Support SVG sources (closes #68)

The fetcher accepts image/svg+xml; internal/magic detects SVG. orig of
an SVG at the orig size is the SVG unchanged, cached, with a
Content-Security-Policy allowing only its styles and data: images; at
another size, ErrSVGNotOriginalSize (400). Any other format renders it
at the least size covering the output, at most 8192 on a side.
Rendering refuses an SVG with a side over 8192 times the other
(ErrSVGTooThin, 415), and the orig size of one above 8192 on a side
(ErrSVGTooLargeForOrigSize, 400). Child processes of pixad read an
SVG's size and render it, killed after 20 seconds and limited to 1 GiB;
any failure there is ErrSVGRenderFailed (415). Tests pinning the SVG
refusal now expect SVG.

Model: opus-5-5

.gitignore

@@ -70,4 +70,3 @@ vendor/
 # Local dev configs
 config.yml
 config.yaml
-config.dev.yml

Makefile

@@ -79,12 +79,15 @@ docker-versioned:
 docker-test:
 	@script/test
 
-# Run local dev server in Docker
+# Run local dev server in Docker, with debug on and a new random signing
+# key each run. Log in to the page with the key it prints.
 devserver: docker-versioned devserver-stop
+	@key="$$(head -c 32 /dev/urandom | base64)" && \
 	docker run -d --name pixad-dev -p 8080:8080 \
-		-v $(CURDIR)/config.dev.yml:/etc/pixa/config.yml:ro \
-		pixa:latest
-	@echo "pixad running at http://localhost:8080"
+		-e PIXA_SIGNING_KEY="$$key" -e PIXA_DEBUG=true \
+		pixa:latest >/dev/null && \
+	echo "pixad running at http://localhost:8080" && \
+	echo "signing key: $$key"
 
 # Stop dev server
 devserver-stop:

README.md

@@ -15,9 +15,10 @@ In this document, the upstream host is the server that holds an original image.
 ## Getting Started
 
 ```bash
-# clone and build
+# clone, install what compiling needs, and build
 git clone https://git.eeqj.de/sneak/pixa.git
 cd pixa
+script/bootstrap --cgo
 make build
 
 # make a config file from the example and put a real signing key in it
@@ -26,18 +27,30 @@ openssl rand -base64 32   # prints a new signing key
 $EDITOR config.yml        # replace the signing_key placeholder with it
 ./bin/pixad --config config.yml
 
-# or build and run the Docker image
+# or build and run the Docker image with that signing key
 make docker
-docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
+docker run -p 8080:8080 -e PIXA_SIGNING_KEY='the new signing key' pixa:latest
 ```
 
-`make build` compiles pixa on your machine into `bin/pixad`. `pixad` is the
-server program. Compiling needs the libraries `script/bootstrap --cgo` installs
-(see Deployment).
+`script/bootstrap --cgo` installs Go, a C compiler and the image libraries pixa
+needs (see Deployment). `make build` then compiles pixa on your machine into
+`bin/pixad`. `pixad` is the server program.
 
 The signing key is pixa's one required secret. It must be 32 characters or
 longer. The placeholder in the example file is refused at startup.
 
+A container takes its settings from environment variables (see Configuration).
+Only `PIXA_SIGNING_KEY` is required. Without it, the container exits at startup
+and names the variable. Every other setting has a built-in default. A config
+file mounted at `/etc/pixa/config.yml` is read when present. An environment
+variable wins over the same setting in the file.
+
+pixa's login page is at `http://localhost:8080/`. Logging in there works only
+with `debug: true` in `config.yml`, or `-e PIXA_DEBUG=true` for the container.
+Otherwise pixa refuses the login with 403, because the page was opened over
+plain HTTP. Use `debug` only on your own machine. A pixa that others use serves
+its pages over HTTPS through a reverse proxy (see Deployment).
+
 Once pixa runs, it serves images through URLs of this form:
 
 ```
@@ -60,11 +73,25 @@ The example config allowlists no host. Every image on an allowlisted host is
 served to anyone, without a signature, at any size and in any format. So
 allowlist only hosts you control, where nobody else can put images.
 
-A container takes its settings from environment variables (see Configuration).
-Only `PIXA_SIGNING_KEY` is required. Without it, the container exits at startup
-and names the variable. Every other setting has a built-in default. A config
-file mounted at `/etc/pixa/config.yml` is read when present. An environment
-variable wins over the same setting in the file.
+To see pixa serve an image, sign a URL for an image on any public HTTPS server.
+Run this in a second terminal while pixa runs, with your own signing key and
+image:
+
+```bash
+key='the new signing key'
+image_host=cdn.example.com    # a public HTTPS server
+image_path=/photos/cat.jpg    # an image on it
+exp=$(($(date +%s) + 3600))   # the URL expires in one hour
+sig=$(printf '%s' "$image_host:$image_path::400:300:webp:$exp:85:cover" |
+    openssl dgst -sha256 -hmac "$key" -binary | base64 | tr '+/' '-_')
+curl --fail-with-body -o cat.webp \
+    "http://localhost:8080/v1/image/$image_host$image_path/400x300.webp?sig=$sig&exp=$exp"
+```
+
+`cat.webp` is then the image at 400x300. If pixa refuses the request, curl
+reports an error and `cat.webp` holds pixa's error message instead. The image
+path must not hold a `%`, a `:` or a query. Signed URLs explains how to sign
+those.
 
 ## Deployment
 
@@ -182,7 +209,8 @@ run under. Its app for pixa needs:
   is half the CPUs pixa sees times 4.2 GiB, plus 64 times 50 MiB. On a host
   where pixa sees 48 CPUs, that is 24 times 4.2 GiB plus 3.1 GiB, about 104 GiB.
   Set `PIXA_MAX_CONCURRENT_PROCESSING` and `PIXA_UPSTREAM_CONNECTIONS` so that
-  this fits the memory the container may use.
+  this fits the memory the container may use. An SVG source is rendered in a
+  process of its own, which can use up to 1 GiB more.
 - **Temporary files:** while pixa processes a large source, libvips writes the
   decoded pixels to a file in `/tmp`. The file is removed from the directory
   once it is written, so a listing of `/tmp` does not show it. It keeps its
@@ -233,8 +261,8 @@ These terms are the names the code uses:
 - **Variant**: the image pixa serves for an image request, made from its source
   with its transform.
 - **Variant key**: the SHA-256 hash of a variant's source, size, format, quality
-  and fit. It names the variant's file and its row in the database, and gives
-  its `ETag`.
+  and fit. It names the variant's file and its row in the database. The
+  variant's `ETag` is made from it and the file's modification time.
 - **Signed URL**: a `/v1/image/` URL whose `sig` parameter signs its image
   request and its expiry `exp` with `signing_key`.
 - **Encrypted URL**: a `/v1/e/` URL whose token holds its image request and any
@@ -249,8 +277,8 @@ These terms are the names the code uses:
     - a check of a cached image failed in any way (see Cache TTL).
 - **Image cache**: the part of pixa that keeps the sources, the variants and the
   negative cache (see Storage).
-- **Cache TTL**: how long pixa serves a cached source and its variants before it
-  checks the source with upstream again: 24 hours (see Cache TTL).
+- **Cache TTL**: how long pixa uses a cached source and serves its variants
+  before it checks the source with upstream again: 24 hours (see Cache TTL).
 - **Disk cache limit** (`cache_max_bytes`): the size in bytes that the cached
   sources and variants may reach together.
 - **Image proxy**: the part of pixa that serves a variant from the image cache
@@ -285,6 +313,7 @@ to any method the list does not give for it.
     - 304 when `If-None-Match` matches the image's `ETag`.
     - 400 for a URL or parameter that is not valid.
     - 400 when the format is `auto` and the `Accept` header is not valid.
+    - 400 for `orig` of an SVG source at a size other than `orig`.
     - 406 when the format is `auto` and `Accept` allows none of the formats it
       chooses from.
     - 401 for a missing or wrong signature.
@@ -305,13 +334,28 @@ to any method the list does not give for it.
       format its `Content-Type` names.
     - 415 when the upstream sends an image whose first bytes libvips reads as
       another format.
+    - 415 for an SVG source with one side more than 8192 times the other, in any
+      format but `orig`.
+    - 415 for an SVG source that is not valid, or that takes more than 20
+      seconds or 1 GiB of memory to render.
     - 502 when the upstream answers with an error status.
     - 502 for 5 minutes after that for the same source URL, when pixa has
       neither the image nor its source in its cache.
     - 502 likewise for 5 minutes after a body whose first bytes are not those of
       its `Content-Type`.
+    - 502 when the upstream host, or a host it redirects to, has no address.
+    - 502 when the upstream redirects to a URL that is not `https`, or redirects
+      10 times.
+    - 502 when the fetch fails before the upstream answers, such as on a refused
+      connection or a failed TLS handshake.
     - 503 when pixa is busy (see Limits).
     - 503 in maintenance mode.
+    - 504 when the address lookup of the upstream host, or of a host it
+      redirects to, times out.
+    - 504 when the connection to the upstream host, or to a host it redirects
+      to, times out.
+    - 504 when the upstream has not finished the TLS handshake within 10
+      seconds.
     - 504 when the upstream has not sent its response headers within
       `upstream_fetch_timeout`.
     - 500 when that time runs out while the image itself is still arriving.
@@ -322,6 +366,8 @@ to any method the list does not give for it.
     - 304 when `If-None-Match` matches the image's `ETag`.
     - 400 for a token that does not decrypt.
     - 400 for a token that asks for a size or fit that is not valid.
+    - 400 for a token that asks for `orig` of an SVG source at a size other than
+      `orig`.
     - 410 once the token has expired.
     - 400 when the token's format is `auto` and the `Accept` header is not
       valid.
@@ -376,6 +422,10 @@ instead. Plain HTTP is for development on the browser's own machine. The login
 session cookie is always marked `Secure`. Over plain HTTP a browser keeps such a
 cookie only for its own machine (`localhost`), if at all.
 
+A form refused with 403 gets a page that names the cause and what to do about
+it. pixa logs a `form refused` warning with the request ID and the form-token
+library's reason, such as `origin invalid`.
+
 The image routes answer the errors listed for them with a JSON body holding
 `error`, `status` and `timestamp`.
 
@@ -460,10 +510,11 @@ outside ASCII. pixa signs and caches the query in that form. So
 `img%3Fname=my%20cat` and `img%3Fname=my%2520cat` in the image URL are the same
 source query, `name=my%20cat`.
 
-**Sizes.** The largest width or height is 8192, for the source and for the
-output. A size with one side `0` has that side worked out from the other side
-and the source's proportions. The side is rounded down, but is at least 1. When
-it would come out above 8192, the request is refused with 400. The fit modes:
+**Sizes.** The largest width or height is 8192, for the output and for a source
+in any format but SVG. A size with one side `0` has that side worked out from
+the other side and the source's proportions. The side is rounded down, but is at
+least 1. When it would come out above 8192, the request is refused with 400. The
+fit modes:
 
 - `cover`: scale the image to cover the size, keeping its proportions, and cut
   out the centre. The output is exactly the size.
@@ -483,9 +534,9 @@ applies to the format `orig` when the source is AVIF, and to `auto` when it
 chooses AVIF. So a client whose `Accept` names `image/avif` but not `image/jxl`
 is refused such a size.
 
-**Source formats.** The source 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
+**Source formats.** The source may be JPEG, PNG, GIF, WebP, AVIF, JPEG XL or
+SVG. 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` is
 refused with 415. So is a body whose first bytes are not those of the format its
 `Content-Type` names: 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
@@ -494,6 +545,20 @@ need that fetch. An image whose first bytes libvips reads as another format is
 refused with 415 too. An SVG whose first bytes pass the check for AVIF is one
 example.
 
+**SVG sources.** `orig` of an SVG source is the SVG itself, unchanged, and is
+cached like any other image. It is given only at the `orig` size, since pixa
+does not resize an SVG as SVG. `orig` of an SVG at any other size is refused
+with 400. A browser that opens an SVG from pixa applies its styles and draws its
+embedded `data:` images, but runs none of its scripts and loads nothing else.
+
+In any format but `orig`, pixa renders an SVG source at the requested size, so
+an enlarged SVG stays sharp. Rendering fetches no URL and reads no file that the
+SVG refers to. In any format but `orig`, an SVG at the `orig` size is rendered
+at its own size. In any format but `orig`, an SVG above 8192 on a side is
+refused with 400 at the `orig` size. In any format but `orig`, an SVG with one
+side more than 8192 times the other is refused with 415. Rendered with no side
+above 8192, its shorter side would be under 1 pixel.
+
 **The format `auto`.** With `auto`, pixa chooses the format for each request
 from its `Accept` header, in this order:
 
@@ -522,7 +587,9 @@ the image after pixa would refuse the URL. A URL with no expiry gets one year.
 `immutable` only stops a client revalidating while its copy is fresh. The
 response also carries `X-Pixa-Cache`: `HIT` for an image served from the cache,
 and `MISS` for one made for this request or older than 24 hours (see Cache TTL).
-The `ETag` is the first 16 characters of the variant key, in quotes.
+The `ETag` is made from the first 16 characters of the variant key and the
+variant file's modification time (see Cache TTL). So a variant made again from a
+changed source gets a new `ETag`.
 
 ### Signed URLs
 
@@ -702,8 +769,9 @@ written percent-encoded.
 
 ### Image Metadata
 
-pixa decodes and re-encodes every image it serves, and removes all metadata from
-the output. This cannot be turned off. The metadata removed is:
+pixa decodes and re-encodes every image it serves, except `orig` of an SVG
+source, which is the SVG unchanged. It removes all metadata from every image it
+re-encodes. This cannot be turned off. The metadata removed is:
 
 - EXIF, which can hold the GPS position, the camera make, model and serial
   number, the capture time and an embedded thumbnail;
@@ -712,14 +780,16 @@ the output. This cannot be turned off. The metadata removed is:
 
 The rest of the rules:
 
-- The `orig` format means the source's own format, not the source's bytes. An
-  `orig` image is re-encoded and stripped like any other.
+- For a source in any format but SVG, the `orig` format means the source's own
+  format, not the source's bytes. An `orig` image is re-encoded and stripped
+  like any other. For an SVG source, `orig` is the SVG's own bytes, its metadata
+  included.
 - An image with an EXIF orientation is turned upright first, so it displays the
   same without the tag. A requested size applies to the upright image.
 - An image with an ICC profile is first converted to sRGB, the standard colour
   space, since clients show an image with no profile as sRGB. Colours outside
   sRGB are clipped: the most saturated ones in a Display P3 photo, for example.
-- JPEG XL output is the one exception, with libvips 8.16 and later. libvips then
+- JPEG XL output is an exception, with libvips 8.16 and later. libvips then
   writes an EXIF block of its own, which pixa cannot stop. The block holds the
   image's size and fixed values: an orientation of 1 and a resolution of 72 dpi.
   Nothing in it comes from the source. The Docker image has libvips 8.16, so its
@@ -754,11 +824,11 @@ through, gives no way to stop an encode once it has begun. So an encode keeps
 its slot until it finishes, which can be after `downstream_timeout`.
 
 **Encode times.** The limits on image size bound how long a conversion can take.
-They are 8192 on each side of the source and of the output, and 16,777,216
-pixels for an AVIF output. Decoding an 8192x8192 source takes up to about 11
-seconds. The WebP and JPEG XL encoders run below libvips' default effort. AVIF
-runs at the lowest effort `govips` can set. With one libvips thread, as pixa
-runs it, encoding 8192x8192 random pixels takes up to about:
+They are 8192 on each side of the output and of a source in any format but SVG,
+and 16,777,216 pixels for an AVIF output. Decoding an 8192x8192 source takes up
+to about 11 seconds. The WebP and JPEG XL encoders run below libvips' default
+effort. AVIF runs at the lowest effort `govips` can set. With one libvips
+thread, as pixa runs it, encoding 8192x8192 random pixels takes up to about:
 
 - 1 second as JPEG;
 - 15 seconds as PNG;
@@ -771,7 +841,8 @@ about 34 seconds with an alpha channel, and 22 without. At 8192x8192 and quality
 85 it would take about 96 seconds with an alpha channel, and 52 without. The 34
 seconds hold at any quality and at 8 or 16 bits per sample. No encode within the
 limits takes more than about 40 seconds. The default `downstream_timeout` is 60
-seconds.
+seconds. An SVG source is rendered in a process of its own, which pixa stops
+after 20 seconds and limits to 1 GiB of memory.
 
 **Processing and connections.** `max_concurrent_processing` images are decoded
 and encoded at once. A request that finds every slot in use waits up to 10
@@ -894,16 +965,17 @@ also removes the expired rows of the negative cache every five minutes, at most
 
 ### Cache TTL
 
-pixa serves a cached source and its variants for 24 hours after upstream last
-sent the source or confirmed it unchanged. After that, the next request for an
-image of that source checks the source with upstream first:
+pixa uses a cached source for 24 hours after upstream last sent it or confirmed
+it unchanged. During that time it serves the source's variants from the cache.
+After that, the next request for an image of that source checks the source with
+upstream first:
 
 - pixa asks upstream to send the source only if it changed. It sends the
   source's `ETag` as `If-None-Match` and its `Last-Modified` as
   `If-Modified-Since`, when upstream gave them. With neither, upstream sends the
   whole image again.
-- When upstream answers 304 Not Modified, pixa serves the cached source and its
-  variants for another 24 hours. Nothing is downloaded or converted again.
+- When upstream answers 304 Not Modified, pixa uses the cached source and serves
+  its variants for another 24 hours. Nothing is downloaded or converted again.
 - When upstream sends the image, it replaces the cached source. Each variant
   made from the old image is converted again from the new one when it is next
   requested.
@@ -913,8 +985,8 @@ image of that source checks the source with upstream first:
   often it serves the image.
 
 pixa evicts a source and its variants 30 days after upstream last sent the
-source or confirmed it unchanged. So a source whose checks keep failing is
-served for at most 30 days. Eviction to stay under the disk cache limit can
+source or confirmed it unchanged. So pixa uses a source whose checks keep
+failing for at most 30 days. Eviction to stay under the disk cache limit can
 remove images sooner.
 
 That eviction can remove a source and keep its variants. Each variant's row
@@ -1164,10 +1236,33 @@ Each item below is one setting, named by its config key:
     - the Go runtime and process metrics
     - `pixa_cache_hits_total`: image requests served from the cache
     - `pixa_cache_misses_total`: image requests not served from the cache
+    - `pixa_source_cache_hits_total`: variants made or checked whose source was
+      in the cache
+    - `pixa_source_cache_misses_total`: variants made or checked whose source
+      was not in the cache
+    - `pixa_cache_usage_bytes`: the bytes of sources and variants in the cache,
+      as eviction last read them; eviction runs after each store and every five
+      minutes
+    - `pixa_cache_max_bytes`: the disk cache limit, `cache_max_bytes`
+    - `pixa_cache_evictions_total`: sources and variants removed by eviction
+    - `pixa_cache_evicted_bytes_total`: the bytes those evictions freed
     - `pixa_upstream_fetches_total`: fetches of a source image that read any
       bytes
     - `pixa_upstream_fetch_bytes_total`: the bytes those fetches read
+    - `pixa_upstream_fetch_duration_seconds`: a histogram of how long those
+      fetches took, up to the last byte of the response
+    - `pixa_upstream_fetch_errors_total`: fetches of a source image that failed,
+      before or while reading the response
     - `pixa_transforms_total`: images converted
+    - `pixa_transform_duration_seconds`: a histogram of how long those
+      conversions took, including the wait for a processing slot
+    - `pixa_transforms_in_progress`: images being converted or waiting for a
+      processing slot
+
+    pixa measures the `pixa_` metrics whether or not `/metrics` is served. They
+    have no labels, so their number never grows with the hosts and paths pixa
+    serves. Requests that share work (see Limits) count one source cache hit or
+    miss between them.
