemented = errors.New("purge not implemented")
+	errCacheRequired      = errors.New("cache is required")
+	errSigningKeyRequired = errors.New("signing key is required")
 )
 
 // NewService creates a new image service.
@@ -189,11 +189,6 @@ func (s *Service) Warm(ctx context.Context, req *ImageRequest) error {
 	return err
 }
 
-// Purge removes a cached image. Purging is not implemented yet.
-func (s *Service) Purge(_ context.Context, _ *ImageRequest) error {
-	return errPurgeNotImplemented
-}
-
 // Stats returns cache statistics.
 func (s *Service) Stats(ctx context.Context) (*CacheStats, error) {
 	return s.cache.Stats(ctx)
@@ -467,6 +462,7 @@ func (s *Service) fetchAndProcess(
 
 	// Log upstream fetch details
 	s.log.Info("upstream fetched",
+		"request_id", middleware.GetReqID(ctx),
 		"host", req.SourceHost,
 		"path", req.SourcePath,
 		"bytes", fetchBytes,
@@ -551,6 +547,7 @@ func (s *Service) processAndStore(
 	}
 
 	s.log.Info("image converted",
+		"request_id", middleware.GetReqID(ctx),
 		"host", req.SourceHost,
 		"path", req.SourcePath,
 		"src_format", processResult.InputFormat,

internal/imgcache/storage.go

@@ -564,7 +564,8 @@ func (s *VariantStorage) Exists(key VariantKey) bool {
 	return err == nil
 }
 
-// Delete removes content at the given key.
+// Delete removes the content at the given key together with its .meta
+// sidecar file. A missing file is not an error.
 func (s *VariantStorage) Delete(key VariantKey) error {
 	path := s.keyToPath(key)
 
@@ -573,18 +574,7 @@ func (s *VariantStorage) Delete(key VariantKey) error {
 		return fmt.Errorf("failed to delete content: %w", err)
 	}
 
-	return nil
-}
-
-// DeleteWithMeta removes the content at the given key together with
-// its .meta sidecar file. A missing file is not an error.
-func (s *VariantStorage) DeleteWithMeta(key VariantKey) error {
-	err := s.Delete(key)
-	if err != nil {
-		return err
-	}
-
-	metaPath := s.keyToPath(key) + ".meta"
+	metaPath := path + ".meta"
 
 	err = os.Remove(metaPath)
 	if err != nil && !os.IsNotExist(err) {

internal/imgcache/storage_internal_test.go

@@ -438,3 +438,73 @@ func TestVariantStorage_StoreLogsFailedMetaWrite(t *testing.T) {
 		t.Errorf("log missing %s; got %q", want, logBuf.String())
 	}
 }
+
+// storeTestVariant stores one variant, with its .meta file, in a new
+// VariantStorage and returns the storage and the variant's key.
+func storeTestVariant(t *testing.T) (*VariantStorage, VariantKey) {
+	t.Helper()
+
+	storage, err := NewVariantStorage(t.TempDir(), slog.New(slog.DiscardHandler))
+	if err != nil {
+		t.Fatalf("NewVariantStorage() error = %v", err)
+	}
+
+	key := CacheKey(&ImageRequest{SourceHost: testHostCDN, SourcePath: testPathCat})
+
+	_, err = storage.Store(key, bytes.NewReader([]byte("variant data")), "image/webp")
+	if err != nil {
+		t.Fatalf("Store() error = %v", err)
+	}
+
+	return storage, key
+}
+
+// TestVariantStorage_DeleteRemovesMeta verifies that Delete removes the
+// variant's .meta file along with the variant file.
+func TestVariantStorage_DeleteRemovesMeta(t *testing.T) {
+	t.Parallel()
+
+	storage, key := storeTestVariant(t)
+	metaPath := storage.keyToPath(key) + ".meta"
+
+	_, err := os.Stat(metaPath)
+	if err != nil {
+		t.Fatalf("Store() wrote no .meta file: %v", err)
+	}
+
+	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")
+	}
+
+	_, err = os.Stat(metaPath)
+	if !os.IsNotExist(err) {
+		t.Errorf(".meta file left after Delete() (stat err=%v)", err)
+	}
+}
+
+// TestVariantStorage_DeleteWithoutMeta verifies that Delete succeeds for a
+// variant whose .meta file is missing.
+func TestVariantStorage_DeleteWithoutMeta(t *testing.T) {
+	t.Parallel()
+
+	storage, key := storeTestVariant(t)
+
+	err := os.Remove(storage.keyToPath(key) + ".meta")
+	if err != nil {
+		t.Fatalf("removing .meta file: %v", err)
+	}
+
+	err = storage.Delete(key)
+	if err != nil {
+		t.Fatalf("Delete() error = %v, want nil", err)
+	}
+
+	if storage.Exists(key) {
+		t.Error("Exists() = true after delete, want false")
+	}
+}

