Container makes /var/lib/pixa usable before starting pixad (closes #159)

The entrypoint now creates /var/lib/pixa if it is missing. When the
directory or one of its top-level entries belongs to another user or
group, it gives the whole tree to pixad (uid and gid 65532); it then
sets the directory's mode to 750 and runs the server as pixad as
before. Data left by a run under another uid is taken over this way.
Only the top level is checked, so a normal start does not walk the
cache; the tree is changed deepest first, so an interrupted start is
finished by the next one.

"Running under upaas" in README.md no longer tells the operator to
create or chown the host directory.

Model: opus-5-5

Dockerfile

@@ -3,13 +3,14 @@
 # golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
 FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60 AS lint
 
-RUN apk add --no-cache make build-base vips-dev libheif-dev pkgconfig
-
 WORKDIR /src
 
-# Copy go mod files first for better layer caching
+# script/bootstrap installs the build dependencies and downloads the Go
+# modules. Only script/, go.mod and go.sum are copied first, so this
+# layer is reused until one of them changes.
+COPY script/ ./script/
 COPY go.mod go.sum ./
-RUN go mod download
+RUN script/bootstrap
 
 # Copy source code
 COPY . .
@@ -28,20 +29,12 @@ FROM golang:1.25.4-alpine@sha256:d3f0cf7723f3429e3f9ed846243970b20a2de7bae6a5b66
 # Depend on lint stage passing
 COPY --from=lint /src/go.sum /dev/null
 
-ARG VERSION=dev
-
-# Install build dependencies for CGO image libraries
-RUN apk add --no-cache \
-    build-base \
-    vips-dev \
-    libheif-dev \
-    pkgconfig
-
 WORKDIR /src
 
-# Copy go mod files first for better layer caching
+# Build dependencies and Go modules, as in the lint stage
+COPY script/ ./script/
 COPY go.mod go.sum ./
-RUN GOTOOLCHAIN=auto go mod download
+RUN script/bootstrap
 
 # Copy source code
 COPY . .
@@ -49,8 +42,14 @@ COPY . .
 # Run tests
 RUN make test
 
-# Build with CGO enabled
+# VERSION is declared here, not earlier: a new value reruns only the
-RUN CGO_ENABLED=1 GOTOOLCHAIN=auto go build -ldflags "-X main.Version=${VERSION}" -o /pixad ./cmd/pixad
+# build, not script/bootstrap or the tests. CGO stays enabled for
+# govips; -trimpath keeps build paths out of the binary, and -s -w
+# leave out the symbol table and debug information.
+ARG VERSION=dev
+RUN CGO_ENABLED=1 GOTOOLCHAIN=auto go build -trimpath \
+    -ldflags "-s -w -X main.Version=${VERSION}" \
+    -o /pixad ./cmd/pixad
 
 # Runtime stage
 # alpine:3.21, 2026-02-25
@@ -68,8 +67,12 @@ RUN apk add --no-cache \
 COPY --from=builder /pixad /usr/local/bin/pixad
 COPY deploy/docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
 
-# Create non-root user, config directory, and data directory
+# Create non-root user, config directory, and data directory. pixad
-RUN adduser -D -H -s /sbin/nologin pixad && \
+# gets uid and gid 65532, which host login and system accounts do not
+# use: a bind-mounted /var/lib/pixa is given to pixad, and on the host
+# it must not belong to a person's account.
+RUN addgroup -g 65532 pixad && \
+    adduser -D -H -s /sbin/nologin -u 65532 -G pixad pixad && \
     mkdir -p /var/lib/pixa /etc/pixa && \
     chown pixad:pixad /var/lib/pixa
 

Dockerfile.lint

@@ -6,16 +6,16 @@
 # golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
 FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60
 
-# pixa is CGO/libvips: the type-aware linters compile every package, so
-# this image needs the same C libraries the build does.
-RUN apk add --no-cache build-base vips-dev libheif-dev pkgconfig
-
 WORKDIR /src
 
-# Modules first for layer caching; go.mod/go.sum settle this layer's
+# pixa is CGO/libvips: the type-aware linters compile every package, so
-# result, so it may safely be reused between runs.
+# this image needs the same C libraries the build does. script/bootstrap
+# installs them and downloads the Go modules. Only script/, go.mod and
+# go.sum are copied first; they settle this layer's result, so it may
+# safely be reused between runs.
+COPY script/ ./script/
 COPY go.mod go.sum ./
-RUN go mod download
+RUN script/bootstrap
 
 COPY . .
 

README.md

@@ -40,9 +40,8 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
 
 - **Port:** pixa listens on container port `8080`.
 - **Volume:** container path `/var/lib/pixa`, where pixa keeps its
-  database and cache. upaas bind-mounts the host path it is given and
+  database and cache. Creating the host directory when it is missing is
-  does not create it, so the host directory must exist before the first
+  upaas's job, tracked in https://git.eeqj.de/sneak/upaas/issues/235.
-  deploy.
 - **Environment variables:**
     - `PIXA_SIGNING_KEY` (required): secret for signed and encrypted URLs
       and login, 32+ characters, for example from
@@ -58,8 +57,6 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
   `healthy`. The probe uses the port from `PORT` (default `8080`), so a
   port changed only in a mounted config file is not seen by it: change
   the port with `PORT`.
-- **First run:** create the host directory. It may be owned by root: the
-  container gives it to its `pixad` user when it starts.
 
 ## Rationale
 
@@ -84,7 +81,10 @@ prevent abuse, and allowlisted source hosts for open access.
 
 Multiple source paths may reference the same content blob; the
 database tracks references rather than using filesystem refcounting.
-In-process caching of request-to-output mappings targets 1-5k r/s.
+Toward a target of 1-5k r/s, pixa keeps in memory the content types of
+the 10,000 transformed images most recently cached or served, so a
+cache hit on one of them reads only the image file from disk and not
+the metadata file stored beside it.
 
 ### Routes
 
@@ -100,16 +100,29 @@ than once, is refused with 400.
 - `<format>`: one of `orig`, `png`, `jpeg`, `webp`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)
 
+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),
+`max-age` is the whole seconds left until then, at most one year, so no browser
+or proxy cache keeps the image after pixa would refuse the URL. A URL with no
+expiry gets one year. `immutable` only stops a client revalidating while its
+copy is fresh.
+
 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
 refused with 429 and a `Retry-After` header. Behind a reverse proxy the client
-address comes from `X-Forwarded-For` only when the proxy's address is in
+address comes from `X-Forwarded-For` only when the address pixa sees for
-`trusted_proxies`; otherwise all users behind the proxy are counted as one
+requests that come through the proxy is in `trusted_proxies`; otherwise all
-client. With the default `trusted_proxies` (the RFC 1918 ranges), a client
+users behind the proxy are counted as one client. That address is not always
-with a private address can choose the address it is counted by through its own
+the proxy's own: a proxy on the Docker host that connects to pixa over
-`X-Forwarded-For`, whether it connects directly or through the proxy, because
+`127.0.0.1` is seen as the gateway of the container's Docker network, such as
-its own address is trusted too. Setting `trusted_proxies` to the proxy's own
+`172.17.0.1` on the default bridge, and one that connects through another of the
-address closes this.
+host's addresses is seen with that address. To be sure, read it as `remoteIP` in
+pixa's request log while it is not in `trusted_proxies` (see `trusted_proxies`
+under Configuration). With the default `trusted_proxies` (the RFC 1918 ranges),
+a client with a private address can choose the address it is counted by through
+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.
 
 ### Image Metadata
 
@@ -163,19 +176,27 @@ Where:
   outside), or `cover` when the URL has no `fit`; a request whose `fit` is
   anything else, an empty `fit=` included, is refused with 400
 
-**Example:** resize `https://cdn.example.com/photos/cat.jpg` to 800x600
+The URL's `sig` is the HMAC-SHA256 result in base64url (the URL-safe alphabet
-WebP with expiration 1704067200, default quality and fit:
+of RFC 4648) with the trailing `=` padding kept, 44 characters in all. pixa
+compares it exactly, so a signature encoded without padding, as Node's
+`base64url` and Go's `base64.RawURLEncoding` do, is refused with 401.
+
+**Example:** with the signing key `example-signing-key-for-documentation`,
+resize `https://cdn.example.com/photos/cat.jpg` to 800x600 WebP with
+expiration 1704067200, default quality and fit:
 
 1. Build input:
    `cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200:85:cover`
-2. Compute HMAC-SHA256 with your secret key
+2. Compute HMAC-SHA256 of it with the signing key
-3. Base64URL-encode the result
+3. Base64URL-encode the result, keeping the `=` padding:
+   `-ay7KHpfqmtIGbibDGbUuBDkymi-Ymdn0NkC6j5EJag=`
 4. URL:
-   `/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=<base64url>&exp=1704067200`
+   `/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=-ay7KHpfqmtIGbibDGbUuBDkymi-Ymdn0NkC6j5EJag=&exp=1704067200`
 
 For the same image at quality 40 with fit `contain`, the input ends in
-`:40:contain` and the URL is
+`:40:contain`, the signature is `5IwXUx6vf7yefhaUvFzgXZvG2o0Df4RJxPTK3pKq5VU=`,
-`/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=<base64url>&exp=1704067200&q=40&fit=contain`.
+and the URL is
+`/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=5IwXUx6vf7yefhaUvFzgXZvG2o0Df4RJxPTK3pKq5VU=&exp=1704067200&q=40&fit=contain`.
 
 **Allowlist patterns:**
 
@@ -212,6 +233,8 @@ variables set by the file's `env:` section are checked the same way.
 | `PIXA_TRUSTED_PROXIES`               | `trusted_proxies`               | CIDR ranges of proxies whose `X-Forwarded-For` is believed; default RFC 1918 |
 | `PIXA_ALLOW_HTTP`                    | `allow_http`                    | Allow plain-HTTP upstreams, for testing only; default `false`                |
 | `PIXA_UPSTREAM_CONNECTIONS_PER_HOST` | `upstream_connections_per_host` | Concurrent connections per upstream host; default `20`                       |
+| `PIXA_UPSTREAM_CONNECTIONS`          | `upstream_connections`          | Concurrent connections to all upstream hosts together; default `64`          |
+| `PIXA_MAX_CONCURRENT_PROCESSING`     | `max_concurrent_processing`     | Images processed at once; default the number of CPUs                         |
 | `PIXA_UPSTREAM_FETCH_TIMEOUT`        | `upstream_fetch_timeout`        | Time allowed for one fetch from an upstream host; default `30s`              |
 | `PIXA_UPSTREAM_MAX_RESPONSE_SIZE`    | `upstream_max_response_size`    | Largest upstream response accepted, in bytes; default 50 MiB                 |
 | `PIXA_DOWNSTREAM_TIMEOUT`            | `downstream_timeout`            | Time allowed for answering one client request; default `60s`                 |
@@ -220,7 +243,7 @@ variables set by the file's `env:` section are checked the same way.
 | `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
 | `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
 | `PIXA_DEBUG`                         | `debug`                         | Debug logging and plain-HTTP local development; default `false`              |
-| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | Maintenance flag reported by the health check; default `false`               |
+| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | Answer image requests with 503; the health check stays 200; default `false`  |
 
 Key settings in more detail:
 
@@ -248,20 +271,46 @@ Key settings in more detail:
   `172.16.0.0/12`, `192.168.0.0/16`), since pixa is deployed behind a
   proxy on a private network; an explicitly empty list (`[]`) trusts no
   one, and an explicit list replaces the default. An invalid CIDR aborts
-  startup. Set this to your proxy's address range if it is not already
+  startup. Set this to the address pixa sees for requests that come through
-  covered by the defaults
+  your proxy, such as `172.17.0.1/32`, when the defaults do not cover it, or
+  to trust nothing else (see the login limit under Routes). For a proxy on
+  the Docker host that connects to pixa over `127.0.0.1`, that address is the
+  gateway of the container's Docker network (`172.17.0.1` on the default
+  bridge), not the proxy's own address; a proxy that connects through another of
+  the host's addresses is seen with that address. To be sure which address it
+  is, set this to `[]` (or `PIXA_TRUSTED_PROXIES` to empty), send a request
+  through the proxy, and read `remoteIP` in pixa's request log line for it
 - `upstream_fetch_timeout` — time allowed for one fetch from an upstream
   host, as a duration such as `30s` (the default) or `2m`
 - `upstream_max_response_size` — largest upstream response accepted, in
   bytes; default `52428800` (50 MiB). It also limits the image data pixa
   decodes
 - `downstream_timeout` — time allowed for answering one client request, as a
-  duration; default `60s`. The upstream fetch counts toward it, so keep it
+  duration; default `60s`. The upstream fetch counts toward it, and so do the
-  longer than `upstream_fetch_timeout`
+  waits for an upstream connection and for a processing slot (up to 10 seconds
+  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)
+- `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
+  finds all of them in use waits up to 10 seconds for one to free up; if none
+  does, and `downstream_timeout` has not ended first, the request is answered
+  503 with the error `server busy, try again later`
+- `max_concurrent_processing` — the most images decoded and encoded at once;
+  default the number of CPUs pixa can use (`GOMAXPROCS`), which follows a
+  container's CPU limit. A request that finds all of them in use waits up to 10
+  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
 
 See `config.example.yml` for all options with defaults.
 

TODO.md

@@ -25,11 +25,81 @@ The disk cache is now size-bounded with LRU eviction
 
 # Next Step
 
-P1: rate limit global concurrent upstream fetches to prevent resource
+P2: security: referer blacklist
-exhaustion
 
 # Completed Steps
 
+- 2026-09-29 the container makes `/var/lib/pixa` usable by itself (closes
+  #159): `deploy/docker-entrypoint.sh` creates the directory if it is missing,
+  gives the directory and everything in it to `pixad` when the directory or one
+  of its top-level entries belongs to another user or group, sets its mode to
+  `750`, then runs the server as `pixad`; data left by an earlier run under
+  another uid is taken over this way; "Running under upaas" in `README.md` no
+  longer tells the operator to create or chown the host directory.
+- 2026-09-29 variant content types kept in memory (closes #70):
+  `Cache.metaCache` holds the content types of up to 10,000 variants in an LRU
+  (`github.com/hashicorp/golang-lru/v2`), filled by `StoreVariant` and by
+  `GetVariant` after it reads a `.meta` file, where a type `StoreVariant` added
+  meanwhile is kept over the one read, and never with the
+  `application/octet-stream` served for a variant without one; for a variant it
+  holds, `GetVariant` skips the `.meta` read, still opening the variant file and
+  taking the size from it; eviction removes the entry before deleting the files,
+  and `GetVariant` removes it when the file will not open; the cap is a
+  constant, not a setting; the unused `variantMeta` type is gone; `README.md`
+  describes it.
+- 2026-09-29 maintenance mode refuses image requests (closes #71): while
+  `maintenance_mode` is on, `/v1/image/` and `/v1/e/` answer 503 with a
+  `Retry-After` header and the JSON error body, from one middleware in
+  `internal/server/routes.go`; the health check stays 200 and reports
+  `maintenance_mode`, as the image's Docker `HEALTHCHECK` requests it and upaas
+  marks a deploy failed when its container is unhealthy; the login and URL
+  generator pages and `/metrics` keep working; documented in `README.md` and
+  `config.example.yml`.
+- 2026-09-29 bound concurrent image processing and upstream fetches (closes
+  #64): `max_concurrent_processing` (default the number of CPUs pixa can use)
+  limits the images decoded and encoded at once, and `upstream_connections`
+  (default 64) the connections to all upstream hosts together, on top of
+  `upstream_connections_per_host`; a fetch holds its connection until its image
+  has been processed, and a request whose source is cached reads it only once it
+  has a processing slot; a request that finds either limit reached waits up to
+  10 seconds for a free one, then gets 503 `server busy, try again later`;
+  libvips runs one worker thread per image with its operation cache off;
+  documented in `README.md` and `config.example.yml`.
+- 2026-09-29 Dockerfiles install through `script/bootstrap` (closes #95): the
+  `Dockerfile` lint and build stages and `Dockerfile.lint` copy `script/`,
+  `go.mod` and `go.sum`, then run `script/bootstrap` in place of their own
+  `apk add` lines, so the build dependencies are listed in one place;
+  `script/bootstrap` now also installs a C compiler when `gcc` is missing; the
+  build uses `-trimpath` and `-s -w` and keeps `CGO_ENABLED=1` for govips;
+  `ARG VERSION` sits just above the build, so a new version reruns neither
+  `script/bootstrap` nor the tests.
+- 2026-09-29 migrations at the path `REPO_POLICIES.md` sets (closes #96): the
+  migration files moved, contents unchanged, from `internal/database/schema/`
+  to `internal/db/migrations/` as `000_migration.sql` and `001_schema.sql`; the
+  `internal/db/migrations` package embeds them and `internal/database` reads
+  them through its `FS()`; the `internal/database` package itself stays; the
+  version still comes from the filename prefix, so a database that has recorded
+  versions 0 and 1 runs neither again.
+- 2026-09-29 `trusted_proxies` advice and signature padding in `README.md`
+  (closes #150): the login-limit paragraph, the `trusted_proxies` entry and
+  `config.example.yml` say to set `trusted_proxies` to the address pixa sees for
+  requests that come through the proxy, which the request log shows as
+  `remoteIP` while it is not trusted; for a proxy on the Docker host that
+  connects over `127.0.0.1` that is the Docker network's gateway, not the
+  proxy's own address; the signature section says `sig` is base64url with the
+  `=` padding kept, and gives the example's `sig` for a stated signing key.
+- 2026-09-29 fixed uid and gid for `pixad` (closes #151): the image creates the
+  `pixad` group with gid 65532 and the `pixad` user with uid 65532, instead of
+  the first free uid 1000, so a bind-mounted `/var/lib/pixa` given to `pixad`
+  is not owned on the host by a person's login account; the first-run step of
+  "Running under upaas" in `README.md` names the uid and gid.
+- 2026-09-29 `max-age` never outlives an expiring URL (closes #63): both image
+  routes build `Cache-Control` from the request's `Expires`, which an encrypted
+  URL's expiry now fills too; `max-age` is one year, or the whole seconds left
+  until the `exp` of a `/v1/image/` URL or the expiry of an encrypted URL when
+  that is sooner, never negative; an allowlisted host's URL that has an `exp`
+  follows it too; `immutable` stays, as freshness now ends at the expiry;
+  documented in `README.md`.
 - 2026-09-28 add the four settings `README.md` documented but pixa did not
   have, which aborted startup as unknown keys (closes #61):
   `access_control_allow_origin` (default `*`, the CORS origin),
@@ -41,6 +111,18 @@ exhaustion
   `https` origin as `README.md` describes it; an invalid value
   aborts startup naming the key and the value; documented in
   `config.example.yml` and `README.md`.
+- 2026-09-28 cache stats report real numbers (closes #56): `Cache.Stats`
+  counts the cached source images and processed variants (`source_content`
+  plus `variant_content`) and takes their size from `Cache.UsageBytes`,
+  instead of reading `request_cache` and `output_content`, which nothing
+  writes; those two tables are left in the schema; a disabled disk cache
+  reports no items and no size. A hit is counted even when the request
+  context has ended. A miss is counted after it is served or fails, also
+  when the request context has ended by then, with the bytes it read from
+  upstream, so `upstream_fetch_count` and `upstream_fetch_bytes` move,
+  including for an upstream body that fails partway or a fetched source
+  that then fails the magic byte check; `transform_count` counts each image
+  the image processor transcodes.
 - 2026-09-28 strip metadata from processed images (closes #82): every output is
   exported with govips' `StripMetadata`, so it carries no EXIF, XMP, IPTC or ICC
   profile; the image is first turned upright with `AutoRotate` (before sizes are
@@ -269,7 +351,6 @@ exhaustion
 # Future Steps
 
