Share one fetch and transcode among concurrent misses (closes #65)

Requests that missed the same variant at once each fetched and
transcoded it. They now share one call through
golang.org/x/sync/singleflight, keyed on the variant cache key. The
first request processes the variant with a context that does not end
with its own; the others wait for its result, holding no connection or
processing slot, and return as soon as their own context ends. The
processing request waits even then, as before. Each request counts one
miss; the processing counts its fetch and transcode once. A panic
while processing becomes an error instead of stopping pixad.

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
-RUN CGO_ENABLED=1 GOTOOLCHAIN=auto go build -ldflags "-X main.Version=${VERSION}" -o /pixad ./cmd/pixad
+# VERSION is declared here, not earlier: a new value reruns only the
+# build, not script/bootstrap or the tests. CGO stays enabled for
+# govips; -trimpath keeps build paths out of the binary, and -s -w
+# leave out the symbol table and debug information.
+ARG VERSION=dev
+RUN CGO_ENABLED=1 GOTOOLCHAIN=auto go build -trimpath \
+    -ldflags "-s -w -X main.Version=${VERSION}" \
+    -o /pixad ./cmd/pixad
 
 # Runtime stage
 # alpine:3.21, 2026-02-25

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
-# result, so it may safely be reused between runs.
+# pixa is CGO/libvips: the type-aware linters compile every package, so
+# 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
-  does not create it, so the host directory must exist before the first
-  deploy.
+  database and cache. Creating the host directory when it is missing is
+  upaas's job, tracked in https://git.eeqj.de/sneak/upaas/issues/235.
 - **Environment variables:**
     - `PIXA_SIGNING_KEY` (required): secret for signed and encrypted URLs
       and login, 32+ characters, for example from
@@ -58,10 +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, owned by root or by uid
-  `65532` and gid `65532`. The server runs as the container's `pixad`
-  user, which has that uid and gid, and the container gives the
-  directory to `pixad` when it starts.
 
 ## Rationale
 
@@ -112,6 +107,13 @@ 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.
 
+When several requests for the same image, size, format, quality and fit miss
+the cache at once, they share one upstream fetch (or one read of the cached
+source) and one transcode: the first request does the work, and the others wait
+for its image or its error, holding no upstream connection or processing slot
+of their own. A waiting request stops waiting when its own client goes away;
+the work goes on for the others.
+
 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
@@ -238,15 +240,28 @@ 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`                 |
+| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read responses: `*` or one origin; default `*`        |
 | `PIXA_METRICS_USERNAME`              | `metrics.username`              | Username for `/metrics`, which is served only when both are set              |
 | `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
 | `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
 | `PIXA_DEBUG`                         | `debug`                         | Debug logging and plain-HTTP local development; default `false`              |
-| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | 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:
 
-- `access_control_allow_origin` — CORS origin
+- `access_control_allow_origin` — the origin a browser lets read pixa's
+  responses, sent as the CORS `Access-Control-Allow-Origin` header: `*`, the
+  default, is any site; otherwise one `http` or `https` origin such as
+  `https://example.com`, whose host is a lowercase host name (letters,
+  digits, hyphens and dots, with a letter in its last part) or an IP address
+  (IPv6 in brackets, in its shortest form), with an optional port 1-65535
+  that has no leading zero and is not the scheme's default. Any other value,
+  including another scheme such as a browser extension's, aborts startup
 - `allowlist_hosts` — list of allowed upstream hosts
 - `blocked_networks` — list of CIDR ranges to refuse for SSRF protection,
   added to the always-enforced built-in ranges (loopback, private,
@@ -272,13 +287,37 @@ Key settings in more detail:
   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` — timeout for origin requests
-- `upstream_max_response_size` — max origin response size
-- `downstream_timeout` — client response timeout
+- `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, 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
 - `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

@@ -25,20 +25,65 @@ The disk cache is now size-bounded with LRU eviction
 
 # Next Step
 
-P1: rate limit global concurrent upstream fetches to prevent resource
-exhaustion
+P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-09-29 share concurrent misses (closes #65): requests that miss the same
+  variant at once (the same cache key, so quality and fit included) share one
+  upstream fetch or cached source read and one transcode through
+  `golang.org/x/sync/singleflight`; the first request's processing runs with a
+  context that does not end with its own, and the others wait for its image or
+  error holding no upstream connection or processing slot, and stop waiting
+  when their own context ends; the request doing the processing waits for it
+  even then, as before; a request whose context has already ended starts
+  nothing; each request counts one miss, and the processing counts its fetch
+  and transcode once; a panic while processing becomes an error for every
+  waiting request instead of stopping pixad; documented in `README.md`.
+- 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; for a variant it holds, `Lookup`
-  skips the check of the disk and `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.
+  `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
@@ -66,6 +111,17 @@ exhaustion
   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),
+  `upstream_fetch_timeout` (default `30s`), `upstream_max_response_size`
+  (default 50 MiB) and `downstream_timeout` (default `60s`, both the
+  server's write timeout and the per-request timeout); each has a
+  `PIXA_` variable; durations are positive Go duration strings, the size a
+  whole number of bytes up to 1 GiB, the origin `*` or one `http` or
+  `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`,
@@ -306,7 +362,6 @@ exhaustion
 # Future Steps
 
 - P2: security
-    - referer blacklist
     - per-IP rate limiting on the image routes
     - per-origin rate limiting
 - P2: HTTP response handling
@@ -315,7 +370,6 @@ exhaustion
     - X-Request-ID propagation
 - P2: auto format selection (format=auto based on Accept header)
 - P2: configuration
-    - add all configuration options from README
     - YAML config file support
 - P2: operational
     - optional Sentry error reporting

config.example.yml

@@ -8,10 +8,20 @@
 # this file's env: section is set while the file loads, so it overrides
 # both the environment the process was started with and this file's own
 # key.
+#
+# Durations are Go duration strings such as 30s or 2m and must be
+# positive; a bare number has no unit and aborts startup. Sizes are a
+# whole number of bytes.
 
 # 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
@@ -67,6 +77,42 @@ 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
+
+# Largest upstream response accepted, in bytes, at most 1073741824
+# (1 GiB) (default: 52428800, 50 MiB)
+upstream_max_response_size: 52428800
+
+# Time allowed for answering one client request (default: 60s). The
+# 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
+# Access-Control-Allow-Origin header: "*" (the default) is any site;
+# otherwise one http or https origin such as https://example.com, whose
+# host is a lowercase host name (letters, digits, hyphens and dots, with a
+# letter in its last part) or an IP address (IPv6 in brackets, in its
+# shortest form), with an optional port 1-65535 that has no leading zero
+# and is not the scheme's default. Any other value, including another
+# scheme such as a browser extension's, aborts startup.
+access_control_allow_origin: "*"
+
 # Maximum disk cache size in bytes. Explicit values are used exactly as
 # given; 0 disables the disk cache entirely (every request fetches and
 # processes uncached). When omitted, the default is 75% of the free

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
-# bind-mounted there keeps its host owner, often root, and pixad could
-# not write to it. The server itself always runs as pixad.
+# root only to make /var/lib/pixa usable by pixad: a host directory
+# bind-mounted there keeps its host owner, often root, and data from an
+# earlier run may belong to another uid. The server itself always runs
+# as pixad.
 set -eu
 
 main() {
-    if [ "$(stat -c %U /var/lib/pixa)" != pixad ]; then
-        chown pixad:pixad /var/lib/pixa
+    mkdir -p /var/lib/pixa
+    # Only the directory and its top-level entries are checked, so a
+    # normal start does not walk the cache. -depth gives each directory
+    # to pixad after its contents, so a start stopped part way leaves
+    # something at the top for the next start to find; -h changes a
+    # symlink itself, never the file it points to.
+    if [ -n "$(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

@@ -20,6 +20,7 @@ require (
 	github.com/spf13/cobra v1.10.2
 	go.uber.org/fx v1.24.0
 	golang.org/x/crypto v0.41.0
+	golang.org/x/sync v0.19.0
 	modernc.org/sqlite v1.42.2
 )
 
@@ -134,7 +135,6 @@ require (
 	golang.org/x/image v0.34.0 // indirect
 	golang.org/x/net v0.43.0 // indirect
 	golang.org/x/oauth2 v0.30.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.36.0 // indirect
 	golang.org/x/term v0.34.0 // indirect
 	golang.org/x/text v0.32.0 // indirect

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

@@ -10,9 +10,11 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
+	"time"
 