 
     A request's `handler` label is the pattern of the route it matches, such as
     `/v1/image/*`, never its path. Requests that match no route share the label

cmd/pixad/main.go

@@ -1,10 +1,21 @@
-// Package main is the entry point for the pixad image proxy server.
+// Package main is the entry point for the pixad image proxy server. pixad
+// also runs as a child process of itself, to read the size of an SVG source
+// or to render it (see imageprocessor.RunAsSVGChildProcess).
 package main
 
-import "sneak.berlin/go/pixa/internal/app"
+import (
+	"os"
+
+	"sneak.berlin/go/pixa/internal/app"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+)
 
 var Version string //nolint:gochecknoglobals // set by ldflags
 
 func main() {
+	if imageprocessor.IsSVGChildProcess(os.Args[1:]) {
+		os.Exit(imageprocessor.RunAsSVGChildProcess(os.Args[1:]))
+	}
+
 	app.Run(Version)
 }

configs/config.example.yml

@@ -158,8 +158,9 @@ access_control_allow_origin: "*"
 # Sentry DSN for error reporting (default: empty, which turns it off)
 sentry_dsn: ""
 
-# Username and password for /metrics, set together (default: unset). Metrics
-# are measured and /metrics is served only when both are set.
+# Username and password for /metrics, set together (default: unset). pixa
+# serves /metrics, and measures HTTP requests, only when both are set. It
+# measures the pixa_ metrics either way.
 # metrics:
 #   username: "admin"
 #   password: "secret"

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/prometheus/client_golang v1.23.2
 	github.com/slok/go-http-metrics v0.13.0
 	github.com/spf13/cobra v1.10.2
+	go.uber.org/dig v1.19.0
 	go.uber.org/fx v1.24.0
 	golang.org/x/crypto v0.41.0
 	golang.org/x/sync v0.19.0
@@ -126,7 +127,6 @@ require (
 	go.opentelemetry.io/otel v1.36.0 // indirect
 	go.opentelemetry.io/otel/metric v1.36.0 // indirect
 	go.opentelemetry.io/otel/trace v1.36.0 // indirect
-	go.uber.org/dig v1.19.0 // indirect
 	go.uber.org/multierr v1.11.0 // indirect
 	go.uber.org/zap v1.27.0 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect

internal/app/app.go

@@ -2,13 +2,16 @@
 package app
 
 import (
+	"context"
 	"fmt"
+	"io"
 	"os"
 	"os/signal"
 	"syscall"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/spf13/cobra"
+	"go.uber.org/dig"
 	"go.uber.org/fx"
 	"sneak.berlin/go/pixa/internal/config"
 	"sneak.berlin/go/pixa/internal/database"
@@ -53,7 +56,10 @@ func run(_ *cobra.Command, _ []string) {
 	// A write to a closed stdout or stderr must not end the process.
 	signal.Ignore(syscall.SIGPIPE)
 
-	fx.New(
+	app := fx.New(
+		// pixad logs with slog only; runApp prints a startup or shutdown
+		// error itself.
+		fx.NopLogger,
 		fx.Provide(
 			config.New,
 			database.New,
@@ -71,5 +77,40 @@ func run(_ *cobra.Command, _ []string) {
 			func(log *logger.Logger) { log.Identify() },
 			func(*server.Server) {},
 		),
-	).Run()
+	)
+
+	exitCode := runApp(app, os.Stderr)
+	if exitCode != 0 {
+		os.Exit(exitCode)
+	}
+}
+
+// runApp starts app, waits for a signal or a shutdown request, stops app,
+// and returns the status pixad exits with, as fx.App.Run does. It prints
+// a start or stop error to stderr, once, as the error that caused it:
+// dig.RootCause drops the frames fx and dig wrap around it.
+func runApp(app *fx.App, stderr io.Writer) int {
+	startCtx, cancelStart := context.WithTimeout(context.Background(), app.StartTimeout())
+	defer cancelStart()
+
+	err := app.Start(startCtx)
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, dig.RootCause(err))
+
+		return 1
+	}
+
+	shutdown := <-app.Wait()
+
+	stopCtx, cancelStop := context.WithTimeout(context.Background(), app.StopTimeout())
+	defer cancelStop()
+
+	err = app.Stop(stopCtx)
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, dig.RootCause(err))
+
+		return 1
+	}
+
+	return shutdown.ExitCode
 }

internal/app/app_internal_test.go

@@ -0,0 +1,65 @@
+package app
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"testing"
+
+	"go.uber.org/fx"
+)
+
+var errTestStopFailed = errors.New("test stop hook failed")
+
+// TestRunAppPrintsStopErrorOnce checks that when a stop hook fails, runApp
+// prints the hook's error once and returns exit status 1.
+func TestRunAppPrintsStopErrorOnce(t *testing.T) {
+	t.Parallel()
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Invoke(func(lifecycle fx.Lifecycle, shutdowner fx.Shutdowner) {
+			lifecycle.Append(fx.Hook{
+				// Ask to shut down as soon as the app has started, so
+				// that runApp goes on to stop it.
+				OnStart: func(context.Context) error { return shutdowner.Shutdown() },
+				OnStop:  func(context.Context) error { return errTestStopFailed },
+			})
+		}),
+	)
+
+	var stderr bytes.Buffer
+
+	exitCode := runApp(app, &stderr)
+	if exitCode != 1 {
+		t.Errorf("runApp() = %d, want 1", exitCode)
+	}
+
+	want := errTestStopFailed.Error() + "\n"
+	if stderr.String() != want {
+		t.Errorf("stderr = %q, want %q", stderr.String(), want)
+	}
+}
+
+// TestRunAppReturnsShutdownExitCode checks that runApp returns the exit
+// code a shutdown request names.
+func TestRunAppReturnsShutdownExitCode(t *testing.T) {
+	t.Parallel()
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Invoke(func(lifecycle fx.Lifecycle, shutdowner fx.Shutdowner) {
+			lifecycle.Append(fx.Hook{
+				OnStart: func(context.Context) error {
+					return shutdowner.Shutdown(fx.ExitCode(3))
+				},
+			})
+		}),
+	)
+
+	exitCode := runApp(app, io.Discard)
+	if exitCode != 3 {
+		t.Errorf("runApp() = %d, want 3", exitCode)
+	}
+}

internal/app/app_test.go

@@ -0,0 +1,177 @@
+package app_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"os"
+	"os/exec"
+	"strconv"
+	"syscall"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/app"
+)
+
+// These tests run pixad in a child process: the test binary itself,
+// started again with runPixadInChildEnvVar set, which makes TestMain run
+// pixad instead of the tests.
+const runPixadInChildEnvVar = "TEST_RUN_PIXAD_IN_CHILD"
+
+const validTestSigningKey = "test-signing-key-for-app-tests-0123456789"
+
+func TestMain(m *testing.M) {
+	if os.Getenv(runPixadInChildEnvVar) == "1" {
+		os.Args = []string{"pixad"}
+
+		app.Run("test")
+		os.Exit(0)
+	}
+
+	os.Exit(m.Run())
+}
+
+// pixadCommand returns a command that runs pixad with only the given
+// environment, in an empty directory with an empty HOME, so that no
+// config file is found.
+func pixadCommand(t *testing.T, env ...string) (*exec.Cmd, *bytes.Buffer) {
+	t.Helper()
+
+	testBinary := os.Args[0]
+	cmd := exec.CommandContext(t.Context(), testBinary) //nolint:gosec // not tainted
+	cmd.Dir = t.TempDir()
+	// A test binary built with -cover warns on stderr at exit when
+	// GOCOVERDIR is not set.
+	cmd.Env = append([]string{
+		runPixadInChildEnvVar + "=1",
+		"HOME=" + t.TempDir(),
+		"GOCOVERDIR=" + t.TempDir(),
+	}, env...)
+
+	var stderr bytes.Buffer
+
+	cmd.Stderr = &stderr
+
+	return cmd, &stderr
+}
+
+// TestRunPrintsStartupErrorOnce checks that an invalid setting is
+// printed once, as the line that names it, and nothing else goes to
+// stderr.
+func TestRunPrintsStartupErrorOnce(t *testing.T) {
+	t.Parallel()
+
+	cmd, stderr := pixadCommand(t, "PIXA_SIGNING_KEY=short")
+
+	err := cmd.Run()
+
+	var exitErr *exec.ExitError
+	if !errors.As(err, &exitErr) || exitErr.ExitCode() != 1 {
+		t.Fatalf("pixad: got %v, want exit status 1; stderr:\n%s", err, stderr)
+	}
+
+	want := `config key "signing_key" (environment variable PIXA_SIGNING_KEY): ` +
+		"value too short: must be at least 32 characters, got 5\n"
+	if stderr.String() != want {
+		t.Errorf("stderr:\n%s\nwant:\n%s", stderr, want)
+	}
+}
+
+// TestRunWritesNothingToStderrOnStartAndStop starts pixad, waits until
+// it answers its health check, stops it with SIGTERM, and checks that it
+// wrote nothing to stderr.
+func TestRunWritesNothingToStderrOnStartAndStop(t *testing.T) {
+	t.Parallel()
+
+	port := freePort(t)
+	cmd, stderr := pixadCommand(t,
+		"PORT="+strconv.Itoa(port),
+		"PIXA_STATE_DIR="+t.TempDir(),
+		"PIXA_SIGNING_KEY="+validTestSigningKey,
+	)
+
+	err := cmd.Start()
+	if err != nil {
+		t.Fatalf("starting pixad: %v", err)
+	}
+
+	waitForHealthCheck(t, cmd, port, stderr)
+
+	err = cmd.Process.Signal(syscall.SIGTERM)
+	if err != nil {
+		t.Fatalf("signalling pixad: %v", err)
+	}
+
+	err = cmd.Wait()
+	if err != nil {
+		t.Fatalf("pixad: got %v, want exit status 0; stderr:\n%s", err, stderr)
+	}
+
+	if stderr.Len() != 0 {
+		t.Errorf("stderr is not empty:\n%s", stderr)
+	}
+}
+
+// freePort returns a TCP port on the loopback address that nothing
+// listens on.
+func freePort(t *testing.T) int {
+	t.Helper()
+
+	var listenConfig net.ListenConfig
+
+	listener, err := listenConfig.Listen(t.Context(), "tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("finding a free port: %v", err)
+	}
+
+	addr, ok := listener.Addr().(*net.TCPAddr)
+	if !ok {
+		t.Fatalf("listener address %v is not a TCP address", listener.Addr())
+	}
+
+	err = listener.Close()
+	if err != nil {
+		t.Fatalf("closing listener: %v", err)
+	}
+
+	return addr.Port
+}
+
+// waitForHealthCheck waits up to 30 seconds for pixad, started as cmd, to
+// answer its health check on port. If it does not, it stops pixad and
+// fails the test with what pixad wrote to stderr.
+func waitForHealthCheck(t *testing.T, cmd *exec.Cmd, port int, stderr *bytes.Buffer) {
+	t.Helper()
+
+	url := fmt.Sprintf("http://127.0.0.1:%d/.well-known/healthcheck.json", port)
+	deadline := time.Now().Add(30 * time.Second)
+
+	for time.Now().Before(deadline) {
+		req, err := http.NewRequestWithContext(t.Context(), http.MethodGet, url, nil)
+		if err != nil {
+			t.Fatalf("health check request: %v", err)
+		}
+
+		resp, err := http.DefaultClient.Do(req)
+		if err == nil {
+			_ = resp.Body.Close()
+
+			if resp.StatusCode == http.StatusOK {
+				return
+			}
+		}
+
+		time.Sleep(100 * time.Millisecond)
+	}
+
+	// Stop pixad first: stderr is complete, and safe to read, only once
+	// cmd.Wait has returned.
+	_ = cmd.Process.Kill()
+	_ = cmd.Wait()
+
+	t.Fatalf("pixad did not answer its health check on port %d; stderr:\n%s",
+		port, stderr)
+}

internal/handlers/auth_csrf_internal_test.go

@@ -55,7 +55,7 @@ func newCSRFTestRouter(t *testing.T) (*Handlers, http.Handler) {
 		config:      cfg,
 		sessMgr:     session.NewManager(keys.SessionHashKey, keys.SessionBlockKey),
 		encGen:      encurl.NewGenerator(keys.EncryptedURLKey),
-		csrfProtect: newCSRFProtect(keys.CSRFKey, cfg.Debug),
+		csrfProtect: newCSRFProtect(keys.CSRFKey, cfg.Debug, slog.New(slog.DiscardHandler)),
 	}
 
 	r := chi.NewRouter()

internal/handlers/bodylimit_internal_test.go

@@ -43,7 +43,7 @@ func newBodyLimitTestRouter(
 		config:      cfg,
 		sessMgr:     session.NewManager(keys.SessionHashKey, keys.SessionBlockKey),
 		encGen:      encurl.NewGenerator(keys.EncryptedURLKey),
-		csrfProtect: newCSRFProtect(keys.CSRFKey, cfg.Debug),
+		csrfProtect: newCSRFProtect(keys.CSRFKey, cfg.Debug, slog.New(slog.DiscardHandler)),
 	}
 
 	r := chi.NewRouter()

internal/handlers/csrf.go

@@ -1,18 +1,43 @@
 package handlers
 
 import (
+	"bytes"
+	"errors"
 	"html/template"
+	"log/slog"
 	"net/http"
+	"net/url"
 
+	"github.com/go-chi/chi/v5/middleware"
 	"github.com/gorilla/csrf"
 
 	"sneak.berlin/go/pixa/internal/seal"
+	"sneak.berlin/go/pixa/internal/templates"
+)
+
+// The causes the form-refused page names, each with what to do about it.
+const (
+	causePlainHTTP = "This page was opened over plain HTTP while debug is " +
+		"off. Open it over HTTPS, or turn debug on to use plain HTTP on " +
+		"your own machine."
+	causeHTTPSWithDebug = "This page was opened over HTTPS while debug is " +
+		"on. Turn debug off, or open the page over plain HTTP."
+	causeOtherHost = "The form was sent from another host than the one in " +
+		"the request's Host header. A reverse proxy in front of pixa must " +
+		"pass the Host header on unchanged."
+	causeNoReferer = "The browser sent neither an Origin nor a Referer " +
+		"header, and over HTTPS pixa needs one to check where the form " +
+		"came from. Let the browser send the Referer header to this site."
+	causeStaleToken = "The form token is missing or out of date. Reload " +
+		"the page and submit the form again."
 )
 
 // newCSRFProtect builds the CSRF-protection middleware for the
 // state-mutating HTML form routes. The token lives in its own cookie,
 // independent of the session cookie, so it also protects the login POST
-// where no session exists yet (login CSRF). key signs that cookie.
+// where no session exists yet (login CSRF). key signs that cookie. A
+// refused form is answered by the handler that handleFormRefused returns.
+// log gets a warning with the library's reason, such as "origin invalid".
 //
 // When plaintext is true (local HTTP development), requests are marked
 // plaintext so the library neither demands an https Referer nor sets a
@@ -20,13 +45,14 @@ import (
 // service runs behind a TLS-terminating proxy, so plaintext is false and
 // the library enforces its https Referer origin check.
 func newCSRFProtect(
-	key [seal.KeySize]byte, plaintext bool,
+	key [seal.KeySize]byte, plaintext bool, log *slog.Logger,
 ) func(http.Handler) http.Handler {
 	protect := csrf.Protect(
 		key[:],
 		csrf.Path("/"),
 		csrf.Secure(!plaintext),
 		csrf.SameSite(csrf.SameSiteStrictMode),
+		csrf.ErrorHandler(handleFormRefused(plaintext, log)),
 	)
 
 	if !plaintext {
@@ -42,6 +68,84 @@ func newCSRFProtect(
 	}
 }
 
+// handleFormRefused returns the handler the CSRF middleware calls when it
+// refuses a request. It logs the library's reason with the request ID and
+// answers 403 with a page naming the cause and what to do. A reason that is
+// not a refusal, such as a failure to make a token, answers 500.
+func handleFormRefused(plaintext bool, log *slog.Logger) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		reason := csrf.FailureReason(r)
+		requestID := middleware.GetReqID(r.Context())
+
+		cause := formRefusedCause(r, reason, plaintext)
+		if cause == "" {
+			log.Error("CSRF middleware failed",
+				"request_id", requestID, "error", reason)
+			http.Error(w, "Internal server error",
+				http.StatusInternalServerError)
+
+			return
+		}
+
+		log.Warn("form refused", "request_id", requestID, "reason", reason)
+
+		var page bytes.Buffer
+
+		err := templates.Render(&page, "form_refused.html",
+			struct{ Cause string }{Cause: cause})
+		if err != nil {
+			log.Error("failed to render form refused template", "error", err)
+			http.Error(w, "Internal server error",
+				http.StatusInternalServerError)
+
+			return
+		}
+
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		w.WriteHeader(http.StatusForbidden)
+
+		_, err = page.WriteTo(w)
+		if err != nil {
+			log.Error("failed to write form refused page", "error", err)
+		}
+	})
+}
+
+// formRefusedCause returns the cause the form-refused page names for the
+// CSRF middleware's reason, or "" when the reason is not a refusal.
+func formRefusedCause(r *http.Request, reason error, plaintext bool) string {
+	switch {
+	case errors.Is(reason, csrf.ErrNoToken), errors.Is(reason, csrf.ErrBadToken):
+		return causeStaleToken
+	case errors.Is(reason, csrf.ErrNoReferer):
+		return causeNoReferer
+	case errors.Is(reason, csrf.ErrBadOrigin):
+		return pageAddressCause(r.Header.Get("Origin"), r.Host, plaintext)
+	case errors.Is(reason, csrf.ErrBadReferer):
+		return pageAddressCause(r.Referer(), r.Host, plaintext)
+	default:
+		return ""
+	}
+}
+
+// pageAddressCause returns the cause for a form whose page address, the
+// Origin or Referer header, the CSRF middleware refused. The middleware
+// expects the request's own host, with https when plaintext is false and
+// http when it is true. So on the request's own host, only the scheme was
+// wrong.
+func pageAddressCause(pageAddress, host string, plaintext bool) string {
+	page, err := url.Parse(pageAddress)
+	if err != nil || page.Host != host {
+		return causeOtherHost
+	}
+
+	if plaintext {
+		return causeHTTPSWithDebug
+	}
+
+	return causePlainHTTP
+}
+
 // CSRF returns the CSRF-protection middleware for the login and generator
 // form routes.
 func (s *Handlers) CSRF() func(http.Handler) http.Handler {

internal/handlers/csrf_refused_internal_test.go

@@ -0,0 +1,205 @@
+package handlers
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"html/template"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5/middleware"
+	"github.com/gorilla/csrf"
+
+	"sneak.berlin/go/pixa/internal/seal"
+)
+
+// refusedFormRequestID is the request ID of the requests postRefusedForm
+// sends, which the warning for a refused form must carry.
+const refusedFormRequestID = "test-request-id"
+
+// TestFormRefusedNamesCause verifies that a form the CSRF middleware refuses
+// answers 403 with a page naming the cause, and logs a warning with the
+// request ID and the middleware's reason.