 - P2: security
-    - referer blacklist
     - per-IP rate limiting on the image routes
     - per-origin rate limiting
 - P2: HTTP response handling

config.example.yml

@@ -16,6 +16,12 @@
 # Server settings
 port: 8080
 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.
 maintenance_mode: false
 
 # Data directory for SQLite database and cache files
@@ -54,7 +60,13 @@ allowlist_hosts:
 # 172.16.0.0/12, 192.168.0.0/16), since pixa is deployed behind a proxy on
 # a private network. An explicitly empty list ([]) trusts no one; an
 # explicit list replaces the default. An invalid CIDR aborts startup.
-# Uncomment to override the defaults with your proxy's address range.
+# Uncomment to override the defaults with the address pixa sees for
+# requests that come through your proxy. That is not always the proxy's own
+# address: a proxy on the Docker host that connects over 127.0.0.1 is seen
+# as the gateway of the container's Docker network (172.17.0.1 on the
+# default bridge), and one that connects through another host address is
+# seen with that address. To be sure, look it up in the request log as the
+# trusted_proxies entry in README.md describes.
 # trusted_proxies:
 #   - 10.0.0.0/8
 #   - 2001:db8::/32
@@ -65,6 +77,19 @@ allow_http: false
 # Maximum concurrent connections per upstream host (default: 20)
 upstream_connections_per_host: 20
 
+# Maximum concurrent connections to all upstream hosts together, on top of
+# the per-host limit (default: 64). A fetch holds its connection until its
+# image has been processed. A fetch that finds none free waits up to 10
+# seconds for one, and if none frees up the request is answered 503, unless
+# downstream_timeout has ended first.
+upstream_connections: 64
+
+# Maximum number of images decoded and encoded at once (default: the
+# number of CPUs pixa can use, which follows a container's CPU limit). A
+# request that finds none free waits up to 10 seconds for one, and if none
+# frees up it is answered 503, unless downstream_timeout has ended first.
+# max_concurrent_processing: 4
+
 # Time allowed for one fetch from an upstream host (default: 30s)
 upstream_fetch_timeout: 30s
 
@@ -72,8 +97,10 @@ upstream_fetch_timeout: 30s
 # (1 GiB) (default: 52428800, 50 MiB)
 upstream_max_response_size: 52428800
 
-# Time allowed for answering one client request, the upstream fetch
+# Time allowed for answering one client request (default: 60s). The
-# included, so keep it longer than upstream_fetch_timeout (default: 60s)
+# upstream fetch counts toward it, and so do the waits for an upstream
+# connection and for a processing slot (up to 10 seconds each), so keep it
+# longer than upstream_fetch_timeout plus 20 seconds.
 downstream_timeout: 60s
 
 # The origin a browser lets read pixa's responses, sent as the CORS

deploy/docker-entrypoint.sh

@@ -1,14 +1,22 @@
 #!/bin/sh
 # deploy/docker-entrypoint.sh: the Docker image's ENTRYPOINT. It runs as
-# root only to give /var/lib/pixa to pixad: a host directory
+# root only to make /var/lib/pixa usable by pixad: a host directory
-# bind-mounted there keeps its host owner, often root, and pixad could
+# bind-mounted there keeps its host owner, often root, and data from an
-# not write to it. The server itself always runs as pixad.
+# earlier run may belong to another uid. The server itself always runs
+# as pixad.
 set -eu
 
 main() {
-    if [ "$(stat -c %U /var/lib/pixa)" != pixad ]; then
+    mkdir -p /var/lib/pixa
-        chown pixad:pixad /var/lib/pixa
+    # Only the directory and its top-level entries are checked, so a
+    # normal start does not walk the cache. -depth gives each directory
+    # to pixad after its contents, so a start stopped part way leaves
+    # something at the top for the next start to find; -h changes a
+    # symlink itself, never the file it points to.
+    if [ -n "$(find /var/lib/pixa -maxdepth 1 \( ! -user pixad -o ! -group pixad \))" ]; then
+        find /var/lib/pixa -depth -exec chown -h pixad:pixad {} +
     fi
+    chmod 750 /var/lib/pixa
     exec su-exec pixad /usr/local/bin/pixad "$@"
 }
 

go.mod

@@ -14,6 +14,7 @@ require (
 	github.com/go-chi/httprate v0.16.0
 	github.com/gorilla/csrf v1.7.3
 	github.com/gorilla/securecookie v1.1.2
+	github.com/hashicorp/golang-lru/v2 v2.0.7
 	github.com/prometheus/client_golang v1.23.2
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/cobra v1.10.2

go.sum

@@ -228,6 +228,8 @@ github.com/hashicorp/go-version v1.2.1/go.mod h1:fltr4n8CU8Ke44wwGCBoEymUuxUHl09
 github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/golang-lru v0.5.4 h1:YDjusn29QI/Das2iO9M0BHnIbxPeyuCHsjMW+lJfyTc=
 github.com/hashicorp/golang-lru v0.5.4/go.mod h1:iADmTwqILo4mZ8BN3D2Q6+9jd8WM5uGBxy+E8yxSoD4=
+github.com/hashicorp/golang-lru/v2 v2.0.7 h1:a+bsQ5rvGLjzHuww6tVxozPZFVghXaHOwFs4luLUK2k=
+github.com/hashicorp/golang-lru/v2 v2.0.7/go.mod h1:QeFd9opnmA6QUJc5vARoKUSoFhyfM2/ZepoAG6RGpeM=
 github.com/hashicorp/hcl v1.0.1-vault-7 h1:ag5OxFVy3QYTFTJODRzTKVZ6xvdfLLCA1cy/Y6xGI0I=
 github.com/hashicorp/hcl v1.0.1-vault-7/go.mod h1:XYhtn6ijBSAj6n4YqAaf7RBPS4I06AItNorpy+MoQNM=
 github.com/hashicorp/logutils v1.0.0/go.mod h1:QIAnNjmIWmVIIkWDTG1z5v++HQmx9WQRO+LraFDTW64=

internal/config/concurrency_limits_internal_test.go

@@ -0,0 +1,162 @@
+package config
+
+import (
+	"runtime"
+	"testing"
+)
+
+// The variables that set the two concurrency limits.
+const (
+	testMaxConcurrentProcessingVar = "PIXA_MAX_CONCURRENT_PROCESSING"
+	testUpstreamConnectionsVar     = "PIXA_UPSTREAM_CONNECTIONS"
+)
+
+// TestOmittedConcurrencyLimitsUseDefaults checks that an omitted
+// max_concurrent_processing is the number of CPUs Go uses and an omitted
+// upstream_connections is 64.
+func TestOmittedConcurrencyLimitsUseDefaults(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine)
+	if err != nil {
+		t.Fatalf("minimal config should be valid, got error: %v", err)
+	}
+
+	if c.MaxConcurrentProcessing != runtime.GOMAXPROCS(0) {
+		t.Errorf("MaxConcurrentProcessing = %d, want %d, one per CPU",
+			c.MaxConcurrentProcessing, runtime.GOMAXPROCS(0))
+	}
+
+	if c.UpstreamConnections != 64 {
+		t.Errorf("UpstreamConnections = %d, want 64", c.UpstreamConnections)
+	}
+}
+
+// TestExplicitConcurrencyLimitsAreUsed checks that valid values for the
+// two limits are used as given.
+func TestExplicitConcurrencyLimitsAreUsed(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+
+		"max_concurrent_processing: 3\nupstream_connections: 10\n")
+	if err != nil {
+		t.Fatalf("valid config should load, got error: %v", err)
+	}
+
+	if c.MaxConcurrentProcessing != 3 {
+		t.Errorf("MaxConcurrentProcessing = %d, want 3", c.MaxConcurrentProcessing)
+	}
+
+	if c.UpstreamConnections != 10 {
+		t.Errorf("UpstreamConnections = %d, want 10", c.UpstreamConnections)
+	}
+}
+
+// TestInvalidConcurrencyLimitAbortsStartup checks that a limit that is
+// not a whole number of at least 1, or is null, aborts startup naming the
+// key and the value, and the variable too where the value could have come
+// from it.
+func TestInvalidConcurrencyLimitAbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	processing := keyMaxConcurrentProcessing
+	connections := keyUpstreamConnections
+
+	runAbortCases(t, []abortCase{
+		{
+			name: "max_concurrent_processing zero",
+			yaml: signingKeyLine + processing + ": 0\n",
+			wantErrSubstrings: []string{
+				processing, testMaxConcurrentProcessingVar, "value 0",
+			},
+		},
+		{
+			name: "max_concurrent_processing negative",
+			yaml: signingKeyLine + processing + ": -2\n",
+			wantErrSubstrings: []string{
+				processing, testMaxConcurrentProcessingVar, "value -2",
+			},
+		},
+		{
+			name: "max_concurrent_processing not a number",
+			yaml: signingKeyLine + processing + ": lots\n",
+			wantErrSubstrings: []string{
+				processing, testMaxConcurrentProcessingVar, "lots",
+			},
+		},
+		{
+			name:              "max_concurrent_processing fractional",
+			yaml:              signingKeyLine + processing + ": 1.5\n",
+			wantErrSubstrings: []string{processing, "1.5"},
+		},
+		{
+			name:              "max_concurrent_processing null",
+			yaml:              signingKeyLine + processing + ": null\n",
+			wantErrSubstrings: []string{processing, nullValueText},
+		},
+		{
+			name: "upstream_connections zero",
+			yaml: signingKeyLine + connections + ": 0\n",
+			wantErrSubstrings: []string{
+				connections, testUpstreamConnectionsVar, "value 0",
+			},
+		},
+		{
+			name: "upstream_connections negative",
+			yaml: signingKeyLine + connections + ": -5\n",
+			wantErrSubstrings: []string{
+				connections, testUpstreamConnectionsVar, "value -5",
+			},
+		},
+		{
+			name: "upstream_connections not a number",
+			yaml: signingKeyLine + connections + ": many\n",
+			wantErrSubstrings: []string{
+				connections, testUpstreamConnectionsVar, "many",
+			},
+		},
+		{
+			name:              "upstream_connections null",
+			yaml:              signingKeyLine + connections + ": null\n",
+			wantErrSubstrings: []string{connections, nullValueText},
+		},
+	})
+}
+
+// TestConcurrencyLimitsFromEnvironment checks that the two variables set
+// the limits over the config file, and that an invalid value in either
+// aborts startup naming the variable and the value.
+func TestConcurrencyLimitsFromEnvironment(t *testing.T) {
+	t.Setenv(testMaxConcurrentProcessingVar, "3")
+	t.Setenv(testUpstreamConnectionsVar, "10")
+
+	c, err := configFromYAML(t, signingKeyLine+
+		"max_concurrent_processing: 5\nupstream_connections: 50\n")
+	if err != nil {
+		t.Fatalf("limits from the environment should load: %v", err)
+	}
+
+	if c.MaxConcurrentProcessing != 3 || c.UpstreamConnections != 10 {
+		t.Errorf("limits = %d and %d, want 3 and 10 from the environment",
+			c.MaxConcurrentProcessing, c.UpstreamConnections)
+	}
+
+	cases := []struct {
+		variable string
+		value    string
+	}{
+		{testMaxConcurrentProcessingVar, "lots"},
+		{testMaxConcurrentProcessingVar, "0"},
+		{testUpstreamConnectionsVar, "-1"},
+		{testUpstreamConnectionsVar, "ten"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.variable+"="+tc.value, func(t *testing.T) {
+			t.Setenv(tc.variable, tc.value)
+
+			_, err := configFromYAML(t, signingKeyLine)
+			wantStartupError(t, err, tc.variable, tc.value)
+		})
+	}
+}

internal/config/config.go

@@ -10,6 +10,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -25,6 +26,7 @@ const (
 	DefaultPort                       = 8080
 	DefaultStateDir                   = "/var/lib/pixa"
 	DefaultUpstreamConnectionsPerHost = 20
+	DefaultUpstreamConnections        = 64
 	DefaultAccessControlAllowOrigin   = "*"
 	DefaultUpstreamFetchTimeout       = 30 * time.Second
 	DefaultUpstreamMaxResponseSize    = 50 << 20 // 50 MiB
@@ -46,6 +48,8 @@ const (
 	keyAllowlistHosts             = "allowlist_hosts"
 	keyAllowHTTP                  = "allow_http"
 	keyUpstreamConnectionsPerHost = "upstream_connections_per_host"
+	keyUpstreamConnections        = "upstream_connections"
+	keyMaxConcurrentProcessing    = "max_concurrent_processing"
 	keyCacheMaxBytes              = "cache_max_bytes"
 	keyBlockedNetworks            = "blocked_networks"
 	keyTrustedProxies             = "trusted_proxies"
@@ -79,7 +83,7 @@ var (
 	errNotAValidURL      = errors.New("not a valid URL")
 	errPortOutOfRange    = errors.New("outside the valid port range")
 	errSizeOutOfRange    = errors.New("outside the accepted range")
-	errTooFewConnections = errors.New("must be at least 1")
+	errMustBeAtLeastOne  = errors.New("must be at least 1")
 	errValueTooShort     = errors.New("value too short")
 	errPlaceholderKey    = errors.New(
 		"is the placeholder from config.example.yml; " +
@@ -126,6 +130,12 @@ type Config struct {
 	AllowHTTP                  bool     // Allow non-TLS upstream (testing only)
 	UpstreamConnectionsPerHost int      // Max concurrent connections per upstream host
 
+	// UpstreamConnections is the most concurrent connections to all
+	// upstream hosts together, on top of the per-host limit.
+	// MaxConcurrentProcessing is the most images processed at once.
+	UpstreamConnections     int
+	MaxConcurrentProcessing int
+
 	// UpstreamFetchTimeout is the time allowed for one fetch from an
 	// upstream host. UpstreamMaxResponseSize is the largest upstream
 	// response accepted, in bytes, and also the image processor's input
@@ -226,14 +236,12 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 // unparseable or invalid is an error: defaults apply only to omitted
 // keys, never to invalid explicit values.
 func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
-	if sc != nil {
 	err := validateKnownKeys(sc)
 	if err != nil {
 		return nil, err
 	}
-	}
 
