Keep WebP, AVIF and JPEG XL encodes at the limits under 40 seconds (closes #272)

WebP is saved at effort 2 and JPEG XL at effort 4, and an AVIF output of
more than 16,777,216 pixels is refused with 400 before anything is
decoded, as AVIF is already at the lowest effort govips can set. With
one libvips thread, at any quality, 8192x8192 random pixels with an
alpha channel take up to about 23 seconds as WebP and 27 as JPEG XL, and
4096x4096, 8192x2048 or 2048x8192 up to about 34 as AVIF. Resizing with
fit=cover already decodes the input, so the output's dimensions are now
worked out before resizing, in outputDimensions. Reading the input moves
into readInput to keep Process within the linter's limits.
TestImageProcessor_WebPAtDefaultEffort becomes TestImageProcessor_WebPAtEffort2, as #280 (comment) allows.

Model: opus-5-5

AGENTS.md

@@ -14,7 +14,11 @@ These rules MUST be followed at all times, it is very important.
   without asking first. In almost all cases, the code should be changed, NOT the
   tests. If you think the test needs to be changed, make your case for that and
   ask for permission to proceed, then stop. You need explicit user approval to
-  modify existing tests. (You do not need user approval for writing NEW tests.)
+  modify existing tests. A bug fix may change an existing test without asking
+  when that test pins exactly the behavior the filed bug calls wrong, and the
+  change moves it to the fixed behavior and nothing else; any other change to an
+  existing test still needs explicit user approval. (You do not need user
+  approval for writing NEW tests.)
 
 - When linting, assume the linter config is CORRECT, and that each item output
   by the linter is something that legitimately needs fixing in the code.

Dockerfile

@@ -1,12 +1,16 @@
-# Every stage declares ARG CACHEBUST first: script/ passes a new value
+# In every stage ARG CACHEBUST comes right after the steps that install
-# on every build, which reruns every RUN step after it. The steps that
+# system packages and download Go's modules, or right after FROM in a
-# compile mount Go's build cache, which survives from build to build.
+# stage with neither, and every step that lints, tests or compiles
+# comes after it. script/ passes a new value on every build, which
+# reruns every RUN step after it. Docker keeps the layers above it
+# until the base image, their own lines or go.mod and go.sum change.
+# The steps that compile mount Go's build cache, and golangci-lint run
+# also mounts golangci-lint's; both survive from build to build.
 
 # Lint phase. script/lint builds it alone. The linter is run directly:
 # `make lint` and script/lint are themselves a docker build.
 # golangci/golangci-lint:v2.14.0, 2026-10-15
 FROM golangci/golangci-lint@sha256:ad862ba6b3798cbe0fd9fd7408d498fd74fbd2623a92406b2fd3898faf0bf98f AS lint
-ARG CACHEBUST
 
 # The linter compiles every package, and govips needs the libvips
 # headers for that. REPO_POLICIES.md has the lint phase install them
@@ -18,25 +22,29 @@ RUN apt-get update \
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
+ARG CACHEBUST
 COPY . .
 RUN --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/root/.cache/golangci-lint \
     golangci-lint run --config .golangci.yml ./...
 
 # Test phase. script/test builds it alone. -race needs cgo and so a C
 # compiler, which the Debian Go image ships and the alpine one does not.
 # golang:1.25.4-trixie, 2026-10-15
 FROM golang@sha256:a02d35efc036053fdf0da8c15919276bf777a80cbfda6a35c5e9f087e652adfc AS test
-ARG CACHEBUST
+
+# govips compiles against the libvips and libheif headers, and the tests
+# need libvips' JPEG XL support; libvips-dev brings all three. The tests
+# also encode AVIF, which needs libheif's AVIF encoder,
+# libheif-plugin-aomenc: Debian only recommends it, so it is named here.
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends libvips-dev libheif-plugin-aomenc \
+    && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /src
-
-# script/bootstrap --cgo installs the build dependencies (the libvips
-# and libheif headers, and libvips' JPEG XL support, which the tests
-# need) and downloads the Go modules.
-COPY script/ ./script/
 COPY go.mod go.sum ./
-RUN script/bootstrap --cgo
+RUN go mod download
-
+ARG CACHEBUST
 COPY . .
 
 # Without -v first; on a failure, again with -v for the details, and
@@ -52,22 +60,23 @@ RUN --mount=type=cache,target=/root/.cache/go-build \
       go test -count=1 -timeout 90s -race -parallel 4 -v ./...; exit 1; }
 
 # Build stage. Nothing is wanted from the two phases above: these copies
-# make BuildKit build them first, so this stage runs only when lint and
+# make BuildKit build them first, so nothing after them runs unless lint
-# test passed.
+# and test passed.
 # golang:1.25.4-alpine3.22, 2026-02-25; the runtime stage uses Alpine 3.22 too
 FROM golang:1.25.4-alpine3.22@sha256:d3f0cf7723f3429e3f9ed846243970b20a2de7bae6a5b66fc5914e228d831bbb AS builder
+
+# git for the version, and for govips a C compiler, pkg-config and the
+# libvips and libheif headers
+RUN apk add --no-cache git build-base pkgconfig vips-dev libheif-dev
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
 ARG CACHEBUST
 
 COPY --from=lint /src/go.sum /dev/null
 COPY --from=test /src/go.sum /dev/null
 
-WORKDIR /src
-
-# Build dependencies, git among them, and Go modules, as in the test
-# phase
-COPY script/ ./script/
-COPY go.mod go.sum ./
-RUN script/bootstrap --cgo
 # A tar-stream context keeps the sender's file owners, which git refuses.
 RUN git config --system --add safe.directory /src
 
@@ -102,7 +111,6 @@ RUN --mount=type=cache,target=/root/.cache/go-build \
 # the libvips and musl it was built with.
 # alpine:3.22, 2026-10-08
 FROM alpine:3.22@sha256:5291449c3df73caf6ed85e649dec1b9e818b39a5d8c871e97afc13e9cd5e8fa8
-ARG CACHEBUST
 
 # Install runtime dependencies only. vips-jxl is libvips' JPEG XL
 # support, without which pixad does not start.
@@ -113,6 +121,7 @@ RUN apk add --no-cache \
     ca-certificates \
     tzdata \
     su-exec
+ARG CACHEBUST
 
 # Copy binary from builder
 COPY --from=builder /pixad /usr/local/bin/pixad

README.md

@@ -102,8 +102,8 @@ it uses libvips through CGO. pixad does not start unless libvips has its JPEG XL
 support, which on Alpine is the `vips-jxl` package and which the nix and brew
 packages of libvips include, as do the apt ones from Debian 12 and Ubuntu 24.04
 on. Building pixa also needs the development files of libvips and libheif,
-`pkg-config` and a C compiler. `script/bootstrap --cgo` installs all of these,
+`pkg-config` and a C compiler. `script/bootstrap --cgo` installs all of these;
-as the `Dockerfile` does where it compiles pixa. Plain `script/bootstrap`, which
+the `Dockerfile` installs its own packages. Plain `script/bootstrap`, which
 `script/setup` and `script/cibuild` run, installs git, make and Go, and Node,
 Yarn and the prettier pinned in `yarn.lock` for formatting the markdown, but
 none of the C libraries: the checks compile pixa in Docker. Docker itself must
@@ -247,9 +247,11 @@ path under `/v1/` answers 200, in maintenance mode too.
   redirects to, is `localhost`, ends in `.localhost` or `.local`, or has an
   address in a blocked network (see `blocked_networks`); 429 over
   `image_requests_per_minute` (below); 415 when the upstream answers with a
-  `Content-Type` that is not one of the source formats (below), or with an image
+  `Content-Type` that is not one of the source formats (below), with a body
-  whose first bytes libvips reads as another format; 502 when the upstream
+  whose first bytes are not those of the format its `Content-Type` names, or
-  answered with an error status, and for 5 minutes after that for an image of
+  with an image whose first bytes libvips reads as another format; 502 when the
+  upstream answered with an error status, and for 5 minutes after that, or after
+  a body whose first bytes are not those of its `Content-Type`, for an image of
   the same source URL that pixa can neither serve from its cache nor make from
   the source image still in its cache; 503 when pixa is busy or in maintenance
   mode; 500 for any other failure.
@@ -355,11 +357,22 @@ either side may be; so is a size with `fit=outside`, which scales the image to
 cover the size and so past it on one side, when that side would come out
 above 8192.
 
+An AVIF image of more than 16,777,216 pixels, the number 4096x4096 and 8192x2048
+have, is refused with 400 before anything is decoded, as its encode could take
+more than 40 seconds (see below). That includes `orig` of an AVIF source, and
+`auto` when it chooses AVIF, so a client whose `Accept` names `image/avif` but
+not `image/jxl` is refused such a size.
+
 The source image may be JPEG, PNG, GIF, WebP, AVIF or JPEG XL. The upstream's
 `Content-Type` must name its format, and the image's first bytes must match it.
 An upstream answering with any other `Content-Type`, SVG included, is refused
-with 415, and so is an image whose first bytes libvips reads as another format,
+with 415. So is a body whose first bytes are not those of the format its
-such as an SVG whose first bytes pass the check for AVIF.
+`Content-Type` names, such as a JPEG sent as `image/png`, text sent as an image,
+or an empty body. pixa then does not fetch that source URL again for 5 minutes,
+as after an upstream error status, and meanwhile answers 502 for an image that
+would need that fetch. An image whose first bytes libvips reads as another
+format, such as an SVG whose first bytes pass the check for AVIF, is refused
+with 415 too.
 