 	"git.eeqj.de/sneak/smartconfig"
 	"go.uber.org/fx"
@@ -24,6 +26,11 @@ const (
 	DefaultPort                       = 8080
 	DefaultStateDir                   = "/var/lib/pixa"
 	DefaultUpstreamConnectionsPerHost = 20
+	DefaultUpstreamConnections        = 64
+	DefaultAccessControlAllowOrigin   = "*"
+	DefaultUpstreamFetchTimeout       = 30 * time.Second
+	DefaultUpstreamMaxResponseSize    = 50 << 20 // 50 MiB
+	DefaultDownstreamTimeout          = 60 * time.Second
 )
 
 // Configuration key names.
@@ -41,9 +48,15 @@ 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"
+	keyAccessControlAllowOrigin   = "access_control_allow_origin"
+	keyUpstreamFetchTimeout       = "upstream_fetch_timeout"
+	keyUpstreamMaxResponseSize    = "upstream_max_response_size"
+	keyDownstreamTimeout          = "downstream_timeout"
 )
 
 // placeholderSigningKey is the dummy signing_key shipped in
@@ -69,7 +82,8 @@ var (
 	errEmptyEntry        = errors.New("contains an empty entry")
 	errNotAValidURL      = errors.New("not a valid URL")
 	errPortOutOfRange    = errors.New("outside the valid port range")
-	errTooFewConnections = errors.New("must be at least 1")
+	errSizeOutOfRange    = errors.New("outside the accepted range")
+	errMustBeAtLeastOne  = errors.New("must be at least 1")
 	errValueTooShort     = errors.New("value too short")
 	errPlaceholderKey    = errors.New(
 		"is the placeholder from config.example.yml; " +
@@ -86,6 +100,10 @@ var (
 	errNotBareHostname = errors.New(
 		"must be a bare hostname without scheme, path, or whitespace")
 	errNoHostnameLabels = errors.New("contains no hostname labels")
+	errNotADuration     = errors.New("not a duration such as 30s or 2m")
+	errMustBePositive   = errors.New("must be positive")
+	errNotAnOrigin      = errors.New(
+		`not "*" or an origin such as https://example.com`)
 )
 
 // Params defines dependencies for Config.
@@ -112,6 +130,29 @@ 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
+	// limit.
+	UpstreamFetchTimeout    time.Duration
+	UpstreamMaxResponseSize int64
+
+	// AccessControlAllowOrigin is the origin the CORS middleware allows
+	// to read responses: "*" for any, or one origin such as
+	// https://example.com.
+	AccessControlAllowOrigin string
+
+	// DownstreamTimeout bounds how long answering one client request may
+	// take. It is both the HTTP server's write timeout and the deadline
+	// of the per-request timeout middleware.
+	DownstreamTimeout time.Duration
+
 	// BlockedNetworks are operator-supplied CIDR ranges to refuse in
 	// addition to the built-in SSRF blocklist. Enforced by the upstream
 	// fetcher's dialer; the built-in ranges always apply.
@@ -195,14 +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 := validateKnownKeys(sc)
+	if err != nil {
+		return nil, err
 	}
 
-	err := validateAllowlistHostsValue(sc)
+	err = validateAllowlistHostsValue(sc)
 	if err != nil {
 		return nil, err
 	}
@@ -240,6 +279,20 @@ 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(
+			keyUpstreamMaxResponseSize, DefaultUpstreamMaxResponseSize),
+		AccessControlAllowOrigin: loader.stringVal(
+			keyAccessControlAllowOrigin, DefaultAccessControlAllowOrigin),
+		DownstreamTimeout: loader.durationVal(
+			keyDownstreamTimeout, DefaultDownstreamTimeout),
 		CacheMaxBytes:   loader.int64Val(keyCacheMaxBytes, 0),
 		BlockedNetworks: blockedNetworks,
 		TrustedProxies:  trustedProxies,
@@ -282,8 +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() {
@@ -353,8 +411,10 @@ func isKnownConfigKey(key string) bool {
 	switch key {
 	case keyDebug, keyMaintenanceMode, keyPort, keyStateDir, keySentryDSN,
 		keyDBURL, keyMetrics, keySigningKey, keyAllowlistHosts, keyAllowHTTP,
-		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, keyBlockedNetworks,
-		keyTrustedProxies, "env":
+		keyUpstreamConnectionsPerHost, keyUpstreamConnections,
+		keyMaxConcurrentProcessing, keyCacheMaxBytes, keyBlockedNetworks,
+		keyTrustedProxies, keyAccessControlAllowOrigin, keyUpstreamFetchTimeout,
+		keyUpstreamMaxResponseSize, keyDownstreamTimeout, "env":
 		return true
 	}
 
@@ -379,9 +439,15 @@ 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",
+		keyAccessControlAllowOrigin:   "PIXA_ACCESS_CONTROL_ALLOW_ORIGIN",
+		keyUpstreamFetchTimeout:       "PIXA_UPSTREAM_FETCH_TIMEOUT",
+		keyUpstreamMaxResponseSize:    "PIXA_UPSTREAM_MAX_RESPONSE_SIZE",
+		keyDownstreamTimeout:          "PIXA_DOWNSTREAM_TIMEOUT",
 	}
 }
 
@@ -518,10 +584,9 @@ func (c *Config) validate() error {
 			settingName(keyPort), c.Port, errPortOutOfRange, maxPort)
 	}
 
-	if c.UpstreamConnectionsPerHost < 1 {
-		return fmt.Errorf("%s: value %d %w",
-			settingName(keyUpstreamConnectionsPerHost),
-			c.UpstreamConnectionsPerHost, errTooFewConnections)
+	err = c.validateConcurrencyLimits()
+	if err != nil {
+		return err
 	}
 
 	if c.StateDir == "" {
@@ -535,6 +600,11 @@ func (c *Config) validate() error {
 			settingName(keyCacheMaxBytes), c.CacheMaxBytes, errMustNotBeNegative)
 	}
 
+	err = c.validateUpstreamMaxResponseSize()
+	if err != nil {
+		return err
+	}
+
 	for _, host := range c.AllowlistHosts {
 		err := validateAllowlistHost(host)
 		if err != nil {
@@ -556,6 +626,106 @@ func (c *Config) validate() error {
 			errMustBeSetTogether)
 	}
 
+	return c.validateAccessControlAllowOrigin()
+}
+
+// validateUpstreamMaxResponseSize checks that upstream_max_response_size
+// is from 1 byte to 1 GiB. An upstream response is read whole into
+// memory, and the image processor reads one byte past this limit, which
+// must not overflow.
+func (c *Config) validateUpstreamMaxResponseSize() error {
+	const maxUpstreamMaxResponseSize = 1 << 30 // 1 GiB
+	if c.UpstreamMaxResponseSize < 1 ||
+		c.UpstreamMaxResponseSize > maxUpstreamMaxResponseSize {
+		return fmt.Errorf("%s: value %d is %w 1-%d",
+			settingName(keyUpstreamMaxResponseSize), c.UpstreamMaxResponseSize,
+			errSizeOutOfRange, maxUpstreamMaxResponseSize)
+	}
+
+	return nil
+}
+
+// validateAccessControlAllowOrigin accepts "*" or an origin exactly as a browser
+// sends it: http or https, an IP address as netip writes it or a lowercase name
+// with a letter in its last part, and an optional port 1-65535, not the default.
+func (c *Config) validateAccessControlAllowOrigin() error {
+	origin := c.AccessControlAllowOrigin
+	if origin == "*" {
+		return nil
+	}
+
+	errOrigin := fmt.Errorf("%s: value %q is %w",
+		settingName(keyAccessControlAllowOrigin), origin, errNotAnOrigin)
+
+	parsed, err := url.Parse(origin)
+	if err != nil {
+		return errOrigin
+	}
+
+	defaultPort := map[string]string{"http": "80", "https": "443"}[parsed.Scheme]
+	if defaultPort == "" {
+		return errOrigin
+	}
+
+	const letters = "abcdefghijklmnopqrstuvwxyz"
+
+	host := parsed.Hostname()
+	lastPart := host[strings.LastIndex(host, ".")+1:]
+
+	addr, err := netip.ParseAddr(host)
+
+	switch {
+	case err == nil && addr.Is6():
+		host = "[" + addr.String() + "]"
+	case err == nil:
+		host = addr.String()
+	case strings.Trim(host, letters+"0123456789-.") != "": // a character other than these
+		return errOrigin
+	case !strings.ContainsAny(lastPart, letters):
+		return errOrigin
+	}
+
+	// The value must be exactly the origin rebuilt from its parts.
+	rebuilt := parsed.Scheme + "://" + host
+
+	port := parsed.Port()
+	if port != "" {
+		_, err := strconv.ParseUint(port, 10, 16)
+		if err != nil || port[0] == '0' || port == defaultPort {
+			return errOrigin
+		}
+
+		rebuilt += ":" + port
+	}
+
+	if rebuilt != origin {
+		return errOrigin
+	}
+
+	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
 }
 