-	err := validateAllowlistHostsValue(sc)
+	err = validateAllowlistHostsValue(sc)
 	if err != nil {
 		return nil, err
 	}
@@ -271,6 +279,12 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		AllowHTTP:       loader.boolVal(keyAllowHTTP, false),
 		UpstreamConnectionsPerHost: loader.intVal(
 			keyUpstreamConnectionsPerHost, DefaultUpstreamConnectionsPerHost),
+		UpstreamConnections: loader.intVal(
+			keyUpstreamConnections, DefaultUpstreamConnections),
+		// Decoding and encoding are CPU-bound, so the default is one image
+		// per CPU Go uses, which follows a container's CPU limit.
+		MaxConcurrentProcessing: loader.intVal(
+			keyMaxConcurrentProcessing, runtime.GOMAXPROCS(0)),
 		UpstreamFetchTimeout: loader.durationVal(
 			keyUpstreamFetchTimeout, DefaultUpstreamFetchTimeout),
 		UpstreamMaxResponseSize: loader.int64Val(
@@ -321,8 +335,13 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 // being silently ignored, and rejects keys that are explicitly set to
 // null: a null is a SET value, never an omission, so it must not
 // silently take the default. The env section is permitted because
-// smartconfig consumes it for environment variable injection.
+// smartconfig consumes it for environment variable injection. A nil sc
+// means no config file, which has no keys to check.
 func validateKnownKeys(sc *smartconfig.Config) error {
+	if sc == nil {
+		return nil
+	}
+
 	var unknown, nullKeys []string
 
 	for key, value := range sc.Data() {
@@ -392,7 +411,8 @@ func isKnownConfigKey(key string) bool {
 	switch key {
 	case keyDebug, keyMaintenanceMode, keyPort, keyStateDir, keySentryDSN,
 		keyDBURL, keyMetrics, keySigningKey, keyAllowlistHosts, keyAllowHTTP,
-		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, keyBlockedNetworks,
+		keyUpstreamConnectionsPerHost, keyUpstreamConnections,
+		keyMaxConcurrentProcessing, keyCacheMaxBytes, keyBlockedNetworks,
 		keyTrustedProxies, keyAccessControlAllowOrigin, keyUpstreamFetchTimeout,
 		keyUpstreamMaxResponseSize, keyDownstreamTimeout, "env":
 		return true
@@ -419,6 +439,8 @@ func envVarNames() map[string]string {
 		keyAllowlistHosts:             "PIXA_ALLOWLIST_HOSTS",
 		keyAllowHTTP:                  "PIXA_ALLOW_HTTP",
 		keyUpstreamConnectionsPerHost: "PIXA_UPSTREAM_CONNECTIONS_PER_HOST",
+		keyUpstreamConnections:        "PIXA_UPSTREAM_CONNECTIONS",
+		keyMaxConcurrentProcessing:    "PIXA_MAX_CONCURRENT_PROCESSING",
 		keyCacheMaxBytes:              "PIXA_CACHE_MAX_BYTES",
 		keyBlockedNetworks:            "PIXA_BLOCKED_NETWORKS",
 		keyTrustedProxies:             "PIXA_TRUSTED_PROXIES",
@@ -562,10 +584,9 @@ func (c *Config) validate() error {
 			settingName(keyPort), c.Port, errPortOutOfRange, maxPort)
 	}
 
-	if c.UpstreamConnectionsPerHost < 1 {
+	err = c.validateConcurrencyLimits()
-		return fmt.Errorf("%s: value %d %w",
+	if err != nil {
-			settingName(keyUpstreamConnectionsPerHost),
+		return err
-			c.UpstreamConnectionsPerHost, errTooFewConnections)
 	}
 
 	if c.StateDir == "" {
@@ -684,6 +705,30 @@ func (c *Config) validateAccessControlAllowOrigin() error {
 	return nil
 }
 
+// validateConcurrencyLimits checks that the two upstream connection limits
+// and the image processing limit are at least 1.
+func (c *Config) validateConcurrencyLimits() error {
+	if c.UpstreamConnectionsPerHost < 1 {
+		return fmt.Errorf("%s: value %d %w",
+			settingName(keyUpstreamConnectionsPerHost),
+			c.UpstreamConnectionsPerHost, errMustBeAtLeastOne)
+	}
+
+	if c.UpstreamConnections < 1 {
+		return fmt.Errorf("%s: value %d %w",
+			settingName(keyUpstreamConnections),
+			c.UpstreamConnections, errMustBeAtLeastOne)
+	}
+
+	if c.MaxConcurrentProcessing < 1 {
+		return fmt.Errorf("%s: value %d %w",
+			settingName(keyMaxConcurrentProcessing),
+			c.MaxConcurrentProcessing, errMustBeAtLeastOne)
+	}
+
+	return nil
+}
+
 // validateAllowlistHost checks that an allowlist_hosts entry is a bare
 // hostname, optionally with a leading dot for suffix matching. URLs,
 // paths, and whitespace indicate a misconfigured entry. An entry with

internal/config/env_internal_test.go

@@ -67,6 +67,8 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 	t.Setenv("PIXA_ALLOWLIST_HOSTS", "s3.sneak.cloud,.example.com")
 	t.Setenv("PIXA_ALLOW_HTTP", "true")
 	t.Setenv("PIXA_UPSTREAM_CONNECTIONS_PER_HOST", "5")
+	t.Setenv("PIXA_UPSTREAM_CONNECTIONS", "10")
+	t.Setenv("PIXA_MAX_CONCURRENT_PROCESSING", "3")
 	t.Setenv("PIXA_CACHE_MAX_BYTES", "1024")
 	t.Setenv("PIXA_BLOCKED_NETWORKS", "203.0.113.0/24")
 	t.Setenv("PIXA_TRUSTED_PROXIES", "192.0.2.0/24")
@@ -93,6 +95,8 @@ func TestEnvironmentSetsEveryKey(t *testing.T) {
 		AllowlistHosts:             []string{testHostS3, ".example.com"},
 		AllowHTTP:                  true,
 		UpstreamConnectionsPerHost: 5,
+		UpstreamConnections:        10,
+		MaxConcurrentProcessing:    3,
 		CacheMaxBytes:              1024,
 		cacheMaxBytesExplicit:      true,
 		BlockedNetworks:            []netip.Prefix{netip.MustParsePrefix("203.0.113.0/24")},

internal/database/database.go

@@ -4,9 +4,9 @@ package database
 import (
 	"context"
 	"database/sql"
-	"embed"
 	"errors"
 	"fmt"
+	"io/fs"
 	"log/slog"
 	"path/filepath"
 	"sort"
@@ -15,14 +15,12 @@ import (
 
 	"go.uber.org/fx"
 	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/db/migrations"
 	"sneak.berlin/go/pixa/internal/logger"
 
 	_ "modernc.org/sqlite" // SQLite driver registration
 )
 
-//go:embed schema/*.sql
-var schemaFS embed.FS
-
 // bootstrapVersion is the migration that creates the schema_migrations
 // table itself. It is applied before the normal migration loop.
 const bootstrapVersion = 0
@@ -113,29 +111,29 @@ func New(lc fx.Lifecycle, params Params) (*Database, error) {
 	return s, nil
 }
 
-// collectMigrations reads the embedded schema directory and returns
+// collectMigrations reads the embedded migrations directory and returns
 // migration filenames sorted lexicographically.
 func collectMigrations() ([]string, error) {
-	entries, err := schemaFS.ReadDir("schema")
+	entries, err := fs.ReadDir(migrations.FS(), ".")
 	if err != nil {
-		return nil, fmt.Errorf("failed to read schema directory: %w", err)
+		return nil, fmt.Errorf("failed to read migrations directory: %w", err)
 	}
 
-	var migrations []string
+	var filenames []string
 
 	for _, entry := range entries {
 		if !entry.IsDir() && strings.HasSuffix(entry.Name(), ".sql") {
-			migrations = append(migrations, entry.Name())
+			filenames = append(filenames, entry.Name())
 		}
 	}
 
-	sort.Strings(migrations)
+	sort.Strings(filenames)
 
-	return migrations, nil
+	return filenames, nil
 }
 
 // bootstrapMigrationsTable ensures the schema_migrations table exists
-// by applying 000.sql if the table is missing.
+// by applying 000_migration.sql if the table is missing.
 func bootstrapMigrationsTable(ctx context.Context, db *sql.DB, log *slog.Logger) error {
 	var tableExists int
 
@@ -150,9 +148,9 @@ func bootstrapMigrationsTable(ctx context.Context, db *sql.DB, log *slog.Logger)
 		return nil
 	}
 
-	content, err := schemaFS.ReadFile("schema/000.sql")
+	content, err := fs.ReadFile(migrations.FS(), "000_migration.sql")
 	if err != nil {
-		return fmt.Errorf("failed to read bootstrap migration 000.sql: %w", err)
+		return fmt.Errorf("failed to read bootstrap migration 000_migration.sql: %w", err)
 	}
 
 	if log != nil {
@@ -177,12 +175,12 @@ func ApplyMigrations(ctx context.Context, db *sql.DB, log *slog.Logger) error {
 		return err
 	}
 
-	migrations, err := collectMigrations()
+	filenames, err := collectMigrations()
 	if err != nil {
 		return err
 	}
 
-	for _, migration := range migrations {
+	for _, migration := range filenames {
 		version, parseErr := ParseMigrationVersion(migration)
 		if parseErr != nil {
 			return parseErr
@@ -208,7 +206,7 @@ func ApplyMigrations(ctx context.Context, db *sql.DB, log *slog.Logger) error {
 		}
 
 		// Read and apply migration.
-		content, readErr := schemaFS.ReadFile(filepath.Join("schema", migration))
+		content, readErr := fs.ReadFile(migrations.FS(), migration)
 		if readErr != nil {
 			return fmt.Errorf("failed to read migration %s: %w", migration, readErr)
 		}

internal/database/migrations_internal_test.go

@@ -0,0 +1,54 @@
+package database
+
+import (
+	"bytes"
+	"database/sql"
+	"log/slog"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	_ "modernc.org/sqlite" // SQLite driver registration
+)
+
+// TestApplyMigrations_SecondRunAppliesNothing applies the migrations twice
+// to one database file, as happens when pixad starts again on the database
+// it created, and checks that the second run applies none of them.
+// ApplyMigrations logs a message starting with "applying" before it runs
+// any migration, the bootstrap one included.
+func TestApplyMigrations_SecondRunAppliesNothing(t *testing.T) {
+	t.Parallel()
+
+	ctx := t.Context()
+
+	db, err := sql.Open("sqlite", filepath.Join(t.TempDir(), "state.sqlite3"))
+	if err != nil {
+		t.Fatalf("failed to open test db: %v", err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	var firstLog bytes.Buffer
+
+	err = ApplyMigrations(ctx, db, slog.New(slog.NewTextHandler(&firstLog, nil)))
+	if err != nil {
+		t.Fatalf("first ApplyMigrations failed: %v", err)
+	}
+
+	if !strings.Contains(firstLog.String(), "applying") {
+		t.Fatalf("first ApplyMigrations logged no applied migration:\n%s",
+			firstLog.String())
+	}
+
+	var secondLog bytes.Buffer
+
+	err = ApplyMigrations(ctx, db, slog.New(slog.NewTextHandler(&secondLog, nil)))
+	if err != nil {
+		t.Fatalf("second ApplyMigrations failed: %v", err)
+	}
+
+	if strings.Contains(secondLog.String(), "applying") {
+		t.Errorf("second ApplyMigrations ran a migration again:\n%s",
+			secondLog.String())
+	}
+}

internal/db/migrations/migrations.go

@@ -0,0 +1,15 @@
+// Package migrations provides the embedded SQL migration files.
+package migrations
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed *.sql
+var files embed.FS
+
+// FS returns the embedded filesystem containing the migration files.
+func FS() fs.FS {
+	return files
+}

internal/encurl/encurl.go

@@ -103,7 +103,8 @@ func (g *Generator) Parse(token string) (*Payload, error) {
 }
 
 // ToImageRequest converts the payload to an ImageRequest.
-// Applies default values for omitted optional fields.
+// Applies default values for omitted optional fields. An ExpiresAt of 0, a URL
+// that never expires, gives the zero Expires.
 func (p *Payload) ToImageRequest() *imgcache.ImageRequest {
 	format := p.Format
 	if format == "" {
@@ -120,6 +121,11 @@ func (p *Payload) ToImageRequest() *imgcache.ImageRequest {
 		fitMode = DefaultFitMode
 	}
 
+	var expires time.Time
+	if p.ExpiresAt != 0 {
+		expires = time.Unix(p.ExpiresAt, 0)
+	}
+
 	return &imgcache.ImageRequest{
 		SourceHost:  p.SourceHost,
 		SourcePath:  p.SourcePath,
@@ -131,6 +137,7 @@ func (p *Payload) ToImageRequest() *imgcache.ImageRequest {
 		Format:  format,
 		Quality: quality,
 		FitMode: fitMode,
+		Expires: expires,
 	}
 }
 

internal/handlers/handlers.go

@@ -113,6 +113,7 @@ func (s *Handlers) initImageService() error {
 		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost
 	}
 
+	fetcherCfg.MaxConnections = s.config.UpstreamConnections
 	fetcherCfg.BlockedNetworks = s.config.BlockedNetworks
 
 	// Create the service
@@ -121,6 +122,7 @@ func (s *Handlers) initImageService() error {
 		FetcherConfig:           fetcherCfg,
 		SigningKey:              s.config.SigningKey,
 		Allowlist:               s.config.AllowlistHosts,
+		MaxConcurrentProcessing: s.config.MaxConcurrentProcessing,
 		Logger:                  s.log,
 	})
 	if err != nil {

internal/handlers/image.go

@@ -12,6 +12,7 @@ import (
 	"github.com/go-chi/chi/v5"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
@@ -217,9 +218,35 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, httpfetcher.ErrTooManyConnections) ||
+		errors.Is(err, imageprocessor.ErrTooManyImages) {
+		s.respondError(w, "server busy, try again later",
+			http.StatusServiceUnavailable)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 
+// cacheControl returns the Cache-Control header for an image served through a
+// URL that expires at expires, or never when expires is the zero time. A cache
+// may keep the image for a year, but not past the URL's expiry, after which
+// pixa refuses the URL. The seconds left are rounded down and never negative.
+// immutable only stops revalidation while the image is fresh, so it also ends
+// at the expiry.
+func cacheControl(expires time.Time) string {
+	const oneYear = 365 * 24 * time.Hour
+
+	maxAge := oneYear
+
+	if !expires.IsZero() {
+		maxAge = min(maxAge, max(time.Until(expires), 0))
+	}
+
+	return fmt.Sprintf("public, max-age=%d, immutable", int64(maxAge/time.Second))
+}
+
 // writeImageResponse writes headers and streams the image content,
 // handling conditional and HEAD requests.
 func (s *Handlers) writeImageResponse(
@@ -235,7 +262,7 @@ func (s *Handlers) writeImageResponse(
 	}
 
 	// Cache control headers
-	w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
+	w.Header().Set("Cache-Control", cacheControl(req.Expires))
 	w.Header().Set("X-Pixa-Cache", string(resp.CacheStatus))
 
 	if resp.ETag != "" {

internal/handlers/image_cache_control_internal_test.go

@@ -0,0 +1,203 @@
+package handlers
+
+import (
+	"image/color"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// photoPath is the path of the JPEG that newSignedHostServer serves.
+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) {
+	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("imgcache.NewCache() error = %v", err)
+	}
+
+	jpegData := generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255})
+
+	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
+		Cache: cache,
+		Fetcher: newMockFetcher(fstest.MapFS{
+			signedHost + photoPath: &fstest.MapFile{Data: jpegData},
+		}),
+		SigningKey: testSigningKey,
+	})
+	if err != nil {
+		t.Fatalf("imgcache.NewService() error = %v", err)
+	}
+
+	encGen, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:    slog.New(slog.DiscardHandler),
+		imgSvc: svc,
+		encGen: encGen,
+	}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+	r.Get("/v1/e/{token}/*", h.HandleImageEnc())
+
+	return h, r
+}
+
+// getMaxAge sends a GET for target to srv, requires a 200, and returns the
+// max-age of the response's Cache-Control header, which must read
+// "public, max-age=<seconds>, immutable".
+func getMaxAge(t *testing.T, srv http.Handler, target string) int {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	rec := httptest.NewRecorder()
+
+	srv.ServeHTTP(rec, req)
+
+	header := rec.Header().Get("Cache-Control")
+	t.Logf("GET %s: %d, Cache-Control: %s", target, rec.Code, header)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	value, hasPrefix := strings.CutPrefix(header, "public, max-age=")
+	value, hasSuffix := strings.CutSuffix(value, ", immutable")
+
+	maxAge, err := strconv.Atoi(value)
+	if !hasPrefix || !hasSuffix || err != nil {
+		t.Fatalf("Cache-Control = %q, want public, max-age=<seconds>, immutable",
+			header)
+	}
+
+	return maxAge
+}
+
+// TestHandleImage_SignedURL_MaxAgeEndsAtExp verifies that an image served
+// through a signed URL expiring in 60 seconds may be cached for at most those
+// 60 seconds. The lower bound of 50 shows the max-age is the time left, not 0.
+func TestHandleImage_SignedURL_MaxAgeEndsAtExp(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newSignedHostServer(t)
+
+	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)
+	}
+
+	maxAge := getMaxAge(t, srv, signedURL)
+	if maxAge < 50 || maxAge > 60 {
+		t.Errorf("max-age = %d, want 50 to 60", maxAge)
+	}
+}
+
+// TestHandleImage_AllowlistedHost_MaxAge verifies the max-age of an image from
+// an allowlisted host, which is served without checking sig or exp. A URL with
+// no exp may be cached for a year. A URL whose exp has passed is the one request
+// that reaches the header after its expiry, and must get 0, never less.
+func TestHandleImage_AllowlistedHost_MaxAge(t *testing.T) {
+	t.Parallel()
+
+	pastExp := strconv.FormatInt(time.Now().Add(-time.Hour).Unix(), 10)
+
+	tests := []struct {
+		name       string
+		query      string
+		wantMaxAge int
+	}{
+		{"no exp", "", 31536000},
+		{"exp already past", "?exp=" + pastExp, 0},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			fix := setupTestHandler(t)
+
+			r := chi.NewRouter()
+			r.Get("/v1/image/*", fix.handler.HandleImage())
+
+			maxAge := getMaxAge(t, r,
+				"/v1/image/"+fix.goodHost+"/images/photo.jpg/50x50.jpeg"+tt.query)
+			if maxAge != tt.wantMaxAge {
+				t.Errorf("max-age = %d, want %d", maxAge, tt.wantMaxAge)
+			}
+		})
+	}
+}
+
+// TestHandleImageEnc_MaxAge verifies that an image served through an encrypted
+// URL with a 60 second TTL may be cached for at most those 60 seconds, that one
+// with a two-year TTL may be cached for a year, and that one made without a
+// TTL, which never expires, may be cached for a year.
+func TestHandleImageEnc_MaxAge(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		expiresAt   int64
+		wantAtLeast int
+		wantAtMost  int
+	}{
+		{"60 second TTL", time.Now().Add(time.Minute).Unix(), 50, 60},
+		{"two-year TTL", time.Now().Add(2 * 365 * 24 * time.Hour).Unix(), 31536000, 31536000},
+		{"no TTL", 0, 31536000, 31536000},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			h, srv := newSignedHostServer(t)
+
+			token, err := h.encGen.Generate(&encurl.Payload{
+				SourceHost: signedHost,
+				SourcePath: photoPath,
+				Width:      50,
+				Height:     50,
+				Format:     imgcache.FormatJPEG,
+				ExpiresAt:  tt.expiresAt,
+			})
+			if err != nil {
+				t.Fatalf("Generate() error = %v", err)
+			}
+
+			maxAge := getMaxAge(t, srv, "/v1/e/"+token+"/img.jpg")
+			if maxAge < tt.wantAtLeast || maxAge > tt.wantAtMost {
+				t.Errorf("max-age = %d, want %d to %d",
+					maxAge, tt.wantAtLeast, tt.wantAtMost)
+			}
+		})
+	}
+}

internal/handlers/image_signature_internal_test.go

@@ -14,9 +14,9 @@ import (
 )
 