 With the format `auto`, pixa chooses the format for each request from its
 `Accept` header, in this order:
@@ -404,14 +417,18 @@ transcode that stops this way is answered 503 with the error
 ended. govips gives pixa no way to stop libvips once an encode has begun, so an
 encode keeps its slot until it finishes, which can be after
 `downstream_timeout`. The limit of 8192 on each side of the source and of the
-output bounds how long that takes. With one libvips thread, as pixad runs it,
+output, and for AVIF the limit of 16,777,216 pixels, bound how long that takes.
-encoding 8192x8192 random pixels takes up to about 1 second as JPEG, 15 as PNG,
+With one libvips thread, as pixad runs it, encoding 8192x8192 random pixels
-16 as GIF, 63 as WebP, 110 as AVIF and 96 as JPEG XL, the slowest of 8 or 16
+takes up to about 1 second as JPEG, 15 as PNG and 16 as GIF, the slowest of 8 or
-bits per sample, with or without an alpha channel. WebP, AVIF and JPEG XL are
+16 bits per sample, with or without an alpha channel. The WebP and JPEG XL
-that slow only with an alpha channel: 8-bit RGB takes about 44, 53 and 28
+encoders run below libvips' default effort, and AVIF at the lowest effort govips
-seconds. Decoding such a source adds up to about 11 seconds. A WebP, AVIF or
+can set, so that no encode takes more than 40 seconds of the default
-JPEG XL encode can therefore hold its slot past the default `downstream_timeout`
+`downstream_timeout` of 60: at any quality and 8 or 16 bits per sample,
-of 60 seconds.
+8192x8192 random pixels with an alpha channel take up to about 23 seconds as
+WebP and 27 as JPEG XL, and 4096x4096, 8192x2048 or 2048x8192 up to about 34 as
+AVIF, which at quality 85 would take about 96 at 8192x8192. Without an alpha
+channel they take up to about 16, 13 and 22 seconds. Decoding such a source adds
+up to about 11 seconds.
 
 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
@@ -463,13 +480,13 @@ nor change what it asks for.
    sent with an empty format, or none, also makes a JPEG XL URL. Width and
    height both empty or `0` keep the original size; if only one of them is empty
    or `0`, that side is scaled to keep the image's proportions. A URL whose
-   scaled side would come out above 8192 answers 400, and so does a source URL
+   scaled side would come out above 8192 answers 400, and so does an AVIF URL of
-   whose path holds a `..` segment. So does a source URL that the image URL
+   more than 16,777,216 pixels or a source URL whose path holds a `..` segment.
-   would fetch written another way, and the page then shows the URL pixa would
+   So does a source URL that the image URL would fetch written another way, and
-   fetch: one with a user name and password, which pixa does not send, or one
+   the page then shows the URL pixa would fetch: one with a user name and
-   whose path holds `%2F`, fetched as `/`, or `(`, fetched as `%28`, as pixa
+   password, which pixa does not send, or one whose path holds `%2F`, fetched as
-   keeps the source path decoded and fetches it percent-encoded again (see
+   `/`, or `(`, fetched as `%28`, as pixa keeps the source path decoded and
-   Routes).
+   fetches it percent-encoded again (see Routes).
 3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
    it expires. `<host>` is the host the page was opened on, and the URL starts
    with `http` instead while `debug` is on. The name after the token is ignored
@@ -713,16 +730,22 @@ Key settings in more detail:
   omitted key defaults to the RFC 1918 private ranges (`10.0.0.0/8`,
   `172.16.0.0/12`, `192.168.0.0/16`), since pixa is deployed behind a proxy on a
   private network; an explicitly empty list (`[]`) trusts no one, and an
-  explicit list replaces the default. An invalid CIDR aborts startup. Set this
+  explicit list replaces the default. An invalid CIDR aborts startup. An IPv6
-  to the address pixa sees for requests that come through your proxy, such as
+  address in the IPv4-mapped form (`::ffff:a.b.c.d`) is matched only as the IPv4
-  `172.17.0.1/32`, when the defaults do not cover it, or to trust nothing else
+  address it holds, so an IPv6 range such as `::/0` trusts neither it nor a
-  (see the login limit and `image_requests_per_minute` under Routes). For a
+  plain IPv4 address. An entry written in that form is read as the IPv4 range it
-  proxy on the Docker host that connects to pixa over `127.0.0.1`, that address
+  covers, so `::ffff:10.0.0.0/104` trusts what `10.0.0.0/8` trusts; such an
-  is the gateway of the container's Docker network (`172.17.0.1` on the default
+  entry shorter than `/96` covers more than IPv4 addresses and aborts startup.
-  bridge), not the proxy's own address; a proxy that connects through another of
+  Set this to the address pixa sees for requests that come through your proxy,
-  the host's addresses is seen with that address. To be sure which address it
+  such as `172.17.0.1/32`, when the defaults do not cover it, or to trust
-  is, set this to `[]` (or `PIXA_TRUSTED_PROXIES` to empty), send a request
+  nothing else (see the login limit and `image_requests_per_minute` under
-  through the proxy, and read `remoteIP` in pixa's request log line for it
+  Routes). For a proxy on the Docker host that connects to pixa over
+  `127.0.0.1`, that address is the gateway of the container's Docker network
+  (`172.17.0.1` on the default bridge), not the proxy's own address; a proxy
+  that connects through another of the host's addresses is seen with that
+  address. To be sure which address it is, set this to `[]` (or
+  `PIXA_TRUSTED_PROXIES` to empty), send a request through the proxy, and read
+  `remoteIP` in pixa's request log line for it
 - `upstream_fetch_timeout` — time allowed for one fetch from an upstream host,
   as a duration such as `30s` (the default) or `2m`
 - `upstream_max_response_size` — largest upstream response accepted, in bytes;
@@ -831,19 +854,25 @@ provide:
   `script/precommit`
 
 Every `docker build` in these scripts passes a new value of the `CACHEBUST`
-build argument, which every stage of the `Dockerfile` declares first, so every
+build argument, which every stage of the `Dockerfile` declares, so every `RUN`
-`RUN` step, the lint and test phases included, runs on every build instead of
+step after that line, the lint and test steps included, runs on every build
-coming from the build cache. Go's build cache is kept from build to build: the
+instead of coming from the build cache. Above the line are the steps that
-steps that compile mount it, so a package whose inputs are unchanged is not
+install system packages and download the Go modules; Docker keeps their layers
-compiled again, and both `go test` runs pass `-count=1` so that no stored test
+until the base image, the step itself, `go.mod` or `go.sum` changes. Go's build
-result is replayed. The `test` phase runs on the Debian Go image, with Debian's
+cache is kept from build to build: the steps that compile mount it, so a package
-libvips and libheif; the image pixa ships is built and runs on Alpine.
+whose inputs are unchanged is not compiled again, and both `go test` runs pass
-`script/check`, `script/cibuild`, `script/docker`, `script/lint`, `script/test`,
+`-count=1` so that no stored test result is replayed. The lint step also mounts
-`script/setup` and `script/install-precommit` are the standard copies from
+golangci-lint's cache, which reuses a package's findings only when its files,
-`sneak/prompts`, kept identical to them. `script/fmt` and `script/fmt-check` are
+those of the packages it imports, `go.mod`, the linters and their settings in
-the standard copies with pixa's `gofmt` step kept before prettier. prettier
+`.golangci.yml`, and the golangci-lint version are all unchanged. The `test`
-formats the markdown only, not the HTML templates, as it cannot parse a Go
+phase runs on the Debian Go image, with Debian's libvips and libheif; the image
-template action inside a tag.
+pixa ships is built and runs on Alpine. `script/check`, `script/cibuild`,
+`script/docker`, `script/lint`, `script/test`, `script/setup` and
+`script/install-precommit` are the standard copies from `sneak/prompts`, kept
+identical to them. `script/fmt` and `script/fmt-check` are the standard copies
+with pixa's `gofmt` step kept before prettier. prettier formats the markdown
+only, not the HTML templates, as it cannot parse a Go template action inside a
+tag.
 
 ## Load Test
 

REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-10-15
+last_modified: 2026-10-18
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -159,8 +159,11 @@ style conventions are in separate documents:
     one's findings, and a host-global lock in `$TMPDIR` makes concurrent runs
     exit non-zero with `parallel golangci-lint is running` — a status a caller
     cannot tell from real findings. Both have produced wrong verdicts in this
-    org, in both directions. A container has its own golangci-lint cache, its
+    org, in both directions. A container has its own `TMPDIR`, so the lock is
-    own `TMPDIR` and a digest-pinned binary, so neither is reachable.
+    out of reach. The lint phase does keep golangci-lint's cache between check
+    builds (below), but it lints every checkout at `/src`, and a finding is
+    taken from that cache only for the same files, settings and golangci-lint
+    version.
 