@@ -673,6 +843,19 @@ func (l *strictLoader) int64Val(key string, defaultVal int64) int64 {
 	return val
 }
 
+func (l *strictLoader) durationVal(key string, defaultVal time.Duration) time.Duration {
+	if l.err != nil {
+		return 0
+	}
+
+	val, err := getDuration(l.sc, key, defaultVal)
+	if err != nil {
+		l.err = err
+	}
+
+	return val
+}
+
 func (l *strictLoader) boolVal(key string, defaultVal bool) bool {
 	if l.err != nil {
 		return false
@@ -794,6 +977,42 @@ func getInt64(sc *smartconfig.Config, key string, defaultVal int64) (int64, erro
 	}
 }
 
+// getDuration returns the duration value for key, or defaultVal if the
+// key is omitted. A present value must be a positive Go duration string
+// such as "30s" or "2m", read with time.ParseDuration; a bare number has
+// no unit and is an error, as is an explicit null.
+func getDuration(
+	sc *smartconfig.Config, key string, defaultVal time.Duration,
+) (time.Duration, error) {
+	raw, ok := lookupValue(sc, key)
+	if !ok {
+		return defaultVal, nil
+	}
+
+	if raw == nil {
+		return 0, errNullConfigValue(key)
+	}
+
+	str, ok := raw.(string)
+	if !ok {
+		return 0, fmt.Errorf("config key %q: value %v (%T) is %w",
+			key, raw, raw, errNotADuration)
+	}
+
+	parsed, err := time.ParseDuration(strings.TrimSpace(str))
+	if err != nil {
+		return 0, fmt.Errorf("%s: value %q is %w",
+			settingName(key), str, errNotADuration)
+	}
+
+	if parsed <= 0 {
+		return 0, fmt.Errorf("%s: value %q %w",
+			settingName(key), str, errMustBePositive)
+	}
+
+	return parsed, nil
+}
+
 // getBool returns the boolean value for key, or defaultVal if the key
 // is omitted. A present value that is not a boolean (or a ParseBool-able
 // string), or is explicitly null, is an error; numbers are not accepted

internal/config/config_validation_internal_test.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"git.eeqj.de/sneak/smartconfig"
 )
@@ -599,3 +600,244 @@ func TestEnsureStateDirFailsOnUncreatablePath(t *testing.T) {
 		t.Errorf("error %q does not name the offending key state_dir", err.Error())
 	}
 }
+
+// TestOmittedOriginTimeoutsAndSizeUseDefaults checks that the CORS
+// origin, the upstream fetch timeout, the upstream response size limit
+// and the downstream timeout default to the values pixa used before they
+// could be configured.
+func TestOmittedOriginTimeoutsAndSizeUseDefaults(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.AccessControlAllowOrigin != "*" {
+		t.Errorf("AccessControlAllowOrigin = %q, want *", c.AccessControlAllowOrigin)
+	}
+
+	if c.UpstreamFetchTimeout != 30*time.Second {
+		t.Errorf("UpstreamFetchTimeout = %v, want 30s", c.UpstreamFetchTimeout)
+	}
+
+	if c.UpstreamMaxResponseSize != 50<<20 {
+		t.Errorf("UpstreamMaxResponseSize = %d, want %d (50 MiB)",
+			c.UpstreamMaxResponseSize, 50<<20)
+	}
+
+	if c.DownstreamTimeout != 60*time.Second {
+		t.Errorf("DownstreamTimeout = %v, want 60s", c.DownstreamTimeout)
+	}
+}
+
+// TestExplicitOriginTimeoutsAndSizeAreUsed checks that valid values for
+// the CORS origin, the two timeouts and the response size limit are used
+// as given. The size is the largest accepted, 1 GiB.
+func TestExplicitOriginTimeoutsAndSizeAreUsed(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+`
+access_control_allow_origin: https://app.example.com
+upstream_fetch_timeout: 10s
+upstream_max_response_size: 1073741824
+downstream_timeout: 2m
+`)
+	if err != nil {
+		t.Fatalf("valid config should load, got error: %v", err)
+	}
+
+	if c.AccessControlAllowOrigin != "https://app.example.com" {
+		t.Errorf("AccessControlAllowOrigin = %q, want https://app.example.com",
+			c.AccessControlAllowOrigin)
+	}
+
+	if c.UpstreamFetchTimeout != 10*time.Second {
+		t.Errorf("UpstreamFetchTimeout = %v, want 10s", c.UpstreamFetchTimeout)
+	}
+
+	if c.UpstreamMaxResponseSize != 1073741824 {
+		t.Errorf("UpstreamMaxResponseSize = %d, want 1073741824",
+			c.UpstreamMaxResponseSize)
+	}
+
+	if c.DownstreamTimeout != 2*time.Minute {
+		t.Errorf("DownstreamTimeout = %v, want 2m", c.DownstreamTimeout)
+	}
+}
+
+// TestOriginWithPortOrAnyOriginIsAccepted checks the other accepted forms
+// of access_control_allow_origin: "*", an origin with a port, and origins
+// whose host is an IPv4 or IPv6 address.
+func TestOriginWithPortOrAnyOriginIsAccepted(t *testing.T) {
+	t.Parallel()
+
+	for _, origin := range []string{
+		"*", "http://localhost:3000", "http://192.0.2.1", "http://[2001:db8::1]:8080",
+	} {
+		c, err := configFromYAML(t, signingKeyLine+
+			"access_control_allow_origin: \""+origin+"\"\n")
+		if err != nil {
+			t.Fatalf("origin %q should be accepted, got error: %v", origin, err)
+		}
+
+		if c.AccessControlAllowOrigin != origin {
+			t.Errorf("AccessControlAllowOrigin = %q, want %q",
+				c.AccessControlAllowOrigin, origin)
+		}
+	}
+}
+
+// invalidTimeoutCases are configs where upstream_fetch_timeout or
+// downstream_timeout is not a positive Go duration string; each must
+// abort startup naming the key and the value.
+func invalidTimeoutCases() []abortCase {
+	return []abortCase{
+		{
+			name:              "upstream_fetch_timeout not a duration",
+			yaml:              signingKeyLine + "upstream_fetch_timeout: soon\n",
+			wantErrSubstrings: []string{keyUpstreamFetchTimeout, "soon"},
+		},
+		{
+			name:              "upstream_fetch_timeout number without a unit",
+			yaml:              signingKeyLine + "upstream_fetch_timeout: 45\n",
+			wantErrSubstrings: []string{keyUpstreamFetchTimeout, "45"},
+		},
+		{
+			name:              "upstream_fetch_timeout zero",
+			yaml:              signingKeyLine + "upstream_fetch_timeout: 0s\n",
+			wantErrSubstrings: []string{keyUpstreamFetchTimeout, "0s"},
+		},
+		{
+			name:              "upstream_fetch_timeout negative",
+			yaml:              signingKeyLine + "upstream_fetch_timeout: -5s\n",
+			wantErrSubstrings: []string{keyUpstreamFetchTimeout, "-5s"},
+		},
+		{
+			name:              "upstream_fetch_timeout null",
+			yaml:              signingKeyLine + "upstream_fetch_timeout: null\n",
+			wantErrSubstrings: []string{keyUpstreamFetchTimeout, nullValueText},
+		},
+		{
+			name:              "downstream_timeout not a duration",
+			yaml:              signingKeyLine + "downstream_timeout: 1 minute\n",
+			wantErrSubstrings: []string{keyDownstreamTimeout, "1 minute"},
+		},
+		{
+			name:              "downstream_timeout zero",
+			yaml:              signingKeyLine + "downstream_timeout: 0s\n",
+			wantErrSubstrings: []string{keyDownstreamTimeout, "0s"},
+		},
+		{
+			name:              "downstream_timeout negative",
+			yaml:              signingKeyLine + "downstream_timeout: -1m\n",
+			wantErrSubstrings: []string{keyDownstreamTimeout, "-1m"},
+		},
+		{
+			name:              "downstream_timeout null",
+			yaml:              signingKeyLine + "downstream_timeout:\n",
+			wantErrSubstrings: []string{keyDownstreamTimeout, nullValueText},
+		},
+	}
+}
+
+// invalidSizeAndOriginCases are configs where upstream_max_response_size
+// is not a whole number of bytes from 1 to 1 GiB, or
+// access_control_allow_origin is neither "*" nor an origin; each must
+// abort startup naming the key and the value.
+func invalidSizeAndOriginCases() []abortCase {
+	badOrigins := []string{
+		"",                           // empty
+		"example.com",                // no scheme
+		"https://example.com/images", // a path
+		"https://example.com/",       // a trailing slash
+		// The CORS middleware reads a * inside an origin as a pattern
+		// that lets other sites read responses.
+		"https://*",
+		"https://*.example.com",
+		"https://*example.com",
+		"https://a.com,b.com",       // two hosts
+		"https://example.com:",      // an empty port
+		"https://:8443",             // no host
+		"https://example.com:0",     // a port below 1
+		"https://example.com:99999", // a port above 65535
+		"https://exämple.com",       // a host name that is not ASCII
+		"https://example.com:443",   // the default port for https
+		"http://example.com:80",     // the default port for http
+		"https://example.com:08080", // a port with a leading zero
+		"https://01.2.3.4",          // an IPv4 address with a leading zero
+		"https://10.0.0",            // an IPv4 address with three parts
+		"https://192.168.1.256",     // an IPv4 address part above 255
+		"https://example.123",       // a host name whose last part is a number
+		"https://[0:0:0:0:0:0:0:1]", // an IPv6 address not in its shortest form
+		"file://example.com",        // a scheme other than http or https
+		"HTTPS://example.com",       // a scheme in upper case
+		"https://Example.com",       // a host name in upper case
+	}
+
+	cases := make([]abortCase, 0, len(badOrigins))
+	for _, origin := range badOrigins {
+		cases = append(cases, abortCase{
+			name: "access_control_allow_origin " + origin,
+			yaml: signingKeyLine +
+				"access_control_allow_origin: \"" + origin + "\"\n",
+			wantErrSubstrings: []string{keyAccessControlAllowOrigin, origin},
+		})
+	}
+
+	return append(cases, []abortCase{
+		{
+			name:              "access_control_allow_origin null",
+			yaml:              signingKeyLine + "access_control_allow_origin: null\n",
+			wantErrSubstrings: []string{keyAccessControlAllowOrigin, nullValueText},
+		},
+		{
+			name:              "upstream_max_response_size with a unit",
+			yaml:              signingKeyLine + "upstream_max_response_size: 50MB\n",
+			wantErrSubstrings: []string{keyUpstreamMaxResponseSize, "50MB"},
+		},
+		{
+			name:              "upstream_max_response_size fractional",
+			yaml:              signingKeyLine + "upstream_max_response_size: 1.5\n",
+			wantErrSubstrings: []string{keyUpstreamMaxResponseSize, "1.5"},
+		},
+		{
+			name:              "upstream_max_response_size zero",
+			yaml:              signingKeyLine + "upstream_max_response_size: 0\n",
+			wantErrSubstrings: []string{keyUpstreamMaxResponseSize, "0"},
+		},
+		{
+			name:              "upstream_max_response_size negative",
+			yaml:              signingKeyLine + "upstream_max_response_size: -1\n",
+			wantErrSubstrings: []string{keyUpstreamMaxResponseSize, "-1"},
+		},
+		{
+			name:              "upstream_max_response_size null",
+			yaml:              signingKeyLine + "upstream_max_response_size: null\n",
+			wantErrSubstrings: []string{keyUpstreamMaxResponseSize, nullValueText},
+		},
+		{
+			name:              "upstream_max_response_size above 1 GiB",
+			yaml:              signingKeyLine + "upstream_max_response_size: 1073741825\n",
+			wantErrSubstrings: []string{keyUpstreamMaxResponseSize, "1073741825"},
+		},
+		{
+			name: "upstream_max_response_size largest 64-bit integer",
+			yaml: signingKeyLine +
+				"upstream_max_response_size: 9223372036854775807\n",
+			wantErrSubstrings: []string{
+				keyUpstreamMaxResponseSize, "9223372036854775807",
+			},
+		},
+	}...)
+}
+
+// TestInvalidOriginTimeoutOrSizeAbortsStartup verifies the
+// no-silent-fallback rule for the CORS origin, the two timeouts and the
+// response size limit: a value that does not parse or is out of range
+// aborts startup naming the key and the value.
+func TestInvalidOriginTimeoutOrSizeAbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	runAbortCases(t, append(invalidTimeoutCases(), invalidSizeAndOriginCases()...))
+}