 // signedHost is not on the allowlist setupTestHandler builds, so a request
-// for it needs a valid signature. No image is served for it: a request that
+// for it needs a valid signature. setupTestHandler serves no image for it: a
-// passes the signature check gets 502 from the failed fetch, and one that
+// request that passes the signature check gets 502 from the failed fetch, and
-// fails the check gets 401.
+// one that fails the check gets 401.
 const signedHost = "signed.example.com"
 
 // getImage sends a GET for target to the image route of fix and returns the

internal/handlers/imageenc.go

@@ -12,6 +12,7 @@ import (
 
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/imgcache"
 )
 
@@ -89,8 +90,8 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 			w.Header().Set("Content-Length", strconv.FormatInt(resp.ContentLength, 10))
 		}
 
-		// Cache headers - encrypted URLs can be cached since they're immutable
+		// Cache headers: max-age ends at the URL's expiry
-		w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
+		w.Header().Set("Cache-Control", cacheControl(req.Expires))
 		w.Header().Set("X-Pixa-Cache", string(resp.CacheStatus))
 
 		// Stream the response
@@ -124,6 +125,10 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 		s.respondError(w, "upstream error", http.StatusBadGateway)
 	case errors.Is(err, httpfetcher.ErrUpstreamTimeout):
 		s.respondError(w, "upstream timeout", http.StatusGatewayTimeout)
+	case errors.Is(err, httpfetcher.ErrTooManyConnections),
+		errors.Is(err, imageprocessor.ErrTooManyImages):
+		s.respondError(w, "server busy, try again later",
+			http.StatusServiceUnavailable)
 	default:
 		s.log.Error("image request failed", "error", err)
 		s.respondError(w, "internal error", http.StatusInternalServerError)

internal/handlers/server_busy_internal_test.go

@@ -0,0 +1,49 @@
+package handlers
+
+import (
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// TestServerBusyAnswers503 checks that both image routes answer 503 with
+// a clear error when the image service gives up waiting for a free upstream
+// connection or processing slot, wrapped as the service wraps them.
+func TestServerBusyAnswers503(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	req := &imgcache.ImageRequest{SourceHost: "img.example.com", SourcePath: "/a.jpg"}
+
+	for _, err := range []error{
+		fmt.Errorf("upstream fetch failed: %w", httpfetcher.ErrTooManyConnections),
+		fmt.Errorf("image processing failed: %w", imageprocessor.ErrTooManyImages),
+	} {
+		plain := httptest.NewRecorder()
+		h.respondImageError(plain, req, err)
+
+		encrypted := httptest.NewRecorder()
+		h.handleImageError(encrypted, err)
+
+		for route, rec := range map[string]*httptest.ResponseRecorder{
+			"/v1/image/": plain, "/v1/e/": encrypted,
+		} {
+			if rec.Code != http.StatusServiceUnavailable {
+				t.Errorf("%s for %v: status = %d, want %d",
+					route, err, rec.Code, http.StatusServiceUnavailable)
+			}
+
+			if !strings.Contains(rec.Body.String(), "server busy, try again later") {
+				t.Errorf("%s for %v: body = %q, want the server busy error",
+					route, err, rec.Body.String())
+			}
+		}
+	}
+}

internal/httpfetcher/httpfetcher.go

@@ -1,5 +1,6 @@
 // Package httpfetcher fetches content from upstream HTTP origins with SSRF
-// protection, per-host connection limits, and content-type validation.
+// protection, connection limits per host and for all hosts together, and
+// content-type validation.
 package httpfetcher
 
 import (
@@ -28,8 +29,13 @@ const (
 	DefaultIdleConnTimeout       = 90 * time.Second
 	DefaultMaxRedirects          = 10
 	DefaultMaxConnectionsPerHost = 20
+	DefaultMaxConnections        = 64
 )
 
+// ConnectionWaitTimeout is how long Fetch waits for a free connection when
+// MaxConnections fetches are already in progress.
+const ConnectionWaitTimeout = 10 * time.Second
+
 // MIME content types.
 const (
 	contentTypeJPEG        = "image/jpeg"
@@ -70,6 +76,7 @@ var (
 	ErrInvalidContentType = errors.New("invalid or unsupported content type")
 	ErrUpstreamError      = errors.New("upstream server error")
 	ErrUpstreamTimeout    = errors.New("upstream request timeout")
+	ErrTooManyConnections = errors.New("too many concurrent upstream connections")
 )
 
 // Internal fetcher errors.
@@ -122,6 +129,9 @@ type Config struct {
 	AllowHTTP bool
 	// MaxConnectionsPerHost limits concurrent connections to each upstream host.
 	MaxConnectionsPerHost int
+	// MaxConnections limits concurrent connections to all upstream hosts
+	// together.
+	MaxConnections int
 	// BlockedNetworks are operator-supplied CIDR ranges refused by the
 	// dialer, in addition to the always-enforced built-in ranges.
 	BlockedNetworks []netip.Prefix
@@ -143,15 +153,22 @@ func DefaultConfig() *Config {
 		},
 		AllowHTTP:             false,
 		MaxConnectionsPerHost: DefaultMaxConnectionsPerHost,
+		MaxConnections:        DefaultMaxConnections,
 	}
 }
 
-// HTTPFetcher implements Fetcher with SSRF protection and per-host connection limits.
+// 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
+	// allHostsSemaphore has one slot per connection allowed to all hosts
+	// together (config.MaxConnections).
+	allHostsSemaphore chan struct{}
+	// connectionWaitTimeout is ConnectionWaitTimeout; tests shorten it.
+	connectionWaitTimeout time.Duration
 }
 
 // New creates a new HTTPFetcher with SSRF protection.
@@ -195,10 +212,15 @@ func New(config *Config) *HTTPFetcher {
 		client:                client,
 		config:                config,
 		hostSems:              make(map[string]chan struct{}),
+		allHostsSemaphore:     make(chan struct{}, config.MaxConnections),
+		connectionWaitTimeout: ConnectionWaitTimeout,
 	}
 }
 
-// Fetch retrieves content from the given URL with SSRF protection.
+// Fetch retrieves content from the given URL with SSRF protection. When
+// MaxConnections fetches are already in progress, it waits up to
+// ConnectionWaitTimeout for one to finish, then fails with
+// ErrTooManyConnections.
 func (f *HTTPFetcher) Fetch(ctx context.Context, url string) (*FetchResult, error) {
 	// Validate URL before making request
 	err := validateURL(ctx, url, f.config.AllowHTTP)
@@ -206,24 +228,17 @@ func (f *HTTPFetcher) Fetch(ctx context.Context, url string) (*FetchResult, erro
 		return nil, err
 	}
 
-	// Extract host for rate limiting
+	release, err := f.acquireConnection(ctx, extractHost(url))
-	host := extractHost(url)
+	if err != nil {
-
+		return nil, err
-	// Acquire semaphore slot for this host
-	sem := f.getHostSemaphore(host)
-	select {
-	case sem <- struct{}{}:
-		// Acquired slot
-	case <-ctx.Done():
-		return nil, ctx.Err()
 	}
 
-	// If we fail before returning a result, release the slot
+	// If we fail before returning a result, release the connection
 	success := false
 
 	defer func() {
 		if !success {
-			<-sem
+			release()
 		}
 	}()
 
@@ -267,17 +282,52 @@ func (f *HTTPFetcher) Fetch(ctx context.Context, url string) (*FetchResult, erro
 		return nil, fmt.Errorf("upstream request failed: %w", err)
 	}
 
-	result, err := f.buildResult(resp, remoteAddr, fetchDuration, sem)
+	result, err := f.buildResult(resp, remoteAddr, fetchDuration, release)
 	if err != nil {
 		return nil, err
 	}
 
-	// Mark success so defer doesn't release the semaphore
+	// Mark success so defer doesn't release the connection; closing the
+	// result's Content does
 	success = true
 
 	return result, nil
 }
 
+// acquireConnection takes a slot for host, then one of the slots shared by
+// all hosts, and returns the func that gives both back. The host's slot
+// comes first, so fetches queued for one busy host hold no shared slot.
+// Only the wait for a shared slot is bounded: after connectionWaitTimeout
+// it fails with ErrTooManyConnections.
+func (f *HTTPFetcher) acquireConnection(
+	ctx context.Context, host string,
+) (func(), error) {
+	hostSem := f.getHostSemaphore(host)
+
+	select {
+	case hostSem <- struct{}{}:
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+
+	select {
+	case f.allHostsSemaphore <- struct{}{}:
+	case <-time.After(f.connectionWaitTimeout):
+		<-hostSem
+
+		return nil, ErrTooManyConnections
+	case <-ctx.Done():
+		<-hostSem
+
+		return nil, ctx.Err()
+	}
+
+	return func() {
+		<-hostSem
+		<-f.allHostsSemaphore
+	}, nil
+}
+
 // getHostSemaphore returns the semaphore for a host, creating it if necessary.
 func (f *HTTPFetcher) getHostSemaphore(host string) chan struct{} {
 	f.hostSemMu.Lock()
@@ -293,12 +343,12 @@ func (f *HTTPFetcher) getHostSemaphore(host string) chan struct{} {
 }
 
 // buildResult validates the upstream response and assembles a FetchResult
-// whose Content releases the host semaphore slot when closed.
+// whose Content calls release when closed.
 func (f *HTTPFetcher) buildResult(
 	resp *http.Response,
 	remoteAddr string,
 	fetchDuration time.Duration,
-	sem chan struct{},
+	release func(),
 ) (*FetchResult, error) {
 	// Extract HTTP version (strip "HTTP/" prefix)
 	httpVersion := strings.TrimPrefix(resp.Proto, "HTTP/")
@@ -333,7 +383,7 @@ func (f *HTTPFetcher) buildResult(
 	}
 
 	return &FetchResult{
-		Content:         &semaphoreReleasingReadCloser{limitedBody, resp.Body, sem},
+		Content:         &semaphoreReleasingReadCloser{limitedBody, resp.Body, release},
 		ContentLength:   resp.ContentLength,
 		ContentType:     contentType,
 		Headers:         resp.Header,
@@ -574,17 +624,18 @@ func (r *limitedReader) Read(p []byte) (int, error) {
 	return n, err
 }
 
-// semaphoreReleasingReadCloser releases a semaphore slot when closed.
+// semaphoreReleasingReadCloser releases the fetch's connection slots when
+// closed.
 type semaphoreReleasingReadCloser struct {
 	*limitedReader
 
 	closer  io.Closer
-	sem    chan struct{}
+	release func()
 }
 
 func (r *semaphoreReleasingReadCloser) Close() error {
 	err := r.closer.Close()
-	<-r.sem // Release semaphore slot
+	r.release()
 
 	return err
 }

internal/httpfetcher/max_connections_internal_test.go

@@ -0,0 +1,165 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"net"
+	"strconv"
+	"testing"
+	"time"
+)
+
+// imageURLOnPort is the fake upstream's image route on testPublicHost at
+// port. Each port is a different host to the per-host limit, while the test
+// dialer sends every port to the one test server.
+func imageURLOnPort(port int) string {
+	return "http://" + net.JoinHostPort(testPublicHost, strconv.Itoa(port)) +
+		"/image"
+}
+
+func TestDefaultConfigMaxConnections(t *testing.T) {
+	t.Parallel()
+
+	if got := DefaultConfig().MaxConnections; got != DefaultMaxConnections {
+		t.Errorf("MaxConnections = %d, want %d", got, DefaultMaxConnections)
+	}
+}
+
+// TestFetchLimitsConnectionsToAllHostsTogether checks that MaxConnections
+// counts the fetches to every host together, apart from the per-host
+// limit: with MaxConnections at 2 and two responses open from two hosts, a
+// fetch from a third host, which has nothing open, waits the whole wait
+// timeout and fails with ErrTooManyConnections. Closing one response lets
+// it through.
+func TestFetchLimitsConnectionsToAllHostsTogether(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxConnections = 2
+
+	f, _ := newServerFetcher(t, srv, cfg)
+	f.connectionWaitTimeout = 100 * time.Millisecond
+
+	first, err := f.Fetch(testContext(t), imageURLOnPort(81))
+	if err != nil {
+		t.Fatalf("first Fetch() error = %v", err)
+	}
+
+	second, err := f.Fetch(testContext(t), imageURLOnPort(82))
+	if err != nil {
+		t.Fatalf("second Fetch() error = %v", err)
+	}
+
+	defer func() { _ = second.Content.Close() }()
+
+	start := time.Now()
+
+	_, err = f.Fetch(testContext(t), imageURLOnPort(83))
+	if !errors.Is(err, ErrTooManyConnections) {
+		t.Fatalf("third Fetch() error = %v, want ErrTooManyConnections", err)
+	}
+
+	if waited := time.Since(start); waited < f.connectionWaitTimeout {
+		t.Errorf("third Fetch() failed after %v, before waiting %v",
+			waited, f.connectionWaitTimeout)
+	}
+
+	if held := semLen(f, testPublicHost+":83"); held != 0 {
+		t.Errorf("the refused fetch kept its host's slot: %d held", held)
+	}
+
+	err = first.Content.Close()
+	if err != nil {
+		t.Fatalf("close first body: %v", err)
+	}
+
+	third, err := f.Fetch(testContext(t), imageURLOnPort(83))
+	if err != nil {
+		t.Fatalf("Fetch() after a response was closed: error = %v", err)
+	}
+
+	_ = third.Content.Close()
+}
+
+// TestFetchFreesHostSlotWhenContextEndsWaitingForConnection checks that a
+// fetch whose request context ends while it waits for a connection shared
+// by all hosts gives its host's slot back. With MaxConnections at 1 and one
+// response open, a fetch from another host takes that host's slot and waits;
+// its context ends long before the 10 second wait timeout.
+func TestFetchFreesHostSlotWhenContextEndsWaitingForConnection(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxConnections = 1
+
+	f, _ := newServerFetcher(t, srv, cfg)
+
+	first, err := f.Fetch(testContext(t), imageURLOnPort(81))
+	if err != nil {
+		t.Fatalf("first Fetch() error = %v", err)
+	}
+
+	defer func() { _ = first.Content.Close() }()
+
+	ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
+	defer cancel()
+
+	_, err = f.Fetch(ctx, imageURLOnPort(82))
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("second Fetch() error = %v, want context.DeadlineExceeded", err)
+	}
+
+	if held := semLen(f, testPublicHost+":82"); held != 0 {
+		t.Errorf("the fetch kept its host's slot after its context ended: "+
+			"%d held", held)
+	}
+}
+
+// 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.
+func TestFetchReleasesConnectionOnError(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name string
+		url  string
+		want error
+	}{
+		{"upstream answers 500", upstreamURL("/status/500"), ErrUpstreamError},
+		{"upstream sends HTML", upstreamURL("/html"), ErrInvalidContentType},
+		// 198.51.100.7 (TEST-NET-2) passes the SSRF checks, and the test
+		// dialer refuses every host but testPublicHost.
+		{"connecting fails", "http://198.51.100.7/image", errUnexpectedDial},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			srv := startUpstream(t)
+
+			cfg := DefaultConfig()
+			cfg.MaxConnections = 1
+
+			f, _ := newServerFetcher(t, srv, cfg)
+			f.connectionWaitTimeout = 100 * time.Millisecond
+
+			_, err := f.Fetch(testContext(t), tc.url)
+			if !errors.Is(err, tc.want) {
+				t.Fatalf("Fetch() error = %v, want %v", err, tc.want)
+			}
+
+			if held := len(f.allHostsSemaphore); held != 0 {
+				t.Fatalf("connection still held after the error: %d held", held)
+			}
+
+			res := fetchImage(t, f, "/image")
+			_ = res.Content.Close()
+		})
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -7,7 +7,9 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"runtime"
 	"sync"
+	"time"
 