 - **Any build that runs checks is given a new `CACHEBUST` value.** Docker
   invalidates a `COPY` layer only when the copied content changes, so on an
@@ -170,15 +173,21 @@ style conventions are in separate documents:
   id, so no two builds share a value, not even the three in one
   `script/cibuild`. `script/lint`, `script/test`, `script/cibuild` and
   `script/docker` are the four, and there is no fifth — `script/check` runs the
-  two gate phases and `script/fmt-check`, and builds no image of its own. Every
+  two gate phases and `script/fmt-check`, and builds no image of its own. In
-  stage of the `Dockerfile` declares `ARG CACHEBUST` as its first line after
+  every stage of the `Dockerfile`, `ARG CACHEBUST` comes right after the steps
-  `FROM`, and a new value reruns every `RUN` step after that line; `COPY` steps
+  that install system packages and download Go's modules, or right after `FROM`
-  are compared by content and need nothing. A stage without the `ARG` line is
+  in a stage with neither, and every step that lints, tests or compiles comes
-  served from cache on an unchanged tree, so a repository re-vendors the scripts
+  after it. A step that installs anything else as well, such as a
-  and adds the `ARG` lines in one commit. The scripts do not pass `--no-cache`:
+  `script/bootstrap` that also installs node packages, comes after it too. A new
-  it reruns every step too, but it also empties every cache mount, and Go's
+  value reruns every `RUN` step after that line; `COPY` steps are compared by
-  build cache, which the canonical Go `Dockerfile` below mounts so that compiled
+  content and need nothing. Docker keeps the layers above the line until the
-  packages survive between check builds, would start empty every time. A bare
+  base image, their own lines or the files they copy change. A stage without the
+  `ARG` line is served from cache on an unchanged tree, so a repository
+  re-vendors the scripts and adds the `ARG` lines in one commit. The scripts do
+  not pass `--no-cache`: it reruns every step too, but it also builds the layers
+  above the line again and empties every cache mount, so Go's build cache and
+  golangci-lint's, which the canonical Go `Dockerfile` below mounts so that they
+  survive between check builds, would start empty every time. A bare
   `docker build .` is not evidence that anything ran: a sub-second build
   reporting success is a cache hit, not a result. Never invalidate by pruning —
   `docker builder prune` and friends destroy a build cache shared with every
@@ -194,19 +203,25 @@ style conventions are in separate documents:
   `Dockerfile`:
 
     ```dockerfile
-    # Every stage declares ARG CACHEBUST first: script/ passes a new value
+    # In every stage ARG CACHEBUST comes right after the steps that install
-    # on every build, which reruns every RUN step after it. The steps that
+    # system packages and download Go's modules, or right after FROM in a
-    # compile mount Go's build cache, which survives from build to build.
+    # stage with neither, and every step that lints, tests or compiles
+    # comes after it. script/ passes a new value on every build, which
+    # reruns every RUN step after it. Docker keeps the layers above it
+    # until the base image, their own lines or go.mod and go.sum change.
+    # The steps that compile mount Go's build cache, and golangci-lint run
+    # also mounts golangci-lint's; both survive from build to build.
 
     # Lint phase
     # golangci/golangci-lint:v2.x.x, YYYY-MM-DD
     FROM golangci/golangci-lint@sha256:... AS lint
-    ARG CACHEBUST
     WORKDIR /src
     COPY go.mod go.sum ./
     RUN go mod download
+    ARG CACHEBUST
     COPY . .
     RUN --mount=type=cache,target=/root/.cache/go-build \
+        --mount=type=cache,target=/root/.cache/golangci-lint \
         golangci-lint run --config .golangci.yml ./...
 
     # Test phase. -race needs cgo and so a C compiler, which the Debian Go
@@ -214,10 +229,10 @@ style conventions are in separate documents:
     # cache also holds test results, which go test would otherwise replay.
     # golang:1.x, YYYY-MM-DD
     FROM golang@sha256:... AS test
-    ARG CACHEBUST
     WORKDIR /src
     COPY go.mod go.sum ./
     RUN go mod download
+    ARG CACHEBUST
     COPY . .
     RUN --mount=type=cache,target=/root/.cache/go-build \
         go test -count=1 -timeout 90s -race -cover ./... || \
@@ -225,19 +240,19 @@ style conventions are in separate documents:
           go test -count=1 -timeout 90s -race -v ./...; exit 1; }
 
     # Build stage. Nothing is wanted from either phase above; the copies
-    # are what make BuildKit build them first, so this stage cannot run
+    # are what make BuildKit build them first, so nothing after them can
-    # unless lint and test passed.
+    # run unless lint and test passed.
     # golang:1.x-alpine, YYYY-MM-DD
     FROM golang@sha256:... AS builder
-    ARG CACHEBUST
-    COPY --from=lint /src/go.sum /dev/null
-    COPY --from=test /src/go.sum /dev/null
     RUN apk add --no-cache git
-    # A tar-stream context keeps the sender's file owners, which git refuses.
-    RUN git config --system --add safe.directory /src
     WORKDIR /src
     COPY go.mod go.sum ./
     RUN go mod download
+    ARG CACHEBUST
+    COPY --from=lint /src/go.sum /dev/null
+    COPY --from=test /src/go.sum /dev/null
+    # A tar-stream context keeps the sender's file owners, which git refuses.
+    RUN git config --system --add safe.directory /src
     COPY . .
 
     # The VERSION build arg when one is given, otherwise
@@ -274,16 +289,26 @@ style conventions are in separate documents:
     - Keep the runtime stage last, and if you add a stage after it, give it the
       same two copies. A plain `docker build .` builds the last stage's chain
       and nothing else.
-    - Only Go's build cache is mounted (`/root/.cache/go-build`, where Go keeps
+    - Check builds keep these, and none of them can hand back a stale result:
-      it for root), and only on the steps that compile: `golangci-lint run`,
+        - The layers above `ARG CACHEBUST`: the system packages and Go's
-      both `go test` runs and `go build`. It keeps compiled packages between
+          modules. Docker installs the packages again when the base image or the
-      check builds, and Go reuses one only when all its inputs are unchanged, so
+          install line changes, and downloads the modules again when `go.mod` or
-      it saves the compile without skipping a check. Nothing else is kept:
+          `go.sum` changes. Go checks every module against `go.sum` as it
-      golangci-lint's own cache would replay stored findings in place of a lint
+          downloads it, and no later build can change a kept layer. The modules
-      run, and Go builds from whatever files a kept module cache holds without
+          are kept in a layer rather than a cache mount because a cache mount is
-      checking them again (only `go mod verify` does), so a file changed there
+          shared by every build on the host, and Go builds from whatever files
-      would go unnoticed. Go's build cache also holds test results, which is why
+          it holds without checking them again (only `go mod verify` does), so a
-      both `go test` runs carry `-count=1`.
+          file changed there would go unnoticed.
+        - Go's build cache (`/root/.cache/go-build`, where Go keeps it for
+          root), mounted on the steps that compile: `golangci-lint run`, both
+          `go test` runs and `go build`. Go reuses a compiled package only when
+          all its inputs are unchanged. The cache also holds test results, which
+          is why both `go test` runs carry `-count=1`.
+        - golangci-lint's cache (`/root/.cache/golangci-lint`, where the pinned
+          image keeps it), mounted on `golangci-lint run`. golangci-lint reuses
+          a package's findings only when the package's files and those of every
+          package it imports, `go.mod`, the linters and their settings in
+          `.golangci.yml`, and the golangci-lint version are all unchanged.
     - If the project uses `//go:embed` directives that reference build artifacts
       (e.g. a web frontend compiled in a separate stage), the lint phase must
       create placeholder files so the embed directives resolve. Example:
@@ -292,12 +317,20 @@ style conventions are in separate documents:
       in the lint phase. The `golangci/golangci-lint` image is Debian-based and
       has no `apk`, so install with `apt-get` under the Debian package name
       (`libvips-dev`, where alpine says `vips-dev`), and delete the package
-      lists in the same `RUN`, so the layer does not keep them:
+      lists in the same `RUN`, so the layer does not keep them. The test phase,
+      on the Debian Go image, installs the libraries its tests need the same
+      way. Either install goes right after `FROM`, above `ARG CACHEBUST`, so
+      Docker keeps it until the base image or the install line changes:
 
         ```dockerfile
+        FROM golangci/golangci-lint@sha256:... AS lint
         RUN apt-get update \
             && apt-get install -y --no-install-recommends libvips-dev \
             && rm -rf /var/lib/apt/lists/*
+        WORKDIR /src
+        COPY go.mod go.sum ./
+        RUN go mod download
+        ARG CACHEBUST
         ```
 