+func TestFormRefusedNamesCause(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		debug      bool
+		origin     string
+		referer    string
+		wantCause  string
+		wantReason error
+	}{
+		{
+			name:       "plain HTTP origin with debug off",
+			origin:     "http://example.com",
+			wantCause:  causePlainHTTP,
+			wantReason: csrf.ErrBadOrigin,
+		},
+		{
+			name:       "plain HTTP referer with debug off",
+			referer:    "http://example.com/",
+			wantCause:  causePlainHTTP,
+			wantReason: csrf.ErrBadReferer,
+		},
+		{
+			name:       "origin on another host",
+			origin:     "https://proxy.example.net",
+			wantCause:  causeOtherHost,
+			wantReason: csrf.ErrBadOrigin,
+		},
+		{
+			name:       "referer on another host",
+			referer:    "https://proxy.example.net/",
+			wantCause:  causeOtherHost,
+			wantReason: csrf.ErrBadReferer,
+		},
+		{
+			name:       "HTTPS origin with debug on",
+			debug:      true,
+			origin:     "https://example.com",
+			wantCause:  causeHTTPSWithDebug,
+			wantReason: csrf.ErrBadOrigin,
+		},
+		{
+			name:       "no origin or referer with debug off",
+			wantCause:  causeNoReferer,
+			wantReason: csrf.ErrNoReferer,
+		},
+		{
+			name:       "missing token",
+			debug:      true,
+			origin:     "http://example.com",
+			wantCause:  causeStaleToken,
+			wantReason: csrf.ErrNoToken,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			rec, logLine := postRefusedForm(t, tt.debug, tt.origin, tt.referer,
+				nil, "")
+			checkFormRefused(t, rec, logLine, tt.wantCause, tt.wantReason)
+		})
+	}
+}
+
+// TestFormRefusedNamesCauseForForeignToken verifies that a form whose token
+// does not match its CSRF cookie answers 403 with a page naming
+// causeStaleToken, and logs a warning with the request ID and the
+// middleware's reason.
+func TestFormRefusedNamesCauseForForeignToken(t *testing.T) {
+	t.Parallel()
+
+	// The router and postRefusedForm derive their CSRF key from
+	// testSigningKey, so cookiesA is valid there and only tokenB is wrong.
+	_, router := newCSRFTestRouter(t)
+	cookiesA, _ := csrfCredentials(t, router, nil)
+	_, tokenB := csrfCredentials(t, router, nil)
+
+	rec, logLine := postRefusedForm(t, true, "http://example.com", "",
+		cookiesA, tokenB)
+	checkFormRefused(t, rec, logLine, causeStaleToken, csrf.ErrBadToken)
+}
+
+// postRefusedForm sends a POST with the given cookies to the CSRF
+// middleware built for debug. It adds the form token and the Origin and
+// Referer headers when they are not empty. It returns the response and the
+// one JSON log line the middleware wrote.
+func postRefusedForm(
+	t *testing.T, debug bool, origin, referer string,
+	cookies []*http.Cookie, token string,
+) (*httptest.ResponseRecorder, map[string]any) {
+	t.Helper()
+
+	keys, err := seal.DeriveKeys(testSigningKey)
+	if err != nil {
+		t.Fatalf("seal.DeriveKeys() error = %v", err)
+	}
+
+	var logs bytes.Buffer
+
+	log := slog.New(slog.NewJSONHandler(&logs, nil))
+	reached := http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+		t.Error("the refused form reached the handler behind the middleware")
+	})
+	srv := newCSRFProtect(keys.CSRFKey, debug, log)(reached)
+
+	form := url.Values{loginKeyField: {"x"}}
+	if token != "" {
+		form.Set(csrfTokenField, token)
+	}
+
+	ctx := context.WithValue(context.Background(),
+		middleware.RequestIDKey, refusedFormRequestID)
+	req := httptest.NewRequestWithContext(ctx, http.MethodPost,
+		"http://example.com/", strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	for _, c := range cookies {
+		req.AddCookie(c)
+	}
+
+	if origin != "" {
+		req.Header.Set("Origin", origin)
+	}
+
+	if referer != "" {
+		req.Header.Set("Referer", referer)
+	}
+
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, req)
+
+	var logLine map[string]any
+
+	err = json.Unmarshal(logs.Bytes(), &logLine)
+	if err != nil {
+		t.Fatalf("decoding log line %q: %v", logs.String(), err)
+	}
+
+	return rec, logLine
+}
+
+// checkFormRefused checks that rec is a 403 page naming wantCause and that
+// logLine is a warning carrying the request ID and wantReason.
+func checkFormRefused(
+	t *testing.T, rec *httptest.ResponseRecorder, logLine map[string]any,
+	wantCause string, wantReason error,
+) {
+	t.Helper()
+
+	if rec.Code != http.StatusForbidden {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusForbidden)
+	}
+
+	body := rec.Body.String()
+	t.Logf("body: %s", body)
+
+	if !strings.Contains(body, template.HTMLEscapeString(wantCause)) {
+		t.Errorf("page does not name the cause %q", wantCause)
+	}
+
+	t.Logf("log line: %v", logLine)
+
+	if logLine["level"] != "WARN" || logLine["msg"] != "form refused" {
+		t.Errorf("log line = %v, want a WARN \"form refused\"", logLine)
+	}
+
+	if logLine["request_id"] != refusedFormRequestID {
+		t.Errorf("logged request_id = %v, want %q",
+			logLine["request_id"], refusedFormRequestID)
+	}
+
+	if logLine["reason"] != wantReason.Error() {
+		t.Errorf("logged reason = %v, want %q", logLine["reason"], wantReason)
+	}
+}

internal/handlers/handlers.go

@@ -56,14 +56,16 @@ type Handlers struct {
 
 // New creates a new Handlers instance.
 func New(_ fx.Lifecycle, params Params) *Handlers {
+	log := params.Logger.Get()
+
 	return &Handlers{
-		log:              params.Logger.Get(),
+		log:              log,
 		hc:               params.Healthcheck,
 		config:           params.Config,
 		imgSvc:           params.ImageService,
 		sessMgr:          params.SessionManager,
 		encGen:           params.EncryptedURLGenerator,
-		csrfProtect:      newCSRFProtect(params.Keys.CSRFKey, params.Config.Debug),
+		csrfProtect:      newCSRFProtect(params.Keys.CSRFKey, params.Config.Debug, log),
 		signer:           signature.New(params.Config.SigningKey),
 		allowlist:        allowlist.New(params.Config.AllowlistHosts),
 		refererBlocklist: allowlist.New(params.Config.RefererBlocklist),

internal/handlers/handlers_internal_test.go

@@ -257,6 +257,90 @@ func TestHandleImage_ConditionalRequest_IfNoneMatch_DifferentETag(t *testing.T)
 	}
 }
 
+// TestHandleImage_ConditionalRequest_EarlierETagGetsChangedImage serves an
+// image, changes it upstream, and requests it again with the ETag first
+// served, once the cache TTL has passed. The changed image must be served,
+// not 304 Not Modified.
+func TestHandleImage_ConditionalRequest_EarlierETagGetsChangedImage(t *testing.T) {
+	t.Parallel()
+
+	const host = "goodhost.example.com"
+
+	upstream := fstest.MapFS{
+		host + photoPath: &fstest.MapFile{
+			Data: generateTestJPEG(t, 100, 100, color.RGBA{255, 0, 0, 255}),
+		},
+	}
+
+	// A cache TTL of one nanosecond has passed by the second request
+	cache, err := imgcache.NewCache(setupTestDB(t), imgcache.Config{
+		StateDir:    t.TempDir(),
+		NegativeTTL: 5 * time.Minute,
+		CacheTTL:    time.Nanosecond,
+		MaxBytes:    1 << 30,
+	}, prometheus.NewRegistry())
+	if err != nil {
+		t.Fatalf("imgcache.NewCache() error = %v", err)
+	}
+
+	svc, err := imageproxy.New(&imageproxy.Config{
+		Cache:   cache,
+		Fetcher: newMockFetcher(upstream),
+		Processor: imageprocessor.New(imageprocessor.Params{
+			MaxInputBytes:           50 << 20,
+			MaxConcurrentProcessing: 2,
+		}),
+		MaxResponseSize: 50 << 20,
+		Logger:          slog.New(slog.DiscardHandler),
+	}, prometheus.NewRegistry())
+	if err != nil {
+		t.Fatalf("imageproxy.New() error = %v", err)
+	}
+
+	h := &Handlers{
+		imgSvc:    svc,
+		log:       slog.New(slog.DiscardHandler),
+		signer:    signature.New(testSigningKey),
+		allowlist: allowlist.New([]string{host}),
+	}
+
+	r := chi.NewRouter()
+	r.Get("/v1/image/*", h.HandleImage())
+
+	target := "/v1/image/" + host + photoPath + "/50x50.jpeg"
+
+	first := httptest.NewRecorder()
+	r.ServeHTTP(first,
+		httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil))
+
+	etag := first.Header().Get("ETag")
+	if first.Code != http.StatusOK || etag == "" {
+		t.Fatalf("first request: status = %d, ETag = %q, want %d and an ETag",
+			first.Code, etag, http.StatusOK)
+	}
+
+	upstream[host+photoPath] = &fstest.MapFile{
+		Data: generateTestJPEG(t, 100, 100, color.RGBA{0, 0, 255, 255}),
+	}
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, target, nil)
+	req.Header.Set("If-None-Match", etag)
+
+	second := httptest.NewRecorder()
+	r.ServeHTTP(second, req)
+	t.Logf("first ETag %s; with it after the change: status %d, ETag %s, %d bytes",
+		etag, second.Code, second.Header().Get("ETag"), second.Body.Len())
+
+	if second.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d with the changed image",
+			second.Code, http.StatusOK)
+	}
+
+	if bytes.Equal(second.Body.Bytes(), first.Body.Bytes()) {
+		t.Error("the image served is the one made before the change")
+	}
+}
+
 func TestHandleImage_ETagHeader(t *testing.T) {
 	t.Parallel()
 

internal/handlers/image.go

@@ -234,12 +234,8 @@ func (s *Handlers) HandleTooManyRequests() http.HandlerFunc {
 func (s *Handlers) respondImageError(
 	w http.ResponseWriter, req *imagerequest.Request, 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) {
+	// The image service has logged this as a refused source
+	if isFirstBytesMismatch(err) {
 		s.respondError(w, "unsupported source format",
 			http.StatusUnsupportedMediaType)
 
@@ -254,7 +250,9 @@ func (s *Handlers) respondImageError(
 	switch {
 	case errors.Is(err, httpfetcher.ErrSSRFBlocked):
 		status, message = http.StatusForbidden, "forbidden"
-	case errors.Is(err, httpfetcher.ErrUpstreamError):
+	case errors.Is(err, httpfetcher.ErrUpstreamError),
+		errors.Is(err, httpfetcher.ErrInvalidHost),
+		errors.Is(err, httpfetcher.ErrUpstreamRequestFailed):
 		status, message = http.StatusBadGateway, "upstream error"
 	case errors.Is(err, httpfetcher.ErrUpstreamTimeout):
 		status, message = http.StatusGatewayTimeout, "upstream timeout"
@@ -267,11 +265,29 @@ func (s *Handlers) respondImageError(
 		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat):
 		status = http.StatusUnsupportedMediaType
 		message = "unsupported source format"
+	case errors.Is(err, imageprocessor.ErrSVGTooThin):
+		status = http.StatusUnsupportedMediaType
+		message = fmt.Sprintf("SVG source too thin to render: one side is "+
+			"more than %d times the other", imageprocessor.MaxInputDimension)
+	case errors.Is(err, imageprocessor.ErrSVGRenderFailed):
+		status = http.StatusUnsupportedMediaType
+		message = fmt.Sprintf("SVG source could not be rendered: it is not "+
+			"valid, or it takes more than %d seconds or %d GiB of memory to "+
+			"render", int(imageprocessor.SVGRenderTimeout.Seconds()),
+			imageprocessor.MaxSVGRenderMemory/bytesPerGiB)
 	case errors.Is(err, imageprocessor.ErrOutputTooLarge):
 		status = http.StatusBadRequest
 		message = fmt.Sprintf("size too large for the source: the side "+
 			"computed from its proportions is above %d",
 			imageprocessor.MaxOutputDimension)
+	case errors.Is(err, imageprocessor.ErrSVGNotOriginalSize):
+		status = http.StatusBadRequest
+		message = "an SVG source is returned as SVG only at the orig size; " +
+			"ask for another format to resize it"
+	case errors.Is(err, imageprocessor.ErrSVGTooLargeForOrigSize):
+		status = http.StatusBadRequest
+		message = fmt.Sprintf("SVG source above %d on a side: give a size "+
+			"to render it at", imageprocessor.MaxOutputDimension)
 	case errors.Is(err, imageprocessor.ErrAVIFOutputTooLarge):
 		status = http.StatusBadRequest
 		message = fmt.Sprintf("size too large for AVIF: the output "+
@@ -295,6 +311,18 @@ func (s *Handlers) respondImageError(
 	s.respondError(w, message, status)
 }
 
+// bytesPerGiB is the number of bytes in a GiB.
+const bytesPerGiB = 1 << 30
+
+// isFirstBytesMismatch reports whether err refuses an upstream body whose
+// first bytes are not those of the format its Content-Type names. That is the
+// upstream's error, not pixa's.
+func isFirstBytesMismatch(err error) bool {
+	return errors.Is(err, magic.ErrMagicByteMismatch) ||
+		errors.Is(err, magic.ErrUnknownFormat) ||
+		errors.Is(err, magic.ErrNotEnoughData)
+}
+
 // cacheControl returns the Cache-Control header for an image served through a
 // URL that expires at expires, or never when expires is the zero time. A cache
 // may keep the image for a year, but not past the URL's expiry, after which
@@ -313,6 +341,26 @@ func cacheControl(expires time.Time) string {
 	return fmt.Sprintf("public, max-age=%d, immutable", int64(maxAge/time.Second))
 }
 
+// svgContentSecurityPolicyValue is the Content-Security-Policy header value
+// of an SVG response, in place of the middleware's. A browser that opens the
+// SVG applies its styles and draws its embedded data: images, but runs none
+// of its scripts and loads nothing else. The styles need unsafe-inline, as an
+// SVG keeps them inline, in style elements and attributes.
+const svgContentSecurityPolicyValue = "default-src 'none'; " +
+	"style-src 'unsafe-inline'; " +
+	"img-src data:; " +
+	"frame-ancestors 'none'; " +
+	"sandbox"
+
+// setSVGContentSecurityPolicy sets the Content-Security-Policy in header to
+// svgContentSecurityPolicyValue when contentType is that of SVG. Any other
+// response keeps the middleware's policy.
+func setSVGContentSecurityPolicy(header http.Header, contentType string) {
+	if contentType == imagerequest.FormatSVG.MIMEType() {
+		header.Set("Content-Security-Policy", svgContentSecurityPolicyValue)
+	}
+}
+
 // refuseBlockedReferer answers 403 with a JSON error when the request's Referer
 // names a host on referer_blocklist, and reports whether it answered. A request
 // with no Referer, or one that does not parse as a URL with a host, is not
@@ -395,6 +443,7 @@ func (s *Handlers) serveImage(
 
 	// Set response headers
 	w.Header().Set("Content-Type", resp.ContentType)
+	setSVGContentSecurityPolicy(w.Header(), resp.ContentType)
 
 	if resp.ContentLength > 0 {
 		w.Header().Set("Content-Length", strconv.FormatInt(resp.ContentLength, 10))

internal/handlers/image_cache_control_internal_test.go

@@ -185,13 +185,13 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 
 	tests := []struct {
 		name        string
-		expires     time.Time
+		ttl         time.Duration
 		wantAtLeast int
 		wantAtMost  int
 	}{
-		{"60 second TTL", time.Now().Add(time.Minute), 50, 60},
-		{"two-year TTL", time.Now().Add(2 * 365 * 24 * time.Hour), 31536000, 31536000},
-		{"no TTL", time.Time{}, 31536000, 31536000},
+		{"60 second TTL", time.Minute, 50, 60},
+		{"two-year TTL", 2 * 365 * 24 * time.Hour, 31536000, 31536000},
+		{"no TTL", 0, 31536000, 31536000},
 	}
 
 	for _, tt := range tests {
@@ -200,6 +200,14 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 
 			h, srv := newSignedHostServer(t, slog.New(slog.DiscardHandler))
 
+			// The expiry is set here, not in the table: a parallel subtest
+			// can wait behind other tests for more than the 10 seconds the
+			// 60 second case allows.
+			var expires time.Time
+			if tt.ttl != 0 {
+				expires = time.Now().Add(tt.ttl)
+			}
+
 			token, err := h.encGen.Generate(&imagerequest.Request{
 				Source: imagerequest.Source{
 					Host: signedHost,
@@ -209,7 +217,7 @@ func TestHandleImageEnc_MaxAge(t *testing.T) {
 					Size:   imagerequest.Size{Width: 50, Height: 50},
 					Format: imagerequest.FormatJPEG,
 				},
-			}, tt.expires)
+			}, expires)
 			if err != nil {
 				t.Fatalf("Generate() error = %v", err)
 			}

internal/handlers/image_errors_internal_test.go

@@ -12,6 +12,7 @@ import (
 	"net/http/httptest"
 	"slices"
 	"strings"
+	"syscall"
 	"testing"
 	"testing/fstest"
 	"time"
@@ -461,13 +462,20 @@ func TestImageRoutes_SameErrorAnswers(t *testing.T) {
 		{"upstream error status",
 			fmt.Errorf("%w: status 404", httpfetcher.ErrUpstreamError),
 			http.StatusBadGateway, slog.LevelWarn},
+		{"upstream host with no address",
+			fmt.Errorf("%w: cdn.example.com", httpfetcher.ErrInvalidHost),
+			http.StatusBadGateway, slog.LevelWarn},
+		{"connection refused",
+			fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamRequestFailed,
+				syscall.ECONNREFUSED),
+			http.StatusBadGateway, slog.LevelWarn},
 		{"no response headers within the fetch timeout",
 			httpfetcher.ErrUpstreamTimeout, http.StatusGatewayTimeout,
 			slog.LevelWarn},
 		{"no free upstream connection", httpfetcher.ErrTooManyConnections,
 			http.StatusServiceUnavailable, slog.LevelWarn},
 		{"Content-Type not a source format",
-			fmt.Errorf("%w: image/svg+xml", httpfetcher.ErrInvalidContentType),
+			fmt.Errorf("%w: image/tiff", httpfetcher.ErrInvalidContentType),
 			http.StatusUnsupportedMediaType, slog.LevelWarn},
 		{"any other failure", io.ErrUnexpectedEOF,
 			http.StatusInternalServerError, slog.LevelError},

internal/handlers/unsupported_source_internal_test.go

@@ -15,7 +15,7 @@ import (
 
 // TestUnsupportedSourceFormatAnswers415 checks that respondImageError, which
 // answers the errors of both image routes, answers 415 with a clear error when
-// the upstream answers with a content type pixa does not accept, such as SVG,
+// the upstream answers with a content type pixa does not accept, such as TIFF,
 // and when libvips does not read a source as the format its first bytes
 // show, wrapped as the service wraps them.
 func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
@@ -23,11 +23,11 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 
 	h := &Handlers{log: slog.New(slog.DiscardHandler)}
 	req := &imagerequest.Request{
-		Source: imagerequest.Source{Host: "img.example.com", Path: "/a.svg"},
+		Source: imagerequest.Source{Host: "img.example.com", Path: "/a.tiff"},
 	}
 
 	for _, err := range []error{
-		fmt.Errorf("upstream fetch failed: %w: image/svg+xml",
+		fmt.Errorf("upstream fetch failed: %w: image/tiff",
 			httpfetcher.ErrInvalidContentType),
 		fmt.Errorf("image processing failed: %w: libvips does not read the input as avif",
 			imageprocessor.ErrUnsupportedInputFormat),
@@ -47,3 +47,67 @@ func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
 		}
 	}
 }
+
+// TestSVGTooThinAnswers415 checks that respondImageError answers 415 with a
+// clear error for an SVG source too thin to render, wrapped as the service
+// wraps it.