 	"github.com/davidbyttow/govips/v2/vips"
 )
@@ -17,11 +19,21 @@ import (
 //nolint:gochecknoglobals // package-level sync.Once for one-time vips init
 var vipsOnce sync.Once
 
-// initVips initializes libvips with quiet logging.
+// initVips initializes libvips with quiet logging, one worker thread per
+// image and no operation cache. Process already works on one image per CPU
+// by default, so more threads per image would only compete for the CPUs.
+// Each request decodes different source bytes, so the operation cache
+// would rarely be hit and would hold memory outside MaxConcurrentProcessing;
+// repeated requests are served from pixa's disk cache instead.
 func initVips() {
 	vipsOnce.Do(func() {
 		vips.LoggingSettings(nil, vips.LogLevelError)
-		vips.Startup(nil)
+		vips.Startup(&vips.Config{
+			ConcurrencyLevel: 1,
+			MaxCacheSize:     0,
+			MaxCacheMem:      0,
+			MaxCacheFiles:    0,
+		})
 	})
 }
 
@@ -106,9 +118,23 @@ var ErrInputDataTooLarge = errors.New("input data exceeds maximum allowed size")
 // not supported.
 var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
 
+// ErrTooManyImages is returned when MaxConcurrentProcessing images are being
+// processed and none finishes within ProcessingWaitTimeout.
+var ErrTooManyImages = errors.New("too many images being processed at once")
+
+// ProcessingWaitTimeout is how long Process waits for a free slot when
+// MaxConcurrentProcessing images are already being processed.
+const ProcessingWaitTimeout = 10 * time.Second
+
 // ImageProcessor implements image transformation using libvips via govips.
 type ImageProcessor struct {
 	maxInputBytes int64
+	// processingSemaphore has one slot per image that may be processed at
+	// once. Process holds a slot from before it reads its input until it
+	// returns, so the input, the decoded image and the output all count.
+	processingSemaphore chan struct{}
+	// processingWaitTimeout is ProcessingWaitTimeout; tests shorten it.
+	processingWaitTimeout time.Duration
 }
 
 // Params holds configuration for creating an ImageProcessor.
@@ -117,6 +143,9 @@ type Params struct {
 	// MaxInputBytes is the maximum allowed input size in bytes.
 	// If <= 0, DefaultMaxInputBytes is used.
 	MaxInputBytes int64
+	// MaxConcurrentProcessing is the most images processed at once.
+	// If <= 0, the number of CPUs Go uses (runtime.GOMAXPROCS(0)) is used.
+	MaxConcurrentProcessing int
 }
 
 // New creates a new image processor with the given parameters.
@@ -129,17 +158,34 @@ func New(params Params) *ImageProcessor {
 		maxInputBytes = DefaultMaxInputBytes
 	}
 
+	maxConcurrentProcessing := params.MaxConcurrentProcessing
+	if maxConcurrentProcessing <= 0 {
+		maxConcurrentProcessing = runtime.GOMAXPROCS(0)
+	}
+
 	return &ImageProcessor{
 		maxInputBytes:         maxInputBytes,
+		processingSemaphore:   make(chan struct{}, maxConcurrentProcessing),
+		processingWaitTimeout: ProcessingWaitTimeout,
 	}
 }
 
-// Process transforms an image according to the request.
+// Process transforms an image according to the request. When
+// MaxConcurrentProcessing images are already being processed, it waits up
+// to ProcessingWaitTimeout for one to finish, then fails with
+// ErrTooManyImages.
 func (p *ImageProcessor) Process(
-	_ context.Context,
+	ctx context.Context,
 	input io.Reader,
 	req *Request,
 ) (*Result, error) {
+	release, err := p.acquireSlot(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	defer release()
+
 	// Read input with a size limit to prevent unbounded memory consumption.
 	// We read at most maxInputBytes+1 so we can detect if the input exceeds
 	// the limit without consuming additional memory.
@@ -285,6 +331,29 @@ func FormatToMIME(format Format) string {
 	}
 }
 
+// acquireSlot takes a slot in processingSemaphore, waiting at most
+// processingWaitTimeout for one to free up, and returns the func that gives
+// it back. A free slot is taken even when ctx has ended; only the wait for
+// one stops when ctx ends, as the rest of Process does not check ctx.
+func (p *ImageProcessor) acquireSlot(ctx context.Context) (func(), error) {
+	release := func() { <-p.processingSemaphore }
+
+	select {
+	case p.processingSemaphore <- struct{}{}:
+		return release, nil
+	default:
+	}
+
+	select {
+	case p.processingSemaphore <- struct{}{}:
+		return release, nil
+	case <-time.After(p.processingWaitTimeout):
+		return nil, ErrTooManyImages
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
 // detectFormat returns the format string from a vips image.
 func (p *ImageProcessor) detectFormat(img *vips.ImageRef) string {
 	format := img.Format()

internal/imageprocessor/max_concurrent_processing_internal_test.go

@@ -0,0 +1,302 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+	"testing/iotest"
+	"time"
+)
+
+// errTestReadFailed is the error the unreadable test input returns.
+var errTestReadFailed = errors.New("test input cannot be read")
+
+// readingCounter counts the Process calls reading their input at the same
+// time and remembers the most there ever were.
+type readingCounter struct {
+	mu      sync.Mutex
+	reading int
+	most    int
+}
+
+func (c *readingCounter) start() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.reading++
+	c.most = max(c.most, c.reading)
+}
+
+func (c *readingCounter) stop() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.reading--
+}
+
+func (c *readingCounter) mostReading() int {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	return c.most
+}
+
+// gatedReader is a Process input. Its first Read counts the call in,
+// reports it on entered and blocks until gate is closed; it counts the call
+// out when it returns io.EOF. Process reads its input only while it holds a
+// processing slot, so the count never goes above MaxConcurrentProcessing.
+type gatedReader struct {
+	data    *bytes.Reader
+	gate    <-chan struct{}
+	entered chan<- struct{}
+	counter *readingCounter
+	started bool
+}
+
+func (r *gatedReader) Read(p []byte) (int, error) {
+	if !r.started {
+		r.started = true
+		r.counter.start()
+
+		r.entered <- struct{}{}
+
+		<-r.gate
+	}
+
+	n, err := r.data.Read(p)
+	if errors.Is(err, io.EOF) {
+		r.counter.stop()
+	}
+
+	return n, err
+}
+
+// smallJPEGRequest asks for a 5x5 JPEG.
+func smallJPEGRequest() *Request {
+	return &Request{
+		Size:    Size{Width: 5, Height: 5},
+		Format:  FormatJPEG,
+		Quality: 85,
+		FitMode: FitCover,
+	}
+}
+
+// processInBackground runs Process on reader in a new goroutine and sends
+// its error on results.
+func processInBackground(
+	proc *ImageProcessor, reader *gatedReader, results chan<- error,
+) {
+	go func() {
+		result, err := proc.Process(context.Background(), reader, smallJPEGRequest())
+		if err == nil {
+			_ = result.Content.Close()
+		}
+
+		results <- err
+	}()
+}
+
+// waitForEntries fails the test unless count Process calls report on
+// entered within a few seconds.
+func waitForEntries(t *testing.T, entered <-chan struct{}, count int) {
+	t.Helper()
+
+	for range count {
+		select {
+		case <-entered:
+		case <-time.After(5 * time.Second):
+			t.Fatal("Process calls did not start reading their input")
+		}
+	}
+}
+
+func TestNewDefaultsMaxConcurrentProcessingToCPUs(t *testing.T) {
+	t.Parallel()
+
+	for _, limit := range []int{0, -1} {
+		proc := New(Params{MaxConcurrentProcessing: limit})
+		if got := cap(proc.processingSemaphore); got != runtime.GOMAXPROCS(0) {
+			t.Errorf("MaxConcurrentProcessing %d: %d slots, want %d, one per CPU",
+				limit, got, runtime.GOMAXPROCS(0))
+		}
+	}
+
+	proc := New(Params{MaxConcurrentProcessing: 3})
+	if got := cap(proc.processingSemaphore); got != 3 {
+		t.Errorf("MaxConcurrentProcessing 3: %d slots, want 3", got)
+	}
+}
+
+// TestProcessNeverExceedsMaxConcurrentProcessing starts more Process calls
+// than MaxConcurrentProcessing allows and holds the first ones inside
+// Process until the test lets them go. No more than the limit may be
+// working at once, and the calls held back must wait for a slot and then
+// succeed.
+func TestProcessNeverExceedsMaxConcurrentProcessing(t *testing.T) {
+	t.Parallel()
+
+	const (
+		limit = 2
+		calls = 6
+	)
+
+	proc := New(Params{MaxConcurrentProcessing: limit})
+	input := createTestJPEG(t, 50, 50)
+
+	counter := &readingCounter{}
+	gate := make(chan struct{})
+	entered := make(chan struct{}, calls)
+	results := make(chan error, calls)
+
+	openGate := sync.OnceFunc(func() { close(gate) })
+	t.Cleanup(openGate)
+
+	for range calls {
+		processInBackground(proc, &gatedReader{
+			data: bytes.NewReader(input), gate: gate, entered: entered,
+			counter: counter,
+		}, results)
+	}
+
+	waitForEntries(t, entered, limit)
+
+	// A call beyond the limit would start reading its input now.
+	select {
+	case <-entered:
+		t.Fatalf("a Process call started while %d were already working", limit)
+	case <-time.After(100 * time.Millisecond):
+	}
+
+	openGate()
+
+	for range calls {
+		err := <-results
+		if err != nil {
+			t.Errorf("Process() error = %v, want nil once a slot is free", err)
+		}
+	}
+
+	if most := counter.mostReading(); most > limit {
+		t.Errorf("%d Process calls worked at once, want at most %d", most, limit)
+	}
+}
+
+// TestProcessWaitsThenFailsWhenNoSlotFrees holds the only slot and checks
+// that another call waits the whole wait timeout, then fails with
+// ErrTooManyImages instead of processing anyway.
+func TestProcessWaitsThenFailsWhenNoSlotFrees(t *testing.T) {
+	t.Parallel()
+
+	proc := New(Params{MaxConcurrentProcessing: 1})
+	proc.processingWaitTimeout = 100 * time.Millisecond
+
+	input := createTestJPEG(t, 10, 10)
+
+	gate := make(chan struct{})
+	entered := make(chan struct{}, 1)
+	held := make(chan error, 1)
+
+	openGate := sync.OnceFunc(func() { close(gate) })
+	t.Cleanup(openGate)
+
+	processInBackground(proc, &gatedReader{
+		data: bytes.NewReader(input), gate: gate, entered: entered,
+		counter: &readingCounter{},
+	}, held)
+	waitForEntries(t, entered, 1)
+
+	start := time.Now()
+
+	_, err := proc.Process(context.Background(), bytes.NewReader(input),
+		smallJPEGRequest())
+	if !errors.Is(err, ErrTooManyImages) {
+		t.Fatalf("Process() error = %v, want ErrTooManyImages", err)
+	}
+
+	if waited := time.Since(start); waited < proc.processingWaitTimeout {
+		t.Errorf("Process() failed after %v, before waiting %v",
+			waited, proc.processingWaitTimeout)
+	}
+
+	openGate()
+
+	err = <-held
+	if err != nil {
+		t.Errorf("Process() holding the slot: error = %v, want nil", err)
+	}
+}
+
+// TestProcessReleasesSlotOnError checks that Process gives its slot back
+// when it fails, whether it fails early or late: with one slot, the slot
+// must be free after the failure and the next call must succeed.
+func TestProcessReleasesSlotOnError(t *testing.T) {
+	t.Parallel()
+
+	valid := createTestJPEG(t, 10, 10)
+
+	unsupported := smallJPEGRequest()
+	unsupported.Format = "bmp"
+
+	cases := []struct {
+		name  string
+		input io.Reader
+		req   *Request
+		// want is the error Process must return; nil means any error.
+		want error
+	}{
+		{
+			name:  "input cannot be read",
+			input: iotest.ErrReader(errTestReadFailed),
+			req:   smallJPEGRequest(),
+			want:  errTestReadFailed,
+		},
+		{
+			name:  "input over the byte limit",
+			input: bytes.NewReader(createTestJPEG(t, 800, 600)),
+			req:   smallJPEGRequest(),
+			want:  ErrInputDataTooLarge,
+		},
+		{
+			name:  "input not an image",
+			input: strings.NewReader("not an image"),
+			req:   smallJPEGRequest(),
+		},
+		{
+			name:  "output format not supported",
+			input: bytes.NewReader(valid),
+			req:   unsupported,
+			want:  ErrUnsupportedOutputFormat,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			proc := New(Params{MaxInputBytes: 4096, MaxConcurrentProcessing: 1})
+			proc.processingWaitTimeout = 100 * time.Millisecond
+
+			_, err := proc.Process(context.Background(), tc.input, tc.req)
+			if err == nil || (tc.want != nil && !errors.Is(err, tc.want)) {
+				t.Fatalf("Process() error = %v, want %v", err, tc.want)
+			}
+
+			if held := len(proc.processingSemaphore); held != 0 {
+				t.Fatalf("slot still held after the error: %d held", held)
+			}
+
+			result, err := proc.Process(context.Background(), bytes.NewReader(valid),
+				smallJPEGRequest())
+			if err != nil {
+				t.Fatalf("Process() after the error = %v, want nil", err)
+			}
+
+			_ = result.Content.Close()
+		})
+	}
+}

internal/imgcache/cache.go

@@ -14,6 +14,7 @@ import (
 	"sync"
 	"time"
 
+	lru "github.com/hashicorp/golang-lru/v2"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 )
 
@@ -26,6 +27,10 @@ var (
 // HTTP status code for successful fetch.
 const httpStatusOK = 200
 
+// metaCacheSize is how many variants' content types metaCache holds. A
+// variant not among them is served as before, reading its .meta file.
+const metaCacheSize = 10000
+
 // CacheConfig holds cache configuration.
 type CacheConfig struct {
 	StateDir    string
@@ -49,12 +54,6 @@ type CacheConfig struct {
 	Logger *slog.Logger
 }
 
-// variantMeta stores content type for fast cache hits without reading .meta file.
-type variantMeta struct {
-	ContentType string
-	Size        int64
-}
-
 // Cache implements the caching layer for the image proxy.
 type Cache struct {
 	db          *sql.DB
@@ -76,9 +75,10 @@ type Cache struct {
 	evictionStarted  bool
 	evictionStopOnce sync.Once
 
-	// In-memory cache of variant metadata (content type, size) to avoid
+	// metaCache holds the content types of the variants most recently
-	// reading .meta files
+	// stored or served, so a hit does not read the variant's .meta file.
-	metaCache map[VariantKey]variantMeta
+	// It never stands in for the variant file, which is always opened.
+	metaCache *lru.Cache[VariantKey, string]
 
 	// contentLocks serializes StoreSource and evictSourceBlob per
 	// content hash, closing the race window between an eviction's row
@@ -101,6 +101,11 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 		log = slog.Default()
 	}
 
+	metaCache, err := lru.New[VariantKey, string](metaCacheSize)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create variant content type cache: %w", err)
+	}
+
 	c := &Cache{
 		db:               db,
 		config:           config,
@@ -109,7 +114,7 @@ func NewCache(db *sql.DB, config CacheConfig) (*Cache, error) {
 		evictionPressure: make(chan struct{}, 1),
 		evictionStop:     make(chan struct{}),
 		evictionDone:     make(chan struct{}),
-		metaCache:        make(map[VariantKey]variantMeta),
+		metaCache:        metaCache,
 		contentLocks:     newContentLock(),
 	}
 
@@ -177,13 +182,30 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 	}, nil
 }
 
-// GetVariant returns a reader, size, and content type for a cached variant.
+// GetVariant returns a reader, size, and content type for a cached
+// variant. The content type comes from metaCache, or else from the
+// variant's .meta file and is then kept in metaCache. A variant with
+// no .meta file is served as application/octet-stream, which is not
+// kept.
 func (c *Cache) GetVariant(cacheKey VariantKey) (io.ReadCloser, int64, string, error) {
 	if c.disabled {
 		return nil, 0, "", ErrNotFound
 	}
 
-	return c.variants.LoadWithMeta(cacheKey)
+	contentType, known := c.metaCache.Get(cacheKey)
+	if !known {
+		return c.loadVariantWithMeta(cacheKey)
+	}
+
+	reader, size, err := c.variants.LoadWithSize(cacheKey)
+	if err != nil {
+		// The file is gone, e.g. deleted outside pixa
+		c.metaCache.Remove(cacheKey)
+
+		return nil, 0, "", err
+	}
+
+	return reader, size, contentType, nil
 }
 
 // StoreSource stores fetched source content and metadata. On a
@@ -286,6 +308,8 @@ func (c *Cache) StoreVariant(
 		return err
 	}
 
+	c.metaCache.Add(cacheKey, contentType)
+
 	_, err = c.db.ExecContext(ctx, `
 		INSERT INTO variant_content (cache_key, size_bytes, content_type)
 		VALUES (?, ?, ?)
@@ -383,13 +407,16 @@ func (c *Cache) GetSourceMetadataID(
 	return id, nil
 }
 
-// GetSourceContent returns a reader for cached source content by its hash.
+// GetSourceContent returns a reader for cached source content by its hash,
-func (c *Cache) GetSourceContent(contentHash ContentHash) (io.ReadCloser, error) {
+// and the content's size in bytes.
+func (c *Cache) GetSourceContent(
+	contentHash ContentHash,
+) (io.ReadCloser, int64, error) {
 	if c.disabled {
-		return nil, ErrNotFound
+		return nil, 0, ErrNotFound
 	}
 