     - `.dockerignore` lets `.git` into the build context. It keeps out every git

TODO.md

@@ -30,6 +30,38 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-20 a WebP, AVIF or JPEG XL encode at the limits takes at most about 34
+  seconds (closes #272): with one libvips thread, 8192x8192 random pixels with
+  an alpha channel took about 61 seconds to encode as WebP, 81 as JPEG XL and,
+  at quality 85, 96 as AVIF, so with the default `downstream_timeout` of 60
+  seconds such an image was never delivered, yet its encode held a processing
+  slot to the end. WebP is now saved at effort 2 and JPEG XL at effort 4, which
+  take up to about 23 and 27 seconds at any quality. AVIF was already at the
+  lowest effort govips can set, so an AVIF output of more than 16,777,216 pixels
+  is refused with 400 before anything is decoded; 4096x4096, 8192x2048 or
+  2048x8192 takes up to about 34 seconds at any quality. Resizing with
+  `fit=cover` already decodes the input, so the output's dimensions are worked
+  out before resizing. `README.md` gives the limit and the new times.
+- 2026-10-20 a source whose first bytes do not match its `Content-Type` answers
+  415 (closes #253): a body whose first bytes are not those of the format its
+  `Content-Type` names, such as a JPEG sent as `image/png`, text, or a body
+  under 12 bytes, answered 500 on both image routes and was logged as an error
+  of pixa's, and each request for it fetched the whole body again. Both routes
+  now answer 415, and the image service logs it at warning level as a refused
+  source. It also gets a negative cache entry, as an upstream error status does,
+  so the source URL is not fetched again for 5 minutes, and meanwhile an image
+  that would need that fetch answers 502.
+- 2026-10-20 `AGENTS.md`: a bug fix may change an existing test without asking
+  when that test pins exactly the behavior the filed bug calls wrong, and the
+  change moves it to the fixed behavior and nothing else (closes #280).
+- 2026-10-18 re-vendored from `sneak/prompts` commit `0653579` (closes #221):
+  each `Dockerfile` stage installs its system packages and downloads the Go
+  modules above `ARG CACHEBUST`, so check builds keep them, and the lint step
+  keeps golangci-lint's cache.
+- 2026-10-19 a `trusted_proxies` entry in the IPv4-mapped form trusted nothing;
+  it is now read as the IPv4 range it covers by the same code that reads
+  `blocked_networks`, and such an entry shorter than `/96` aborts startup
+  (closes #303).
 - 2026-10-18 the next reconciliation pass waits at least ten times as long as
   the last one took, so passes on a large cache no longer run back to back
   (closes #250).

internal/config/config.go

@@ -272,7 +272,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		return nil, err
 	}
 
-	blockedNetworks, err := parseBlockedNetworks(sc)
+	blockedNetworks, err := parseCIDRList(sc, keyBlockedNetworks)
 	if err != nil {
 		return nil, err
 	}
@@ -1249,6 +1249,13 @@ func defaultTrustedProxies() []netip.Prefix {
 // wrong type, an empty entry, a non-string entry, or an unparseable CIDR
 // aborts startup naming the key and the offending value; the default
 // (an empty list) applies only to an omitted key.
+//
+// An entry written in the IPv4-mapped form (::ffff:a.b.c.d) is kept as the
+// IPv4 range it covers: ::ffff:10.0.0.0/104 is kept as 10.0.0.0/8. Both
+// blocked_networks and trusted_proxies are matched against an IPv4-mapped
+// address as the IPv4 address it holds, so the entry as written would match
+// nothing. Such an entry shorter than /96 covers more than the IPv4
+// addresses and aborts startup.
 func parseCIDRList(sc *smartconfig.Config, key string) ([]netip.Prefix, error) {
 	raw, ok := lookupValue(sc, key)
 	if !ok {
@@ -1264,6 +1271,10 @@ func parseCIDRList(sc *smartconfig.Config, key string) ([]netip.Prefix, error) {
 		return nil, err
 	}
 
+	// An IPv4-mapped address is the 96 bits of ::ffff:0:0/96 followed by
+	// the 32 bits of the IPv4 address it holds.
+	const mappedPrefixBits = 96
+
 	prefixes := make([]netip.Prefix, 0, len(entries))
 
 	for _, entry := range entries {
@@ -1273,40 +1284,17 @@ func parseCIDRList(sc *smartconfig.Config, key string) ([]netip.Prefix, error) {
 				settingName(key), entry, errNotAValidCIDR)
 		}
 
-		prefixes = append(prefixes, prefix)
+		if prefix.Addr().Is4In6() && prefix.Bits() < mappedPrefixBits {
-	}
-
-	return prefixes, nil
-}
-
-// parseBlockedNetworks parses blocked_networks with parseCIDRList, and keeps
-// an entry written in the IPv4-mapped form (::ffff:a.b.c.d) as the IPv4
-// range it covers: ::ffff:10.0.0.0/104 is kept as 10.0.0.0/8. The fetcher
-// matches an IPv4-mapped address as the IPv4 address it holds, so the entry
-// as written would match nothing. Such an entry shorter than /96 covers more
-// than the IPv4 addresses and aborts startup.
-func parseBlockedNetworks(sc *smartconfig.Config) ([]netip.Prefix, error) {
-	prefixes, err := parseCIDRList(sc, keyBlockedNetworks)
-	if err != nil {
-		return nil, err
-	}
-
-	// An IPv4-mapped address is the 96 bits of ::ffff:0:0/96 followed by
-	// the 32 bits of the IPv4 address it holds.
-	const mappedPrefixBits = 96
-
-	for i, prefix := range prefixes {
-		if !prefix.Addr().Is4In6() {
-			continue
-		}
-
-		if prefix.Bits() < mappedPrefixBits {
 			return nil, fmt.Errorf("%s: value %q is %w",
-				settingName(keyBlockedNetworks), prefix, errMappedTooShort)
+				settingName(key), entry, errMappedTooShort)
 		}
 
-		prefixes[i] = netip.PrefixFrom(prefix.Addr().Unmap(),
+		if prefix.Addr().Is4In6() {
-			prefix.Bits()-mappedPrefixBits)
+			prefix = netip.PrefixFrom(prefix.Addr().Unmap(),
+				prefix.Bits()-mappedPrefixBits)
+		}
+
+		prefixes = append(prefixes, prefix)
 	}
 
 	return prefixes, nil

internal/config/trusted_proxies_internal_test.go

@@ -83,3 +83,41 @@ func TestTrustedProxiesInvalidAbortsStartup(t *testing.T) {
 		},
 	})
 }
+
+// TestTrustedProxiesMappedEntryIsIPv4Range checks that an entry written in
+// the IPv4-mapped form (::ffff:a.b.c.d) is kept as the IPv4 range it covers,
+// since the client address is matched as the IPv4 address it holds. /96, the
+// shortest such entry, covers every IPv4 address.
+func TestTrustedProxiesMappedEntryIsIPv4Range(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine+
+		`trusted_proxies: ["::ffff:10.0.0.0/104", "::ffff:0:0/96"]`+"\n")
+	if err != nil {
+		t.Fatalf("IPv4-mapped trusted_proxies entries should load: %v", err)
+	}
+
+	got := make([]string, len(c.TrustedProxies))
+	for i, p := range c.TrustedProxies {
+		got[i] = p.String()
+	}
+
+	if joined := strings.Join(got, ","); joined != "10.0.0.0/8,0.0.0.0/0" {
+		t.Errorf("TrustedProxies = %v, want 10.0.0.0/8 and 0.0.0.0/0", got)
+	}
+}
+
+// TestTrustedProxiesMappedEntryShorterThan96AbortsStartup checks that an
+// entry in the IPv4-mapped form shorter than /96, which covers more than the
+// IPv4 addresses, aborts startup with an error naming the key and the value.
+func TestTrustedProxiesMappedEntryShorterThan96AbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	runAbortCases(t, []abortCase{
+		{
+			name:              "mapped-95",
+			yaml:              signingKeyLine + `trusted_proxies: ["::ffff:0:0/95"]` + "\n",
+			wantErrSubstrings: []string{keyTrustedProxies, "::ffff:0:0/95"},
+		},
+	})
+}

internal/handlers/avif_too_large_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/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// TestAVIFOutputTooLargeAnswers400 checks that respondImageError and
+// handleImageError, which answer the errors of the two image routes, answer
+// 400 naming the AVIF pixel limit when the image processor refuses an AVIF
+// output above it, wrapped as the service wraps it. The request's source only
+// goes into a log line, so it is left empty.
+func TestAVIFOutputTooLargeAnswers400(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	err := fmt.Errorf("image processing failed: %w: 4097x4096",
+		imageprocessor.ErrAVIFOutputTooLarge)
+	wantBody := fmt.Sprintf("above %d pixels", imageprocessor.MaxAVIFOutputPixels)
+
+	plain := httptest.NewRecorder()
+	h.respondImageError(plain, &imgcache.ImageRequest{}, err)
+
+	encrypted := httptest.NewRecorder()
+	h.handleImageError(encrypted, err)
+
+	for name, rec := range map[string]*httptest.ResponseRecorder{
+		"respondImageError": plain, "handleImageError": encrypted,
+	} {
+		t.Logf("%s: status = %d, body = %q", name, rec.Code, rec.Body.String())
+
+		if rec.Code != http.StatusBadRequest {
+			t.Errorf("%s: status = %d, want %d",
+				name, rec.Code, http.StatusBadRequest)
+		}
+
+		if !strings.Contains(rec.Body.String(), wantBody) {
+			t.Errorf("%s: body = %q, want it to say %q",
+				name, rec.Body.String(), wantBody)
+		}
+	}
+}