internal/middleware/middleware.go

@@ -2,9 +2,12 @@
 package middleware
 
 import (
+	"context"
+	"crypto/rand"
 	"log/slog"
 	"net/http"
 	"net/netip"
+	"regexp"
 	"time"
 
 	basicauth "github.com/99designs/basicauth-go"
@@ -115,6 +118,32 @@ func (s *Middleware) RateLimit(
 		})
 }
 
+// requestIDPattern is what a request's own X-Request-Id must look like to be
+// kept as its ID: 1 to 64 letters, digits, '-', '_' or '.'.
+var requestIDPattern = regexp.MustCompile(`^[A-Za-z0-9._-]{1,64}$`)
+
+// RequestID returns a middleware that gives each request an ID and sends it as
+// the X-Request-Id response header, so a client can quote it when reporting a
+// problem. The ID is the request's own X-Request-Id when that matches
+// requestIDPattern, and otherwise a random one, which tells nothing about the
+// machine or the traffic. It is stored in the request context under chi's
+// RequestIDKey, where the logging middleware, the handlers and the upstream
+// fetch read it.
+func (s *Middleware) RequestID() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			id := r.Header.Get(middleware.RequestIDHeader)
+			if !requestIDPattern.MatchString(id) {
+				id = rand.Text()
+			}
+
+			w.Header().Set(middleware.RequestIDHeader, id)
+			ctx := context.WithValue(r.Context(), middleware.RequestIDKey, id)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
 type loggingResponseWriter struct {
 	http.ResponseWriter
 

internal/middleware/middleware_internal_test.go

@@ -1,11 +1,16 @@
 package middleware
 
 import (
+	"bytes"
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
+	"strings"
 	"testing"
 
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
 	"sneak.berlin/go/pixa/internal/config"
 )
 
@@ -56,6 +61,203 @@ func TestCORSAnswersWithConfiguredOrigin(t *testing.T) {
 	}
 }
 