internal/config/env_internal_test.go

@@ -8,6 +8,7 @@ import (
 	"slices"
 	"strings"
 	"testing"
+	"time"
 
 	"sneak.berlin/go/pixa/internal/globals"
 	"sneak.berlin/go/pixa/internal/logger"
@@ -66,9 +67,15 @@ 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")
+	t.Setenv("PIXA_ACCESS_CONTROL_ALLOW_ORIGIN", "https://app.example.com")
+	t.Setenv("PIXA_UPSTREAM_FETCH_TIMEOUT", "10s")
+	t.Setenv("PIXA_UPSTREAM_MAX_RESPONSE_SIZE", "1048576")
+	t.Setenv("PIXA_DOWNSTREAM_TIMEOUT", "2m")
 
 	c, err := newFromSmartConfig(nil)
 	if err != nil {
@@ -88,10 +95,16 @@ 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")},
 		TrustedProxies:             []netip.Prefix{netip.MustParsePrefix("192.0.2.0/24")},
+		AccessControlAllowOrigin:   "https://app.example.com",
+		UpstreamFetchTimeout:       10 * time.Second,
+		UpstreamMaxResponseSize:    1048576,
+		DownstreamTimeout:          2 * time.Minute,
 	}
 
 	if !reflect.DeepEqual(*c, want) {
@@ -280,6 +293,30 @@ func TestInvalidDebugFromEnvironmentAbortsStartup(t *testing.T) {
 	wantStartupError(t, err, "PIXA_DEBUG", "maybe")
 }
 
+// TestInvalidOriginTimeoutOrSizeFromEnvironmentAbortsStartup checks that
+// an invalid CORS origin, timeout or response size limit in its variable
+// aborts startup naming the variable and the value.
+func TestInvalidOriginTimeoutOrSizeFromEnvironmentAbortsStartup(t *testing.T) {
+	cases := []struct {
+		variable string
+		value    string
+	}{
+		{"PIXA_ACCESS_CONTROL_ALLOW_ORIGIN", "example.com"},
+		{"PIXA_UPSTREAM_FETCH_TIMEOUT", "soon"},
+		{"PIXA_UPSTREAM_MAX_RESPONSE_SIZE", "50MB"},
+		{"PIXA_DOWNSTREAM_TIMEOUT", "0s"},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.variable, func(t *testing.T) {
+			t.Setenv(tc.variable, tc.value)
+
+			_, err := configFromYAML(t, signingKeyLine)
+			wantStartupError(t, err, tc.variable, tc.value)
+		})
+	}
+}
+
 // TestConfigFileAloneBehavesAsBefore checks that with no variables set
 // (TestMain unsets them) the config file's values are used and omitted
 // keys take their defaults.

internal/handlers/handlers.go

@@ -106,20 +106,24 @@ func (s *Handlers) initImageService() error {
 	// Create the fetcher config
 	fetcherCfg := httpfetcher.DefaultConfig()
 	fetcherCfg.AllowHTTP = s.config.AllowHTTP
+	fetcherCfg.Timeout = s.config.UpstreamFetchTimeout
+	fetcherCfg.MaxResponseSize = s.config.UpstreamMaxResponseSize
 
 	if s.config.UpstreamConnectionsPerHost > 0 {
 		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost
 	}
 
+	fetcherCfg.MaxConnections = s.config.UpstreamConnections
 	fetcherCfg.BlockedNetworks = s.config.BlockedNetworks
 
 	// Create the service
 	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
-		Cache:         cache,
-		FetcherConfig: fetcherCfg,
-		SigningKey:    s.config.SigningKey,
-		Allowlist:     s.config.AllowlistHosts,
-		Logger:        s.log,
+		Cache:                   cache,
+		FetcherConfig:           fetcherCfg,
+		SigningKey:              s.config.SigningKey,
+		Allowlist:               s.config.AllowlistHosts,
+		MaxConcurrentProcessing: s.config.MaxConcurrentProcessing,
+		Logger:                  s.log,
 	})
 	if err != nil {
 		return err

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"
 )
 
@@ -217,6 +218,14 @@ 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)
 }
 

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"
 )
 
@@ -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.
@@ -192,13 +209,18 @@ func New(config *Config) *HTTPFetcher {
 	}
 
 	return &HTTPFetcher{
-		client:   client,
-		config:   config,
-		hostSems: make(map[string]chan struct{}),
+		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
-	host := extractHost(url)
-
-	// Acquire semaphore slot for this host
-	sem := f.getHostSemaphore(host)
-	select {
-	case sem <- struct{}{}:
-		// Acquired slot
-	case <-ctx.Done():
-		return nil, ctx.Err()
+	release, err := f.acquireConnection(ctx, extractHost(url))
+	if err != nil {
+		return nil, 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{}
+	closer  io.Closer
+	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,
+		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