internal/handlers/first_bytes_internal_test.go

@@ -0,0 +1,180 @@
+package handlers
+
+import (
+	"bytes"
+	"encoding/json"
+	"image/color"
+	"log/slog"
+	"net/http"
+	"slices"
+	"testing"
+	"testing/fstest"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// mismatchedSourcePath is the path of the source that
+// TestFirstBytesNotMatchingContentType serves on allowlistedHost. The mock
+// fetcher names a file's Content-Type from its extension, so the source is
+// sent as image/png.
+const mismatchedSourcePath = "/images/source.png"
+
+// newRoutesFetchingThrough returns both image routes, and the Handlers behind
+// them, whose image service fetches through fetcher and has allowlistedHost on
+// its allowlist. The handlers and the image service log to log.
+func newRoutesFetchingThrough(
+	t *testing.T, fetcher httpfetcher.Fetcher, log *slog.Logger,
+) (*Handlers, http.Handler) {
+	t.Helper()
+
+	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.CacheConfig{
+		StateDir:    t.TempDir(),
+		NegativeTTL: 5 * time.Minute,
+	})
+	if err != nil {
+		t.Fatalf("imgcache.NewCache() error = %v", err)
+	}
+
+	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
+		Cache:      cache,
+		Fetcher:    fetcher,
+		SigningKey: testSigningKey,
+		Allowlist:  []string{allowlistedHost},
+		Logger:     log,
+	})
+	if err != nil {
+		t.Fatalf("imgcache.NewService() error = %v", err)
+	}
+
+	encGen, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator() error = %v", err)
+	}
+
+	h := &Handlers{log: log, imgSvc: svc, encGen: encGen}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+	r.Get("/v1/e/{token}/*", h.HandleImageEnc())
+
+	return h, r
+}
+
+// mismatchedSourcePlainURL returns the /v1/image/ URL of mismatchedSourcePath
+// on allowlistedHost at 50x50 as a JPEG.
+func mismatchedSourcePlainURL(*testing.T, *Handlers) string {
+	return "/v1/image/" + allowlistedHost + mismatchedSourcePath + "/50x50.jpeg"
+}
+
+// mismatchedSourceEncURL returns an encrypted /v1/e/ URL, made with h's
+// generator, of mismatchedSourcePath on allowlistedHost at 50x50 as a JPEG.
+func mismatchedSourceEncURL(t *testing.T, h *Handlers) string {
+	t.Helper()
+
+	token, err := h.encGen.Generate(&encurl.Payload{
+		SourceHost: allowlistedHost,
+		SourcePath: mismatchedSourcePath,
+		Width:      50,
+		Height:     50,
+		Format:     imgcache.FormatJPEG,
+	})
+	if err != nil {
+		t.Fatalf("Generate() error = %v", err)
+	}
+
+	return "/v1/e/" + token + "/img.jpg"
+}
+
+// loggedLevels reads the JSON log lines in logs and returns the messages
+// logged at each level.
+func loggedLevels(t *testing.T, logs *bytes.Buffer) map[string][]string {
+	t.Helper()
+
+	t.Logf("logged:\n%s", logs.String())
+
+	messages := make(map[string][]string)
+
+	dec := json.NewDecoder(logs)
+	for dec.More() {
+		var line struct {
+			Level string `json:"level"`
+			Msg   string `json:"msg"`
+		}
+
+		err := dec.Decode(&line)
+		if err != nil {
+			t.Fatalf("decoding log line: %v", err)
+		}
+
+		messages[line.Level] = append(messages[line.Level], line.Msg)
+	}
+
+	return messages
+}
+
+// TestFirstBytesNotMatchingContentType checks that both image routes answer
+// 415 for an upstream body whose first bytes are not those of the format its
+// Content-Type names, and log it as a refused source, not as an error of
+// pixa's. The next request for that source must answer 502 without fetching
+// it again, as after an upstream error status.
+func TestFirstBytesNotMatchingContentType(t *testing.T) {
+	t.Parallel()
+
+	bodies := map[string][]byte{
+		"JPEG sent as image/png":          generateTestJPEG(t, 10, 10, color.Black),
+		"text sent as image/png":          []byte("This is just text, not a PNG"),
+		"body too short to name a format": []byte("short"),
+	}
+
+	// The source's URL on each image route.
+	sourceURLs := map[string]func(*testing.T, *Handlers) string{
+		"plain URL":     mismatchedSourcePlainURL,
+		"encrypted URL": mismatchedSourceEncURL,
+	}
+
+	for bodyName, body := range bodies {
+		for urlName, sourceURL := range sourceURLs {
+			t.Run(urlName+", "+bodyName, func(t *testing.T) {
+				t.Parallel()
+
+				fetcher := &countingFetcher{
+					Fetcher: httpfetcher.NewMock(fstest.MapFS{
+						allowlistedHost + mismatchedSourcePath: &fstest.MapFile{
+							Data: body,
+						},
+					}),
+				}
+
+				var logs bytes.Buffer
+
+				h, routes := newRoutesFetchingThrough(t, fetcher,
+					slog.New(slog.NewJSONHandler(&logs, nil)))
+				target := sourceURL(t, h)
+
+				checkErrorBody(t, sendGet(t, routes, target),
+					http.StatusUnsupportedMediaType, "unsupported source format")
+
+				levels := loggedLevels(t, &logs)
+				if len(levels[slog.LevelError.String()]) != 0 {
+					t.Errorf("logged at error level: %q",
+						levels[slog.LevelError.String()])
+				}
+
+				if !slices.Contains(levels[slog.LevelWarn.String()], "source refused") {
+					t.Error(`no "source refused" line logged at warning level`)
+				}
+
+				checkErrorBody(t, sendGet(t, routes, target),
+					http.StatusBadGateway, "upstream error")
+
+				if n := fetcher.fetches.Load(); n != 1 {
+					t.Errorf("upstream fetched %d times, want 1", n)
+				}
+			})
+		}
+	}
+}

internal/handlers/image.go

@@ -14,6 +14,7 @@ import (
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/magic"
 )
 
 // HandleImage handles the main image proxy route:
@@ -220,6 +221,18 @@ func (s *Handlers) HandleTooManyRequests() http.HandlerFunc {
 func (s *Handlers) respondImageError(
 	w http.ResponseWriter, req *imgcache.ImageRequest, err error,
 ) {
+	// An upstream body whose first bytes are not those of the format its
+	// Content-Type names is the upstream's error, not pixa's: the image
+	// service has logged it as a refused source.
+	if errors.Is(err, magic.ErrMagicByteMismatch) ||
+		errors.Is(err, magic.ErrUnknownFormat) ||
+		errors.Is(err, magic.ErrNotEnoughData) {
+		s.respondError(w, "unsupported source format",
+			http.StatusUnsupportedMediaType)
+
+		return
+	}
+
 	s.log.Error("failed to get image",
 		"host", req.SourceHost,
 		"path", req.SourcePath,
@@ -264,6 +277,14 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, imageprocessor.ErrAVIFOutputTooLarge) {
+		s.respondError(w, fmt.Sprintf("size too large for AVIF: the output "+
+			"would be above %d pixels", imageprocessor.MaxAVIFOutputPixels),
+			http.StatusBadRequest)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 

internal/handlers/imageenc.go

@@ -15,6 +15,7 @@ import (
 	"sneak.berlin/go/pixa/internal/httpfetcher"
 	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/imgcache"
+	"sneak.berlin/go/pixa/internal/magic"
 )
 
 // HandleImageEnc handles requests to /v1/e/{token}/* for encrypted
@@ -163,13 +164,20 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 		s.respondError(w, "server busy, try again later",
 			http.StatusServiceUnavailable)
 	case errors.Is(err, httpfetcher.ErrInvalidContentType),
-		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat):
+		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat),
+		errors.Is(err, magic.ErrMagicByteMismatch),
+		errors.Is(err, magic.ErrUnknownFormat),
+		errors.Is(err, magic.ErrNotEnoughData):
 		s.respondError(w, "unsupported source format",
 			http.StatusUnsupportedMediaType)
 	case errors.Is(err, imageprocessor.ErrOutputTooLarge):
 		s.respondError(w, fmt.Sprintf("size too large for the source: the side "+
 			"computed from its proportions is above %d",
 			imageprocessor.MaxOutputDimension), http.StatusBadRequest)
+	case errors.Is(err, imageprocessor.ErrAVIFOutputTooLarge):
+		s.respondError(w, fmt.Sprintf("size too large for AVIF: the output "+
+			"would be above %d pixels", imageprocessor.MaxAVIFOutputPixels),
+			http.StatusBadRequest)
 	default:
 		s.log.Error("image request failed", "error", err)
 		s.respondError(w, "internal error", http.StatusInternalServerError)