-	return c.srcContent.Load(contentHash)
+	return c.srcContent.LoadWithSize(contentHash)
 }
 
 // CleanExpired removes expired entries from the cache.
@@ -419,20 +446,22 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 		return nil, fmt.Errorf("failed to get cache stats: %w", err)
 	}
 
-	// Get actual item count and total size from content tables
+	// Count and size the cached source images and processed variants. A
-	err = c.db.QueryRowContext(ctx,
+	// disabled cache holds none, whatever rows an earlier run left.
-		`SELECT COUNT(*) FROM request_cache`,
+	if !c.disabled {
-	).Scan(&stats.TotalItems)
+		err = c.db.QueryRowContext(ctx, `
+			SELECT (SELECT COUNT(*) FROM source_content)
+			     + (SELECT COUNT(*) FROM variant_content)
+		`).Scan(&stats.TotalItems)
 		if err != nil {
 			c.log.Warn("failed to count cache items for stats", "error", err)
 		}
 
-	err = c.db.QueryRowContext(ctx,
+		stats.TotalSizeBytes, err = c.UsageBytes(ctx)
-		`SELECT COALESCE(SUM(size_bytes), 0) FROM output_content`,
-	).Scan(&stats.TotalSizeBytes)
 		if err != nil {
 			c.log.Warn("failed to sum cache size for stats", "error", err)
 		}
+	}
 
 	// Compute hit rate as a ratio
 	if stats.HitCount+stats.MissCount > 0 {
@@ -481,6 +510,44 @@ func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64)
 	}
 }
 
+// IncrementTransformCount counts one image transcoded by the image processor.
+func (c *Cache) IncrementTransformCount(ctx context.Context) {
+	_, err := c.db.ExecContext(ctx, `
+		UPDATE cache_stats
+		SET transform_count = transform_count + 1,
+		    last_updated_at = CURRENT_TIMESTAMP
+		WHERE id = 1
+	`)
+	if err != nil {
+		c.log.Warn("failed to count transform", "error", err)
+	}
+}
+
+// loadVariantWithMeta is GetVariant for a variant metaCache does not
+// hold: it reads the content type from the variant's .meta file and
+// keeps it in metaCache, unless a StoreVariant has put one there
+// meanwhile, as the store's is newer. A read that finds no .meta file,
+// as one can between a store's writing of the variant file and of its
+// .meta file, serves application/octet-stream and keeps nothing, so
+// metaCache only ever holds a type read from a .meta file or passed to
+// StoreVariant.
+func (c *Cache) loadVariantWithMeta(
+	cacheKey VariantKey,
+) (io.ReadCloser, int64, string, error) {
+	reader, size, contentType, err := c.variants.LoadWithMeta(cacheKey)
+	if err != nil {
+		return nil, 0, "", err
+	}
+
+	if contentType == "" {
+		return reader, size, fallbackContentType, nil
+	}
+
+	c.metaCache.ContainsOrAdd(cacheKey, contentType)
+
+	return reader, size, contentType, nil
+}
+
 // writeMetadataSidecar writes the JSON metadata sidecar of a stored source.
 // A failure is logged and is otherwise non-fatal; the metadata is in the
 // database.

internal/imgcache/eviction.go

@@ -39,8 +39,8 @@ const tempFilePrefix = ".tmp-"
 // to each variant file.
 const variantMetaSuffix = ".meta"
 
-// fallbackContentType is recorded when a reconciled variant file has
+// fallbackContentType is the content type given to a variant file that
-// no readable .meta sidecar.
+// has no readable .meta sidecar, when it is served or reconciled.
 const fallbackContentType = "application/octet-stream"
 
 // UsageBytes returns the total number of bytes of cache content
@@ -271,7 +271,9 @@ func (c *Cache) sourceCandidates(ctx context.Context) ([]evictionCandidate, erro
 
 // evictVariant removes one variant: accounting row first, then the
 // content and .meta files, so the database never references a deleted
-// file.
+// file. The metaCache entry goes before the files; a GetVariant that
+// read them just before may put it back, and the next GetVariant then
+// fails to open the file and removes it again.
 func (c *Cache) evictVariant(ctx context.Context, cacheKey VariantKey) error {
 	_, err := c.db.ExecContext(ctx,
 		`DELETE FROM variant_content WHERE cache_key = ?`, string(cacheKey))
@@ -279,6 +281,8 @@ func (c *Cache) evictVariant(ctx context.Context, cacheKey VariantKey) error {
 		return fmt.Errorf("failed to delete variant accounting row: %w", err)
 	}
 
+	c.metaCache.Remove(cacheKey)
+
 	err = c.variants.DeleteWithMeta(cacheKey)
 	if err != nil {
 		return err

internal/imgcache/imgcache.go

@@ -95,7 +95,8 @@ type ImageRequest struct {
 	FitMode FitMode
 	// Signature is the HMAC signature for non-allowlisted hosts
 	Signature string
-	// Expires is the signature expiration timestamp
+	// Expires is when the URL expires: the exp of a signed URL, or the expiry
+	// of an encrypted URL; the zero time if it has none
 	Expires time.Time
 	// AllowHTTP indicates whether HTTP (non-TLS) is allowed for this request
 	AllowHTTP bool
@@ -162,9 +163,11 @@ type ImageCache interface {
 
 // CacheStats contains cache statistics
 type CacheStats struct {
-	// TotalItems is the number of cached items
+	// TotalItems is the number of cached source images plus processed
+	// variants
 	TotalItems int64
-	// TotalSizeBytes is the total size of cached content
+	// TotalSizeBytes is the total size of cached source images and
+	// processed variants
 	TotalSizeBytes int64
 	// HitCount is the number of cache hits
 	HitCount int64

internal/imgcache/max_concurrent_processing_internal_test.go

@@ -0,0 +1,125 @@
+package imgcache
+
+import (
+	"image/color"
+	"image/jpeg"
+	"io"
+	"os"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+)
+
+// widthOnlyRequest asks for the test photo at width, its height scaled to
+// keep the photo's aspect ratio.
+func widthOnlyRequest(fixtures *TestFixtures, width int) *ImageRequest {
+	return &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: testPathPhoto,
+		Size:       Size{Width: width},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+}
+
+// holdProcessingSlot takes one of proc's processing slots and returns the
+// func that gives it back. Process takes its slot before it reads its input,
+// so once it has read a byte from the pipe it holds the slot, until the pipe
+// is closed.
+func holdProcessingSlot(
+	t *testing.T, proc *imageprocessor.ImageProcessor,
+) func() {
+	t.Helper()
+
+	input, feed := io.Pipe()
+
+	go func() {
+		_, _ = proc.Process(t.Context(), input, &imageprocessor.Request{})
+	}()
+
+	_, err := feed.Write([]byte{0})
+	if err != nil {
+		t.Fatalf("Process call to hold the slot did not start: %v", err)
+	}
+
+	release := func() { _ = feed.Close() }
+	t.Cleanup(release)
+
+	return release
+}
+
+// TestService_Get_WaitsForSlotBeforeReadingCachedSource checks that a
+// request whose source is cached holds none of it while it waits for a
+// processing slot: it reads the cached file only once it has a slot. With
+// the only slot held, a request for a new width of the cached 100x100 photo
+// waits; the cached file is then rewritten as a 100x50 image before the slot
+// is freed, so the request must answer with that image scaled to 40x20.
+func TestService_Get_WaitsForSlotBeforeReadingCachedSource(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+	svc.processor = imageprocessor.New(
+		imageprocessor.Params{MaxConcurrentProcessing: 1},
+	)
+
+	// A first request caches the photo as a source.
+	resp, err := svc.Get(t.Context(), widthOnlyRequest(fixtures, 50))
+	if err != nil {
+		t.Fatalf("first Get() error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	contentHash, _, err := svc.cache.LookupSource(t.Context(),
+		widthOnlyRequest(fixtures, 50))
+	if err != nil || contentHash == "" {
+		t.Fatalf("LookupSource() = %q, %v; want the cached source",
+			contentHash, err)
+	}
+
+	release := holdProcessingSlot(t, svc.processor)
+
+	var (
+		waited    *ImageResponse
+		waitedErr error
+	)
+
+	done := make(chan struct{})
+
+	go func() {
+		defer close(done)
+
+		waited, waitedErr = svc.Get(t.Context(), widthOnlyRequest(fixtures, 40))
+	}()
+
+	// Give the request time to reach the slot: had it read the cached source
+	// before waiting, it would have read it by now.
+	time.Sleep(100 * time.Millisecond)
+
+	err = os.WriteFile(svc.cache.srcContent.hashToPath(contentHash),
+		generateTestJPEG(t, 100, 50, color.RGBA{0, 0, 255, 255}), 0o600)
+	if err != nil {
+		t.Fatalf("failed to rewrite the cached source: %v", err)
+	}
+
+	release()
+	<-done
+
+	if waitedErr != nil {
+		t.Fatalf("Get() error = %v", waitedErr)
+	}
+
+	defer func() { _ = waited.Content.Close() }()
+
+	output, err := jpeg.DecodeConfig(waited.Content)
+	if err != nil {
+		t.Fatalf("failed to decode the response: %v", err)
+	}
+
+	if output.Width != 40 || output.Height != 20 {
+		t.Errorf("response is %dx%d, want 40x20: the request read the cached "+
+			"source before it had a processing slot", output.Width, output.Height)
+	}
+}

internal/imgcache/metacache_internal_test.go

@@ -0,0 +1,349 @@
+package imgcache
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"sync"
+	"testing"
+	"time"
+)
+
+// webpRequest returns a request for a 100x100 WebP variant of path.
+func webpRequest(path string) *ImageRequest {
+	return &ImageRequest{
+		SourceHost: testHostCDN,
+		SourcePath: path,
+		Size:       Size{Width: 100, Height: 100},
+		Format:     FormatWebP,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+}
+
+// assertVariantServed checks that GetVariant serves key with the given
+// content and the image/webp content type storeEvictionTestVariant stores.
+func assertVariantServed(t *testing.T, cache *Cache, key VariantKey, content []byte) {
+	t.Helper()
+
+	reader, size, contentType, err := cache.GetVariant(key)
+	if err != nil {
+		t.Fatalf("GetVariant(%s) error = %v", key, err)
+	}
+
+	defer func() { _ = reader.Close() }()
+
+	got, err := io.ReadAll(reader)
+	if err != nil {
+		t.Fatalf("reading variant %s: %v", key, err)
+	}
+
+	if !bytes.Equal(got, content) {
+		t.Errorf("GetVariant(%s) content = %q, want %q", key, got, content)
+	}
+
+	if size != int64(len(content)) {
+		t.Errorf("GetVariant(%s) size = %d, want %d", key, size, len(content))
+	}
+
+	if contentType != testContentTypeWebP {
+		t.Errorf("GetVariant(%s) content type = %q, want %q",
+			key, contentType, testContentTypeWebP)
+	}
+}
+
+// assertVariantNotFound checks that GetVariant refuses key with
+// ErrNotFound.
+func assertVariantNotFound(t *testing.T, cache *Cache, key VariantKey) {
+	t.Helper()
+
+	reader, _, _, err := cache.GetVariant(key)
+	if err == nil {
+		_ = reader.Close()
+	}
+
+	if !errors.Is(err, ErrNotFound) {
+		t.Errorf("GetVariant(%s) error = %v, want ErrNotFound", key, err)
+	}
+}
+
+// assertLookupMisses checks that Lookup reports request as a miss.
+func assertLookupMisses(t *testing.T, cache *Cache, request *ImageRequest) {
+	t.Helper()
+
+	lookup, err := cache.Lookup(t.Context(), request)
+	if err != nil {
+		t.Fatalf("Lookup(%s) error = %v", request.SourcePath, err)
+	}
+
+	if lookup.Hit {
+		t.Errorf("Lookup(%s) is a hit, want a miss", request.SourcePath)
+	}
+}
+
+// renameFile renames the file at from to to.
+func renameFile(t *testing.T, from, to string) {
+	t.Helper()
+
+	err := os.Rename(from, to)
+	if err != nil {
+		t.Fatalf("renaming %s: %v", from, err)
+	}
+}
+
+// TestSecondHitDoesNotReadMetaFile checks that once a variant has been
+// stored or read, a hit takes its content type from memory: with the
+// .meta file deleted, GetVariant must still return the stored content
+// type rather than the application/octet-stream it uses without one.
+func TestSecondHitDoesNotReadMetaFile(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+	content := []byte("webp variant bytes")
+
+	storeEvictionTestVariant(t, cache, testVariantKeyOne, content)
+
+	// A second Cache on the same state directory starts with nothing in
+	// memory, as pixad does after a restart, so its first read uses the
+	// .meta file.
+	restarted, err := NewCache(cache.db, cache.config)
+	if err != nil {
+		t.Fatalf("NewCache() error = %v", err)
+	}
+
+	assertVariantServed(t, restarted, testVariantKeyOne, content)
+
+	err = os.Remove(cache.variants.keyToPath(testVariantKeyOne) + ".meta")
+	if err != nil {
+		t.Fatalf("removing .meta file: %v", err)
+	}
+
+	assertVariantServed(t, cache, testVariantKeyOne, content)
+	assertVariantServed(t, restarted, testVariantKeyOne, content)
+}
+
+// TestReadDuringStoreKeepsStoredContentType checks that a GetVariant
+// which began before StoreVariant finished cannot replace the content
+// type the store kept in memory. Such a read can find the variant file
+// but not yet its .meta file, and so gets application/octet-stream. The
+// test deletes the .meta file after the store, then runs the part of
+// GetVariant that comes after its check of memory.
+func TestReadDuringStoreKeepsStoredContentType(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+	content := []byte("webp variant bytes")
+
+	storeEvictionTestVariant(t, cache, testVariantKeyOne, content)
+
+	err := os.Remove(cache.variants.keyToPath(testVariantKeyOne) + ".meta")
+	if err != nil {
+		t.Fatalf("removing .meta file: %v", err)
+	}
+
+	reader, _, contentType, err := cache.loadVariantWithMeta(testVariantKeyOne)
+	if err != nil {
+		t.Fatalf("loadVariantWithMeta(%s) error = %v", testVariantKeyOne, err)
+	}
+
+	_ = reader.Close()
+
+	t.Logf("the read without a .meta file got content type %q", contentType)
+
+	assertVariantServed(t, cache, testVariantKeyOne, content)
+}
+
+// TestReadOfOlderMetaFileKeepsStoredContentType checks that a read
+// which got its content type from a .meta file that StoreVariant had
+// not yet rewritten cannot replace the type the store kept in memory.
+// The test writes such a .meta file, with a different content type,
+// after the store, then runs the part of GetVariant that comes after
+// its check of memory.
+func TestReadOfOlderMetaFileKeepsStoredContentType(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+	content := []byte("webp variant bytes")
+
+	storeEvictionTestVariant(t, cache, testVariantKeyOne, content)
+
+	olderMeta, err := json.Marshal(VariantMeta{
+		ContentType: testContentTypeJPEG,
+		Size:        int64(len(content)),
+	})
+	if err != nil {
+		t.Fatalf("encoding .meta file: %v", err)
+	}
+
+	metaPath := cache.variants.keyToPath(testVariantKeyOne) + ".meta"
+
+	err = os.WriteFile(metaPath, olderMeta, StorageFilePerm)
+	if err != nil {
+		t.Fatalf("writing .meta file: %v", err)
+	}
+
+	reader, _, contentType, err := cache.loadVariantWithMeta(testVariantKeyOne)
+	if err != nil {
+		t.Fatalf("loadVariantWithMeta(%s) error = %v", testVariantKeyOne, err)
+	}
+
+	_ = reader.Close()
+
+	if contentType != testContentTypeJPEG {
+		t.Fatalf("loadVariantWithMeta(%s) content type = %q, want %q from the .meta file",
+			testVariantKeyOne, contentType, testContentTypeJPEG)
+	}
+
+	kept, _ := cache.metaCache.Get(testVariantKeyOne)
+	if kept != testContentTypeWebP {
+		t.Errorf("content type in memory = %q, want the stored %q",
+			kept, testContentTypeWebP)
+	}
+}
+
+// TestFailedReadDuringStoreKeepsStoredContentType checks that a read
+// which found no .meta file cannot leave application/octet-stream in
+// memory, even when another read has removed the content type
+// StoreVariant kept there. In this order: the store; a read that finds
+// the variant in memory but cannot open its file, and so removes it
+// from memory; a read that opened the variant file before the store
+// wrote its .meta file. Later hits must get the stored content type.
+func TestFailedReadDuringStoreKeepsStoredContentType(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+	content := []byte("webp variant bytes")
+	variantPath := cache.variants.keyToPath(testVariantKeyOne)
+	metaPath := variantPath + ".meta"
+
+	storeEvictionTestVariant(t, cache, testVariantKeyOne, content)
+
+	renameFile(t, variantPath, variantPath+".hidden")
+	assertVariantNotFound(t, cache, testVariantKeyOne)
+	renameFile(t, variantPath+".hidden", variantPath)
+
+	renameFile(t, metaPath, metaPath+".hidden")
+
+	reader, _, contentType, err := cache.GetVariant(testVariantKeyOne)
+	if err != nil {
+		t.Fatalf("GetVariant(%s) error = %v", testVariantKeyOne, err)
+	}
+
+	_ = reader.Close()
+
+	t.Logf("the read without a .meta file got content type %q", contentType)
+
+	renameFile(t, metaPath+".hidden", metaPath)
+
+	assertVariantServed(t, cache, testVariantKeyOne, content)
+}
+
+// TestEvictedVariantIsNotServed checks that a variant the evictor
+// removed is a miss and cannot be read, although it had been stored
+// and served before.
+func TestEvictedVariantIsNotServed(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1500)
+
+	oldRequest := webpRequest("/old.jpg")
+	newRequest := webpRequest("/new.jpg")
+	oldKey := CacheKey(oldRequest)
+	newKey := CacheKey(newRequest)
+	oldContent := bytes.Repeat([]byte{0x01}, 1000)
+	newContent := bytes.Repeat([]byte{0x02}, 1000)
+
+	storeEvictionTestVariant(t, cache, oldKey, oldContent)
+	storeEvictionTestVariant(t, cache, newKey, newContent)
+	assertVariantServed(t, cache, oldKey, oldContent)
+	setVariantLastAccessed(t, cache, oldKey, time.Now().Add(-time.Hour))
+
+	err := cache.EvictToLimit(t.Context())
+	if err != nil {
+		t.Fatalf("EvictToLimit() error = %v", err)
+	}
+
+	assertLookupMisses(t, cache, oldRequest)
+	assertVariantNotFound(t, cache, oldKey)
+	assertVariantServed(t, cache, newKey, newContent)
+}
+
+// TestVariantDeletedFromDiskIsNotServed checks that a variant whose
+// file was deleted by something other than the evictor cannot be read,
+// and is a miss afterwards.
+func TestVariantDeletedFromDiskIsNotServed(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+
+	request := webpRequest("/deleted.jpg")
+	key := CacheKey(request)
+	content := []byte("webp variant bytes")
+
+	storeEvictionTestVariant(t, cache, key, content)
+	assertVariantServed(t, cache, key, content)
+
+	err := os.Remove(cache.variants.keyToPath(key))
+	if err != nil {
+		t.Fatalf("removing variant file: %v", err)
+	}
+
+	assertVariantNotFound(t, cache, key)
+	assertLookupMisses(t, cache, request)
+}
+
+// TestConcurrentVariantStoreReadAndEvict stores, reads and evicts
+// variants from several goroutines at once, for the race detector.
+func TestConcurrentVariantStoreReadAndEvict(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<20)
+	ctx := t.Context()
+
+	var wg sync.WaitGroup
+
+	for goroutine := range 8 {
+		wg.Go(func() {
+			key := VariantKey(fmt.Sprintf("aabbccdd01%02d", goroutine))
+			content := []byte(key)
+
+			for range 20 {
+				err := cache.StoreVariant(
+					ctx, key, bytes.NewReader(content), testContentTypeWebP)
+				if err != nil {
+					t.Errorf("StoreVariant(%s) error = %v", key, err)
+
+					return
+				}
+
+				reader, _, contentType, err := cache.GetVariant(key)
+				if err != nil {
+					t.Errorf("GetVariant(%s) error = %v", key, err)
+
+					return
+				}
+
+				_ = reader.Close()
+
+				if contentType != testContentTypeWebP {
+					t.Errorf("GetVariant(%s) content type = %q, want %q",
+						key, contentType, testContentTypeWebP)
+				}
+
+				err = cache.evictVariant(ctx, key)
+				if err != nil {
+					t.Errorf("evictVariant(%s) error = %v", key, err)
+
+					return
+				}
+
+				assertVariantNotFound(t, cache, key)
+			}
+		})
+	}
+
+	wg.Wait()
+}