@@ -159,15 +159,13 @@ type LookupResult struct {
 	CacheStatus CacheStatus
 }
 
-// Lookup checks if a processed variant exists on disk: a variant held
-// in metaCache counts as present without a check of the disk. Hits
-// touch the variant's LRU timestamp; a disabled cache always misses.
+// Lookup checks if a processed variant exists on disk. Hits touch the
+// variant's LRU timestamp; a disabled cache always misses.
 func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, error) {
 	cacheKey := CacheKey(req)
 
-	// Check memory, then variant storage - no DB needed for cache hits
-	if !c.disabled &&
-		(c.metaCache.Contains(cacheKey) || c.variants.Exists(cacheKey)) {
+	// Check variant storage directly - no DB needed for cache hits
+	if !c.disabled && c.variants.Exists(cacheKey) {
 		c.touchVariant(ctx, cacheKey)
 
 		return &LookupResult{
@@ -186,32 +184,27 @@ func (c *Cache) Lookup(ctx context.Context, req *ImageRequest) (*LookupResult, e
 
 // 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.
+// 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
 	}
 
 	contentType, known := c.metaCache.Get(cacheKey)
-	if known {
-		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
+	if !known {
+		return c.loadVariantWithMeta(cacheKey)
 	}
 
-	reader, size, contentType, err := c.variants.LoadWithMeta(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
 	}
 
-	c.metaCache.Add(cacheKey, contentType)
-
 	return reader, size, contentType, nil
 }
 
@@ -414,13 +407,16 @@ func (c *Cache) GetSourceMetadataID(
 	return id, nil
 }
 
-// GetSourceContent returns a reader for cached source content by its hash.
-func (c *Cache) GetSourceContent(contentHash ContentHash) (io.ReadCloser, error) {
+// GetSourceContent returns a reader for cached source content by its hash,
+// 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.
@@ -475,7 +471,8 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
 	return &stats, nil
 }
 
-// IncrementStats increments cache statistics.
+// IncrementStats counts a cache hit or miss, and an upstream fetch that read
+// fetchBytes bytes, as IncrementUpstreamFetch does.
 func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64) {
 	var err error
 
@@ -499,18 +496,26 @@ func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64)
 		c.log.Warn("failed to count cache hit or miss", "hit", hit, "error", err)
 	}
 
-	if fetchBytes > 0 {
-		_, err = c.db.ExecContext(ctx, `
-			UPDATE cache_stats
-			SET upstream_fetch_count = upstream_fetch_count + 1,
-			    upstream_fetch_bytes = upstream_fetch_bytes + ?,
-			    last_updated_at = CURRENT_TIMESTAMP
-			WHERE id = 1
-		`, fetchBytes)
-		if err != nil {
-			c.log.Warn("failed to count upstream fetch",
-				"fetch_bytes", fetchBytes, "error", err)
-		}
+	c.IncrementUpstreamFetch(ctx, fetchBytes)
+}
+
+// IncrementUpstreamFetch counts one upstream fetch that read fetchBytes bytes.
+// A fetch that read no bytes is not counted.
+func (c *Cache) IncrementUpstreamFetch(ctx context.Context, fetchBytes int64) {
+	if fetchBytes <= 0 {
+		return
+	}
+
+	_, err := c.db.ExecContext(ctx, `
+		UPDATE cache_stats
+		SET upstream_fetch_count = upstream_fetch_count + 1,
+		    upstream_fetch_bytes = upstream_fetch_bytes + ?,
+		    last_updated_at = CURRENT_TIMESTAMP
+		WHERE id = 1
+	`, fetchBytes)
+	if err != nil {
+		c.log.Warn("failed to count upstream fetch",
+			"fetch_bytes", fetchBytes, "error", err)
 	}
 }
 
@@ -527,6 +532,31 @@ func (c *Cache) IncrementTransformCount(ctx context.Context) {
 	}
 }
 
+// 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/concurrent_misses_internal_test.go