+func TestSVGTooThinAnswers415(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	req := &imagerequest.Request{
+		Source: imagerequest.Source{Host: "svg.example.com", Path: "/a.svg"},
+	}
+	err := fmt.Errorf("image processing failed: %w: 81930x10",
+		imageprocessor.ErrSVGTooThin)
+
+	rec := httptest.NewRecorder()
+	h.respondImageError(rec, req, err)
+	t.Logf("for %v: %d %s", err, rec.Code, rec.Body.String())
+
+	if rec.Code != http.StatusUnsupportedMediaType {
+		t.Errorf("status = %d, want %d", rec.Code,
+			http.StatusUnsupportedMediaType)
+	}
+
+	if !strings.Contains(rec.Body.String(), "SVG source too thin to render") {
+		t.Errorf("body = %q, want the SVG too thin error", rec.Body.String())
+	}
+}
+
+// TestSVGRenderFailedAnswers415 checks that respondImageError answers 415,
+// naming the limits, for an SVG source that a child process could not read
+// or render, wrapped as the service wraps it.
+func TestSVGRenderFailedAnswers415(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	err := fmt.Errorf("image processing failed: %w: it took more than 20s",
+		imageprocessor.ErrSVGRenderFailed)
+
+	rec := httptest.NewRecorder()
+	h.respondImageError(rec, &imagerequest.Request{}, err)
+	t.Logf("for %v: %d %s", err, rec.Code, rec.Body.String())
+
+	checkErrorBody(t, rec, http.StatusUnsupportedMediaType,
+		"SVG source could not be rendered: it is not valid, or it takes more "+
+			"than 20 seconds or 1 GiB of memory to render")
+}
+
+// TestSVGTooLargeForOrigSizeAnswers400 checks that respondImageError answers
+// 400, asking for a size, for an SVG source above 8192 on a side asked for at
+// the orig size in a raster format, wrapped as the service wraps it.
+func TestSVGTooLargeForOrigSizeAnswers400(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	err := fmt.Errorf("image processing failed: %w: 10000x10 is above 8192",
+		imageprocessor.ErrSVGTooLargeForOrigSize)
+
+	rec := httptest.NewRecorder()
+	h.respondImageError(rec, &imagerequest.Request{}, err)
+	t.Logf("for %v: %d %s", err, rec.Code, rec.Body.String())
+
+	checkErrorBody(t, rec, http.StatusBadRequest,
+		"SVG source above 8192 on a side: give a size to render it at")
+}

internal/httpfetcher/httpfetcher.go

@@ -69,7 +69,11 @@ 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")
+	// ErrUpstreamRequestFailed is a fetch that failed before the upstream
+	// answered, other than by timeout or cancellation: a failed connection
+	// or TLS handshake, or a redirect the fetcher refuses.
+	ErrUpstreamRequestFailed = errors.New("upstream request failed")
+	ErrTooManyConnections    = errors.New("too many concurrent upstream connections")
 )
 
 // Internal fetcher errors.
@@ -124,6 +128,7 @@ func allowedContentTypes() []string {
 		imagerequest.FormatWebP.MIMEType(),
 		imagerequest.FormatAVIF.MIMEType(),
 		imagerequest.FormatJXL.MIMEType(),
+		imagerequest.FormatSVG.MIMEType(),
 	}
 }
 
@@ -283,11 +288,7 @@ func (f *HTTPFetcher) Fetch(
 	fetchDuration := time.Since(startTime)
 
 	if err != nil {
-		if errors.Is(err, context.DeadlineExceeded) {
-			return nil, ErrUpstreamTimeout
-		}
-
-		return nil, fmt.Errorf("upstream request failed: %w", err)
+		return nil, upstreamRequestError(err)
 	}
 
 	// The deferred release gives the connection back, as no body follows
@@ -316,6 +317,33 @@ func (f *HTTPFetcher) Fetch(
 	return result, nil
 }
 
+// upstreamRequestError returns the error Fetch reports when its upstream
+// request fails with err:
+//   - err wrapped in ErrUpstreamTimeout for a timeout, so the log names what
+//     timed out.
+//   - err, still matching context.Canceled, when the caller cancelled the
+//     fetch.
+//   - err wrapped in ErrUpstreamRequestFailed otherwise.
+func upstreamRequestError(err error) error {
+	var netErr net.Error
+
+	switch {
+	case errors.Is(err, context.DeadlineExceeded),
+		errors.Is(err, ErrUpstreamTimeout),
+		errors.As(err, &netErr) && netErr.Timeout():
+		// A timeout other than the fetch timeout, such as the TLS
+		// handshake timeout, does not match context.DeadlineExceeded. An
+		// address lookup or a connection that timed out is
+		// ErrUpstreamTimeout already, so its log line names that error twice.
+		return fmt.Errorf("%w: %w", ErrUpstreamTimeout, err)
+	case errors.Is(err, context.Canceled):
+		// The caller cancelled the fetch; the upstream did not fail
+		return fmt.Errorf("upstream request cancelled: %w", err)
+	default:
+		return fmt.Errorf("%w: %w", ErrUpstreamRequestFailed, err)
+	}
+}
+
 // requestHeaders returns the headers of a fetch: User-Agent, Accept, the
 // ID of the request the fetch serves, so the fetch can be found in the
 // upstream host's logs, and a non-empty ifNoneMatch or ifModifiedSince.
@@ -505,7 +533,7 @@ func validateURL(ctx context.Context, fetchURL *neturl.URL, allowHTTP bool) erro
 	// Resolve the host to check IP addresses
 	addrs, err := net.DefaultResolver.LookupIPAddr(ctx, host)
 	if err != nil {
-		return fmt.Errorf("%w: %s", ErrInvalidHost, host)
+		return addressLookupError(host, err)
 	}
 
 	private := slices.ContainsFunc(addrs, func(addr net.IPAddr) bool {
@@ -518,6 +546,23 @@ func validateURL(ctx context.Context, fetchURL *neturl.URL, allowHTTP bool) erro
 	return nil
 }
 
+// addressLookupError returns the error for a failed address lookup of host:
+//   - ErrUpstreamTimeout when the lookup timed out.
+//   - err, still matching context.Canceled, when the lookup was cancelled.
+//   - ErrInvalidHost otherwise.
+func addressLookupError(host string, err error) error {
+	var netErr net.Error
+	if errors.As(err, &netErr) && netErr.Timeout() {
+		return fmt.Errorf("%w: address lookup of %s", ErrUpstreamTimeout, host)
+	}
+
+	if errors.Is(err, context.Canceled) {
+		return fmt.Errorf("address lookup cancelled: %w", err)
+	}
+
+	return fmt.Errorf("%w: %s", ErrInvalidHost, host)
+}
+
 // isLocalhost checks if the host is localhost.
 func isLocalhost(host string) bool {
 	host = strings.ToLower(host)
@@ -646,7 +691,7 @@ func dialSSRFSafe(
 	// Resolve the address
 	ips, err := net.DefaultResolver.LookupIP(ctx, "ip", host)
 	if err != nil {
-		return nil, fmt.Errorf("%w: %s", ErrInvalidHost, host)
+		return nil, addressLookupError(host, err)
 	}
 
 	// Check all resolved IPs
@@ -657,7 +702,10 @@ func dialSSRFSafe(
 	}
 
 	// Connect using the first valid IP
-	var dialer net.Dialer
+	var (
+		dialer      net.Dialer
+		lastDialErr error
+	)
 
 	for _, ip := range ips {
 		addr := net.JoinHostPort(ip.String(), port)
@@ -666,9 +714,18 @@ func dialSSRFSafe(
 		if err == nil {
 			return conn, nil
 		}
+
+		lastDialErr = err
 	}
 
-	return nil, fmt.Errorf("%w to %s", errConnectFailed, host)
+	// Only the last address tried decides whether the connection timed out
+	var netErr net.Error
+	if errors.As(lastDialErr, &netErr) && netErr.Timeout() {
+		return nil, fmt.Errorf("%w: connect to %s: %w",
+			ErrUpstreamTimeout, host, lastDialErr)
+	}
+
+	return nil, fmt.Errorf("%w to %s: %w", errConnectFailed, host, lastDialErr)
 }
 
 // limitedReader wraps a reader and refuses a body longer than remaining

internal/httpfetcher/httpfetcher_internal_test.go

@@ -279,7 +279,7 @@ func TestDetectContentTypeFromPath(t *testing.T) {
 		{"foo/bar.gif", imagerequest.FormatGIF.MIMEType()},
 		{"foo/bar.webp", imagerequest.FormatWebP.MIMEType()},
 		{"foo/bar.avif", imagerequest.FormatAVIF.MIMEType()},
-		{"foo/bar.svg", contentTypeSVG},
+		{"foo/bar.svg", imagerequest.FormatSVG.MIMEType()},
 		{"foo/bar.bin", contentTypeOctetStream},
 		{"foo/bar", contentTypeOctetStream},
 	}

internal/httpfetcher/mock.go

@@ -67,11 +67,9 @@ func (m *MockFetcher) Fetch(
 	}, nil
 }
 
-// The content types the mock fetcher gives files in no format pixa reads.
-const (
-	contentTypeSVG         = "image/svg+xml"
-	contentTypeOctetStream = "application/octet-stream"
-)
+// contentTypeOctetStream is the content type the mock fetcher gives files in
+// no format pixa reads.
+const contentTypeOctetStream = "application/octet-stream"
 
 // detectContentTypeFromPath returns the MIME type based on file extension.
 func detectContentTypeFromPath(path string) string {
@@ -91,7 +89,7 @@ func detectContentTypeFromPath(path string) string {
 	case strings.HasSuffix(path, ".jxl"):
 		return imagerequest.FormatJXL.MIMEType()
 	case strings.HasSuffix(path, ".svg"):
-		return contentTypeSVG
+		return imagerequest.FormatSVG.MIMEType()
 	default:
 		return contentTypeOctetStream
 	}

internal/httpfetcher/request_failed_internal_test.go

@@ -0,0 +1,213 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"net"
+	"os"
+	"syscall"
+	"testing"
+	"time"
+)
+
+// TestFetchFailedBeforeAnswer checks the error of a fetch whose connection
+// fails:
+//   - refused: ErrUpstreamRequestFailed.
+//   - timed out: ErrUpstreamTimeout, still matching the dial's error, even for
+//     a timeout error that does not match context.DeadlineExceeded, such as
+//     the TLS handshake timeout.
+//   - cancelled by its caller: context.Canceled, not ErrUpstreamRequestFailed.
+func TestFetchFailedBeforeAnswer(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		dial       func(cancel context.CancelFunc) error
+		wantErrs   []error
+		wantNotErr error
+	}{
+		{
+			name: "connection refused",
+			dial: func(context.CancelFunc) error {
+				return syscall.ECONNREFUSED
+			},
+			wantErrs:   []error{ErrUpstreamRequestFailed},
+			wantNotErr: context.Canceled,
+		},
+		{
+			name: "timed out while connecting",
+			dial: func(context.CancelFunc) error {
+				return os.ErrDeadlineExceeded
+			},
+			wantErrs:   []error{ErrUpstreamTimeout, os.ErrDeadlineExceeded},
+			wantNotErr: ErrUpstreamRequestFailed,
+		},
+		{
+			name: "cancelled while connecting",
+			dial: func(cancel context.CancelFunc) error {
+				cancel()
+
+				return context.Canceled
+			},
+			wantErrs:   []error{context.Canceled},
+			wantNotErr: ErrUpstreamRequestFailed,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx, cancel := context.WithCancel(testContext(t))
+			defer cancel()
+
+			f := New(&Config{
+				Timeout:               30 * time.Second,
+				MaxResponseSize:       50 << 20,
+				AllowHTTP:             true,
+				MaxConnectionsPerHost: 20,
+				MaxConnections:        64,
+				DialContext: func(context.Context, string, string) (net.Conn, error) {
+					return nil, tt.dial(cancel)
+				},
+			})
+
+			_, err := f.Fetch(ctx, upstreamSource("/image"), "", "")
+			t.Logf("Fetch() error: %v", err)
+
+			for _, wantErr := range tt.wantErrs {
+				if !errors.Is(err, wantErr) {
+					t.Errorf("Fetch() error = %v, want %v", err, wantErr)
+				}
+			}
+
+			if errors.Is(err, tt.wantNotErr) {
+				t.Errorf("Fetch() error = %v, must not be %v", err, tt.wantNotErr)
+			}
+		})
+	}
+}
+
+// TestFetchAddressLookupTimedOut checks that a fetch whose address lookup
+// times out fails with ErrUpstreamTimeout, both for the lookup of the source
+// host and for the lookup while connecting. The error must match neither
+// ErrInvalidHost nor ErrUpstreamRequestFailed, which the image routes answer
+// 502. A context whose deadline has passed makes the lookup time out.
+func TestFetchAddressLookupTimedOut(t *testing.T) {
+	t.Parallel()
+
+	checkTimedOut := func(t *testing.T, err error) {
+		t.Helper()
+		t.Logf("Fetch() error: %v", err)
+
+		if !errors.Is(err, ErrUpstreamTimeout) {
+			t.Errorf("Fetch() error = %v, want ErrUpstreamTimeout", err)
+		}
+
+		if errors.Is(err, ErrInvalidHost) || errors.Is(err, ErrUpstreamRequestFailed) {
+			t.Errorf("Fetch() error = %v, must match neither ErrInvalidHost "+
+				"nor ErrUpstreamRequestFailed", err)
+		}
+	}
+
+	t.Run("lookup of the source host", func(t *testing.T) {
+		t.Parallel()
+
+		ctx, cancel := context.WithTimeout(testContext(t), 0)
+		defer cancel()
+
+		f := New(&Config{
+			Timeout:               30 * time.Second,
+			MaxResponseSize:       50 << 20,
+			MaxConnectionsPerHost: 20,
+			MaxConnections:        64,
+		})
+
+		_, err := f.Fetch(ctx, imageSourceOn("cdn.example.com"), "", "")
+		checkTimedOut(t, err)
+	})
+
+	t.Run("lookup while connecting", func(t *testing.T) {
+		t.Parallel()
+
+		// The source host is an address, which validateURL does not look up;
+		// the dialer looks up a name
+		f := New(&Config{
+			Timeout:               30 * time.Second,
+			MaxResponseSize:       50 << 20,
+			MaxConnectionsPerHost: 20,
+			MaxConnections:        64,
+			DialContext: func(ctx context.Context, network, _ string) (net.Conn, error) {
+				ctx, cancel := context.WithTimeout(ctx, 0)
+				defer cancel()
+
+				return dialSSRFSafe(ctx, network, "img.example.com:443", nil)
+			},
+		})
+
+		_, err := f.Fetch(testContext(t), upstreamSource("/image"), "", "")
+		checkTimedOut(t, err)
+	})
+}
+
+// TestFetchConnectionTimedOut checks that a fetch whose connection in
+// dialSSRFSafe times out fails with ErrUpstreamTimeout, which the image
+// routes answer 504. The error must match neither ErrUpstreamRequestFailed
+// nor ErrInvalidHost, which they answer 502. A context whose deadline has
+// passed makes the connection time out without touching the network.
+func TestFetchConnectionTimedOut(t *testing.T) {
+	t.Parallel()
+
+	f := New(&Config{
+		Timeout:               30 * time.Second,
+		MaxResponseSize:       50 << 20,
+		MaxConnectionsPerHost: 20,
+		MaxConnections:        64,
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			ctx, cancel := context.WithTimeout(ctx, 0)
+			defer cancel()
+
+			return dialSSRFSafe(ctx, network, addr, nil)
+		},
+	})
+
+	_, err := f.Fetch(testContext(t), upstreamSource("/image"), "", "")
+	t.Logf("Fetch() error: %v", err)
+
+	if !errors.Is(err, ErrUpstreamTimeout) {
+		t.Errorf("Fetch() error = %v, want ErrUpstreamTimeout", err)
+	}
+
+	if errors.Is(err, ErrUpstreamRequestFailed) || errors.Is(err, ErrInvalidHost) {
+		t.Errorf("Fetch() error = %v, must match neither "+
+			"ErrUpstreamRequestFailed nor ErrInvalidHost", err)
+	}
+}
+
+// TestFetchAddressLookupCancelled checks that a fetch whose address lookup
+// its caller cancelled fails with an error matching context.Canceled, not
+// ErrInvalidHost, which the image routes answer 502.
+func TestFetchAddressLookupCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(testContext(t))
+	cancel()
+
+	f := New(&Config{
+		Timeout:               30 * time.Second,
+		MaxResponseSize:       50 << 20,
+		MaxConnectionsPerHost: 20,
+		MaxConnections:        64,
+	})
+
+	_, err := f.Fetch(ctx, imageSourceOn("cdn.example.com"), "", "")
+	t.Logf("Fetch() error: %v", err)
+
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("Fetch() error = %v, want context.Canceled", err)
+	}
+
+	if errors.Is(err, ErrInvalidHost) {
+		t.Errorf("Fetch() error = %v, must not be ErrInvalidHost", err)
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -1,4 +1,7 @@
 // Package imageprocessor provides image format conversion and resizing using libvips.
+// It reads the size of an SVG source and renders it in child processes, each
+// the running program started again (see RunAsSVGChildProcess). pixa kills
+// them after SVGRenderTimeout, and each may use at most MaxSVGRenderMemory.
 package imageprocessor
 
 import (
@@ -7,6 +10,8 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
+	"strconv"
 	"sync"
 	"time"
 
@@ -113,10 +118,43 @@ var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
 // ErrUnsupportedInputFormat is returned, before the input is decoded, when
 // libvips reads the input's first bytes as another format than its source
 // format, or as none. An SVG whose first bytes pass the first-bytes check for
-// AVIF is one example. A source format other than the six pixa reads is also
-// refused with it.
+// AVIF is one example. A source format other than the seven pixa reads is
+// also refused with it.
 var ErrUnsupportedInputFormat = errors.New("unsupported input format")
 
+// ErrSVGNotOriginalSize is returned when orig is asked of an SVG source at a
+// size other than orig. pixa returns an SVG only as it is; a raster format
+// gives it a size.
+var ErrSVGNotOriginalSize = errors.New(
+	"an SVG is returned as SVG only at the orig size")
+
+// ErrSVGTooThin is returned, before rendering, when an SVG source has one
+// side more than MaxInputDimension times the other. Rendered with its longer
+// side at most MaxInputDimension, its shorter side would be under 1 pixel.
+var ErrSVGTooThin = errors.New("SVG source too thin to render")
+
+// ErrSVGTooLargeForOrigSize is returned, before rendering, when a format other
+// than orig asks for an SVG source at the orig size and the SVG has a side
+// above MaxOutputDimension.
+var ErrSVGTooLargeForOrigSize = errors.New(
+	"SVG source too large to render at its own size")
+
+// SVGRenderTimeout is how long the child processes that read the size of one
+// SVG source and render it may run, counted from the start of the first,
+// before pixa kills them.
+const SVGRenderTimeout = 20 * time.Second
+
+// MaxSVGRenderMemory is the most memory, in bytes, that each child process
+// reading the size of an SVG source or rendering it may use. It is the
+// child's RLIMIT_DATA, which limits its heap and its other private writable
+// memory.
+const MaxSVGRenderMemory = 1 << 30 // 1 GiB
+
+// ErrSVGRenderFailed is returned when a child process fails to read the size
+// of an SVG source or to render it: librsvg cannot read the SVG, or the
+// child takes more than SVGRenderTimeout or MaxSVGRenderMemory.
+var ErrSVGRenderFailed = errors.New("SVG source could not be rendered")
+
 // 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")
@@ -134,6 +172,10 @@ type ImageProcessor struct {
 	processingSemaphore chan struct{}
 	// processingWaitTimeout is ProcessingWaitTimeout; tests shorten it.
 	processingWaitTimeout time.Duration
+	// svgRenderTimeout is SVGRenderTimeout; tests shorten it.
+	svgRenderTimeout time.Duration
+	// maxSVGRenderMemory is MaxSVGRenderMemory; tests lower it.