internal/imgcache/service.go

@@ -43,6 +43,9 @@ type ServiceConfig struct {
 	SigningKey string
 	// Allowlist is the list of hosts that don't require signatures
 	Allowlist []string
+	// MaxConcurrentProcessing is the most images processed at once; zero
+	// uses the image processor's default, one per CPU
+	MaxConcurrentProcessing int
 	// Logger for logging
 	Logger *slog.Logger
 }
@@ -91,9 +94,10 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 	}
 
 	maxResponseSize := fetcherCfg.MaxResponseSize
-	processor := imageprocessor.New(
+	processor := imageprocessor.New(imageprocessor.Params{
-		imageprocessor.Params{MaxInputBytes: maxResponseSize},
+		MaxInputBytes:           maxResponseSize,
-	)
+		MaxConcurrentProcessing: cfg.MaxConcurrentProcessing,
+	})
 
 	return &Service{
 		cache:           cfg.Cache,
@@ -143,7 +147,8 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 			s.log.Error("failed to get cached variant", "key", result.CacheKey, "error", err)
 			// Fall through to re-process
 		} else {
-			s.cache.IncrementStats(ctx, true, 0)
+			// Counted also when the request context has ended meanwhile
+			s.cache.IncrementStats(context.WithoutCancel(ctx), true, 0)
 
 			return &ImageResponse{
 				Content:       reader,
@@ -155,12 +160,15 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 		}
 	}
 
-	// Cache miss - check if we have source content cached
+	// Cache miss - process the cached source or fetch it, then count the
+	// miss with the bytes it fetched from upstream, also when it failed or
+	// the request context has ended meanwhile
 	cacheKey := CacheKey(req)
 
-	s.cache.IncrementStats(ctx, false, 0)
+	response, fetchedBytes, err := s.processFromSourceOrFetch(ctx, req, cacheKey)
+
+	s.cache.IncrementStats(context.WithoutCancel(ctx), false, fetchedBytes)
 
-	response, err := s.processFromSourceOrFetch(ctx, req, cacheKey)
 	if err != nil {
 		return nil, err
 	}
@@ -233,47 +241,47 @@ func (s *Service) GenerateSignedURL(
 		baseURL, path, sig, exp, req.Quality, req.FitMode), nil
 }
 
-// loadCachedSource attempts to load source content from cache, returning nil
+// loadCachedSource opens source content from cache, without reading it, and
-// if the cached data is unavailable or exceeds maxResponseSize.
+// returns it with its size; nil if the cached data is unavailable, empty or
-func (s *Service) loadCachedSource(contentHash ContentHash) []byte {
+// exceeds maxResponseSize.
-	reader, err := s.cache.GetSourceContent(contentHash)
+func (s *Service) loadCachedSource(
+	contentHash ContentHash,
+) (io.ReadCloser, int64) {
+	reader, size, err := s.cache.GetSourceContent(contentHash)
 	if err != nil {
 		s.log.Warn("failed to load cached source, fetching", "error", err)
 
-		return nil
+		return nil, 0
 	}
 
-	// Bound the read to maxResponseSize to prevent unbounded memory use
+	if size > s.maxResponseSize {
-	// from unexpectedly large cached files.
-	limited := io.LimitReader(reader, s.maxResponseSize+1)
-	data, err := io.ReadAll(limited)
 		_ = reader.Close()
 
-	if err != nil {
-		s.log.Warn("failed to read cached source, fetching", "error", err)
-
-		return nil
-	}
-
-	if int64(len(data)) > s.maxResponseSize {
 		s.log.Warn("cached source exceeds max response size, discarding",
 			"hash", contentHash,
 			"max_bytes", s.maxResponseSize,
 		)
 
-		return nil
+		return nil, 0
 	}
 
-	return data
+	if size == 0 {
+		_ = reader.Close()
+
+		return nil, 0
+	}
+
+	return reader, size
 }
 
 // processFromSourceOrFetch processes an image, using cached source content
-// if available.
+// if available. It also returns the number of bytes fetched from upstream,
+// as fetchAndProcess does, or 0 when the cached source was used.
 func (s *Service) processFromSourceOrFetch(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-) (*ImageResponse, error) {
+) (*ImageResponse, int64, error) {
 	// Check if we have cached source content
 	contentHash, _, err := s.cache.LookupSource(ctx, req)
 	if err != nil {
@@ -281,37 +289,38 @@ func (s *Service) processFromSourceOrFetch(
 	}
 
 	var (
-		sourceData []byte
+		source     io.ReadCloser
-		fetchBytes int64
+		sourceSize int64
 	)
 
 	if contentHash != "" {
 		s.log.Debug("using cached source", "hash", contentHash)
-		sourceData = s.loadCachedSource(contentHash)
+		source, sourceSize = s.loadCachedSource(contentHash)
 	}
 
 	// Fetch from upstream if we don't have source data or it's empty
-	if len(sourceData) == 0 {
+	if source == nil {
-		resp, err := s.fetchAndProcess(ctx, req, cacheKey)
+		return s.fetchAndProcess(ctx, req, cacheKey)
-		if err != nil {
-			return nil, err
 	}
 
-		return resp, nil
+	defer func() { _ = source.Close() }()
-	}
 
-	// Process using cached source
+	// Process using cached source; nothing was fetched from upstream. The
-	fetchBytes = int64(len(sourceData))
+	// image processor reads the source only once it has a processing slot,
+	// so a request waiting for one holds none of it in memory.
+	resp, err := s.processAndStore(ctx, req, cacheKey, source, sourceSize)
 
-	return s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+	return resp, 0, err
 }
 
 // fetchAndProcess fetches from upstream, processes, and caches the result.
+// It also returns the number of bytes read from upstream, including when
+// reading the response or a later step fails.
 func (s *Service) fetchAndProcess(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-) (*ImageResponse, error) {
+) (*ImageResponse, int64, error) {
 	// Fetch from upstream
 	sourceURL := req.SourceURL()
 
@@ -330,20 +339,24 @@ func (s *Service) fetchAndProcess(
 			}
 		}
 
-		return nil, fmt.Errorf("upstream fetch failed: %w", err)
+		return nil, 0, fmt.Errorf("upstream fetch failed: %w", err)
 	}
 
+	// Closing the body frees the upstream connection. It is closed only
+	// after processing, so the fetcher's connection limit also bounds the
+	// fetched sources held in memory while their requests wait for a
+	// processing slot.
 	defer func() { _ = fetchResult.Content.Close() }()
 
 	// Read and validate the source content
 	sourceData, err := io.ReadAll(fetchResult.Content)
+	fetchBytes := int64(len(sourceData))
+
 	if err != nil {
-		return nil, fmt.Errorf("failed to read upstream response: %w", err)
+		return nil, fetchBytes, fmt.Errorf("failed to read upstream response: %w", err)
 	}
 
 	// Calculate download bitrate
-	fetchBytes := int64(len(sourceData))
-
 	var downloadRate string
 
 	if fetchResult.FetchDurationMs > 0 {
@@ -368,7 +381,7 @@ func (s *Service) fetchAndProcess(
 	// Validate magic bytes match content type
 	err = magic.ValidateMagicBytes(sourceData, fetchResult.ContentType)
 	if err != nil {
-		return nil, fmt.Errorf("content validation failed: %w", err)
+		return nil, fetchBytes, fmt.Errorf("content validation failed: %w", err)
 	}
 
 	// Store source content
@@ -378,15 +391,20 @@ func (s *Service) fetchAndProcess(
 		// Continue even if caching fails
 	}
 
-	return s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
+	resp, err := s.processAndStore(
+		ctx, req, cacheKey, bytes.NewReader(sourceData), fetchBytes,
+	)
+
+	return resp, fetchBytes, err
 }
 
-// processAndStore processes an image and stores the result.
+// processAndStore processes the image read from source and stores the
+// result.
 func (s *Service) processAndStore(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-	sourceData []byte,
+	source io.Reader,
 	fetchBytes int64,
 ) (*ImageResponse, error) {
 	// Process the image
@@ -399,13 +417,16 @@ func (s *Service) processAndStore(
 		FitMode: imageprocessor.FitMode(req.FitMode),
 	}
 
-	processResult, err := s.processor.Process(ctx, bytes.NewReader(sourceData), processReq)
+	processResult, err := s.processor.Process(ctx, source, processReq)
 	if err != nil {
 		return nil, fmt.Errorf("image processing failed: %w", err)
 	}
 
 	processDuration := time.Since(processStart)
 
+	// Counted also when the request context has ended meanwhile
+	s.cache.IncrementTransformCount(context.WithoutCancel(ctx))
+
 	// Read processed content
 	processedData, err := io.ReadAll(processResult.Content)
 	_ = processResult.Content.Close()

internal/imgcache/stats_internal_test.go

@@ -4,6 +4,9 @@ import (
 	"bytes"
 	"context"
 	"database/sql"
+	"image/color"
+	"io"
+	"io/fs"
 	"log/slog"
 	"math"
 	"strings"
@@ -11,6 +14,7 @@ import (
 	"time"
 
 	"sneak.berlin/go/pixa/internal/database"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
 )
 
 func setupStatsTestDB(t *testing.T) *sql.DB {
@@ -125,7 +129,7 @@ func TestStats_LogsFailedCountQueries(t *testing.T) {
 	}
 