internal/imageprocessor/avif_too_large_internal_test.go

@@ -0,0 +1,84 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"os"
+	"testing"
+)
+
+// TestImageProcessor_RefusesAVIFAboveMaxPixels verifies that an AVIF output
+// of more than MaxAVIFOutputPixels is refused with ErrAVIFOutputTooLarge
+// before anything is decoded, whatever size and fit give it, the source's own
+// size included, and that an AVIF output of at most that many pixels, or one
+// in another format, is not. One input is a 64x32 PNG cut off in its pixel
+// data. The other is testdata/red.avif, a 4x4 AVIF, with the size in its
+// header changed to 4097x4096. libvips reads each input's size from its
+// header but fails when it decodes its pixel data, so a request that gets
+// that failure was not refused, and a refusal came before any decoding.
+func TestImageProcessor_RefusesAVIFAboveMaxPixels(t *testing.T) {
+	t.Parallel()
+
+	whole := createTestPNG(t, 64, 32)
+	cutOff := whole[:len(whole)/2]
+
+	largeAVIF, err := os.ReadFile("testdata/red.avif")
+	if err != nil {
+		t.Fatalf("failed to read test AVIF: %v", err)
+	}
+
+	// The ispe box gives the width and then the height, 4 bytes each, after
+	// the box's type and its 4 bytes of version and flags
+	ispe := bytes.Index(largeAVIF, []byte("ispe"))
+	binary.BigEndian.PutUint32(largeAVIF[ispe+8:], 4097)
+	binary.BigEndian.PutUint32(largeAVIF[ispe+12:], 4096)
+
+	tests := []struct {
+		name          string
+		input         []byte
+		format        Format
+		width, height int
+		fit           FitMode
+		refused       bool
+	}{
+		{"AVIF at the limit", cutOff, FormatAVIF, 4096, 4096, FitCover, false},
+		{"AVIF above the limit", cutOff, FormatAVIF, 4096, 4097, FitCover, true},
+		{"WebP above the AVIF limit", cutOff, FormatWebP, 4096, 4097, FitCover, false},
+		// FitInside does not enlarge, so the output stays 64x32
+		{"AVIF inside a larger size", cutOff, FormatAVIF, 8192, 8192, FitInside, false},
+		// FitOutside scales 64x32 by 128 to cover 8192x1: 8192x4096
+		{"AVIF outside a smaller size", cutOff, FormatAVIF, 8192, 1, FitOutside, true},
+		// With no size, the output keeps the source's 4097x4096
+		{"AVIF of the source's size", largeAVIF, FormatAVIF, 0, 0, FitCover, true},
+		{"orig of an AVIF source", largeAVIF, FormatOriginal, 0, 0, FitCover, true},
+		{"JPEG of the source's size", largeAVIF, FormatJPEG, 0, 0, FitCover, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := New(Params{}).Process(t.Context(), bytes.NewReader(tt.input),
+				&Request{
+					Size:    Size{Width: tt.width, Height: tt.height},
+					Format:  tt.format,
+					FitMode: tt.fit,
+				})
+			t.Logf("Process() error = %v", err)
+
+			if tt.refused {
+				if !errors.Is(err, ErrAVIFOutputTooLarge) {
+					t.Errorf("Process() error = %v, want %v", err, ErrAVIFOutputTooLarge)
+				}
+
+				return
+			}
+
+			if err == nil || errors.Is(err, ErrAVIFOutputTooLarge) {
+				t.Errorf("Process() error = %v, want the failure to decode "+
+					"the input", err)
+			}
+		})
+	}
+}

internal/imageprocessor/encoder_settings_internal_test.go

@@ -77,10 +77,10 @@ func TestImageProcessor_PNGIsCompressed(t *testing.T) {
 	}
 }
 
-// TestImageProcessor_WebPAtDefaultEffort verifies that WebP is saved at
+// TestImageProcessor_WebPAtEffort2 verifies that WebP is saved at effort 2,
-// libvips' default effort, 4: the output is the size of the same image saved
+// not libvips' default of 4: the output is the size of the same image saved
-// at that effort.
+// at effort 2.
-func TestImageProcessor_WebPAtDefaultEffort(t *testing.T) {
+func TestImageProcessor_WebPAtEffort2(t *testing.T) {
 	t.Parallel()
 
 	input := createTestJPEG(t, 200, 150)
@@ -90,14 +90,14 @@ func TestImageProcessor_WebPAtDefaultEffort(t *testing.T) {
 	want, _, err := decode(t, input).ExportWebp(&vips.WebpExportParams{
 		StripMetadata:   true,
 		Quality:         85,
-		ReductionEffort: 4,
+		ReductionEffort: 2,
 	})
 	if err != nil {
 		t.Fatalf("ExportWebp() error = %v", err)
 	}
 
 	if size != int64(len(want)) {
-		t.Errorf("WebP output is %d bytes, want %d, the size at effort 4",
+		t.Errorf("WebP output is %d bytes, want %d, the size at effort 2",
 			size, len(want))
 	}
 }
@@ -143,3 +143,29 @@ func TestImageProcessor_AVIFAtEffort1(t *testing.T) {
 			"and 8 bits", size, len(want))
 	}
 }
+
+// TestImageProcessor_JPEGXLAtEffort4 verifies that JPEG XL is saved at
+// effort 4, not libvips' default of 7: the output is the size of the same
+// image saved at effort 4, with the distance exportJXL gives quality 85.
+func TestImageProcessor_JPEGXLAtEffort4(t *testing.T) {
+	t.Parallel()
+
+	input := createTestJPEG(t, 200, 150)
+
+	size := processedSize(t, input, &Request{Format: FormatJXL, Quality: 85})
+
+	params := vips.NewJxlExportParams()
+	params.Quality = 85
+	params.Distance = jxlDistance(85)
+	params.Effort = 4
+
+	want, _, err := decode(t, input).ExportJxl(params)
+	if err != nil {
+		t.Fatalf("ExportJxl() error = %v", err)
+	}
+
+	if size != int64(len(want)) {
+		t.Errorf("JPEG XL output is %d bytes, want %d, the size at effort 4",
+			size, len(want))
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -142,6 +142,19 @@ const MaxOutputDimension = 8192
 // requested size.
 var ErrOutputTooLarge = errors.New("output image dimensions exceed maximum")
 
+// MaxAVIFOutputPixels is the most pixels an AVIF output may have, so that an
+// encode takes at most 40 seconds of the default downstream_timeout of 60
+// seconds; AVIF is already at the lowest effort govips can set (avifEffort).
+// With one thread, as pixad runs libvips, saving 4096x4096, 8192x2048 or
+// 2048x8192 random pixels with an alpha channel takes up to about 34
+// seconds, the slowest at any quality and at 8 or 16 bits per sample, and
+// 5120x4096 at 8 bits already takes about 36.
+const MaxAVIFOutputPixels = 4096 * 4096
+
+// ErrAVIFOutputTooLarge is returned, before anything is decoded, when an AVIF
+// output would have more than MaxAVIFOutputPixels pixels.
+var ErrAVIFOutputTooLarge = errors.New("AVIF output has too many pixels")
+
 // ErrUnsupportedOutputFormat is returned when the requested output format is
 // not supported.
 var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
@@ -210,7 +223,8 @@ func New(params Params) *ImageProcessor {
 // ErrTooManyImages. When ctx has ended once Process has a slot, or before it
 // encodes, it returns ctx's error and gives the slot back. govips gives pixa
 // no way to stop libvips once an encode has begun, so an encode keeps its
-// slot until it finishes.
+// slot until it finishes. An AVIF output of more than MaxAVIFOutputPixels is
+// refused with ErrAVIFOutputTooLarge before anything is decoded.
 func (p *ImageProcessor) Process(
 	ctx context.Context,
 	input io.Reader,
@@ -223,18 +237,9 @@ func (p *ImageProcessor) Process(
 
 	defer release()
 
-	// Read input with a size limit to prevent unbounded memory consumption.
+	data, err := p.readInput(input)
-	// We read at most maxInputBytes+1 so we can detect if the input exceeds
-	// the limit without consuming additional memory.
-	limited := io.LimitReader(input, p.maxInputBytes+1)
-
-	data, err := io.ReadAll(limited)
 	if err != nil {
-		return nil, fmt.Errorf("failed to read input: %w", err)
+		return nil, err
-	}
-
-	if int64(len(data)) > p.maxInputBytes {
-		return nil, ErrInputDataTooLarge
 	}
 
 	// govips picks the libvips loader from the input's first bytes with
@@ -277,20 +282,28 @@ func (p *ImageProcessor) Process(
 		return nil, err
 	}
 
-	// Resize if needed
-	if targetWidth != origWidth || targetHeight != origHeight {
-		err := p.resize(img, targetWidth, targetHeight, req.FitMode)
-		if err != nil {
-			return nil, fmt.Errorf("failed to resize: %w", err)
-		}
-	}
-
 	// orig is the source's own format; encode refuses an empty format
 	outputFormat := req.Format
 	if outputFormat == FormatOriginal {
 		outputFormat = sourceFormat
 	}
 
+	// Determine output dimensions; too large ones are refused before
+	// resizing, which with FitCover already decodes the input
+	outputWidth, outputHeight, err := outputDimensions(origWidth, origHeight,
+		targetWidth, targetHeight, req.FitMode, outputFormat)
+	if err != nil {
+		return nil, err
+	}
+
+	// Resize if needed
+	if targetWidth != origWidth || targetHeight != origHeight {
+		err := p.resize(img, outputWidth, outputHeight, req.FitMode)
+		if err != nil {
+			return nil, fmt.Errorf("failed to resize: %w", err)
+		}
+	}
+
 	// Encode to target format
 	output, err := p.encode(ctx, img, outputFormat, req.Quality)
 	if err != nil {
@@ -445,6 +458,23 @@ func (p *ImageProcessor) acquireSlot(ctx context.Context) (func(), error) {
 	return release, nil
 }
 