+// TestCORSAnswersPreflightWithConfiguredOrigin checks that the CORS
+// middleware answers a preflight request, which the CORS library handles
+// apart from other requests, the same way: "*" lets any origin read
+// responses and a single origin lets only that origin read them.
+func TestCORSAnswersPreflightWithConfiguredOrigin(t *testing.T) {
+	t.Parallel()
+
+	const appOrigin = "https://app.example.com"
+
+	cases := []struct {
+		configured    string
+		requestOrigin string
+		want          string
+	}{
+		{"*", "https://any.example.com", "*"},
+		{appOrigin, appOrigin, appOrigin},
+		{appOrigin, "https://other.example.com", ""},
+	}
+
+	testHandler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	for _, tc := range cases {
+		mw := &Middleware{
+			log:    slog.Default(),
+			config: &config.Config{AccessControlAllowOrigin: tc.configured},
+		}
+
+		handler := mw.CORS()(testHandler)
+
+		// An OPTIONS request naming the method it asks about is the
+		// preflight a browser sends before some cross-origin requests.
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodOptions, "/v1/image/example.com/a.jpg/1x1.png", nil)
+		req.Header.Set("Origin", tc.requestOrigin)
+		req.Header.Set("Access-Control-Request-Method", http.MethodGet)
+
+		rec := httptest.NewRecorder()
+
+		handler.ServeHTTP(rec, req)
+
+		got := rec.Header().Get("Access-Control-Allow-Origin")
+		if got != tc.want {
+			t.Errorf("configured %q, preflight from %q: "+
+				"Access-Control-Allow-Origin = %q, want %q",
+				tc.configured, tc.requestOrigin, got, tc.want)
+		}
+	}
+}
+
+// TestMetricsAuthRequiresConfiguredCredentials checks that MetricsAuth on
+// its own answers 401 with a challenge to a request without credentials or
+// with a wrong username or password, and lets a request with the configured
+// username and password through. That the router puts it in front of
+// /metrics is not tested.
+func TestMetricsAuthRequiresConfiguredCredentials(t *testing.T) {
+	t.Parallel()
+
+	const (
+		username  = "metricsuser"
+		password  = "metricspass"
+		challenge = `Basic realm="metrics"`
+	)
+
+	// An empty username stands for a request sent without credentials.
+	cases := []struct {
+		name        string
+		username    string
+		password    string
+		wantReached bool
+	}{
+		{"no credentials", "", "", false},
+		{"wrong username", "someone", password, false},
+		{"wrong password", username, "wrongpass", false},
+		{"configured credentials", username, password, true},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			mw := &Middleware{
+				log: slog.Default(),
+				config: &config.Config{
+					MetricsUsername: username,
+					MetricsPassword: password,
+				},
+			}
+
+			reached := false
+			handler := mw.MetricsAuth()(http.HandlerFunc(
+				func(http.ResponseWriter, *http.Request) {
+					reached = true
+				}))
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/metrics", nil)
+
+			if tc.username != "" {
+				req.SetBasicAuth(tc.username, tc.password)
+			}
+
+			rec := httptest.NewRecorder()
+
+			handler.ServeHTTP(rec, req)
+
+			if reached != tc.wantReached {
+				t.Fatalf("request reached /metrics = %v, want %v",
+					reached, tc.wantReached)
+			}
+
+			if tc.wantReached {
+				return
+			}
+
+			if rec.Code != http.StatusUnauthorized {
+				t.Errorf("status = %d, want %d",
+					rec.Code, http.StatusUnauthorized)
+			}
+
+			if got := rec.Header().Get("WWW-Authenticate"); got != challenge {
+				t.Errorf("WWW-Authenticate = %q, want %q", got, challenge)
+			}
+		})
+	}
+}
+
+// TestMetricsRecordsServedRequest checks that the metrics middleware
+// records a request it served, so /metrics reports it. It is the only test
+// in this package that sets up the metrics middleware, which registers with
+// the process-wide Prometheus registry and can do so only once.
+func TestMetricsRecordsServedRequest(t *testing.T) {
+	t.Parallel()
+
+	// The line /metrics shows once one GET /test has been served.
+	const want = `http_request_duration_seconds_count{` +
+		`code="200",handler="/test",method="GET",service=""} 1`
+
+	mw := &Middleware{log: slog.Default(), config: &config.Config{}}
+
+	handler := mw.Metrics()(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}))
+
+	handler.ServeHTTP(httptest.NewRecorder(), httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/test", nil))
+
+	rec := httptest.NewRecorder()
+	promhttp.Handler().ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/metrics", nil))
+
+	if !strings.Contains(rec.Body.String(), want) {
+		t.Errorf("/metrics does not report the GET /test served; "+
+			"want the line %q in:\n%s", want, rec.Body.String())
+	}
+}
+
+// TestLoggingLeavesOutSubmittedSigningKey checks that a login, a POST /
+// whose form carries the signing key, leaves no trace of the key in the
+// request's log line.
+func TestLoggingLeavesOutSubmittedSigningKey(t *testing.T) {
+	t.Parallel()
+
+	const signingKey = "test-signing-key-0123456789abcdef"
+
+	var buf bytes.Buffer
+
+	mw := newTestMiddleware(t, &buf)
+
+	// The handler reads the key from the form, as the login handler does.
+	handler := mw.Logging()(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			if got := r.FormValue("key"); got != signingKey {
+				t.Errorf("key in form = %q, want %q", got, signingKey)
+			}
+
+			w.WriteHeader(http.StatusSeeOther)
+		}))
+
+	form := url.Values{"key": {signingKey}}
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodPost, "/",
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	handler.ServeHTTP(httptest.NewRecorder(), req)
+
+	if !strings.Contains(buf.String(), `"method":"POST"`) {
+		t.Fatalf("no log line for the request; got %q", buf.String())
+	}
+
+	if strings.Contains(buf.String(), signingKey) {
+		t.Errorf("log output contains the signing key; got %q", buf.String())
+	}
+}
+
 func TestSecurityHeaders(t *testing.T) {
 	t.Parallel()
 