@@ -0,0 +1,444 @@
+package imgcache
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"image/jpeg"
+	"io"
+	"io/fs"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/magic"
+)
+
+// arrivalWait is how long a test gives requests it has started to reach the
+// point where they wait for a held fetch.
+const arrivalWait = 100 * time.Millisecond
+
+// heldFetcher counts the fetches it is asked for and holds each one until
+// releaseFetches is called, so that a test can have requests arrive while a
+// fetch is in progress. started receives once for every fetch.
+type heldFetcher struct {
+	upstream    httpfetcher.Fetcher
+	fetches     atomic.Int32
+	started     chan struct{}
+	release     chan struct{}
+	releaseOnce sync.Once
+}
+
+func (f *heldFetcher) Fetch(
+	ctx context.Context, url string,
+) (*httpfetcher.FetchResult, error) {
+	f.fetches.Add(1)
+
+	f.started <- struct{}{}
+
+	select {
+	case <-f.release:
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+
+	return f.upstream.Fetch(ctx, url)
+}
+
+// releaseFetches lets every held fetch, and every later one, go on.
+func (f *heldFetcher) releaseFetches() {
+	f.releaseOnce.Do(func() { close(f.release) })
+}
+
+// setupHeldFetchService returns a test service whose fetches go through a
+// heldFetcher. Its database is limited to one connection: each connection to
+// an in-memory SQLite database opens a new, empty one, so requests running at
+// once must share the connection that holds the schema.
+func setupHeldFetchService(t *testing.T) (*Service, *TestFixtures, *heldFetcher) {
+	t.Helper()
+
+	svc, fixtures := SetupTestService(t)
+	svc.cache.db.SetMaxOpenConns(1)
+
+	fetcher := &heldFetcher{
+		upstream: svc.fetcher,
+		started:  make(chan struct{}, 100),
+		release:  make(chan struct{}),
+	}
+	svc.fetcher = fetcher
+
+	t.Cleanup(fetcher.releaseFetches)
+
+	return svc, fixtures, fetcher
+}
+
+// photoVariant asks for the test photo, 100x100, at 50x25 as a JPEG of the
+// given quality and fit mode. Each call returns a new request, as Get writes
+// to the request it is given.
+func photoVariant(fixtures *TestFixtures, quality int, fit FitMode) *ImageRequest {
+	return &ImageRequest{
+		SourceHost: fixtures.GoodHost,
+		SourcePath: testPathPhoto,
+		Size:       Size{Width: 50, Height: 25},
+		Format:     FormatJPEG,
+		Quality:    quality,
+		FitMode:    fit,
+	}
+}
+
+// getResult is what one Get call returned, with the image read out.
+type getResult struct {
+	image []byte
+	err   error
+}
+
+// startGet calls Get in a goroutine of its own and delivers what it returned
+// on the channel.
+func startGet(
+	ctx context.Context, svc *Service, req *ImageRequest,
+) <-chan getResult {
+	results := make(chan getResult, 1)
+
+	go func() {
+		resp, err := svc.Get(ctx, req)
+		if err != nil {
+			results <- getResult{err: err}
+
+			return
+		}
+
+		defer func() { _ = resp.Content.Close() }()
+
+		image, err := io.ReadAll(resp.Content)
+		results <- getResult{image: image, err: err}
+	}()
+
+	return results
+}
+
+// jpegSize returns the width and height of the JPEG image in data, or 0 and 0
+// if data is not one.
+func jpegSize(data []byte) (int, int) {
+	config, err := jpeg.DecodeConfig(bytes.NewReader(data))
+	if err != nil {
+		return 0, 0
+	}
+
+	return config.Width, config.Height
+}
+
+// TestService_Get_ConcurrentMissesShareOneFetch starts several requests for
+// one uncached variant while the first one's fetch is held. Between them they
+// must fetch the source once and transcode it once, every one must be answered
+// with the same 50x25 JPEG, and each must count one miss.
+func TestService_Get_ConcurrentMissesShareOneFetch(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+
+	const requests = 8
+
+	pending := make([]<-chan getResult, 0, requests)
+	for range requests {
+		pending = append(pending,
+			startGet(t.Context(), svc, photoVariant(fixtures, 85, FitCover)))
+	}
+
+	<-fetcher.started
+	time.Sleep(arrivalWait)
+	fetcher.releaseFetches()
+
+	var first []byte
+
+	for i, results := range pending {
+		got := <-results
+		if got.err != nil {
+			t.Fatalf("request %d: Get() error = %v", i, got.err)
+		}
+
+		if width, height := jpegSize(got.image); width != 50 || height != 25 {
+			t.Errorf("request %d: image is %dx%d, want a 50x25 JPEG", i, width, height)
+		}
+
+		if first == nil {
+			first = got.image
+		} else if !bytes.Equal(got.image, first) {
+			t.Errorf("request %d: image differs from request 0's", i)
+		}
+	}
+
+	if fetches := fetcher.fetches.Load(); fetches != 1 {
+		t.Errorf("%d requests made %d upstream fetches, want 1", requests, fetches)
+	}
+
+	// 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)
+	}
+
+	want := cacheStatsCounters{0, requests, 1, int64(len(photo)), 1}
+
+	if got := readCacheStatsCounters(t, svc.cache); got != want {
+		t.Errorf("counters = %+v, want %+v", got, want)
+	}
+}
+
+// TestService_Get_ConcurrentVariantsStayApart requests three variants of the
+// test photo at once that differ only in quality or fit. Each must be made by
+// a fetch and a transcode of its own, and each answer must be its own variant.
+func TestService_Get_ConcurrentVariantsStayApart(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+
+	cover := startGet(t.Context(), svc, photoVariant(fixtures, 85, FitCover))
+	lowQuality := startGet(t.Context(), svc, photoVariant(fixtures, 40, FitCover))
+	contain := startGet(t.Context(), svc, photoVariant(fixtures, 85, FitContain))
+
+	for range 3 {
+		select {
+		case <-fetcher.started:
+		case <-time.After(5 * time.Second):
+			t.Fatal("fewer fetches started than variants requested: " +
+				"variants differing in quality or fit were merged")
+		}
+	}
+
+	fetcher.releaseFetches()
+
+	images := make(map[string][]byte)
+
+	for _, variant := range []struct {
+		name          string
+		results       <-chan getResult
+		width, height int
+	}{
+		{"q=85 fit=cover", cover, 50, 25},
+		{"q=40 fit=cover", lowQuality, 50, 25},
+		{"q=85 fit=contain", contain, 25, 25},
+	} {
+		got := <-variant.results
+		if got.err != nil {
+			t.Fatalf("%s: Get() error = %v", variant.name, got.err)
+		}
+
+		width, height := jpegSize(got.image)
+		if width != variant.width || height != variant.height {
+			t.Errorf("%s: image is %dx%d, want a %dx%d JPEG", variant.name,
+				width, height, variant.width, variant.height)
+		}
+
+		images[variant.name] = got.image
+	}
+
+	if bytes.Equal(images["q=85 fit=cover"], images["q=40 fit=cover"]) {
+		t.Error("q=40 was answered with the q=85 image")
+	}
+}
+
+// TestService_Get_WaiterStopsWhenItsContextEnds has a second request for a
+// variant join the first one's held fetch, then ends the second request's
+// context. The second request must return at once with the context's error,
+// while the fetch is still held, and the first must still be answered.
+func TestService_Get_WaiterStopsWhenItsContextEnds(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+
+	first := startGet(t.Context(), svc, photoVariant(fixtures, 85, FitCover))
+
+	<-fetcher.started
+
+	waiterCtx, cancelWaiter := context.WithCancel(t.Context())
+	waiter := startGet(waiterCtx, svc, photoVariant(fixtures, 85, FitCover))
+
+	time.Sleep(arrivalWait)
+	cancelWaiter()
+
+	select {
+	case got := <-waiter:
+		if !errors.Is(got.err, context.Canceled) {
+			t.Errorf("waiting request: Get() error = %v, want %v",
+				got.err, context.Canceled)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("waiting request did not return when its context ended")
+	}
+
+	fetcher.releaseFetches()
+
+	got := <-first
+	if got.err != nil {
+		t.Fatalf("first request: Get() error = %v", got.err)
+	}
+
+	if width, height := jpegSize(got.image); width != 50 || height != 25 {
+		t.Errorf("first request: image is %dx%d, want a 50x25 JPEG", width, height)
+	}
+
+	if fetches := fetcher.fetches.Load(); fetches != 1 {
+		t.Errorf("upstream fetches = %d, want 1", fetches)
+	}
+}
+
+// TestService_Get_FirstRequestLeavingKeepsTheWork ends the context of the
+// request whose fetch is held, after a second request has joined it. The fetch
+// and transcode must go on and answer the second request. The first request
+// waits for its own work, as every request did before misses were shared, and
+// is answered too.
+func TestService_Get_FirstRequestLeavingKeepsTheWork(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+
+	firstCtx, cancelFirst := context.WithCancel(t.Context())
+	first := startGet(firstCtx, svc, photoVariant(fixtures, 85, FitCover))
+
+	<-fetcher.started
+
+	second := startGet(t.Context(), svc, photoVariant(fixtures, 85, FitCover))
+
+	time.Sleep(arrivalWait)
+	cancelFirst()
+	fetcher.releaseFetches()
+
+	for name, results := range map[string]<-chan getResult{
+		"first request": first, "second request": second,
+	} {
+		got := <-results
+		if got.err != nil {
+			t.Fatalf("%s: Get() error = %v", name, got.err)
+		}
+
+		if width, height := jpegSize(got.image); width != 50 || height != 25 {
+			t.Errorf("%s: image is %dx%d, want a 50x25 JPEG", name, width, height)
+		}
+	}
+
+	if fetches := fetcher.fetches.Load(); fetches != 1 {
+		t.Errorf("upstream fetches = %d, want 1", fetches)
+	}
+}
+
+// TestService_Get_ConcurrentMissesShareAFailure has several requests for an
+// image that cannot be served arrive while its fetch is held: the one fetch
+// answers all of them with its error. The request after them is answered from
+// the negative cache when the failure is kept there, and fetches again when it
+// is not.
+func TestService_Get_ConcurrentMissesShareAFailure(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		path        string
+		wantErr     error // what every request at once gets
+		wantNextErr error // what the request after them gets
+		wantFetches int32 // fetches once the request after them is answered
+	}{
+		{"upstream answers 404, kept in the negative cache",
+			"/images/missing.jpg", httpfetcher.ErrUpstreamError,
+			ErrNegativeCached, 1},
+		{"source fails the magic byte check, not kept",
+			"/images/text.png", magic.ErrUnknownFormat, magic.ErrUnknownFormat, 2},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			svc, fixtures, fetcher := setupHeldFetchService(t)
+
+			request := func() *ImageRequest {
+				req := photoVariant(fixtures, 85, FitCover)
+				req.SourcePath = tc.path
+
+				return req
+			}
+
+			pending := make([]<-chan getResult, 0, 4)
+			for range 4 {
+				pending = append(pending, startGet(t.Context(), svc, request()))
+			}
+
+			<-fetcher.started
+			time.Sleep(arrivalWait)
+			fetcher.releaseFetches()
+
+			for i, results := range pending {
+				if got := <-results; !errors.Is(got.err, tc.wantErr) {
+					t.Errorf("request %d: Get() error = %v, want %v", i, got.err, tc.wantErr)
+				}
+			}
+
+			_, err := svc.Get(t.Context(), request())
+			if !errors.Is(err, tc.wantNextErr) {
+				t.Errorf("next request: Get() error = %v, want %v", err, tc.wantNextErr)
+			}
+
+			if fetches := fetcher.fetches.Load(); fetches != tc.wantFetches {
+				t.Errorf("upstream fetches = %d, want %d", fetches, tc.wantFetches)
+			}
+		})
+	}
+}
+
+// TestService_Get_EndedRequestFetchesNothing checks that a request whose
+// context has already ended when it misses the cache starts no fetch.
+func TestService_Get_EndedRequestFetchesNothing(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	_, err := svc.Get(ctx, photoVariant(fixtures, 85, FitCover))
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("Get() error = %v, want %v", err, context.Canceled)
+	}
+
+	if fetches := fetcher.fetches.Load(); fetches != 0 {
+		t.Errorf("upstream fetches = %d, want 0", fetches)
+	}
+}
+
+// panickingFetcher panics on every fetch.
+type panickingFetcher struct{}
+
+func (panickingFetcher) Fetch(
+	context.Context, string,
+) (*httpfetcher.FetchResult, error) {
+	panic("upstream fetcher panicked")
+}
+
+// TestService_Get_PanicBecomesAnError checks that a panic while a variant is
+// being made reaches its request as an error naming the panic, instead of
+// being raised again.
+func TestService_Get_PanicBecomesAnError(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := SetupTestService(t)
+	svc.fetcher = panickingFetcher{}
+
+	var err error
+
+	func() {
+		defer func() {
+			if recovered := recover(); recovered != nil {
+				t.Fatalf("Get() panicked: %v", recovered)
+			}
+		}()
+
+		_, err = svc.Get(t.Context(), photoVariant(fixtures, 85, FitCover))
+	}()
+
+	if err == nil || !strings.Contains(err.Error(), "upstream fetcher panicked") {
+		t.Errorf("Get() error = %v, want one naming the panic", err)
+	}
+}