 	_, err = db.ExecContext(t.Context(),
-		`DROP TABLE request_cache; DROP TABLE output_content`)
+		`DROP TABLE source_content; DROP TABLE variant_content`)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -183,3 +187,313 @@ func TestIncrementStats_LogsFailedUpdates(t *testing.T) {
 		}
 	}
 }
+
+// TestStats_TotalsCountSourcesAndVariants verifies that TotalItems and
+// TotalSizeBytes cover the stored source images and processed variants.
+func TestStats_TotalsCountSourcesAndVariants(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	storeEvictionTestSource(t, cache, testHostCDN, testPathCat,
+		bytes.Repeat([]byte{0xAA}, 1000))
+	storeEvictionTestVariant(t, cache, testVariantKeyOne,
+		bytes.Repeat([]byte{0xAB}, 500))
+	storeEvictionTestVariant(t, cache, testVariantKeyTwo,
+		bytes.Repeat([]byte{0xAC}, 250))
+
+	stats, err := cache.Stats(t.Context())
+	if err != nil {
+		t.Fatalf("Stats() error = %v", err)
+	}
+
+	if stats.TotalItems != 3 {
+		t.Errorf("TotalItems = %d, want 3 (1 source, 2 variants)", stats.TotalItems)
+	}
+
+	if stats.TotalSizeBytes != 1750 {
+		t.Errorf("TotalSizeBytes = %d, want 1750 (1000+500+250)",
+			stats.TotalSizeBytes)
+	}
+}
+
+// TestStats_DisabledCacheReportsNoItems verifies that a disabled disk cache
+// reports no items and no size, even when its database still holds the
+// rows of an earlier run with the disk cache enabled.
+func TestStats_DisabledCacheReportsNoItems(t *testing.T) {
+	t.Parallel()
+
+	enabled, _ := newEvictionTestCache(t, 1<<30)
+
+	storeEvictionTestSource(t, enabled, testHostCDN, testPathCat,
+		bytes.Repeat([]byte{0xAA}, 1000))
+	storeEvictionTestVariant(t, enabled, testVariantKeyOne,
+		bytes.Repeat([]byte{0xAB}, 500))
+
+	disabled, err := NewCache(enabled.db, CacheConfig{
+		StateDir:         t.TempDir(),
+		CacheTTL:         time.Hour,
+		NegativeTTL:      5 * time.Minute,
+		DisableDiskCache: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stats, err := disabled.Stats(t.Context())
+	if err != nil {
+		t.Fatalf("Stats() error = %v", err)
+	}
+
+	if stats.TotalItems != 0 || stats.TotalSizeBytes != 0 {
+		t.Errorf("TotalItems = %d, TotalSizeBytes = %d, want 0 and 0",
+			stats.TotalItems, stats.TotalSizeBytes)
+	}
+}
+
+// cacheStatsCounters holds the counters of the cache_stats row, in column
+// order.
+type cacheStatsCounters struct {
+	hitCount           int64
+	missCount          int64
+	upstreamFetchCount int64
+	upstreamFetchBytes int64
+	transformCount     int64
+}
+
+// readCacheStatsCounters reads the counters of the cache_stats row.
+func readCacheStatsCounters(t *testing.T, cache *Cache) cacheStatsCounters {
+	t.Helper()
+
+	var got cacheStatsCounters
+
+	err := cache.db.QueryRowContext(t.Context(), `
+		SELECT hit_count, miss_count, upstream_fetch_count,
+		       upstream_fetch_bytes, transform_count
+		FROM cache_stats WHERE id = 1
+	`).Scan(&got.hitCount, &got.missCount, &got.upstreamFetchCount,
+		&got.upstreamFetchBytes, &got.transformCount)
+	if err != nil {
+		t.Fatalf("failed to read cache_stats: %v", err)
+	}
+
+	return got
+}
+
+// TestService_Get_CountsStats walks Get through a miss that fetches the
+// source, a hit, a miss that reuses the cached source, and two misses whose
+// source cannot be used, checking every cache_stats counter after each.
+func TestService_Get_CountsStats(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+
+	// NewTestFS builds the same files the test service's fetcher serves.
+	testFS, _ := NewTestFS(t)
+
+	photo, err := fs.ReadFile(testFS, fixtures.GoodHostJPEG)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fake, err := fs.ReadFile(testFS, fixtures.InvalidFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	photoBytes, fakeBytes := int64(len(photo)), int64(len(fake))
+
+	// want is hits, misses, upstream fetches, upstream bytes, transforms.
+	steps := []struct {
+		name    string
+		path    string
+		size    int
+		wantErr bool
+		want    cacheStatsCounters
+	}{
+		{"miss that fetches the source", testPathPhoto, 50, false,
+			cacheStatsCounters{0, 1, 1, photoBytes, 1}},
+		{"hit", testPathPhoto, 50, false,
+			cacheStatsCounters{1, 1, 1, photoBytes, 1}},
+		{"miss that reuses the cached source", testPathPhoto, 25, false,
+			cacheStatsCounters{1, 2, 1, photoBytes, 2}},
+		{"miss whose source fails the magic byte check", "/images/fake.jpg", 50, true,
+			cacheStatsCounters{1, 3, 2, photoBytes + fakeBytes, 2}},
+		{"miss whose source is not found", "/images/nonexistent.jpg", 50, true,
+			cacheStatsCounters{1, 4, 2, photoBytes + fakeBytes, 2}},
+	}
+
+	for _, step := range steps {
+		resp, err := svc.Get(t.Context(), &ImageRequest{
+			SourceHost: fixtures.GoodHost,
+			SourcePath: step.path,
+			Size:       Size{Width: step.size, Height: step.size},
+			Format:     FormatJPEG,
+			Quality:    85,
+			FitMode:    FitCover,
+		})
+		if (err != nil) != step.wantErr {
+			t.Fatalf("%s: Get() error = %v, want error %t", step.name, err, step.wantErr)
+		}
+
+		if err == nil {
+			_ = resp.Content.Close()
+		}
+
+		got := readCacheStatsCounters(t, svc.cache)
+		if got != step.want {
+			t.Fatalf("after the %s: counters = %+v, want %+v", step.name, got, step.want)
+		}
+	}
+}
+
+// fakeUpstream answers every fetch with itself as a JPEG body. The body
+// serves data, then calls cancel, when set, and returns err; io.EOF ends
+// the body normally.
+type fakeUpstream struct {
+	data   *bytes.Reader
+	cancel context.CancelFunc
+	err    error
+}
+
+func (u *fakeUpstream) Fetch(
+	context.Context, string,
+) (*httpfetcher.FetchResult, error) {
+	return &httpfetcher.FetchResult{
+		Content:       io.NopCloser(u),
+		ContentLength: -1,
+		ContentType:   testContentTypeJPEG,
+	}, nil
+}
+
+func (u *fakeUpstream) Read(p []byte) (int, error) {
+	if u.data.Len() > 0 {
+		return u.data.Read(p)
+	}
+
+	if u.cancel != nil {
+		u.cancel()
+	}
+
+	return 0, u.err
+}
+
+// TestService_Get_CountsInterruptedMisses checks every cache_stats counter
+// after a miss whose request context ends during or after the upstream
+// fetch, and after a miss whose upstream body is over the size limit.
+func TestService_Get_CountsInterruptedMisses(t *testing.T) {
+	t.Parallel()
+
+	photo := generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255})
+	half := len(photo) / 2
+
+	// want is hits, misses, upstream fetches, upstream bytes, transforms.
+	tests := []struct {
+		name    string
+		served  int   // bytes of the photo the upstream body serves
+		cancel  bool  // whether the body then ends the request context
+		readErr error // what the body then returns
+		wantErr bool
+		want    cacheStatsCounters
+	}{
+		{"request context ends during the fetch", half, true, context.Canceled, true,
+			cacheStatsCounters{0, 1, 1, int64(half), 0}},
+		{"request context ends after the fetch", len(photo), true, io.EOF, false,
+			cacheStatsCounters{0, 1, 1, int64(len(photo)), 1}},
+		{"upstream body over the size limit", half, false,
+			httpfetcher.ErrResponseTooLarge, true,
+			cacheStatsCounters{0, 1, 1, int64(half), 0}},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			svc, fixtures := SetupTestService(t)
+
+			ctx, cancel := context.WithCancel(t.Context())
+			defer cancel()
+
+			upstream := &fakeUpstream{
+				data: bytes.NewReader(photo[:tc.served]),
+				err:  tc.readErr,
+			}
+			if tc.cancel {
+				upstream.cancel = cancel
+			}
+
+			svc.fetcher = upstream
+
+			resp, err := svc.Get(ctx, &ImageRequest{
+				SourceHost: fixtures.GoodHost,
+				SourcePath: testPathPhoto,
+				Size:       Size{Width: 50, Height: 50},
+				Format:     FormatJPEG,
+				Quality:    85,
+				FitMode:    FitCover,
+			})
+			t.Logf("Get() error = %v", err)
+
+			if (err != nil) != tc.wantErr {
+				t.Fatalf("Get() error = %v, want error %t", err, tc.wantErr)
+			}
+
+			if err == nil {
+				_ = resp.Content.Close()
+			}
+
+			got := readCacheStatsCounters(t, svc.cache)
+			if got != tc.want {
+				t.Errorf("counters = %+v, want %+v", got, tc.want)
+			}
+		})
+	}
+}
+
+// TestService_Get_CountsHitAfterRequestEnds checks every cache_stats counter
+// after a hit served with a request context that has already ended: only
+// the hit count moves.
+func TestService_Get_CountsHitAfterRequestEnds(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+
+	req := &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: testPathPhoto,
+		Size:       Size{Width: 50, Height: 50},
+		Format:     FormatJPEG,
+		Quality:    85,
+		FitMode:    FitCover,
+	}
+
+	// A first request caches the variant.
+	resp, err := svc.Get(t.Context(), req)
+	if err != nil {
+		t.Fatalf("first Get() error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	want := readCacheStatsCounters(t, svc.cache)
+	want.hitCount++
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	resp, err = svc.Get(ctx, req)
+	if err != nil {
+		t.Fatalf("Get() with an ended request context: error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	if resp.CacheStatus != CacheHit {
+		t.Fatalf("CacheStatus = %v, want %v", resp.CacheStatus, CacheHit)
+	}
+
+	got := readCacheStatsCounters(t, svc.cache)
+	if got != want {
+		t.Errorf("counters = %+v, want %+v", got, want)
+	}
+}

internal/imgcache/storage.go

@@ -506,32 +506,44 @@ func (s *VariantStorage) Load(key VariantKey) (io.ReadCloser, error) {
 	return f, nil
 }
 
-// LoadWithMeta returns a reader, size, and content type for the content at
+// LoadWithSize returns a reader and file size for the content at the
-// the given key.
+// given key.
-func (s *VariantStorage) LoadWithMeta(
+func (s *VariantStorage) LoadWithSize(key VariantKey) (io.ReadCloser, int64, error) {
-	key VariantKey,
-) (io.ReadCloser, int64, string, error) {
 	path := s.keyToPath(key)
-	metaPath := path + ".meta"
 
 	f, err := os.Open(path) //nolint:gosec // path derived from cache key
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, 0, "", ErrNotFound
+			return nil, 0, ErrNotFound
 		}
 
-		return nil, 0, "", fmt.Errorf("failed to open content: %w", err)
+		return nil, 0, fmt.Errorf("failed to open content: %w", err)
 	}
 
 	stat, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
 
-		return nil, 0, "", fmt.Errorf("failed to stat content: %w", err)
+		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
 	}
 
-	// Load metadata for content type
+	return f, stat.Size(), nil
-	contentType := "application/octet-stream" // fallback
+}
+
+// LoadWithMeta returns a reader, size, and content type for the content at
+// the given key. The content type is read from the .meta file, and is
+// empty when that file is missing or unreadable.
+func (s *VariantStorage) LoadWithMeta(
+	key VariantKey,
+) (io.ReadCloser, int64, string, error) {
+	f, size, err := s.LoadWithSize(key)
+	if err != nil {
+		return nil, 0, "", err
+	}
+
+	var contentType string
+
+	metaPath := s.keyToPath(key) + ".meta"
 
 	metaData, err := os.ReadFile(metaPath) //nolint:gosec // path derived from cache key
 	if err == nil {
@@ -541,7 +553,7 @@ func (s *VariantStorage) LoadWithMeta(
 		}
 	}
 
-	return f, stat.Size(), contentType, nil
+	return f, size, contentType, nil
 }
 
 // Exists checks if content exists at the given key.

internal/imgcache/testutil_internal_test.go

@@ -24,6 +24,7 @@ const (
 	testHostExample       = "example.com"
 	testPathCat           = "/photos/cat.jpg"
 	testContentTypeJPEG   = "image/jpeg"
+	testContentTypeWebP   = "image/webp"
 	testHeaderContentType = "Content-Type"
 )
 

internal/server/login_rate_limit_internal_test.go

@@ -18,6 +18,7 @@ import (
 	"sneak.berlin/go/pixa/internal/database"
 	"sneak.berlin/go/pixa/internal/globals"
 	"sneak.berlin/go/pixa/internal/handlers"
+	"sneak.berlin/go/pixa/internal/healthcheck"
 	"sneak.berlin/go/pixa/internal/logger"
 	"sneak.berlin/go/pixa/internal/middleware"
 )
@@ -71,8 +72,15 @@ func newTestServer(t *testing.T) *Server {
 		t.Fatalf("database.New() error = %v", err)
 	}
 
+	hc, err := healthcheck.New(lc, healthcheck.Params{
+		Globals: &globals.Globals{}, Config: cfg, Logger: log, Database: db,
+	})
+	if err != nil {
+		t.Fatalf("healthcheck.New() error = %v", err)
+	}
+
 	h, err := handlers.New(lc, handlers.Params{
-		Logger: log, Database: db, Config: cfg,
+		Logger: log, Healthcheck: hc, Database: db, Config: cfg,
 	})
 	if err != nil {
 		t.Fatalf("handlers.New() error = %v", err)

internal/server/maintenance_internal_test.go

@@ -0,0 +1,169 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/healthcheck"
+)
+
+// unsignedImagePath is an image URL that carries no signature.
+const unsignedImagePath = "/v1/image/cdn.example.com/cat.jpg/100x100.jpeg"
+
+// TestMaintenanceModeRefusesImageRequests verifies that while maintenance
+// mode is on, both image routes answer 503 Service Unavailable with a
+// Retry-After header and the JSON error body the image handlers send.
+func TestMaintenanceModeRefusesImageRequests(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+	s.config.MaintenanceMode = true
+
+	requests := []struct {
+		method string
+		path   string
+	}{
+		{http.MethodGet, unsignedImagePath},
+		{http.MethodHead, unsignedImagePath},
+		{http.MethodGet, "/v1/e/token/cat.jpg"},
+	}
+
+	for _, tc := range requests {
+		t.Run(tc.method+" "+tc.path, func(t *testing.T) {
+			t.Parallel()
+
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), tc.method, tc.path, nil))
+			t.Logf("status %d, body %s", rec.Code, rec.Body.String())
+
+			if rec.Code != http.StatusServiceUnavailable {
+				t.Fatalf("status = %d, want %d",
+					rec.Code, http.StatusServiceUnavailable)
+			}
+
+			retryAfter := rec.Header().Get("Retry-After")
+
+			seconds, err := strconv.Atoi(retryAfter)
+			if err != nil || seconds <= 0 {
+				t.Errorf("Retry-After = %q, want a positive number of seconds",
+					retryAfter)
+			}
+
+			// A HEAD response carries no body.
+			if tc.method == http.MethodHead {
+				return
+			}
+
+			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("body is not JSON: %v", err)
+			}
+
+			if body.Error == "" || body.Status != http.StatusServiceUnavailable ||
+				body.Timestamp == "" {
+				t.Errorf("body = %+v, want an error, status %d and a timestamp",
+					body, http.StatusServiceUnavailable)
+			}
+		})
+	}
+}
+
+// TestImageRequestsServedWithoutMaintenanceMode verifies that while
+// maintenance mode is off, image requests reach the image handlers instead
+// of the 503. The handlers refuse an unsigned image URL with 401 and a token
+// they cannot decrypt with 400, so either status shows a request got through.
+func TestImageRequestsServedWithoutMaintenanceMode(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+	s.config.MaintenanceMode = false
+
+	requests := []struct {
+		method string
+		path   string
+		want   int
+	}{
+		{http.MethodGet, unsignedImagePath, http.StatusUnauthorized},
+		{http.MethodHead, unsignedImagePath, http.StatusUnauthorized},
+		{http.MethodGet, "/v1/e/token/cat.jpg", http.StatusBadRequest},
+	}
+
+	for _, tc := range requests {
+		t.Run(tc.method+" "+tc.path, func(t *testing.T) {
+			t.Parallel()
+
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), tc.method, tc.path, nil))
+			t.Logf("status %d, body %s", rec.Code, rec.Body.String())
+
+			if rec.Code != tc.want {
+				t.Errorf("status = %d, want %d from the image handler",
+					rec.Code, tc.want)
+			}
+		})
+	}
+}
+
+// TestMaintenanceModeKeepsOtherRoutes verifies that while maintenance mode
+// is on, the health check still answers 200 and reports it, and the login
+// page and /metrics still answer 200. The image's Docker HEALTHCHECK
+// requests the health check: a 503 there would make the container
+// unhealthy, and upaas marks a deploy failed when its container is
+// unhealthy.
+func TestMaintenanceModeKeepsOtherRoutes(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+	s.config.MaintenanceMode = true
+
+	// /metrics is routed only when its username is set.
+	s.config.MetricsUsername = "metrics"
+	s.config.MetricsPassword = "metrics-password"
+	s.SetupRoutes()
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/.well-known/healthcheck.json", nil))
+	t.Logf("health check status %d, body %s", rec.Code, rec.Body.String())
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("health check status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	var health healthcheck.Response
+
+	err := json.NewDecoder(rec.Body).Decode(&health)
+	if err != nil || !health.Maintenance {
+		t.Errorf("health check maintenance_mode = %v (error %v), want true",
+			health.Maintenance, err)
+	}
+
+	rec = httptest.NewRecorder()
+	s.ServeHTTP(rec, clientRequest(t, http.MethodGet, nil, firstClient, ""))
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("login page status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	req := httptest.NewRequestWithContext(t.Context(),
+		http.MethodGet, "/metrics", nil)
+	req.SetBasicAuth(s.config.MetricsUsername, s.config.MetricsPassword)
+
+	rec = httptest.NewRecorder()
+	s.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("/metrics status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}

internal/server/routes.go

@@ -1,7 +1,9 @@
 package server
 
 import (
+	"encoding/json"
 	"net/http"
+	"strconv"
 	"time"
 
 	sentryhttp "github.com/getsentry/sentry-go/http"
@@ -17,6 +19,10 @@ import (
 // make per minute; the next is refused with 429 Too Many Requests.
 const LoginAttemptsPerMinute = 5
 
+// MaintenanceRetryAfterSeconds is the Retry-After, in seconds, sent with
+// the 503 that the image routes answer while maintenance mode is on.
+const MaintenanceRetryAfterSeconds = 300
+
 // SetupRoutes configures all HTTP routes.
 func (s *Server) SetupRoutes() {
 	s.router = chi.NewRouter()
@@ -68,15 +74,23 @@ func (s *Server) SetupRoutes() {
 
 	s.router.Get("/logout", s.h.HandleLogout())
 
+	// Image routes, refused while maintenance mode is on. Only these: the
+	// image's Docker HEALTHCHECK requests the health check, a 503 there
+	// would make the container unhealthy, and upaas marks a deploy failed
+	// when its container is unhealthy.
+	s.router.Group(func(r chi.Router) {
+		r.Use(s.refuseDuringMaintenance)
+
 		// Main image proxy route
 		// /v1/image/<host>/<path>/<width>x<height>.<format>
-	s.router.Get("/v1/image/*", s.h.HandleImage())
+		r.Get("/v1/image/*", s.h.HandleImage())
-	s.router.Head("/v1/image/*", s.h.HandleImage())
+		r.Head("/v1/image/*", s.h.HandleImage())
 
 		// Encrypted image URL route
 		// The trailing filename (e.g., /img.jpg) is ignored but helps
 		// browsers with content type
-	s.router.Get("/v1/e/{token}/*", s.h.HandleImageEnc())
+		r.Get("/v1/e/{token}/*", s.h.HandleImageEnc())
+	})
 
 	// Metrics endpoint with auth
 	if s.config.MetricsUsername != "" {
@@ -86,3 +100,30 @@ func (s *Server) SetupRoutes() {
 		})
 	}
 }
+
+// refuseDuringMaintenance answers a request with 503 Service Unavailable,
+// a Retry-After header and a JSON error body while maintenance mode is on,
+// and passes it on otherwise. The body has the fields of the JSON errors
+// the image handlers send.
+func (s *Server) refuseDuringMaintenance(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !s.MaintenanceMode() {
+			next.ServeHTTP(w, r)
+
+			return
+		}
+
+		w.Header().Set("Retry-After", strconv.Itoa(MaintenanceRetryAfterSeconds))
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusServiceUnavailable)
+
+		err := json.NewEncoder(w).Encode(map[string]any{
+			"error":     "down for maintenance, try again later",
+			"status":    http.StatusServiceUnavailable,
+			"timestamp": time.Now().UTC().Format(time.RFC3339),
+		})
+		if err != nil {
+			s.log.Error("json encode error", "error", err)
+		}
+	})
+}

script/bootstrap

@@ -5,8 +5,10 @@
 # or apk (detected in that order); assumes NOTHING is present (not git,
 # make, or go). The linter is never installed on the host: golangci-lint
 # runs only inside a container, Dockerfile.lint or the Dockerfile lint
-# stage (see script/lint). CGO image libraries (pkg-config, vips,
+# stage (see script/lint). A C compiler and the CGO image libraries
-# libheif) are installed for the govips bindings.
+# (pkg-config, vips, libheif) are installed for the govips bindings.
+# Both Dockerfiles run this script too, so their build dependencies are
+# the ones listed here.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
@@ -53,6 +55,11 @@ missing() {
 
 # CGO dependencies for govips (image processing)
 ensure_cgo_deps() {
+    # cgo compiles with gcc on Linux; build-base and build-essential
+    # also bring the C library headers.
+    if missing gcc; then
+        pkg_install gcc build-essential gcc build-base
+    fi
     if missing pkg-config; then
         pkg_install pkg-config pkg-config pkg-config pkgconfig
     fi