internal/middleware/request_id_internal_test.go

@@ -0,0 +1,118 @@
+package middleware
+
+import (
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5/middleware"
+
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// sendRequestID sends a request through the RequestID middleware, carrying
+// incoming as its X-Request-Id unless that is empty. It returns the ID the
+// next handler found in the request context, which the upstream fetch sends
+// and the log lines carry, and the X-Request-Id of the response.
+func sendRequestID(t *testing.T, incoming string) (string, string) {
+	t.Helper()
+
+	mw := &Middleware{log: slog.Default(), config: &config.Config{}}
+
+	var inContext string
+
+	handler := mw.RequestID()(http.HandlerFunc(
+		func(_ http.ResponseWriter, r *http.Request) {
+			inContext = middleware.GetReqID(r.Context())
+		}))
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", nil)
+	if incoming != "" {
+		req.Header.Set("X-Request-Id", incoming)
+	}
+
+	rec := httptest.NewRecorder()
+	handler.ServeHTTP(rec, req)
+
+	return inContext, rec.Header().Get("X-Request-Id")
+}
+
+// TestRequestIDKeepsShortPlainID verifies that a request's own X-Request-Id of
+// at most 64 letters, digits, '-', '_' or '.' is kept as its ID.
+func TestRequestIDKeepsShortPlainID(t *testing.T) {
+	t.Parallel()
+
+	for _, incoming := range []string{
+		"client-request-id",
+		"A1_b2.c3-d4",
+		strings.Repeat("a", 64),
+	} {
+		inContext, inResponse := sendRequestID(t, incoming)
+
+		if inContext != incoming || inResponse != incoming {
+			t.Errorf("incoming %q: context has %q, response %q, want both %q",
+				incoming, inContext, inResponse, incoming)
+		}
+	}
+}
+
+// TestRequestIDReplacesLongOrUnusualID verifies that a request's own
+// X-Request-Id that is over 64 characters or holds anything but letters,
+// digits, '-', '_' or '.' is neither sent back nor sent upstream: the request
+// gets a fresh ID instead.
+func TestRequestIDReplacesLongOrUnusualID(t *testing.T) {
+	t.Parallel()
+
+	for _, incoming := range []string{
+		strings.Repeat("a", 65),
+		strings.Repeat("a", 9000),
+		"has space",
+		"a/b",
+		"a,b",
+		"<script>",
+		"ünicode",
+	} {
+		inContext, inResponse := sendRequestID(t, incoming)
+		t.Logf("incoming %.20q: made up %q", incoming, inResponse)
+
+		if inResponse == "" || inResponse == incoming {
+			t.Errorf("incoming %.20q: response has %q, want a fresh ID",
+				incoming, inResponse)
+		}
+
+		if inContext != inResponse {
+			t.Errorf("incoming %.20q: context has %q, want the response's %q",
+				incoming, inContext, inResponse)
+		}
+	}
+}
+
+// TestRequestIDMadeUpTellsNothing verifies that the ID made up for a request
+// that sent none differs for every request and does not hold the host name.
+func TestRequestIDMadeUpTellsNothing(t *testing.T) {
+	t.Parallel()
+
+	hostname, err := os.Hostname()
+	if err != nil {
+		t.Fatalf("os.Hostname() error = %v", err)
+	}
+
+	firstInContext, first := sendRequestID(t, "")
+	_, second := sendRequestID(t, "")
+	t.Logf("host %q, made up %q and %q", hostname, first, second)
+
+	if first == "" || first == second {
+		t.Errorf("made up %q and %q, want two different IDs", first, second)
+	}
+
+	if firstInContext != first {
+		t.Errorf("context has %q, want the response's %q", firstInContext, first)
+	}
+
+	if strings.Contains(first, hostname) {
+		t.Errorf("made-up ID %q holds the host name %q", first, hostname)
+	}
+}