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
-// no readable .meta sidecar.
+// fallbackContentType is the content type given to a variant file that
+// 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

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

@@ -2,6 +2,7 @@ package imgcache
 
 import (
 	"bytes"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -83,6 +84,16 @@ func assertLookupMisses(t *testing.T, cache *Cache, request *ImageRequest) {
 	}
 }
 
+// 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
@@ -114,6 +125,122 @@ func TestSecondHitDoesNotReadMetaFile(t *testing.T) {
 	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.

internal/imgcache/service.go

@@ -8,9 +8,11 @@ import (
 	"io"
 	"log/slog"
 	"net/url"
+	"runtime/debug"
 	"time"
 
 	"github.com/dustin/go-humanize"
+	"golang.org/x/sync/singleflight"
 	"sneak.berlin/go/pixa/internal/allowlist"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
@@ -29,6 +31,9 @@ type Service struct {
 	log             *slog.Logger
 	allowHTTP       bool
 	maxResponseSize int64
+	// variantsInProgress lets the requests that miss the same variant at the
+	// same time share one fetch and one transcode.
+	variantsInProgress singleflight.Group
 }
 
 // ServiceConfig holds configuration for the image service.
@@ -43,6 +48,9 @@ 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 +99,10 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 	}
 
 	maxResponseSize := fetcherCfg.MaxResponseSize
-	processor := imageprocessor.New(
-		imageprocessor.Params{MaxInputBytes: maxResponseSize},
-	)
+	processor := imageprocessor.New(imageprocessor.Params{
+		MaxInputBytes:           maxResponseSize,
+		MaxConcurrentProcessing: cfg.MaxConcurrentProcessing,
+	})
 
 	return &Service{
 		cache:           cfg.Cache,
@@ -156,14 +165,12 @@ func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, e
 		}
 	}
 
-	// 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)
+	// Cache miss - get the variant, processed once for all the requests that
+	// miss it at the same time, then count this request's miss, also when it
+	// failed or the request context has ended meanwhile
+	response, err := s.processOrWait(ctx, req)
 
-	response, fetchedBytes, err := s.processFromSourceOrFetch(ctx, req, cacheKey)
-
-	s.cache.IncrementStats(context.WithoutCancel(ctx), false, fetchedBytes)
+	s.cache.IncrementStats(context.WithoutCancel(ctx), false, 0)
 
 	if err != nil {
 		return nil, err
@@ -237,82 +244,160 @@ func (s *Service) GenerateSignedURL(
 		baseURL, path, sig, exp, req.Quality, req.FitMode), nil
 }
 
-// loadCachedSource attempts to load source content from cache, returning nil
-// if the cached data is unavailable or exceeds maxResponseSize.
-func (s *Service) loadCachedSource(contentHash ContentHash) []byte {
-	reader, err := s.cache.GetSourceContent(contentHash)
+// errPanicked is returned when processing a variant panicked.
+var errPanicked = errors.New("panic while processing image")
+
+// processOrWait returns the variant req asks for. The first of the requests
+// that miss a variant at the same time processes it, and singleflight hands
+// its result, or its error, to the others: they fetch nothing, read no source
+// and take no upstream connection or processing slot. The processing runs with
+// a context that does not end with the first request's, so the others are
+// still served if that client goes away; the fetch timeout and the waits for a
+// connection and a processing slot still bound it.
+func (s *Service) processOrWait(
+	ctx context.Context, req *ImageRequest,
+) (*ImageResponse, error) {
+	// A request that has already ended starts no processing
+	if ctx.Err() != nil {
+		return nil, ctx.Err()
+	}
+
+	cacheKey := CacheKey(req)
+
+	// Closed when this request's own function runs, which singleflight does
+	// only when no other request is processing the variant
+	processing := make(chan struct{})
+
+	results := s.variantsInProgress.DoChan(string(cacheKey),
+		func() (_ any, err error) {
+			close(processing)
+
+			// singleflight would raise a panic again in a goroutine of its
+			// own, where no handler recovers it, and stop pixad
+			defer func() {
+				recovered := recover()
+				if recovered != nil {
+					s.log.Error("panic while processing image",
+						"host", req.SourceHost, "path", req.SourcePath,
+						"panic", recovered, "stack", string(debug.Stack()))
+
+					err = fmt.Errorf("%w: %v", errPanicked, recovered)
+				}
+			}()
+
+			return s.processFromSourceOrFetch(
+				context.WithoutCancel(ctx), req, cacheKey)
+		})
+
+	var result singleflight.Result
+
+	select {
+	case result = <-results:
+	case <-ctx.Done():
+		select {
+		case <-processing:
+			// This request is processing the variant: it waits for the
+			// result, as every request did before misses were shared
+			result = <-results
+		default:
+			// Another request is processing the variant, or this request's
+			// function has not started yet; the processing goes on without it
+			return nil, ctx.Err()
+		}
+	}
+
+	if result.Err != nil {
+		return nil, result.Err
+	}
+
+	variant, _ := result.Val.(*processedVariant)
+
+	return &ImageResponse{
+		Content:       io.NopCloser(bytes.NewReader(variant.data)),
+		ContentLength: int64(len(variant.data)),
+		ContentType:   variant.contentType,
+		FetchedBytes:  variant.fetchedBytes,
+		ETag:          formatETag(cacheKey),
+	}, nil
+}
+
+// loadCachedSource opens source content from cache, without reading it, and
+// returns it with its size; nil if the cached data is unavailable, empty or
+// exceeds maxResponseSize.
+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
-	// from unexpectedly large cached files.
-	limited := io.LimitReader(reader, s.maxResponseSize+1)
-	data, err := io.ReadAll(limited)
-	_ = reader.Close()
+	if size > s.maxResponseSize {
+		_ = 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. It also returns the number of bytes fetched from upstream,
-// as fetchAndProcess does, or 0 when the cached source was used.
+// if available.
 func (s *Service) processFromSourceOrFetch(
 	ctx context.Context,
 	req *ImageRequest,
 	cacheKey VariantKey,
-) (*ImageResponse, int64, error) {
+) (*processedVariant, error) {
 	// Check if we have cached source content
 	contentHash, _, err := s.cache.LookupSource(ctx, req)
 	if err != nil {
 		s.log.Warn("source lookup failed", "error", err)
 	}
 
-	var sourceData []byte
+	var (
+		source     io.ReadCloser
+		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 {
 		return s.fetchAndProcess(ctx, req, cacheKey)
 	}
 
-	// Process using cached source; nothing was fetched from upstream
-	resp, err := s.processAndStore(
-		ctx, req, cacheKey, sourceData, int64(len(sourceData)),
-	)
+	defer func() { _ = source.Close() }()
 
-	return resp, 0, err
+	// Process using cached source; nothing was fetched from upstream. The
+	// image processor reads the source only once it has a processing slot,
+	// so a request waiting for one holds none of it in memory.
+	return s.processAndStore(ctx, req, cacheKey, source, sourceSize)
 }
 
 // fetchAndProcess fetches from upstream, processes, and caches the result.
-// It also returns the number of bytes read from upstream, including when
+// It counts the fetch with the 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, int64, error) {
+) (*processedVariant, error) {
 	// Fetch from upstream
 	sourceURL := req.SourceURL()
 
@@ -331,17 +416,24 @@ func (s *Service) fetchAndProcess(
 			}
 		}
 
-		return nil, 0, fmt.Errorf("upstream fetch failed: %w", err)
+		return nil, 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))
 
+	// Counted also when the request context has ended meanwhile
+	s.cache.IncrementUpstreamFetch(context.WithoutCancel(ctx), fetchBytes)
+
 	if err != nil {
-		return nil, fetchBytes, fmt.Errorf("failed to read upstream response: %w", err)
+		return nil, fmt.Errorf("failed to read upstream response: %w", err)
 	}
 
 	// Calculate download bitrate
@@ -369,7 +461,7 @@ func (s *Service) fetchAndProcess(
 	// Validate magic bytes match content type
 	err = magic.ValidateMagicBytes(sourceData, fetchResult.ContentType)
 	if err != nil {
-		return nil, fetchBytes, fmt.Errorf("content validation failed: %w", err)
+		return nil, fmt.Errorf("content validation failed: %w", err)
 	}
 
 	// Store source content
@@ -379,19 +471,28 @@ func (s *Service) fetchAndProcess(
 		// Continue even if caching fails
 	}
 