+	maxSVGRenderMemory int
 }
 
 // Params holds configuration for creating an ImageProcessor. Both fields
@@ -153,6 +195,8 @@ func New(params Params) *ImageProcessor {
 		maxInputBytes:         params.MaxInputBytes,
 		processingSemaphore:   make(chan struct{}, params.MaxConcurrentProcessing),
 		processingWaitTimeout: ProcessingWaitTimeout,
+		svgRenderTimeout:      SVGRenderTimeout,
+		maxSVGRenderMemory:    MaxSVGRenderMemory,
 	}
 }
 
@@ -164,7 +208,10 @@ func New(params Params) *ImageProcessor {
 // 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. An AVIF output of more than MaxAVIFOutputPixels is
-// refused with ErrAVIFOutputTooLarge before anything is decoded.
+// refused with ErrAVIFOutputTooLarge before anything is decoded. orig of an
+// SVG source is the SVG as it is, since libvips cannot save SVG; any other
+// format renders it at the output size. Child processes read an SVG's size
+// and render it, as processSVG describes.
 func (p *ImageProcessor) Process(
 	ctx context.Context,
 	input io.Reader,
@@ -183,35 +230,18 @@ func (p *ImageProcessor) Process(
 		return nil, err
 	}
 
-	// Input libvips would not load as sourceFormat is refused before it is
-	// decoded
-	err = checkInputFormat(data, sourceFormat)
+	if sourceFormat == imagerequest.FormatSVG {
+		return p.processSVG(ctx, data, transform)
+	}
+
+	img, err := loadImage(data, sourceFormat)
 	if err != nil {
 		return nil, err
 	}
-
-	// Decode image
-	img, err := vips.NewImageFromBuffer(data)
-	if err != nil {
-		return nil, fmt.Errorf("failed to decode image: %w", err)
-	}
 	defer img.Close()
 
-	// Turn the image upright now: encode strips the EXIF orientation tag,
-	// and sizes below must be worked out on the upright image.
-	err = img.AutoRotate()
-	if err != nil {
-		return nil, fmt.Errorf("failed to auto-rotate: %w", err)
-	}
-
-	// Get original dimensions
-	origWidth := img.Width()
-	origHeight := img.Height()
-
-	// Validate input dimensions to prevent DoS via decompression bombs
-	if origWidth > MaxInputDimension || origHeight > MaxInputDimension {
-		return nil, ErrInputTooLarge
-	}
+	origWidth, origHeight := img.Width(), img.Height()
+	outputFormat := resolveOutputFormat(transform.Format, sourceFormat)
 
 	// Determine target dimensions; too large ones are refused before resizing
 	targetWidth, targetHeight, err := targetDimensions(transform.Size,
@@ -220,12 +250,6 @@ func (p *ImageProcessor) Process(
 		return nil, err
 	}
 
-	// orig is the source's own format; encode refuses an empty format
-	outputFormat := transform.Format
-	if outputFormat == imagerequest.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,
@@ -242,8 +266,104 @@ func (p *ImageProcessor) Process(
 		}
 	}
 
-	// Encode to target format
-	output, err := p.encode(ctx, img, outputFormat, transform.Quality)
+	return p.encodeResult(ctx, img, outputFormat, transform.Quality,
+		origWidth, origHeight)
+}
+
+// WaitForProcessing waits until no image is being processed, or until ctx
+// ends, and returns how many images were still being processed then. It
+// waits by taking each slot in processingSemaphore as it frees up until it
+// holds them all, or until ctx ends, then gives back the slots it took.
+func (p *ImageProcessor) WaitForProcessing(ctx context.Context) int {
+	taken := 0
+
+	defer func() {
+		for range taken {
+			<-p.processingSemaphore
+		}
+	}()
+
+	for taken < cap(p.processingSemaphore) {
+		select {
+		case p.processingSemaphore <- struct{}{}:
+			taken++
+		case <-ctx.Done():
+			return len(p.processingSemaphore) - taken
+		}
+	}
+
+	return 0
+}
+
+// processSVG is Process for an SVG source, data. Apart from the first-bytes
+// check, pixad hands the SVG to libvips only in child processes (see
+// runSVGChildProcess). One reads the SVG's size. For a raster format a second
+// renders it as PNG, which is then resized and encoded as any other image.
+// pixa kills them once svgRenderTimeout has passed since the first one
+// started.
+func (p *ImageProcessor) processSVG(
+	ctx context.Context, data []byte, transform *imagerequest.Transform,
+) (*Result, error) {
+	err := checkInputFormat(data, imagerequest.FormatSVG)
+	if err != nil {
+		return nil, err
+	}
+
+	deadline := time.Now().Add(p.svgRenderTimeout)
+
+	origWidth, origHeight, err := p.readSVGSize(ctx, deadline, data)
+	if err != nil {
+		return nil, err
+	}
+
+	outputFormat := resolveOutputFormat(transform.Format, imagerequest.FormatSVG)
+	if outputFormat == imagerequest.FormatSVG {
+		return svgAsItIs(data, transform.Size, origWidth, origHeight)
+	}
+
+	err = checkSVGOrigSize(transform.Size, origWidth, origHeight)
+	if err != nil {
+		return nil, err
+	}
+
+	targetWidth, targetHeight, err := targetDimensions(transform.Size,
+		origWidth, origHeight)
+	if err != nil {
+		return nil, err
+	}
+
+	outputWidth, outputHeight, err := outputDimensions(origWidth, origHeight,
+		targetWidth, targetHeight, transform.FitMode, outputFormat)
+	if err != nil {
+		return nil, err
+	}
+
+	// The SVG is rendered at about the output size, to stay sharp
+	img, err := p.renderSVG(ctx, deadline, data, origWidth, origHeight,
+		outputWidth, outputHeight)
+	if err != nil {
+		return nil, err
+	}
+	defer img.Close()
+
+	if targetWidth != origWidth || targetHeight != origHeight {
+		err := p.resize(img, outputWidth, outputHeight, transform.FitMode)
+		if err != nil {
+			return nil, fmt.Errorf("failed to resize: %w", err)
+		}
+	}
+
+	return p.encodeResult(ctx, img, outputFormat, transform.Quality,
+		origWidth, origHeight)
+}
+
+// encodeResult encodes img as format at quality and returns it as the result
+// for a source of origWidth x origHeight.
+func (p *ImageProcessor) encodeResult(
+	ctx context.Context, img *vips.ImageRef, format imagerequest.Format,
+	quality, origWidth, origHeight int,
+) (*Result, error) {
+	output, err := p.encode(ctx, img, format, quality)
 	if err != nil {
 		return nil, fmt.Errorf("failed to encode: %w", err)
 	}
@@ -251,7 +371,7 @@ func (p *ImageProcessor) Process(
 	return &Result{
 		Content:       io.NopCloser(bytes.NewReader(output)),
 		ContentLength: int64(len(output)),
-		ContentType:   outputFormat.MIMEType(),
+		ContentType:   format.MIMEType(),
 		Width:         img.Width(),
 		Height:        img.Height(),
 		InputWidth:    origWidth,
@@ -290,29 +410,125 @@ func targetDimensions(
 	return width, height, nil
 }
 
-// WaitForProcessing waits until no image is being processed, or until ctx
-// ends, and returns how many images were still being processed then. It
-// waits by taking each slot in processingSemaphore as it frees up until it
-// holds them all, or until ctx ends, then gives back the slots it took.
-func (p *ImageProcessor) WaitForProcessing(ctx context.Context) int {
-	taken := 0
-
-	defer func() {
-		for range taken {
-			<-p.processingSemaphore
-		}
-	}()
-
-	for taken < cap(p.processingSemaphore) {
-		select {
-		case p.processingSemaphore <- struct{}{}:
-			taken++
-		case <-ctx.Done():
-			return len(p.processingSemaphore) - taken
-		}
+// resolveOutputFormat returns the format of the output for the requested
+// format: sourceFormat for orig, and format itself for any other. encode
+// refuses an empty format.
+func resolveOutputFormat(
+	format, sourceFormat imagerequest.Format,
+) imagerequest.Format {
+	if format == imagerequest.FormatOriginal {
+		return sourceFormat
 	}
 
-	return 0
+	return format
+}
+
+// loadImage refuses data that libvips would not load as sourceFormat, then
+// decodes it and turns the image upright: encode strips the EXIF orientation
+// tag, and sizes must be worked out on the upright image. An image with a
+// side above MaxInputDimension is ErrInputTooLarge, which guards against
+// decompression bombs.
+func loadImage(
+	data []byte, sourceFormat imagerequest.Format,
+) (*vips.ImageRef, error) {
+	err := checkInputFormat(data, sourceFormat)
+	if err != nil {
+		return nil, err
+	}
+
+	img, err := vips.NewImageFromBuffer(data)
+	if err != nil {
+		return nil, fmt.Errorf("failed to decode image: %w", err)
+	}
+
+	err = img.AutoRotate()
+	if err != nil {
+		img.Close()
+
+		return nil, fmt.Errorf("failed to auto-rotate: %w", err)
+	}
+
+	if img.Width() > MaxInputDimension || img.Height() > MaxInputDimension {
+		img.Close()
+
+		return nil, ErrInputTooLarge
+	}
+
+	return img, nil
+}
+
+// svgAsItIs returns the SVG data unchanged, which is orig of an SVG source
+// at the orig size. width x height is the SVG's own size. Any other size is
+// ErrSVGNotOriginalSize.
+func svgAsItIs(
+	data []byte, size imagerequest.Size, width, height int,
+) (*Result, error) {
+	if !size.OriginalSize() {
+		return nil, fmt.Errorf("%w: asked for %dx%d", ErrSVGNotOriginalSize,
+			size.Width, size.Height)
+	}
+
+	return &Result{
+		Content:       io.NopCloser(bytes.NewReader(data)),
+		ContentLength: int64(len(data)),
+		ContentType:   imagerequest.FormatSVG.MIMEType(),
+		Width:         width,
+		Height:        height,
+		InputWidth:    width,
+		InputHeight:   height,
+	}, nil
+}
+
+// checkSVGOrigSize returns ErrSVGTooLargeForOrigSize when size is the orig
+// size and the SVG source, of width x height, has a side above
+// MaxOutputDimension. processSVG calls it after returning orig of the SVG, so
+// the SVG would be rendered at its own size.
+func checkSVGOrigSize(size imagerequest.Size, width, height int) error {
+	if size.OriginalSize() && max(width, height) > MaxOutputDimension {
+		return fmt.Errorf("%w: %dx%d is above %d", ErrSVGTooLargeForOrigSize,
+			width, height, MaxOutputDimension)
+	}
+
+	return nil
+}
+
+// svgRenderSize returns the least size in the proportions of width x height,
+// an SVG's own size, that covers outputWidth x outputHeight, so that resize
+// hardly changes it. Neither side is above MaxInputDimension.
+func svgRenderSize(width, height, outputWidth, outputHeight int) (int, int) {
+	scale := min(
+		max(float64(outputWidth)/float64(width),
+			float64(outputHeight)/float64(height)),
+		float64(MaxInputDimension)/float64(max(width, height)))
+
+	return int(math.Round(float64(width) * scale)),
+		int(math.Round(float64(height) * scale))
+}
+
+// renderSVG renders the SVG data, whose own size is width x height, at
+// svgRenderSize for outputWidth x outputHeight, in a child process that pixa
+// kills at deadline (see runSVGChildProcess). It returns the rendered image,
+// read from the child's PNG. An SVG with one side more than
+// MaxInputDimension times the other is ErrSVGTooThin.
+func (p *ImageProcessor) renderSVG(
+	ctx context.Context, deadline time.Time, data []byte,
+	width, height, outputWidth, outputHeight int,
+) (*vips.ImageRef, error) {
+	if max(width, height) > MaxInputDimension*min(width, height) {
+		return nil, fmt.Errorf("%w: %dx%d", ErrSVGTooThin, width, height)
+	}
+
+	renderWidth, renderHeight := svgRenderSize(width, height,
+		outputWidth, outputHeight)
+
+	png, err := p.runSVGChildProcess(ctx, deadline, data,
+		renderSVGAsPNGArgument, strconv.Itoa(renderWidth),
+		strconv.Itoa(renderHeight))
+	if err != nil {
+		return nil, err
+	}
+
+	return loadImage(png, imagerequest.FormatPNG)
 }
 
 // acquireSlot takes a slot in processingSemaphore, waiting at most
@@ -397,6 +613,8 @@ func vipsImageType(format imagerequest.Format) vips.ImageType {
 		return vips.ImageTypeAVIF
 	case imagerequest.FormatJXL:
 		return vips.ImageTypeJXL
+	case imagerequest.FormatSVG:
+		return vips.ImageTypeSVG
 	case imagerequest.FormatOriginal, imagerequest.FormatAuto:
 		return vips.ImageTypeUnknown
 	default:
@@ -575,7 +793,9 @@ func (p *ImageProcessor) encode(
 	case imagerequest.FormatJXL:
 		return exportJXL(img, quality)
 
-	case imagerequest.FormatOriginal, imagerequest.FormatAuto:
+	// libvips cannot save SVG; Process returns an SVG source as it is
+	case imagerequest.FormatSVG, imagerequest.FormatOriginal,
+		imagerequest.FormatAuto:
 		return nil, fmt.Errorf("%w: %s", ErrUnsupportedOutputFormat, format)
 
 	default:

internal/imageprocessor/imageprocessor_internal_test.go

@@ -19,6 +19,11 @@ import (
 )
 
 func TestMain(m *testing.M) {
+	// The SVG tests start this test binary again as SVG child processes
+	if IsSVGChildProcess(os.Args[1:]) {
+		os.Exit(RunAsSVGChildProcess(os.Args[1:]))
+	}
+
 	initVips()
 
 	code := m.Run()

internal/imageprocessor/source_formats_internal_test.go

@@ -51,7 +51,7 @@ func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
 }
 
 // TestImageProcessor_NotASourceFormatRefused verifies that a source format
-// other than the six, such as orig, auto or none, is refused with
+// other than the seven, such as orig, auto or none, is refused with
 // ErrUnsupportedInputFormat. The input is text, which libvips reads as no
 // format.
 func TestImageProcessor_NotASourceFormatRefused(t *testing.T) {

internal/imageprocessor/svg_child_process.go

@@ -0,0 +1,255 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"strconv"
+	"syscall"
+	"time"
+
+	"github.com/davidbyttow/govips/v2/vips"
+)
+
+// An SVG child process is the running program started again with one of
+// these arguments first, naming what it does. Its command line, after the
+// program's name, is one of:
+//
+//	read-svg-size MEMORY_LIMIT
+//	render-svg-as-png MEMORY_LIMIT WIDTH HEIGHT
+//
+// MEMORY_LIMIT is in bytes. The child reads the SVG on standard input. It
+// writes the SVG's size as WIDTH HEIGHT, or the SVG rendered at WIDTH x
+// HEIGHT as PNG, to standard output.
+const (
+	readSVGSizeArgument    = "read-svg-size"
+	renderSVGAsPNGArgument = "render-svg-as-png"
+)
+
+// errSVGChildProcessArguments is returned for a command line that is not
+// that of an SVG child process.
+var errSVGChildProcessArguments = errors.New(
+	"not the command line of an SVG child process")
+
+// IsSVGChildProcess reports whether args, a process's command line after the
+// program's name, are those of an SVG child process.
+func IsSVGChildProcess(args []string) bool {
+	return len(args) > 0 &&
+		(args[0] == readSVGSizeArgument || args[0] == renderSVGAsPNGArgument)
+}
+
+// RunAsSVGChildProcess runs this process as the SVG child process that args,
+// its command line after the program's name, ask for. It returns the status
+// to exit with: 0 once it has written its result, and 1 after writing an
+// error to standard error, which the parent discards.
+func RunAsSVGChildProcess(args []string) int {
+	err := runAsSVGChildProcess(args, os.Stdin, os.Stdout)
+	if err != nil {
+		_, _ = fmt.Fprintln(os.Stderr, err)
+
+		return 1
+	}
+
+	return 0
+}
+
+// runAsSVGChildProcess does the work of RunAsSVGChildProcess, reading the SVG
+// from stdin and writing the result to stdout. It sets the process's limits
+// before it reads the SVG.
+func runAsSVGChildProcess(
+	args []string, stdin io.Reader, stdout io.Writer,
+) error {
+	numbers, err := parseSVGChildProcessArguments(args)
+	if err != nil {
+		return err
+	}
+
+	err = setSVGChildProcessLimits(numbers[0])
+	if err != nil {
+		return err
+	}
+
+	initVips()
+
+	svg, err := io.ReadAll(stdin)
+	if err != nil {
+		return fmt.Errorf("failed to read the SVG: %w", err)
+	}
+
+	if args[0] == readSVGSizeArgument {
+		return writeSVGSize(svg, stdout)
+	}
+
+	return writeSVGAsPNG(svg, numbers[1], numbers[2], stdout)
+}
+
+// parseSVGChildProcessArguments returns the numbers on the command line of an
+// SVG child process, args: the memory limit, then the width and height for
+// render-svg-as-png.
+func parseSVGChildProcessArguments(args []string) ([]int, error) {
+	wantArgs := 2
+	if len(args) > 0 && args[0] == renderSVGAsPNGArgument {
+		wantArgs = 4
+	}
+
+	if !IsSVGChildProcess(args) || len(args) != wantArgs {
+		return nil, fmt.Errorf("%w: %q", errSVGChildProcessArguments, args)
+	}
+
+	numbers := make([]int, 0, len(args)-1)
+
+	for _, arg := range args[1:] {
+		number, err := strconv.Atoi(arg)
+		if err != nil {
+			return nil, fmt.Errorf("%w: %q: %w", errSVGChildProcessArguments,
+				args, err)
+		}
+
+		numbers = append(numbers, number)
+	}
+
+	return numbers, nil
+}
+
+// setSVGChildProcessLimits sets this process's RLIMIT_DATA to memoryLimit
+// bytes, so that allocating heap or other private writable memory past it
+// fails. RLIMIT_AS would also count the address space the Go runtime reserves
+// but does not use. It also sets RLIMIT_CORE to 0: librsvg crashes when an
+// allocation fails, and the crash would write a core file of up to
+// memoryLimit bytes into pixad's working directory.
+func setSVGChildProcessLimits(memoryLimit int) error {
+	if memoryLimit < 1 {
+		return fmt.Errorf("%w: memory limit %d", errSVGChildProcessArguments,
+			memoryLimit)
+	}
+
+	err := syscall.Setrlimit(syscall.RLIMIT_DATA, &syscall.Rlimit{
+		Cur: uint64(memoryLimit),
+		Max: uint64(memoryLimit),
+	})
+	if err != nil {
+		return fmt.Errorf("failed to limit memory to %d bytes: %w",
+			memoryLimit, err)
+	}
+
+	err = syscall.Setrlimit(syscall.RLIMIT_CORE, &syscall.Rlimit{})
+	if err != nil {
+		return fmt.Errorf("failed to turn off core files: %w", err)
+	}
+
+	return nil
+}
+
+// writeSVGSize writes the size of svg to stdout as WIDTH HEIGHT. librsvg
+// reads the whole SVG for it.
+func writeSVGSize(svg []byte, stdout io.Writer) error {
+	img, err := vips.NewImageFromBuffer(svg)
+	if err != nil {
+		return fmt.Errorf("failed to read the SVG: %w", err)
+	}
+
+	defer img.Close()
+
+	_, err = fmt.Fprintf(stdout, "%d %d\n", img.Width(), img.Height())
+
+	return err
+}
+
+// writeSVGAsPNG renders svg at width x height and writes it to stdout as PNG.
+// libvips' thumbnail has librsvg render the SVG at that size directly, at
+// any scale: govips takes a density only in whole dots per inch, which cannot
+// render an SVG below 1/72 of its own size. Given the SVG as bytes with no
+// file name, librsvg loads no file or URL the SVG refers to.
+func writeSVGAsPNG(svg []byte, width, height int, stdout io.Writer) error {
+	img, err := vips.NewThumbnailWithSizeFromBuffer(svg, width, height,
+		vips.InterestingNone, vips.SizeBoth)
+	if err != nil {
+		return fmt.Errorf("failed to render the SVG: %w", err)
+	}
+
+	defer img.Close()
+
+	png, err := exportPNG(img)
+	if err != nil {
+		return fmt.Errorf("failed to save the SVG as PNG: %w", err)
+	}
+
+	_, err = stdout.Write(png)
+
+	return err
+}
+
+// readSVGSize reads the size of the SVG data in a child process that pixa
+// kills at deadline (see runSVGChildProcess).