+// readInput reads input with a size limit to prevent unbounded memory
+// consumption, refusing more than maxInputBytes with ErrInputDataTooLarge. It
+// reads at most maxInputBytes+1 so it can detect if the input exceeds the
+// limit without consuming additional memory.
+func (p *ImageProcessor) readInput(input io.Reader) ([]byte, error) {
+	data, err := io.ReadAll(io.LimitReader(input, p.maxInputBytes+1))
+	if err != nil {
+		return nil, fmt.Errorf("failed to read input: %w", err)
+	}
+
+	if int64(len(data)) > p.maxInputBytes {
+		return nil, ErrInputDataTooLarge
+	}
+
+	return data, nil
+}
+
 // detectFormat returns the format string for a govips image type.
 func (p *ImageProcessor) detectFormat(imageType vips.ImageType) string {
 	switch imageType {
@@ -469,9 +499,67 @@ func (p *ImageProcessor) detectFormat(imageType vips.ImageType) string {
 	}
 }
 
-// resize resizes the image according to the fit mode. With FitOutside one
+// outputDimensions returns the dimensions resize gives an image of imgWidth x
-// side can come out larger than requested; when it would be above
+// imgHeight for the target dimensions and fit mode, without resizing it, and
-// MaxOutputDimension, resize returns ErrOutputTooLarge without resizing.
+// refuses an output too large for format. With FitOutside one side can come
+// out larger than the target; when it would be above MaxOutputDimension,
+// outputDimensions returns ErrOutputTooLarge. An AVIF output of more than
+// MaxAVIFOutputPixels is ErrAVIFOutputTooLarge.
+func outputDimensions(
+	imgWidth, imgHeight, width, height int, fit FitMode, format Format,
+) (int, int, error) {
+	scaleW := float64(width) / float64(imgWidth)
+	scaleH := float64(height) / float64(imgHeight)
+	outputWidth, outputHeight := width, height
+
+	switch fit {
+	case FitCover, FitFill, "":
+		// Cover crops, and fill stretches, to exactly the target dimensions
+
+	case FitContain:
+		// Fit within the target dimensions, maintaining aspect ratio. Scaled
+		// by the smaller scale, the short side of a thin image can round
+		// down to 0, which libvips refuses as a width, so each side is at
+		// least 1.
+		scale := min(scaleW, scaleH)
+		outputWidth = max(1, int(float64(imgWidth)*scale))
+		outputHeight = max(1, int(float64(imgHeight)*scale))
+
+	case FitInside:
+		// Same as contain, but only shrink
+		scale := min(scaleW, scaleH, 1)
+		outputWidth = max(1, int(float64(imgWidth)*scale))
+		outputHeight = max(1, int(float64(imgHeight)*scale))
+
+	case FitOutside:
+		// The smallest dimension fits, the other may exceed the target. A
+		// side scaled to 1 can come out just under 1 in floating point, as
+		// 49 times 1/49 does, and round down to 0, which libvips refuses as
+		// a width, so each side is at least 1.
+		scale := max(scaleW, scaleH)
+		outputWidth = max(1, int(float64(imgWidth)*scale))
+		outputHeight = max(1, int(float64(imgHeight)*scale))
+
+	default:
+		return 0, 0, fmt.Errorf("%w: %s", ErrInvalidFitMode, fit)
+	}
+
+	// The side FitOutside scales past the target is held to the maximum too
+	if max(outputWidth, outputHeight) > MaxOutputDimension {
+		return 0, 0, fmt.Errorf("%w: %dx%d is above %d", ErrOutputTooLarge,
+			outputWidth, outputHeight, MaxOutputDimension)
+	}
+
+	if format == FormatAVIF && outputWidth*outputHeight > MaxAVIFOutputPixels {
+		return 0, 0, fmt.Errorf("%w: %dx%d is above %d pixels",
+			ErrAVIFOutputTooLarge, outputWidth, outputHeight, MaxAVIFOutputPixels)
+	}
+
+	return outputWidth, outputHeight, nil
+}
+
+// resize resizes the image to width x height, the dimensions
+// outputDimensions gives for the fit mode.
 func (p *ImageProcessor) resize(
 	img *vips.ImageRef, width, height int, fit FitMode,
 ) error {
@@ -480,58 +568,20 @@ func (p *ImageProcessor) resize(
 		// Crop the centre and resize to fill exact dimensions (default)
 		return cropCentreAndResize(img, width, height)
 
-	case FitContain:
-		// Resize to fit within dimensions, maintaining aspect ratio. Scaled
-		// by the smaller scale, the short side of a thin image can round
-		// down to 0, which libvips refuses as a width, so each side is at
-		// least 1.
-		imgW, imgH := img.Width(), img.Height()
-		scaleW := float64(width) / float64(imgW)
-		scaleH := float64(height) / float64(imgH)
-		scale := min(scaleW, scaleH)
-		newW := max(1, int(float64(imgW)*scale))
-		newH := max(1, int(float64(imgH)*scale))
-
-		return img.Thumbnail(newW, newH, vips.InterestingNone)
-
 	case FitFill:
 		// Resize to exact dimensions (may distort)
 		return img.ThumbnailWithSize(width, height, vips.InterestingNone, vips.SizeForce)
 
 	case FitInside:
-		// Same as contain, but only shrink
+		// outputDimensions keeps the dimensions of an image that already fits
-		if img.Width() <= width && img.Height() <= height {
+		if img.Width() == width && img.Height() == height {
-			return nil // Already fits
+			return nil
 		}
 
-		imgW, imgH := img.Width(), img.Height()
+		return img.Thumbnail(width, height, vips.InterestingNone)
-		scaleW := float64(width) / float64(imgW)
-		scaleH := float64(height) / float64(imgH)
-		scale := min(scaleW, scaleH)
-		newW := max(1, int(float64(imgW)*scale))
-		newH := max(1, int(float64(imgH)*scale))
 
-		return img.Thumbnail(newW, newH, vips.InterestingNone)
+	case FitContain, FitOutside:
-
+		return img.Thumbnail(width, height, vips.InterestingNone)
-	case FitOutside:
-		// Resize so smallest dimension fits, may exceed target on other
-		// dimension. A side scaled to 1 can come out just under 1 in
-		// floating point, as 49 times 1/49 does, and round down to 0, which
-		// libvips refuses as a width, so each side is at least 1.
-		imgW, imgH := img.Width(), img.Height()
-		scaleW := float64(width) / float64(imgW)
-		scaleH := float64(height) / float64(imgH)
-		scale := max(scaleW, scaleH)
-		newW := max(1, int(float64(imgW)*scale))
-		newH := max(1, int(float64(imgH)*scale))
-
-		// The dimension that exceeds the target is held to the maximum too
-		if newW > MaxOutputDimension || newH > MaxOutputDimension {
-			return fmt.Errorf("%w: %dx%d is above %d", ErrOutputTooLarge,
-				newW, newH, MaxOutputDimension)
-		}
-
-		return img.Thumbnail(newW, newH, vips.InterestingNone)
 
 	default:
 		return fmt.Errorf("%w: %s", ErrInvalidFitMode, fit)
@@ -667,11 +717,16 @@ func exportGIF(img *vips.ImageRef) ([]byte, error) {
 	return output, err
 }
 
-// webpEffort is libvips' default WebP effort, from 0 (fastest) to 6.
+// webpEffort is the WebP effort, from 0 (fastest) to 6. It is below libvips'
-const webpEffort = 4
+// default of 4 so that an encode takes at most 40 seconds of the default
+// downstream_timeout of 60 seconds. With one thread, as pixad runs libvips,
+// saving an 8192x8192 image of random pixels with an alpha channel takes up
+// to about 23 seconds at 2, and up to about 16 without one, at any quality;
+// with an alpha channel, 3 takes about 50 seconds and 4 about 61.
+const webpEffort = 2
 
-// exportWebP encodes img as lossy WebP at quality and libvips' default
+// exportWebP encodes img as lossy WebP at quality and webpEffort, without
-// effort, without metadata.
+// metadata.
 func exportWebP(img *vips.ImageRef, quality int) ([]byte, error) {
 	output, _, err := img.ExportWebp(&vips.WebpExportParams{
 		StripMetadata:   true,
@@ -683,12 +738,13 @@ func exportWebP(img *vips.ImageRef, quality int) ([]byte, error) {
 }
 
 // avifEffort is the AVIF effort, from 0 (fastest) to 9; 1 is the lowest
-// govips can set. With one thread, as pixad runs libvips, 1 takes about 51
+// govips can set. With one thread, as pixad runs libvips, 1 takes about 52
-// seconds to save an 8192x8192 RGB image of random pixels, and about 110
+// seconds to save an 8192x8192 RGB image of random pixels at quality 85,
-// with an alpha channel, the worst case, against the default
+// and about 96 with an alpha channel, against the default
-// downstream_timeout of 60 seconds. On an image of milder noise, which 1
+// downstream_timeout of 60 seconds, so an AVIF output is held to
-// saves in about 12 seconds, 2 takes nearly a minute and libvips' default,
+// MaxAVIFOutputPixels. On an image of milder noise, which 1 saves in about
-// 4, takes minutes.
+// 12 seconds, 2 takes nearly a minute and libvips' default, 4, takes
+// minutes.
 const avifEffort = 1
 