internal/server/metrics_internal_test.go

@@ -0,0 +1,32 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// TestNoMetricsRecordedWithoutMetricsUsername checks that with no metrics
+// username set the router records nothing about the requests it serves.
+// /metrics is not served then, so the process-wide Prometheus registry is
+// read directly. TestMaintenanceModeKeepsOtherRoutes records into the same
+// registry, but never a GET /robots.txt.
+func TestNoMetricsRecordedWithoutMetricsUsername(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+	s.ServeHTTP(httptest.NewRecorder(), httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/robots.txt", nil))
+
+	rec := httptest.NewRecorder()
+	promhttp.Handler().ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/metrics", nil))
+
+	if strings.Contains(rec.Body.String(), `handler="/robots.txt"`) {
+		t.Errorf("with no metrics username GET /robots.txt was recorded:\n%s",
+			rec.Body.String())
+	}
+}

internal/server/request_id_internal_test.go

@@ -0,0 +1,58 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// requestIDHeader is the header that carries a request's ID.
+const requestIDHeader = "X-Request-Id"
+
+// TestResponsesCarryRequestID verifies that every response, whatever its route
+// and status, carries the request's ID as X-Request-Id, so a client can quote
+// it when reporting a problem: one pixa made up when the request brought none,
+// and the request's own X-Request-Id when it brought one.
+func TestResponsesCarryRequestID(t *testing.T) {
+	t.Parallel()
+
+	const clientRequestID = "client-request-id"
+
+	s := newTestServer(t)
+
+	paths := []string{
+		"/robots.txt",
+		"/no-such-path",
+		unsignedImagePath,
+		encryptedImagePath,
+	}
+
+	for _, path := range paths {
+		t.Run(path, func(t *testing.T) {
+			t.Parallel()
+
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, path, nil))
+			t.Logf("status %d, %s %q",
+				rec.Code, requestIDHeader, rec.Header().Get(requestIDHeader))
+
+			if rec.Header().Get(requestIDHeader) == "" {
+				t.Errorf("response has no %s", requestIDHeader)
+			}
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, path, nil)
+			req.Header.Set(requestIDHeader, clientRequestID)
+
+			rec = httptest.NewRecorder()
+			s.ServeHTTP(rec, req)
+
+			got := rec.Header().Get(requestIDHeader)
+			if got != clientRequestID {
+				t.Errorf("%s = %q, want the request's own %q",
+					requestIDHeader, got, clientRequestID)
+			}
+		})
+	}
+}

internal/server/routes.go

@@ -28,7 +28,7 @@ func (s *Server) SetupRoutes() {
 	s.router = chi.NewRouter()
 
 	s.router.Use(middleware.Recoverer)
-	s.router.Use(middleware.RequestID)
+	s.router.Use(s.mw.RequestID())
 	s.router.Use(s.mw.ClientIP())
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
@@ -97,6 +97,7 @@ func (s *Server) SetupRoutes() {
 			// The trailing filename (e.g., /img.jpg) is ignored but helps
 			// browsers with content type
 			r.Get("/e/{token}/*", s.h.HandleImageEnc())
+			r.Head("/e/{token}/*", s.h.HandleImageEnc())
 		})
 	})
 

internal/server/routes_internal_test.go

@@ -0,0 +1,27 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestEncryptedImageRouteAnswersHEAD verifies that HEAD on the encrypted image
+// route reaches its handler, as GET does, instead of being answered 405 Method
+// Not Allowed. The handler refuses a token it cannot decrypt with 400, so that
+// status shows the request got through.
+func TestEncryptedImageRouteAnswersHEAD(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodHead, encryptedImagePath, nil))
+	t.Logf("status %d", rec.Code)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d from the encrypted image handler",
+			rec.Code, http.StatusBadRequest)
+	}
+}