+func (p *ImageProcessor) readSVGSize(
+	ctx context.Context, deadline time.Time, data []byte,
+) (int, int, error) {
+	output, err := p.runSVGChildProcess(ctx, deadline, data,
+		readSVGSizeArgument)
+	if err != nil {
+		return 0, 0, err
+	}
+
+	var width, height int
+
+	_, err = fmt.Sscan(string(output), &width, &height)
+	if err != nil {
+		return 0, 0, fmt.Errorf("failed to read the size %q an SVG child "+
+			"process wrote: %w", output, err)
+	}
+
+	return width, height, nil
+}
+
+// runSVGChildProcess starts the running program again as an SVG child
+// process, with argument, p.maxSVGRenderMemory and extraArgs as its command
+// line, and waits for it. It gives the child data on standard input,
+// discards its standard error, and returns its standard output. pixa kills
+// the child at deadline. A child that fails or is killed is
+// ErrSVGRenderFailed, unless ctx has ended: runSVGChildProcess then returns
+// ctx's error. Failing to start the child is another error.
+func (p *ImageProcessor) runSVGChildProcess(
+	ctx context.Context, deadline time.Time, data []byte,
+	argument string, extraArgs ...string,
+) ([]byte, error) {
+	executable, err := os.Executable()
+	if err != nil {
+		return nil, fmt.Errorf("failed to find the running program: %w", err)
+	}
+
+	childCtx, cancel := context.WithDeadline(ctx, deadline)
+	defer cancel()
+
+	args := append([]string{argument, strconv.Itoa(p.maxSVGRenderMemory)},
+		extraArgs...)
+
+	//nolint:gosec // the running program, with arguments pixa makes
+	cmd := exec.CommandContext(childCtx, executable, args...)
+	cmd.Stdin = bytes.NewReader(data)
+
+	var stdout bytes.Buffer
+
+	cmd.Stdout = &stdout
+
+	err = cmd.Run()
+
+	var exitError *exec.ExitError
+
+	switch {
+	case err == nil:
+		return stdout.Bytes(), nil
+	case ctx.Err() != nil:
+		return nil, ctx.Err()
+	case childCtx.Err() != nil:
+		return nil, fmt.Errorf("%w: it took more than %v", ErrSVGRenderFailed,
+			p.svgRenderTimeout)
+	case errors.As(err, &exitError):
+		return nil, fmt.Errorf("%w: %w", ErrSVGRenderFailed, err)
+	default:
+		return nil, fmt.Errorf("failed to run an SVG child process: %w", err)
+	}
+}

internal/imageprocessor/svg_internal_test.go

@@ -0,0 +1,503 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/davidbyttow/govips/v2/vips"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// testSVG is a 16x12 SVG, black on its left half and white on its right.
+const testSVG = `<svg xmlns="http://www.w3.org/2000/svg" width="16" height="12">` +
+	`<rect width="16" height="12" fill="white"/>` +
+	`<rect width="8" height="12" fill="black"/></svg>`
+
+// newSVGTestProcessor returns a processor for the SVG tests.
+func newSVGTestProcessor() *ImageProcessor {
+	return New(Params{MaxInputBytes: 1 << 20, MaxConcurrentProcessing: 1})
+}
+
+// processSVG processes svg, as an SVG source, with transform.
+func processSVG(
+	t *testing.T, svg string, transform imagerequest.Transform,
+) (*Result, []byte, error) {
+	t.Helper()
+
+	result, err := newSVGTestProcessor().Process(t.Context(),
+		bytes.NewReader([]byte(svg)), imagerequest.FormatSVG, &transform)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	output, err := io.ReadAll(result.Content)
+	if err != nil {
+		t.Fatalf("reading the output: %v", err)
+	}
+
+	return result, output, nil
+}
+
+// TestImageProcessor_SVGOrigIsTheSVG checks that orig of an SVG at the orig
+// size is the SVG itself, never another format, and that orig at another
+// size is ErrSVGNotOriginalSize.
+func TestImageProcessor_SVGOrigIsTheSVG(t *testing.T) {
+	t.Parallel()
+
+	result, output, err := processSVG(t, testSVG,
+		imagerequest.Transform{Format: imagerequest.FormatOriginal})
+	if err != nil {
+		t.Fatalf("orig of the SVG: %v", err)
+	}
+
+	t.Logf("orig of the SVG: %s, %dx%d", result.ContentType, result.Width,
+		result.Height)
+
+	if result.ContentType != "image/svg+xml" || string(output) != testSVG {
+		t.Errorf("orig of the SVG = %s %q, want image/svg+xml %q",
+			result.ContentType, output, testSVG)
+	}
+
+	_, _, err = processSVG(t, testSVG, imagerequest.Transform{
+		Size:   imagerequest.Size{Width: 32, Height: 24},
+		Format: imagerequest.FormatOriginal,
+	})
+	if !errors.Is(err, ErrSVGNotOriginalSize) {
+		t.Errorf("orig of the SVG at 32x24: error = %v, want %v", err,
+			ErrSVGNotOriginalSize)
+	}
+}
+
+// TestImageProcessor_RendersSVGToEachRasterFormat checks that an SVG is
+// rendered to each raster format at the requested size.
+func TestImageProcessor_RendersSVGToEachRasterFormat(t *testing.T) {
+	t.Parallel()
+
+	for _, format := range []imagerequest.Format{
+		imagerequest.FormatJPEG, imagerequest.FormatPNG, imagerequest.FormatGIF,
+		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
+	} {
+		result, output, err := processSVG(t, testSVG, imagerequest.Transform{
+			Size:   imagerequest.Size{Width: 40, Height: 30},
+			Format: format,
+		})
+		if err != nil {
+			t.Errorf("SVG as %s: %v", format, err)
+
+			continue
+		}
+
+		t.Logf("SVG as %s: %s, %dx%d, %d bytes", format, result.ContentType,
+			result.Width, result.Height, len(output))
+
+		if result.ContentType != format.MIMEType() ||
+			vips.DetermineImageType(output) != vipsImageType(format) ||
+			result.Width != 40 || result.Height != 30 {
+			t.Errorf("SVG as %s = %s, %dx%d; want %s, 40x30", format,
+				result.ContentType, result.Width, result.Height, format.MIMEType())
+		}
+	}
+}
+
+// TestImageProcessor_RendersSVGSharpWhenEnlarged checks that a 16x12 SVG
+// asked for at 1600x1200 is rendered at that size, not rendered at 16x12 and
+// enlarged: 10 pixels either side of the edge between its black and white
+// halves are black and white, where an enlarged image is grey there.
+func TestImageProcessor_RendersSVGSharpWhenEnlarged(t *testing.T) {
+	t.Parallel()
+
+	_, output, err := processSVG(t, testSVG, imagerequest.Transform{
+		Size:   imagerequest.Size{Width: 1600, Height: 1200},
+		Format: imagerequest.FormatPNG,
+	})
+	if err != nil {
+		t.Fatalf("SVG as PNG at 1600x1200: %v", err)
+	}
+
+	img, err := vips.NewImageFromBuffer(output)
+	if err != nil {
+		t.Fatalf("decoding the PNG: %v", err)
+	}
+
+	t.Cleanup(img.Close)
+
+	for x, want := range map[int]float64{790: 0, 810: 255} {
+		point, err := img.GetPoint(x, 600)
+		if err != nil {
+			t.Fatalf("GetPoint(%d, 600): %v", x, err)
+		}
+
+		t.Logf("pixel at %d,600: %v", x, point)
+
+		if point[0] != want {
+			t.Errorf("pixel at %d,600 = %v, want %v", x, point[0], want)
+		}
+	}
+}
+
+// TestImageProcessor_SVGFetchesNothing checks that rendering an SVG draws no
+// image it refers to: one at a URL on a local server, by href or xlink:href,
+// or a local file that exists. Each SVG renders without error, white where
+// the image would be. An SVG that uses an external entity at that server is
+// a separate case. No SVG makes a request to the server.
+func TestImageProcessor_SVGFetchesNothing(t *testing.T) {
+	t.Parallel()
+
+	var requests atomic.Int64
+
+	// Blue is 128 in every pixel of this PNG.
+	png := createTestPNG(t, 4, 4)
+
+	server := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			requests.Add(1)
+			t.Logf("request for %s", r.URL)
+			w.Header().Set("Content-Type", "image/png")
+			_, _ = w.Write(png)
+		}))
+	t.Cleanup(server.Close)
+
+	pngPath := filepath.Join(t.TempDir(), "image.png")
+
+	err := os.WriteFile(pngPath, png, 0o600)
+	if err != nil {
+		t.Fatalf("writing the PNG file: %v", err)
+	}
+
+	for _, reference := range []string{
+		`href="` + server.URL + `/image.png"`,
+		`xlink:href="` + server.URL + `/xlink.png"`,
+		`href="file://` + pngPath + `"`,
+	} {
+		svg := `<svg xmlns="http://www.w3.org/2000/svg" ` +
+			`xmlns:xlink="http://www.w3.org/1999/xlink" width="16" height="12">` +
+			`<rect width="16" height="12" fill="white"/>` +
+			`<image ` + reference + ` width="16" height="12"/></svg>`
+
+		_, output, err := processSVG(t, svg, imagerequest.Transform{
+			Format: imagerequest.FormatPNG,
+		})
+		if err != nil {
+			t.Errorf("SVG with image %s as PNG: %v", reference, err)
+
+			continue
+		}
+
+		img, err := vips.NewImageFromBuffer(output)
+		if err != nil {
+			t.Fatalf("decoding the PNG: %v", err)
+		}
+
+		point, err := img.GetPoint(8, 6)
+		img.Close()
+
+		if err != nil {
+			t.Fatalf("GetPoint(8, 6): %v", err)
+		}
+
+		t.Logf("SVG with image %s: pixel at 8,6 = %v", reference, point)
+
+		if point[2] != 255 {
+			t.Errorf("SVG with image %s: blue at 8,6 = %v, want 255: the "+
+				"image is drawn", reference, point[2])
+		}
+	}
+
+	svg := `<?xml version="1.0"?>` +
+		`<!DOCTYPE svg [<!ENTITY outside SYSTEM "` + server.URL + `/entity">]>` +
+		`<svg xmlns="http://www.w3.org/2000/svg" width="16" height="12">` +
+		`<text y="10">&outside;</text></svg>`
+
+	// librsvg loads no external entity, so it refuses this SVG: outside is
+	// not defined.
+	_, _, err = processSVG(t, svg, imagerequest.Transform{
+		Format: imagerequest.FormatPNG,
+	})
+	t.Logf("SVG with an external entity as PNG: error = %v", err)
+
+	if requests.Load() != 0 {
+		t.Errorf("rendering the SVGs made %d requests, want none",
+			requests.Load())
+	}
+}
+
+// TestImageProcessor_RendersSVGLargerThanMaxInputDimension checks that an
+// SVG whose own size is above MaxInputDimension is limited only by the
+// output checks: it is rendered at a raster size, orig of it is the SVG
+// itself, and a raster format at its own size is ErrSVGTooLargeForOrigSize.
+// The 1000000x1000000 one is rendered at a ten-thousandth of its own size.
+func TestImageProcessor_RendersSVGLargerThanMaxInputDimension(t *testing.T) {
+	t.Parallel()
+
+	for _, svg := range []string{
+		`<svg xmlns="http://www.w3.org/2000/svg" width="10000" height="10">` +
+			`<rect width="10000" height="10"/></svg>`,
+		`<svg xmlns="http://www.w3.org/2000/svg" width="1000000" ` +
+			`height="1000000"><rect width="1000000" height="1000000"/></svg>`,
+	} {
+		result, _, err := processSVG(t, svg, imagerequest.Transform{
+			Size:   imagerequest.Size{Width: 100, Height: 100},
+			Format: imagerequest.FormatPNG,
+		})
+		if err != nil {
+			t.Errorf("%s as PNG at 100x100: %v", svg, err)
+		} else {
+			t.Logf("%s as PNG at 100x100: %dx%d", svg, result.Width, result.Height)
+
+			if result.Width != 100 || result.Height != 100 {
+				t.Errorf("%s as PNG at 100x100 = %dx%d, want 100x100", svg,
+					result.Width, result.Height)
+			}
+		}
+
+		result, output, err := processSVG(t, svg,
+			imagerequest.Transform{Format: imagerequest.FormatOriginal})
+		if err != nil || result.ContentType != imagerequest.FormatSVG.MIMEType() ||
+			string(output) != svg {
+			t.Errorf("orig of %s: error = %v, want the SVG itself", svg, err)
+		}
+
+		_, _, err = processSVG(t, svg,
+			imagerequest.Transform{Format: imagerequest.FormatPNG})
+		if !errors.Is(err, ErrSVGTooLargeForOrigSize) {
+			t.Errorf("%s as PNG at its own size: error = %v, want %v", svg,
+				err, ErrSVGTooLargeForOrigSize)
+		}
+	}
+}
+
+// TestImageProcessor_RefusesSVGTooThin checks that an SVG with one side more
+// than MaxInputDimension times the other is ErrSVGTooThin at a raster size,
+// that one at exactly that proportion is rendered, and that orig of either
+// is the SVG itself.
+func TestImageProcessor_RefusesSVGTooThin(t *testing.T) {
+	t.Parallel()
+
+	tooThin := `<svg xmlns="http://www.w3.org/2000/svg" width="81930" ` +
+		`height="10"><rect width="81930" height="10"/></svg>`
+	atTheLimit := `<svg xmlns="http://www.w3.org/2000/svg" width="81920" ` +
+		`height="10"><rect width="81920" height="10"/></svg>`
+	size := imagerequest.Size{Width: 100, Height: 100}
+
+	_, _, err := processSVG(t, tooThin,
+		imagerequest.Transform{Size: size, Format: imagerequest.FormatPNG})
+	if !errors.Is(err, ErrSVGTooThin) {
+		t.Errorf("81930x10 SVG as PNG at 100x100: error = %v, want %v", err,
+			ErrSVGTooThin)
+	}
+
+	result, _, err := processSVG(t, atTheLimit,
+		imagerequest.Transform{Size: size, Format: imagerequest.FormatPNG})
+	if err != nil {
+		t.Errorf("81920x10 SVG as PNG at 100x100: %v", err)
+	} else if result.Width != 100 || result.Height != 100 {
+		t.Errorf("81920x10 SVG as PNG at 100x100 = %dx%d, want 100x100",
+			result.Width, result.Height)
+	}
+
+	for _, svg := range []string{tooThin, atTheLimit} {
+		result, output, err := processSVG(t, svg,
+			imagerequest.Transform{Format: imagerequest.FormatOriginal})
+		if err != nil || result.ContentType != imagerequest.FormatSVG.MIMEType() ||
+			string(output) != svg {
+			t.Errorf("orig of %s: error = %v, want the SVG itself", svg, err)
+		}
+	}
+}
+
+// testSVGRenderTimeout is the time limit of the SVG time limit tests, short
+// so that they finish in a few seconds. Under the race detector a child
+// process takes about a second to start and read an SVG's size.
+const testSVGRenderTimeout = 3 * time.Second
+
+// processSVGTimed has processor process svg, as an SVG source, with transform,
+// and returns the error and how long it took.
+func processSVGTimed(
+	t *testing.T, processor *ImageProcessor, svg string,
+	transform imagerequest.Transform,
+) (time.Duration, error) {
+	t.Helper()
+
+	start := time.Now()
+	_, err := processor.Process(t.Context(), strings.NewReader(svg),
+		imagerequest.FormatSVG, &transform)
+
+	return time.Since(start), err
+}
+
+// checkProcessesAnotherImage checks that processor, after refusing an SVG,
+// processes a PNG.
+func checkProcessesAnotherImage(t *testing.T, processor *ImageProcessor) {
+	t.Helper()
+
+	_, err := processor.Process(t.Context(),
+		bytes.NewReader(createTestPNG(t, 64, 48)), imagerequest.FormatPNG,
+		&imagerequest.Transform{
+			Size:   imagerequest.Size{Width: 32, Height: 24},
+			Format: imagerequest.FormatJPEG,
+		})
+	if err != nil {
+		t.Errorf("a PNG after the SVG: %v", err)
+	}
+}
+
+// TestImageProcessor_StopsSlowSVGRender checks that an SVG that takes about
+// 12 seconds to render, 600 shapes each drawn with an feTurbulence filter
+// effect, is ErrSVGRenderFailed soon after a time limit of
+// testSVGRenderTimeout, not when the render would end. The processor then
+// processes another image.
+func TestImageProcessor_StopsSlowSVGRender(t *testing.T) {
+	t.Parallel()
+
+	svg := `<svg xmlns="http://www.w3.org/2000/svg" width="256" height="256">` +
+		`<filter id="noise">` +
+		`<feTurbulence baseFrequency="0.05" numOctaves="4"/></filter>` +
+		strings.Repeat(`<rect width="256" height="256" filter="url(#noise)"/>`,
+			600) +
+		`</svg>`
+
+	processor := newSVGTestProcessor()
+	processor.svgRenderTimeout = testSVGRenderTimeout
+
+	elapsed, err := processSVGTimed(t, processor, svg, imagerequest.Transform{
+		Size:   imagerequest.Size{Width: 256, Height: 256},
+		Format: imagerequest.FormatPNG,
+	})
+	t.Logf("slow SVG as PNG: error %v after %v", err, elapsed)
+
+	if !errors.Is(err, ErrSVGRenderFailed) {
+		t.Errorf("slow SVG as PNG: error = %v, want %v", err,
+			ErrSVGRenderFailed)
+	}
+
+	if elapsed > testSVGRenderTimeout+2*time.Second {
+		t.Errorf("slow SVG as PNG: refused after %v, want soon after %v",
+			elapsed, testSVGRenderTimeout)
+	}
+
+	checkProcessesAnotherImage(t, processor)
+}
+
+// TestImageProcessor_StopsSlowSVGSizeRead checks that orig of an SVG whose
+// size takes about 10 seconds to read is ErrSVGRenderFailed soon after a time
+// limit of testSVGRenderTimeout. To read the size, librsvg matches each of the
+// SVG's 2500 CSS rules against each of its 2500 shapes, walking up the 75
+// groups around it. The processor then processes another image.
+func TestImageProcessor_StopsSlowSVGSizeRead(t *testing.T) {
+	t.Parallel()
+
+	var rules strings.Builder
+
+	for i := range 2500 {
+		fmt.Fprintf(&rules, ".group%d rect { fill: red }", i)
+	}
+
+	svg := `<svg xmlns="http://www.w3.org/2000/svg" width="16" height="12">` +
+		`<style>` + rules.String() + `</style>` + strings.Repeat(`<g>`, 75) +
+		strings.Repeat(`<rect width="1" height="1"/>`, 2500) +
+		strings.Repeat(`</g>`, 75) + `</svg>`
+
+	processor := newSVGTestProcessor()
+	processor.svgRenderTimeout = testSVGRenderTimeout
+
+	elapsed, err := processSVGTimed(t, processor, svg,
+		imagerequest.Transform{Format: imagerequest.FormatOriginal})
+	t.Logf("orig of the SVG: error %v after %v", err, elapsed)
+
+	if !errors.Is(err, ErrSVGRenderFailed) {
+		t.Errorf("orig of the SVG: error = %v, want %v", err,
+			ErrSVGRenderFailed)
+	}
+
+	if elapsed > testSVGRenderTimeout+2*time.Second {
+		t.Errorf("orig of the SVG: refused after %v, want soon after %v",
+			elapsed, testSVGRenderTimeout)
+	}
+
+	checkProcessesAnotherImage(t, processor)
+}
+
+// TestImageProcessor_StopsSVGRenderOverMemoryLimit checks that an SVG that
+// needs about 500 MiB more than a child process's own memory to render, a
+// chain of 2000 blurs that each keep their result, is ErrSVGRenderFailed with
+// a memory limit of 512 MiB, and that a plain SVG renders under the same
+// limit. A child process needs about 130 MiB of its own, and about 350 MiB
+// under the race detector, so the chain also renders under MaxSVGRenderMemory.