-	resp, err := s.processAndStore(ctx, req, cacheKey, sourceData, fetchBytes)
-
-	return resp, fetchBytes, err
+	return s.processAndStore(
+		ctx, req, cacheKey, bytes.NewReader(sourceData), fetchBytes,
+	)
 }
 
-// processAndStore processes an image and stores the result.
+// processedVariant is a variant as processAndStore made it. Each request that
+// shared its processing serves it through a reader of its own.
+type processedVariant struct {
+	data         []byte
+	contentType  string
+	fetchedBytes int64
+}
+
+// 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) {
+) (*processedVariant, error) {
 	// Process the image
 	processStart := time.Now()
 
@@ -402,7 +503,7 @@ 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)
 	}
@@ -455,12 +556,10 @@ func (s *Service) processAndStore(
 		// Continue even if caching fails
 	}
 
-	return &ImageResponse{
-		Content:       io.NopCloser(bytes.NewReader(processedData)),
-		ContentLength: outputSize,
-		ContentType:   processResult.ContentType,
-		FetchedBytes:  fetchBytes,
-		ETag:          formatETag(cacheKey),
+	return &processedVariant{
+		data:         processedData,
+		contentType:  processResult.ContentType,
+		fetchedBytes: fetchBytes,
 	}, nil
 }
 

internal/imgcache/storage.go

@@ -531,7 +531,8 @@ func (s *VariantStorage) LoadWithSize(key VariantKey) (io.ReadCloser, int64, err
 }
 
 // LoadWithMeta returns a reader, size, and content type for the content at
-// the given key.
+// 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) {
@@ -540,8 +541,8 @@ func (s *VariantStorage) LoadWithMeta(
 		return nil, 0, "", err
 	}
 
-	// Load metadata for content type
-	contentType := "application/octet-stream" // fallback
+	var contentType string
+
 	metaPath := s.keyToPath(key) + ".meta"
 
 	metaData, err := os.ReadFile(metaPath) //nolint:gosec // path derived from cache key

internal/middleware/middleware.go

@@ -172,7 +172,7 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 // CORS returns a CORS middleware.
 func (s *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{
-		AllowedOrigins:   []string{"*"},
+		AllowedOrigins:   []string{s.config.AccessControlAllowOrigin},
 		AllowedMethods:   []string{"GET", "HEAD", "OPTIONS"},
 		AllowedHeaders:   []string{"Accept", "Authorization", "Content-Type"},
 		ExposedHeaders:   []string{"Link"},

internal/middleware/middleware_internal_test.go

@@ -9,6 +9,53 @@ import (
 	"sneak.berlin/go/pixa/internal/config"
 )
 
+// TestCORSAnswersWithConfiguredOrigin checks that the CORS middleware
+// answers with access_control_allow_origin, where "*" lets any origin read
+// responses and a single origin lets only that origin read them.
+func TestCORSAnswersWithConfiguredOrigin(t *testing.T) {
+	t.Parallel()
+
+	const appOrigin = "https://app.example.com"
+
+	cases := []struct {
+		configured    string
+		requestOrigin string
+		want          string
+	}{
+		{"*", "https://any.example.com", "*"},
+		{appOrigin, appOrigin, appOrigin},
+		{appOrigin, "https://other.example.com", ""},
+	}
+
+	testHandler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	for _, tc := range cases {
+		mw := &Middleware{
+			log:    slog.Default(),
+			config: &config.Config{AccessControlAllowOrigin: tc.configured},
+		}
+
+		handler := mw.CORS()(testHandler)
+
+		req := httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, "/v1/image/example.com/a.jpg/1x1.png", nil)
+		req.Header.Set("Origin", tc.requestOrigin)
+
+		rec := httptest.NewRecorder()
+
+		handler.ServeHTTP(rec, req)
+
+		got := rec.Header().Get("Access-Control-Allow-Origin")
+		if got != tc.want {
+			t.Errorf("configured %q, request from %q: "+
+				"Access-Control-Allow-Origin = %q, want %q",
+				tc.configured, tc.requestOrigin, got, tc.want)
+		}
+	}
+}
+
 func TestSecurityHeaders(t *testing.T) {
 	t.Parallel()
 

internal/server/http.go

@@ -14,7 +14,6 @@ const (
 	// short, so a slowloris client dribbling headers is dropped well
 	// before it ties up a connection for the whole ReadTimeout window.
 	HTTPReadHeaderTimeout = 10 * time.Second
-	HTTPWriteTimeout      = 60 * time.Second
 	// HTTPIdleTimeout bounds how long an idle keep-alive connection is
 	// held open, so idle connections cannot accumulate without limit on a
 	// service targeting high concurrency.
@@ -30,7 +29,7 @@ func (s *Server) newHTTPServer() *http.Server {
 		Addr:              fmt.Sprintf(":%d", s.config.Port),
 		ReadTimeout:       HTTPReadTimeout,
 		ReadHeaderTimeout: HTTPReadHeaderTimeout,
-		WriteTimeout:      HTTPWriteTimeout,
+		WriteTimeout:      s.config.DownstreamTimeout,
 		IdleTimeout:       HTTPIdleTimeout,
 		MaxHeaderBytes:    HTTPMaxHeaderBytes,
 		Handler:           s,

internal/server/http_internal_test.go

@@ -11,11 +11,15 @@ import (
 // carries every hardening timeout wired onto it, including the slowloris
 // defense (ReadHeaderTimeout) and the keep-alive bound (IdleTimeout). This
 // guards against a field being defined but never set on the server, so
-// each assertion compares the server field to its constant.
+// each assertion compares the server field to its constant, or, for
+// WriteTimeout, to downstream_timeout from the config.
 func TestNewHTTPServerTimeouts(t *testing.T) {
 	t.Parallel()
 
-	s := &Server{config: &config.Config{Port: 8080}}
+	s := &Server{config: &config.Config{
+		Port:              8080,
+		DownstreamTimeout: 45 * time.Second,
+	}}
 
 	srv := s.newHTTPServer()
 
@@ -26,7 +30,7 @@ func TestNewHTTPServerTimeouts(t *testing.T) {
 	}{
 		{"ReadTimeout", srv.ReadTimeout, HTTPReadTimeout},
 		{"ReadHeaderTimeout", srv.ReadHeaderTimeout, HTTPReadHeaderTimeout},
-		{"WriteTimeout", srv.WriteTimeout, HTTPWriteTimeout},
+		{"WriteTimeout", srv.WriteTimeout, 45 * time.Second},
 		{"IdleTimeout", srv.IdleTimeout, HTTPIdleTimeout},
 	}
 

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"
 )
@@ -51,11 +52,12 @@ func newTestServer(t *testing.T) *Server {
 
 	stateDir := t.TempDir()
 	cfg := &config.Config{
-		Debug:          true,
-		SigningKey:     testSigningKey,
-		StateDir:       stateDir,
-		DBURL:          "file:" + filepath.Join(stateDir, "state.sqlite3"),
-		TrustedProxies: []netip.Prefix{netip.MustParsePrefix("10.0.0.0/8")},
+		Debug:             true,
+		SigningKey:        testSigningKey,
+		StateDir:          stateDir,
+		DBURL:             "file:" + filepath.Join(stateDir, "state.sqlite3"),
+		TrustedProxies:    []netip.Prefix{netip.MustParsePrefix("10.0.0.0/8")},
+		DownstreamTimeout: config.DefaultDownstreamTimeout,
 	}
 
 	lc := fxtest.NewLifecycle(t)
@@ -70,8 +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()
@@ -33,7 +39,7 @@ func (s *Server) SetupRoutes() {
 	}
 
 	s.router.Use(s.mw.CORS())
-	s.router.Use(middleware.Timeout(HTTPWriteTimeout))
+	s.router.Use(middleware.Timeout(s.config.DownstreamTimeout))
 
 	if s.sentryEnabled {
 		sentryHandler := sentryhttp.New(sentryhttp.Options{
@@ -68,15 +74,23 @@ func (s *Server) SetupRoutes() {
 
 	s.router.Get("/logout", s.h.HandleLogout())
 
-	// Main image proxy route
-	// /v1/image/<host>/<path>/<width>x<height>.<format>
-	s.router.Get("/v1/image/*", s.h.HandleImage())
-	s.router.Head("/v1/image/*", s.h.HandleImage())
+	// 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)
 
-	// 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())
+		// Main image proxy route
+		// /v1/image/<host>/<path>/<width>x<height>.<format>
+		r.Get("/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
+		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,
-# libheif) are installed for the govips bindings.
+# stage (see script/lint). A C compiler and the CGO image libraries
+# (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