 // avifBitdepth is the AVIF bit depth, 8 bits per sample for every image.
@@ -715,10 +771,18 @@ func exportAVIF(img *vips.ImageRef, quality int) ([]byte, error) {
 // JPEG that names none.
 const jxlResolution = 72 / 25.4
 
-// exportJXL encodes img as JPEG XL at quality, with libvips' default effort
+// jxlEffort is the JPEG XL effort, from 1 (fastest) to 9. It is below
-// and without metadata. govips sends libvips a distance, the JPEG XL
+// libvips' default of 7 so that an encode takes at most 40 seconds of the
-// encoder's own measure of quality, along with the quality, and libvips then
+// default downstream_timeout of 60 seconds. With one thread, as pixad runs
-// uses the distance alone, so the quality is also given as a distance.
+// libvips, saving an 8192x8192 image of random pixels with an alpha channel
+// takes up to about 27 seconds at 4, and up to about 13 without one, at any
+// quality; with an alpha channel, 5 takes about 43 seconds and 7 about 81.
+const jxlEffort = 4
+
+// exportJXL encodes img as JPEG XL at quality and jxlEffort, without
+// metadata. govips sends libvips a distance, the JPEG XL encoder's own
+// measure of quality, along with the quality, and libvips then uses the
+// distance alone, so the quality is also given as a distance.
 func exportJXL(img *vips.ImageRef, quality int) ([]byte, error) {
 	// libvips converts a CMYK image to sRGB before it saves WebP, AVIF or
 	// PNG, but cannot save one as JPEG XL. encode has already converted
@@ -757,6 +821,7 @@ func exportJXL(img *vips.ImageRef, quality int) ([]byte, error) {
 	params := vips.NewJxlExportParams()
 	params.Quality = quality
 	params.Distance = jxlDistance(quality)
+	params.Effort = jxlEffort
 
 	output, _, err := toSave.ExportJxl(params)
 	if err != nil {

internal/imgcache/concurrent_misses_internal_test.go

@@ -331,8 +331,7 @@ func TestService_Get_FirstRequestLeavingKeepsTheWork(t *testing.T) {
 // 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
+// the negative cache, where the failure is kept.
-// is not.
 func TestService_Get_ConcurrentMissesShareAFailure(t *testing.T) {
 	t.Parallel()
 
@@ -346,8 +345,8 @@ func TestService_Get_ConcurrentMissesShareAFailure(t *testing.T) {
 		{"upstream answers 404, kept in the negative cache",
 			"/images/missing.jpg", httpfetcher.ErrUpstreamError,
 			ErrNegativeCached, 1},
-		{"source fails the magic byte check, not kept",
+		{"source fails the magic byte check, kept in the negative cache",
-			"/images/text.png", magic.ErrUnknownFormat, magic.ErrUnknownFormat, 2},
+			"/images/text.png", magic.ErrUnknownFormat, ErrNegativeCached, 1},
 	}
 
 	for _, tc := range tests {

internal/imgcache/service.go

@@ -458,13 +458,7 @@ func (s *Service) fetchAndProcess(
 	if err != nil {
 		// Store negative cache for certain errors
 		if isNegativeCacheable(err) {
-			statusCode := extractStatusCode(err)
+			s.storeNegative(ctx, req, extractStatusCode(err), err)
-
-			storeErr := s.cache.StoreNegative(ctx, req, statusCode, err.Error())
-			if storeErr != nil {
-				s.log.Warn("failed to store negative cache entry",
-					"host", req.SourceHost, "path", req.SourcePath, "error", storeErr)
-			}
 		}
 
 		return nil, fmt.Errorf("upstream fetch failed: %w", err)
@@ -510,9 +504,16 @@ func (s *Service) fetchAndProcess(
 		"cipher", fetchResult.TLSCipherSuite,
 	)
 
-	// Validate magic bytes match content type
+	// Validate magic bytes match content type. A body that does not is the
+	// upstream's error, so it gets a negative cache entry, as an upstream
+	// error status does.
 	err = magic.ValidateMagicBytes(sourceData, fetchResult.ContentType)
 	if err != nil {
+		s.log.Warn("source refused", "request_id", middleware.GetReqID(ctx),
+			"host", req.SourceHost, "path", req.SourcePath,
+			"content_type", fetchResult.ContentType, "error", err)
+		s.storeNegative(ctx, req, httpStatusUnsupportedMediaType, err)
+
 		return nil, fmt.Errorf("content validation failed: %w", err)
 	}
 
@@ -528,6 +529,19 @@ func (s *Service) fetchAndProcess(
 	)
 }
 
+// storeNegative stores a negative cache entry for req's source, whose fetch
+// failed with err, so the source is not fetched again until the entry
+// expires. A failure to store the entry is only logged.
+func (s *Service) storeNegative(
+	ctx context.Context, req *ImageRequest, statusCode int, err error,
+) {
+	storeErr := s.cache.StoreNegative(ctx, req, statusCode, err.Error())
+	if storeErr != nil {
+		s.log.Warn("failed to store negative cache entry",
+			"host", req.SourceHost, "path", req.SourcePath, "error", storeErr)
+	}
+}
+
 // 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 {
@@ -642,8 +656,9 @@ func signatureRequest(req *ImageRequest) *signature.Request {
 
 // HTTP status codes for error responses.
 const (
-	httpStatusBadGateway    = 502
+	httpStatusBadGateway           = 502
-	httpStatusInternalError = 500
+	httpStatusInternalError        = 500
+	httpStatusUnsupportedMediaType = 415
 )
 
 // isNegativeCacheable returns true if the error should be cached.

script/bootstrap

@@ -19,9 +19,8 @@
 #                            CGO image libraries (pkg-config, vips
 #                            with its JPEG XL support, libheif) for
 #                            the govips bindings instead of Node: to
-#                            compile pixa, in the Dockerfile's test
+#                            compile pixa outside Docker, as
-#                            phase and build stage, which format
+#                            `make build` does
-#                            nothing
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"

script/cibuild

@@ -2,11 +2,12 @@
 # script/cibuild: run the CI build. It bootstraps first: a CI runner
 # checks out and runs this and nothing else, and script/fmt-check runs
 # the formatter on the host, which a pristine checkout cannot do.
-# A new CACHEBUST value reruns every RUN step, since a cached gate phase
+# A new CACHEBUST value reruns every RUN step after the ARG CACHEBUST
-# is a check that did not run; unlike --no-cache, it keeps Go's build cache.
+# line, since a cached gate phase is a check that did not run; unlike
+# --no-cache, it keeps the layers above that line and the cache mounts.
 # This relies on every stage of the Dockerfile declaring ARG CACHEBUST
-# first; a stage without that line is served from cache on an unchanged
+# before every step that lints, tests or compiles; a stage without that
-# tree, with no warning.
+# line is served from cache on an unchanged tree, with no warning.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"

script/docker

@@ -1,11 +1,12 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
-# A new CACHEBUST value reruns every RUN step, since a cached gate phase
+# A new CACHEBUST value reruns every RUN step after the ARG CACHEBUST
-# is a check that did not run; unlike --no-cache, it keeps Go's build cache.
+# line, since a cached gate phase is a check that did not run; unlike
+# --no-cache, it keeps the layers above that line and the cache mounts.
 # This relies on every stage of the Dockerfile declaring ARG CACHEBUST
-# first; a stage without that line is served from cache on an unchanged
+# before every step that lints, tests or compiles; a stage without that
-# tree, with no warning.
+# line is served from cache on an unchanged tree, with no warning.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"

script/lint

@@ -5,11 +5,13 @@
 # make its answer untrustworthy.
 #
 # The phase is not the last stage in the file, so it is built only when
-# --target names it. A new CACHEBUST value reruns every RUN step, since
+# --target names it. A new CACHEBUST value reruns every RUN step after
-# a cached lint layer is a lint that did not run; unlike --no-cache, it
+# the ARG CACHEBUST line, since a cached lint layer is a lint that did
-# keeps Go's build cache. This relies on every stage of the Dockerfile
+# not run; unlike --no-cache, it keeps the layers above that line and
-# declaring ARG CACHEBUST first; a stage without that line is served
+# the cache mounts. This relies on every stage of the Dockerfile
-# from cache on an unchanged tree, with no warning.
+# declaring ARG CACHEBUST before every step that lints, tests or
+# compiles; a stage without that line is served from cache on an
+# unchanged tree, with no warning.
 # --output type=cacheonly writes no image, since nothing uses one.
 set -eu
 

script/test

@@ -3,11 +3,12 @@
 # and this builds that phase alone, on the same terms as script/lint:
 # --target because a phase that is not the last stage is built only when
 # named, and a new CACHEBUST value because a cached test layer is a test
-# that did not run; unlike --no-cache, it keeps Go's build cache. This
+# that did not run; unlike --no-cache, it keeps the layers above the ARG
-# relies on every stage of the Dockerfile declaring ARG CACHEBUST first;
+# CACHEBUST line and the cache mounts. This relies on every stage of the
-# a stage without that line is served from cache on an unchanged tree,
+# Dockerfile declaring ARG CACHEBUST before every step that lints, tests
-# with no warning. --output type=cacheonly writes no image, since
+# or compiles; a stage without that line is served from cache on an
-# nothing uses one.
+# unchanged tree, with no warning. --output type=cacheonly writes no
+# image, since nothing uses one.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"