+func TestImageProcessor_StopsSVGRenderOverMemoryLimit(t *testing.T) {
+	t.Parallel()
+
+	var blurs strings.Builder
+
+	blurs.WriteString(`<feGaussianBlur stdDeviation="3" result="blur0"/>`)
+
+	for i := 1; i < 2000; i++ {
+		fmt.Fprintf(&blurs,
+			`<feGaussianBlur in="blur%d" stdDeviation="3" result="blur%d"/>`,
+			i-1, i)
+	}
+
+	svg := `<svg xmlns="http://www.w3.org/2000/svg" width="256" height="256">` +
+		`<filter id="blurs" color-interpolation-filters="sRGB">` +
+		blurs.String() + `</filter>` +
+		`<rect width="256" height="256" fill="red" filter="url(#blurs)"/></svg>`
+	size := imagerequest.Size{Width: 256, Height: 256}
+
+	processor := newSVGTestProcessor()
+	processor.maxSVGRenderMemory = 512 << 20
+
+	elapsed, err := processSVGTimed(t, processor, svg,
+		imagerequest.Transform{Size: size, Format: imagerequest.FormatPNG})
+	t.Logf("SVG of 2000 blurs as PNG: error %v after %v", err, elapsed)
+
+	if !errors.Is(err, ErrSVGRenderFailed) {
+		t.Errorf("SVG of 2000 blurs as PNG: error = %v, want %v", err,
+			ErrSVGRenderFailed)
+	}
+
+	_, err = processSVGTimed(t, processor, testSVG,
+		imagerequest.Transform{Size: size, Format: imagerequest.FormatPNG})
+	if err != nil {
+		t.Errorf("plain SVG as PNG after it: %v", err)
+	}
+}
+
+// TestSVGRenderSize checks the size at which an SVG is rendered to cover an
+// output size, and that it is held to MaxInputDimension.
+func TestSVGRenderSize(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                      string
+		width, height             int
+		outputWidth, outputHeight int
+		wantWidth, wantHeight     int
+	}{
+		{"own size", 16, 12, 16, 12, 16, 12},
+		{"100 times larger", 16, 12, 1600, 1200, 1600, 1200},
+		{"wider than the SVG's proportions", 16, 12, 160, 12, 160, 120},
+		{"10 times smaller", 100, 100, 10, 10, 10, 10},
+		{"held to MaxInputDimension", 8192, 1, 8192, 8192, 8192, 1},
+		{"held to MaxInputDimension when enlarged", 1024, 1, 8192, 8192, 8192, 8},
+		{"far above MaxInputDimension", 1000000, 1000000, 100, 100, 100, 100},
+	}
+
+	for _, tt := range tests {
+		gotWidth, gotHeight := svgRenderSize(tt.width, tt.height,
+			tt.outputWidth, tt.outputHeight)
+		if gotWidth != tt.wantWidth || gotHeight != tt.wantHeight {
+			t.Errorf("%s: svgRenderSize(%d, %d, %d, %d) = %dx%d, want %dx%d",
+				tt.name, tt.width, tt.height, tt.outputWidth, tt.outputHeight,
+				gotWidth, gotHeight, tt.wantWidth, tt.wantHeight)
+		}
+	}
+}

internal/imageproxy/imageproxy.go

@@ -37,11 +37,15 @@ type Proxy struct {
 	// same time share one fetch and one transcode.
 	variantsInProgress singleflight.Group
 
-	// The counts of upstream fetches and transforms, made and registered by
-	// New. They are kept in memory, so adding to one waits for nothing.
-	upstreamFetches    prometheus.Counter
-	upstreamFetchBytes prometheus.Counter
-	transforms         prometheus.Counter
+	// The metrics of upstream fetches and transforms, made and registered by
+	// New. They are kept in memory, so updating one waits for nothing.
+	upstreamFetches        prometheus.Counter
+	upstreamFetchBytes     prometheus.Counter
+	upstreamFetchDurations prometheus.Histogram
+	upstreamFetchErrors    prometheus.Counter
+	transforms             prometheus.Counter
+	transformDurations     prometheus.Histogram
+	transformsInProgress   prometheus.Gauge
 }
 
 // Config holds the proxy's configuration.
@@ -62,9 +66,9 @@ type Config struct {
 // errCacheRequired is returned when the proxy is built without a cache.
 var errCacheRequired = errors.New("cache is required")
 
-// New creates a new image proxy and registers its counters on
+// New creates a new image proxy and registers its metrics on
 // metricsRegistry. pixad passes Prometheus' default registry, the one
-// /metrics serves. It fails when metricsRegistry already holds counters of
+// /metrics serves. It fails when metricsRegistry already holds metrics of
 // the same names.
 func New(
 	cfg *Config, metricsRegistry prometheus.Registerer,
@@ -99,18 +103,39 @@ func New(
 			Name: "pixa_upstream_fetch_bytes_total",
 			Help: "Bytes read from upstream by fetches of source images.",
 		}),
+		upstreamFetchDurations: prometheus.NewHistogram(prometheus.HistogramOpts{
+			Name: "pixa_upstream_fetch_duration_seconds",
+			Help: "Duration of the fetches pixa_upstream_fetches_total counts, " +
+				"until the response's last byte.",
+		}),
+		upstreamFetchErrors: prometheus.NewCounter(prometheus.CounterOpts{
+			Name: "pixa_upstream_fetch_errors_total",
+			Help: "Fetches of a source image from upstream that failed, " +
+				"before or while reading the response.",
+		}),
 		transforms: prometheus.NewCounter(prometheus.CounterOpts{
 			Name: "pixa_transforms_total",
 			Help: "Images the image processor transformed.",
 		}),
+		transformDurations: prometheus.NewHistogram(prometheus.HistogramOpts{
+			Name: "pixa_transform_duration_seconds",
+			Help: "Duration of the transforms pixa_transforms_total counts, " +
+				"waiting for a processing slot included.",
+		}),
+		transformsInProgress: prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: "pixa_transforms_in_progress",
+			Help: "Images being transformed or waiting for a processing slot.",
+		}),
 	}
 
-	for _, counter := range []prometheus.Counter{
-		p.upstreamFetches, p.upstreamFetchBytes, p.transforms,
+	for _, metric := range []prometheus.Collector{
+		p.upstreamFetches, p.upstreamFetchBytes, p.upstreamFetchDurations,
+		p.upstreamFetchErrors, p.transforms, p.transformDurations,
+		p.transformsInProgress,
 	} {
-		err = metricsRegistry.Register(counter)
+		err = metricsRegistry.Register(metric)
 		if err != nil {
-			return nil, fmt.Errorf("failed to register the proxy's counters: %w", err)
+			return nil, fmt.Errorf("failed to register the proxy's metrics: %w", err)
 		}
 	}
 
@@ -168,16 +193,16 @@ func (p *Proxy) Get(
 ) (*ImageResponse, error) {
 	cacheKey := imagerequest.CacheKey(req)
 
-	reader, size, contentType, err := p.cache.Variant(cacheKey)
+	cached, err := p.cache.Variant(cacheKey)
 	if err == nil {
 		p.cache.IncrementStats(true)
 
 		return &ImageResponse{
-			Content:       reader,
-			ContentLength: size,
-			ContentType:   contentType,
+			Content:       cached.Content,
+			ContentLength: cached.Size,
+			ContentType:   cached.ContentType,
 			CacheStatus:   CacheHit,
-			ETag:          formatETag(cacheKey),
+			ETag:          formatETag(cacheKey, cached.CheckedAt),
 		}, nil
 	}
 
@@ -288,7 +313,7 @@ func (p *Proxy) processOrWait(
 		ContentLength: int64(len(variant.data)),
 		ContentType:   variant.contentType,
 		FetchedBytes:  variant.fetchedBytes,
-		ETag:          formatETag(cacheKey),
+		ETag:          formatETag(cacheKey, variant.checkedAt),
 	}, nil
 }
 
@@ -341,7 +366,10 @@ func (p *Proxy) loadCachedSource(
 }
 
 // processFromSourceOrFetch makes the variant req asks for, from the cached
-// source when there is one and from a fetched source otherwise.
+// source when there is one and from a fetched source otherwise. It counts a
+// source cache miss when it fetches the source because the source has no row
+// in the cache or its cached file cannot be used, and a source cache hit
+// otherwise.
 //
 // A cached source last checked with upstream CacheTTL or more ago is checked
 // first: upstream is asked to send it only if it changed since. When it
@@ -362,6 +390,8 @@ func (p *Proxy) processFromSourceOrFetch(
 			p.log.Warn("failed to look up cached source, fetching", "error", err)
 		}
 
+		p.cache.IncrementSourceStats(false)
+
 		return p.fetchAndProcess(ctx, req, cacheKey)
 	}
 
@@ -375,16 +405,21 @@ func (p *Proxy) processFromSourceOrFetch(
 		case err != nil:
 			p.recordFailedCheck(ctx, req, err)
 		default:
+			p.cache.IncrementSourceStats(true)
+
 			return p.processFetched(ctx, req, cacheKey, fetched)
 		}
 	}
 
 	variant := p.renewVariant(ctx, cacheKey, cached)
 	if variant != nil {
+		p.cache.IncrementSourceStats(true)
+
 		return variant, nil
 	}
 
 	source, sourceSize, sourceFormat := p.loadCachedSource(ctx, req.Source)
+	p.cache.IncrementSourceStats(source != nil)
 
 	// Fetch from upstream if we don't have source data or it's empty
 	if source == nil {
@@ -439,7 +474,7 @@ func (p *Proxy) renewVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey,
 	source imgcache.SourceInfo,
 ) *processedVariant {
-	reader, _, contentType, err := p.cache.RenewVariant(ctx, cacheKey, source)
+	cached, err := p.cache.RenewVariant(ctx, cacheKey, source)
 	if err != nil {
 		if !errors.Is(err, imgcache.ErrNotFound) {
 			p.log.Warn("cached variant not renewed, making it again",
@@ -449,7 +484,7 @@ func (p *Proxy) renewVariant(
 		return nil
 	}
 
-	return p.readCachedVariant(cacheKey, reader, contentType)
+	return p.readCachedVariant(cacheKey, cached)
 }
 
 // cachedVariantIgnoringTTL returns the cached variant of cacheKey whatever
@@ -460,7 +495,7 @@ func (p *Proxy) renewVariant(
 func (p *Proxy) cachedVariantIgnoringTTL(
 	ctx context.Context, cacheKey imagerequest.VariantKey,
 ) (*processedVariant, imgcache.SourceInfo) {
-	reader, _, contentType, err := p.cache.VariantIgnoringTTL(cacheKey)
+	cached, err := p.cache.VariantIgnoringTTL(cacheKey)
 	if err != nil {
 		if !errors.Is(err, imgcache.ErrNotFound) {
 			p.log.Warn("cached variant not served", "cache_key", cacheKey,
@@ -470,7 +505,7 @@ func (p *Proxy) cachedVariantIgnoringTTL(
 		return nil, imgcache.SourceInfo{}
 	}
 
-	variant := p.readCachedVariant(cacheKey, reader, contentType)
+	variant := p.readCachedVariant(cacheKey, cached)
 	if variant == nil {
 		return nil, imgcache.SourceInfo{}
 	}
@@ -484,15 +519,14 @@ func (p *Proxy) cachedVariantIgnoringTTL(
 	return variant, madeFrom
 }
 
-// readCachedVariant reads the cached variant of cacheKey that reader opened,
-// whose content type is contentType, and closes reader. It returns nil when
-// the variant cannot be read.
+// readCachedVariant reads the cached variant of cacheKey from its opened file,
+// and closes the file. It returns nil when the variant cannot be read.
 func (p *Proxy) readCachedVariant(
-	cacheKey imagerequest.VariantKey, reader io.ReadCloser, contentType string,
+	cacheKey imagerequest.VariantKey, file imgcache.VariantFile,
 ) *processedVariant {
-	defer func() { _ = reader.Close() }()
+	defer func() { _ = file.Content.Close() }()
 
-	data, err := io.ReadAll(reader)
+	data, err := io.ReadAll(file.Content)
 	if err != nil {
 		p.log.Warn("failed to read cached variant", "cache_key", cacheKey,
 			"error", err)
@@ -500,7 +534,11 @@ func (p *Proxy) readCachedVariant(
 		return nil
 	}
 
-	return &processedVariant{data: data, contentType: contentType}
+	return &processedVariant{
+		data:        data,
+		contentType: file.ContentType,
+		checkedAt:   file.CheckedAt,
+	}
 }
 
 // fetchAndProcess fetches the source from upstream, stores it, and makes
@@ -528,7 +566,9 @@ func (p *Proxy) fetchAndProcess(
 	case variant == nil:
 		return nil, err
 	case errors.Is(err, errSourceNotModified):
-		err = p.cache.SetVariantCheckedAt(ctx, cacheKey, time.Now().UTC())
+		variant.checkedAt = time.Now().UTC()
+
+		err = p.cache.SetVariantCheckedAt(ctx, cacheKey, variant.checkedAt)
 		if err != nil {
 			p.log.Warn("failed to renew cached variant", "cache_key", cacheKey,
 				"error", err)
@@ -576,36 +616,32 @@ type fetchedSource struct {
 // fetchSource fetches req's source from upstream and stores it in the cache.
 // A non-empty ifNoneMatch or ifModifiedSince asks upstream to send the source
 // only if it changed since; errSourceNotModified says it did not. It counts
-// the fetch with the bytes read from upstream, including when reading the
-// response or a later step fails. It fetches nothing while the source URL is
-// in the negative cache. The caller closes the returned response.
+// the fetch with the bytes read from upstream and its duration, including
+// when reading the response or a later step fails, and counts a fetch that
+// fails or whose response cannot be read as an upstream fetch error. It
+// fetches nothing while the source URL is in the negative cache. The caller
+// closes the returned response.
 func (p *Proxy) fetchSource(
 	ctx context.Context,
 	req *imagerequest.Request,
 	ifNoneMatch, ifModifiedSince string,
 ) (*fetchedSource, error) {
-	// Check negative cache - skip fetching for recently-failed URLs
-	negHit, err := p.cache.Failure(ctx, req.Source)
+	err := p.refuseNegativeCached(ctx, req)
 	if err != nil {
-		p.log.Warn("negative cache check failed", "error", err)
-	}
-
-	if negHit {
-		p.log.Debug("negative cache hit",
-			"host", req.Source.Host,
-			"path", req.Source.Path,
-		)
-
-		return nil, fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
+		return nil, err
 	}
 
 	// Fetch from upstream
 	p.log.Debug("fetching from upstream", "host", req.Source.Host,
 		"path", req.Source.Path, "query", req.Source.Query)
 
+	fetchStart := time.Now()
+
 	fetchResult, err := p.fetcher.Fetch(ctx, req.Source,
 		ifNoneMatch, ifModifiedSince)
 	if err != nil {
+		p.upstreamFetchErrors.Inc()
+
 		// Store negative cache for certain errors
 		if isNegativeCacheable(err) {
 			p.storeNegative(ctx, req)
@@ -629,9 +665,11 @@ func (p *Proxy) fetchSource(
 	fetchBytes := int64(len(sourceData))
 
 	// Counted also when the request context has ended meanwhile
-	p.incrementUpstreamFetch(fetchBytes)
+	p.incrementUpstreamFetch(fetchBytes, time.Since(fetchStart))
 
 	if err != nil {
+		p.upstreamFetchErrors.Inc()
+
 		_ = fetchResult.Content.Close()
 
 		return nil, fmt.Errorf("failed to read upstream response: %w", err)
@@ -672,6 +710,29 @@ func (p *Proxy) fetchSource(
 	}, nil
 }
 
+// refuseNegativeCached returns an error wrapping ErrNegativeCached while
+// req's source URL is in the negative cache, and nil otherwise. A failure to
+// read the negative cache is only logged, and returns nil.
+func (p *Proxy) refuseNegativeCached(
+	ctx context.Context, req *imagerequest.Request,
+) error {
+	negHit, err := p.cache.Failure(ctx, req.Source)
+	if err != nil {
+		p.log.Warn("negative cache check failed", "error", err)
+	}
+
+	if !negHit {
+		return nil
+	}
+
+	p.log.Debug("negative cache hit",
+		"host", req.Source.Host,
+		"path", req.Source.Path,
+	)
+
+	return fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamError, ErrNegativeCached)
+}
+
 // fetchedSourceInfo returns what the cache keeps about a source upstream
 // just sent: fetched and checked now, with the ETag and Last-Modified
 // headers of fetchResult.
@@ -717,15 +778,16 @@ func (p *Proxy) logUpstreamFetch(
 	)
 }
 
-// incrementUpstreamFetch counts one upstream fetch that read fetchBytes bytes.
-// A fetch that read no bytes is not counted.
-func (p *Proxy) incrementUpstreamFetch(fetchBytes int64) {
+// incrementUpstreamFetch counts one upstream fetch that read fetchBytes bytes
+// and took duration. A fetch that read no bytes is not counted.
+func (p *Proxy) incrementUpstreamFetch(fetchBytes int64, duration time.Duration) {
 	if fetchBytes <= 0 {
 		return
 	}
 
 	p.upstreamFetches.Inc()
 	p.upstreamFetchBytes.Add(float64(fetchBytes))
+	p.upstreamFetchDurations.Observe(duration.Seconds())
 }
 
 // storeNegative stores a negative cache entry for req's source, whose fetch
@@ -745,6 +807,9 @@ type processedVariant struct {
 	data         []byte
 	contentType  string
 	fetchedBytes int64
+	// checkedAt is its source's check time when the variant was made or last
+	// renewed, which its cached file has as its modification time
+	checkedAt time.Time
 }
 
 // processAndStore processes the image read from source, which is in
@@ -764,8 +829,7 @@ func (p *Proxy) processAndStore(
 
 	// ctx has the deadline of the request doing the processing, which can
 	// end while a request waiting for its image has time left
-	processResult, err := p.processor.Process(ctx, source, sourceFormat,
-		&req.Transform)
+	processResult, err := p.transform(ctx, source, sourceFormat, &req.Transform)
 	if errors.Is(err, context.DeadlineExceeded) {
 		return nil, fmt.Errorf("%w: %w", ErrProcessingTimedOut, err)
 	}
@@ -778,6 +842,7 @@ func (p *Proxy) processAndStore(
 
 	// Counted also when the request context has ended meanwhile
 	p.transforms.Inc()
+	p.transformDurations.Observe(processDuration.Seconds())
 
 	// Read processed content
 	processedData, err := io.ReadAll(processResult.Content)
@@ -826,9 +891,25 @@ func (p *Proxy) processAndStore(
 		data:         processedData,
 		contentType:  processResult.ContentType,
 		fetchedBytes: fetchBytes,
+		checkedAt:    sourceInfo.CheckedAt,
 	}, nil
 }
 
+// transform has the image processor transform the image read from source
+// as requested says, and counts it in pixa_transforms_in_progress until the
+// processor returns or panics.
+func (p *Proxy) transform(
+	ctx context.Context,
+	source io.Reader,
+	sourceFormat imagerequest.Format,
+	requested *imagerequest.Transform,
+) (*imageprocessor.Result, error) {
+	p.transformsInProgress.Inc()
+	defer p.transformsInProgress.Dec()
+
+	return p.processor.Process(ctx, source, sourceFormat, requested)
+}
+
 // isNegativeCacheable returns true if the error should be cached.
 func isNegativeCacheable(err error) bool {
 	return errors.Is(err, httpfetcher.ErrUpstreamError)
@@ -837,13 +918,16 @@ func isNegativeCacheable(err error) bool {
 // etagHashLength is the number of hash characters to use for ETags.
 const etagHashLength = 16
 
-// formatETag formats a VariantKey as a quoted ETag value.
-func formatETag(key imagerequest.VariantKey) string {
+// formatETag returns the quoted ETag of the variant of key: the first
+// etagHashLength characters of key, then checkedAt in Unix nanoseconds.
+// checkedAt is its source's check time when the variant was made or last
+// renewed, so a variant made again from new source bytes gets another ETag.
+func formatETag(key imagerequest.VariantKey, checkedAt time.Time) string {
 	hash := string(key)
 	// Use first 16 characters of hash for a shorter but still unique ETag
 	if len(hash) > etagHashLength {
 		hash = hash[:etagHashLength]
 	}
 
-	return `"` + hash + `"`
+	return fmt.Sprintf(`"%s-%d"`, hash, checkedAt.UnixNano())
 }

internal/imageproxy/metrics_internal_test.go

@@ -0,0 +1,213 @@
+package imageproxy
+
+import (
+	"os"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// metricValues holds the source cache, upstream fetch and transform metrics
+// of a proxy and its cache. A histogram is given by its count of
+// observations.
+type metricValues struct {
+	sourceCacheHits   uint64
+	sourceCacheMisses uint64
+	fetchDurations    uint64
+	fetchErrors       uint64
+	transformDuration uint64
+	inProgress        float64
+}
+
+// readMetricValues reads the metrics of svc and its cache that
+// metricValues holds from their metrics registry.
+func readMetricValues(t *testing.T, svc *testProxy) metricValues {
+	t.Helper()
+
+	families, err := svc.registry.Gather()
+	if err != nil {
+		t.Fatalf("failed to gather the metrics: %v", err)
+	}
+
+	var values metricValues
+
+	for _, family := range families {
+		metric := family.GetMetric()[0]
+
+		switch family.GetName() {
+		case "pixa_source_cache_hits_total":
+			values.sourceCacheHits = uint64(metric.GetCounter().GetValue())
+		case "pixa_source_cache_misses_total":
+			values.sourceCacheMisses = uint64(metric.GetCounter().GetValue())
+		case "pixa_upstream_fetch_duration_seconds":
+			values.fetchDurations = metric.GetHistogram().GetSampleCount()
+		case "pixa_upstream_fetch_errors_total":
+			values.fetchErrors = uint64(metric.GetCounter().GetValue())
+		case "pixa_transform_duration_seconds":
+			values.transformDuration = metric.GetHistogram().GetSampleCount()
+		case "pixa_transforms_in_progress":
+			values.inProgress = metric.GetGauge().GetValue()
+		}
+	}
+
+	return values
+}
+
+// TestProxy_Get_CountsSourceCacheFetchesAndTransforms walks Get through a
+// miss that fetches the source, a hit, a miss that uses the cached source
+// and a miss whose source is not found upstream, checking the source cache,
+// upstream fetch and transform metrics after each.
+func TestProxy_Get_CountsSourceCacheFetchesAndTransforms(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := setupTestProxy(t)
+
+	// want is source cache hits, source cache misses, fetch durations
+	// observed, fetch errors, transform durations observed, transforms in
+	// progress.
+	steps := []struct {
+		name    string
+		path    string
+		size    int
+		wantErr bool
+		want    metricValues
+	}{
+		{"miss that fetches the source", testPathPhoto, 50, false,
+			metricValues{0, 1, 1, 0, 1, 0}},
+		{"hit", testPathPhoto, 50, false,
+			metricValues{0, 1, 1, 0, 1, 0}},
+		{"miss that uses the cached source", testPathPhoto, 25, false,
+			metricValues{1, 1, 1, 0, 2, 0}},
+		{"miss whose source is not found", "/images/absent.jpg", 50, true,
+			metricValues{1, 2, 1, 1, 2, 0}},
+	}
+
+	for _, step := range steps {
+		resp, err := svc.Get(t.Context(), &imagerequest.Request{
+			Source: imagerequest.Source{
+				Host: fixtures.GoodHost,
+				Path: step.path,
+			},
+			Transform: imagerequest.Transform{
+				Size:    imagerequest.Size{Width: step.size, Height: step.size},
+				Format:  imagerequest.FormatJPEG,
+				Quality: 85,
+				FitMode: imagerequest.FitCover,
+			},
+		})
+		if (err != nil) != step.wantErr {
+			t.Fatalf("%s: Get() error = %v, want error %t", step.name, err, step.wantErr)
+		}
+
+		if err == nil {
+			_ = resp.Content.Close()
+		}
+
+		got := readMetricValues(t, svc)
+		t.Logf("after the %s: metrics = %+v", step.name, got)
+
+		if got != step.want {
+			t.Fatalf("after the %s: metrics = %+v, want %+v", step.name, got, step.want)
+		}
+	}
+}
+
+// TestProxy_Get_CountsMissingSourceFileAsSourceCacheMiss deletes the file of
+// a cached source but keeps its row. A request for a second variant of that
+// source must fetch the source again, and count a source cache miss, not a
+// hit.
+func TestProxy_Get_CountsMissingSourceFileAsSourceCacheMiss(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := setupTestProxy(t)
+
+	// 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()
+
+	source := widthOnlyRequest(fixtures, 50).Source
+
+	err = os.Remove(sourceFilePath(svc, source.Key()))
+	if err != nil {
+		t.Fatalf("failed to delete the cached source file: %v", err)
+	}
+
+	before := readMetricValues(t, svc)
+
+	resp, err = svc.Get(t.Context(), widthOnlyRequest(fixtures, 25))
+	if err != nil {
+		t.Fatalf("Get() of a second variant error = %v", err)
+	}
+
+	_ = resp.Content.Close()
+
+	after := readMetricValues(t, svc)
+	t.Logf("before the second variant: metrics = %+v; after: %+v", before, after)
+
+	if after.sourceCacheHits != before.sourceCacheHits ||
+		after.sourceCacheMisses != before.sourceCacheMisses+1 {
+		t.Errorf("source cache hits went from %d to %d and misses from %d to %d, "+
+			"want no new hit and one more miss", before.sourceCacheHits,
+			after.sourceCacheHits, before.sourceCacheMisses, after.sourceCacheMisses)
+	}
+}
+
+// TestProxy_Get_CountsTransformWaitingForSlotAsInProgress holds the only
+// processing slot and starts a request that must transform the photo. While
+// the request waits for the slot, pixa_transforms_in_progress must be 1; once
+// the request returns, 0.
+func TestProxy_Get_CountsTransformWaitingForSlotAsInProgress(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures := setupTestProxy(t)
+	svc.processor = imageprocessor.New(
+		imageprocessor.Params{MaxInputBytes: 50 << 20, MaxConcurrentProcessing: 1},
+	)
+
+	release := holdProcessingSlot(t, svc.processor)
+
+	result := startGet(t.Context(), svc, widthOnlyRequest(fixtures, 50))
+
+	// The request fetches the photo first, then waits for the slot.
+	deadline := time.Now().Add(5 * time.Second)
+	for readMetricValues(t, svc).inProgress != 1 && time.Now().Before(deadline) {
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	waiting := readMetricValues(t, svc).inProgress
+	t.Logf("while the request waits for the slot: transforms in progress = %v",
+		waiting)
+
+	select {
+	case got := <-result:
+		t.Fatalf("Get() returned while the only processing slot was held, "+
+			"error = %v", got.err)
+	default:
+	}
+
+	if waiting != 1 {
+		t.Fatalf("transforms in progress = %v while the request waits for the "+
+			"slot, want 1", waiting)
+	}
+
+	release()
+
+	got := <-result
+	if got.err != nil {
+		t.Fatalf("Get() error = %v", got.err)
+	}
+
+	returned := readMetricValues(t, svc).inProgress
+	t.Logf("once the request returned: transforms in progress = %v", returned)
+
+	if returned != 0 {
+		t.Errorf("transforms in progress = %v once the request returned, want 0",
+			returned)
+	}
+}

internal/imageproxy/negative_cache_internal_test.go

@@ -82,15 +82,3 @@ func TestProxy_Get_ServesCachedVariantOfNegativeCachedURL(t *testing.T) {
 			err, ErrNegativeCached)
 	}
 }
-
-func TestProxy_Get_ReturnsErrorForNegativeCachedURL(t *testing.T) {
-	t.Parallel()
-
-	// This test verifies that Proxy.Get() checks the negative cache
-	// We can't easily test the full pipeline without vips, but we can
-	// verify the error type
-	err := ErrNegativeCached
-	if !errors.Is(err, ErrNegativeCached) {
-		t.Error("ErrNegativeCached should be identifiable with errors.Is")
-	}
-}

internal/imagerequest/imagerequest.go

@@ -14,7 +14,8 @@ import (
 var ErrInvalidFitMode = errors.New("invalid fit mode")
 
 // Format is an image format. pixa reads and writes the formats from JPEG to
-// GIF below. In a request, orig and auto stand for one of them.
+// GIF below. It reads SVG and writes it only as orig of an SVG source. In a
+// request, orig and auto stand for another format.
 type Format string
 
 // Supported image formats.
@@ -26,6 +27,7 @@ const (
 	FormatAVIF     Format = "avif"
 	FormatJXL      Format = "jxl"
 	FormatGIF      Format = "gif"
+	FormatSVG      Format = "svg"
 
 	// FormatAuto stands for JPEG XL, AVIF, WebP or JPEG, chosen for each
 	// request from its Accept header once the URL's signature or token has
@@ -50,6 +52,8 @@ func (f Format) MIMEType() string {
 		return "image/jxl"
 	case FormatGIF:
 		return "image/gif"
+	case FormatSVG:
+		return "image/svg+xml"
 	case FormatOriginal, FormatAuto:
 		return ""
 	default:

internal/imgcache/cache.go

@@ -3,8 +3,8 @@
 //   - the variants made from them
 //   - the negative cache: the source URLs whose fetch failed recently
 //
-// Each image is a file with a row in the database. A source and its
-// variants are served until the cache TTL has passed since upstream last sent
+// Each image is a file with a row in the database. A source is used, and its
+// variants are served, until the cache TTL has passed since upstream last sent
 // the source or confirmed it unchanged. Then the source is due to be checked
 // with upstream again (see Stale). Eviction removes:
 //   - the least recently used images, to keep the cache under its size limit
@@ -45,14 +45,14 @@ const cleanExpiredBatchSize = 1000
 const DefaultNegativeTTL = 5 * time.Minute
 
 // DefaultCacheTTL is how long after upstream last sent a cached source, or
-// answered that it had not changed, pixad serves the source and its
+// answered that it had not changed, pixad uses the source and serves its
 // variants without asking upstream again.
 const DefaultCacheTTL = 24 * time.Hour
 
 // EvictionAfterLastCheck is how long after upstream last sent a cached
 // source, or answered that it had not changed, the source and its variants
 // are evicted (see EvictNotCheckedRecently). Until then a source whose checks
-// fail is served.
+// fail is used.
 const EvictionAfterLastCheck = 30 * 24 * time.Hour
 
 // ErrStale is returned by Variant for a variant whose file's modification
@@ -64,7 +64,7 @@ type Config struct {
 	StateDir    string
 	NegativeTTL time.Duration
 
-	// CacheTTL is how long a cached source and its variants are served
+	// CacheTTL is how long a cached source is used, and its variants served,
 	// before upstream is asked again whether the source changed. Zero means
 	// DefaultCacheTTL.
 	CacheTTL time.Duration
@@ -111,10 +111,16 @@ type Cache struct {
 	pendingAccessTimesDone    chan struct{}
 	pendingAccessTimesCancel  context.CancelFunc
 
-	// The counts of cache hits and misses, made and registered by
-	// registerCounters (see counters.go)
-	cacheHits   prometheus.Counter
-	cacheMisses prometheus.Counter
+	// The cache's metrics, made and registered by registerMetrics (see
+	// metrics.go)
+	cacheHits         prometheus.Counter
+	cacheMisses       prometheus.Counter
+	sourceCacheHits   prometheus.Counter
+	sourceCacheMisses prometheus.Counter
+	usageBytes        prometheus.Gauge
+	maxBytes          prometheus.Gauge
+	evictions         prometheus.Counter
+	evictedBytes      prometheus.Counter
 
 	// reconciliationPageSize is the most rows one read of a content table
 	// returns in the reconciliation pass. NewCache sets it to
@@ -128,7 +134,7 @@ type Cache struct {
 	reconciliationReadTestHook func(rows int)
 }
 
-// NewCache creates a new cache instance and registers its counters on
+// NewCache creates a new cache instance and registers its metrics on
 // metricsRegistry. pixad passes Prometheus' default registry, the one
 // /metrics serves. It makes no database call.
 func NewCache(
@@ -157,7 +163,7 @@ func NewCache(
 		reconciliationPageSize: defaultReconciliationPageSize,
 	}
 
-	err := c.registerCounters(metricsRegistry)
+	err := c.registerMetrics(metricsRegistry)
 	if err != nil {
 		return nil, err
 	}
@@ -186,31 +192,43 @@ func NewCache(
 	return c, nil
 }
 
-// Variant opens a cached variant and returns a reader, its size and its
-// content type, read from the variant file's first bytes. It opens that one
-// file and makes no database call: the file's modification time is its
-// source's check time when the variant was made or last renewed (see
-// PutVariant), and a variant whose time is CacheTTL or more ago is ErrStale.
-// A variant it opens has its access time handed to the background writer
-// without waiting (see touchVariant). A variant not in the cache, or any
-// variant of a disabled cache, is ErrNotFound; a variant file whose first
+// VariantFile is the open file of a cached variant, as Variant,
+// VariantIgnoringTTL and RenewVariant return it.
+type VariantFile struct {
+	// Content reads the file. The caller closes it.
+	Content io.ReadCloser
+	// Size is the file's size in bytes.
+	Size int64
+	// ContentType is the MIME type of the variant's format, read from the
+	// file's first bytes.
+	ContentType string
+	// CheckedAt is the file's modification time: its source's check time
+	// when the variant was made or last renewed.
+	CheckedAt time.Time
+}
+
+// Variant opens a cached variant and returns its file (see VariantFile). It
+// opens that one file and makes no database call: the file's modification
+// time is its source's check time when the variant was made or last renewed
+// (see PutVariant), and a variant whose time is CacheTTL or more ago is
+// ErrStale. A variant it opens has its access time handed to the background
+// writer without waiting (see touchVariant). A variant not in the cache, or
+// any variant of a disabled cache, is ErrNotFound; a variant file whose first
 // bytes are not those of an output format is another error.
-func (c *Cache) Variant(
-	cacheKey imagerequest.VariantKey,
-) (io.ReadCloser, int64, string, error) {
+func (c *Cache) Variant(cacheKey imagerequest.VariantKey) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, info, err := c.openVariant(cacheKey)
+	file, info, err := c.variants.Open(string(cacheKey))
 	if err != nil {
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	if c.Stale(info.ModTime()) {
 		_ = file.Close()
 
-		return nil, 0, "", ErrStale
+		return VariantFile{}, ErrStale
 	}
 
 	return c.finishOpeningVariant(cacheKey, file, info)
@@ -221,14 +239,14 @@ func (c *Cache) Variant(
 // cached, before it checks the source with upstream (see LookupVariant).
 func (c *Cache) VariantIgnoringTTL(
 	cacheKey imagerequest.VariantKey,
-) (io.ReadCloser, int64, string, error) {
+) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, info, err := c.openVariant(cacheKey)
+	file, info, err := c.variants.Open(string(cacheKey))
 	if err != nil {
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	return c.finishOpeningVariant(cacheKey, file, info)
@@ -241,41 +259,50 @@ func (c *Cache) VariantIgnoringTTL(
 // ErrNotFound, as is a variant not in the cache.
 func (c *Cache) RenewVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey, source SourceInfo,
-) (io.ReadCloser, int64, string, error) {
+) (VariantFile, error) {
 	if c.config.MaxBytes == 0 {
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
-	file, info, err := c.openVariant(cacheKey)
+	file, info, err := c.variants.Open(string(cacheKey))
 	if err != nil {
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	if info.ModTime().Before(source.FetchedAt) {
 		_ = file.Close()
 
-		return nil, 0, "", ErrNotFound
+		return VariantFile{}, ErrNotFound
 	}
 
 	format, err := readFormat(file)
 	if err != nil {
 		_ = file.Close()
 
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
-	if info.ModTime().Before(source.CheckedAt) {
+	checkedAt := info.ModTime()
+
+	if checkedAt.Before(source.CheckedAt) {
 		err = c.SetVariantCheckedAt(ctx, cacheKey, source.CheckedAt)
 		if err != nil {
 			_ = file.Close()
 
-			return nil, 0, "", err
+			return VariantFile{}, err
 		}
+
+		checkedAt = source.CheckedAt
 	}
 
 	c.touchVariant(cacheKey)
 
-	return file, info.Size(), format.MIMEType(), nil
+	return VariantFile{
+		Content:     file,
+		Size:        info.Size(),
+		ContentType: format.MIMEType(),
+		CheckedAt:   checkedAt,
+	}, nil
 }
 
 // SetVariantCheckedAt records checkedAt as when upstream last sent the source
@@ -406,7 +433,7 @@ func (c *Cache) Source(
 		return nil, 0, "", ErrNotFound
 	}
 
-	file, size, err := c.srcContent.Open(string(key))
+	file, info, err := c.srcContent.Open(string(key))
 	if err != nil {
 		return nil, 0, "", err
 	}
@@ -420,7 +447,7 @@ func (c *Cache) Source(
 
 	c.touchSourceContent(key)
 
-	return file, size, format, nil
+	return file, info.Size(), format, nil
 }
 
 // SourceInfo is what the cache keeps in a cached source's row besides its
@@ -635,17 +662,6 @@ func (c *Cache) CleanExpired(ctx context.Context) error {
 	}
 }
 
-// IncrementStats counts a cache hit or miss. It only adds to the cache's
-// Prometheus counters in memory (see counters.go), so it never waits for the
-// database or fails.
-func (c *Cache) IncrementStats(hit bool) {
-	if hit {
-		c.cacheHits.Inc()
-	} else {
-		c.cacheMisses.Inc()
-	}
-}
-
 // touchVariant hands the access time of a variant served from the cache to
 // the goroutine StartPendingAccessTimeWrites starts, which writes it in its
 // next batch. It never waits: while maxPendingAccessTimes access times of
@@ -694,55 +710,43 @@ func (c *Cache) cleanExpiredBatch(ctx context.Context) (int64, error) {
 	return deleted, nil
 }
 
-// openVariant opens the file of a cached variant and returns it with its
-// information, which gives its size and its modification time.
-func (c *Cache) openVariant(
-	cacheKey imagerequest.VariantKey,
-) (*os.File, fs.FileInfo, error) {
-	file, _, err := c.variants.Open(string(cacheKey))
-	if err != nil {
-		return nil, nil, err
-	}
-
-	info, err := file.Stat()
-	if err != nil {
-		_ = file.Close()
-
-		return nil, nil, fmt.Errorf("failed to stat variant: %w", err)
-	}
-
-	return file, info, nil
-}
-
-// finishOpeningVariant reads the content type of a variant file openVariant
-// opened from its first bytes, hands the variant's access time to the
-// background writer (see touchVariant), and returns the file as Variant does.
-// It closes the file when its first bytes are not those of an output format.
+// finishOpeningVariant returns a variant file that FileStore.Open opened, with
+// its information info, as Variant does. It reads the content type from the
+// file's first bytes and hands the variant's access time to the background
+// writer (see touchVariant). It closes the file when its first bytes are not
+// those of an output format.
 func (c *Cache) finishOpeningVariant(
 	cacheKey imagerequest.VariantKey, file *os.File, info fs.FileInfo,
-) (io.ReadCloser, int64, string, error) {
+) (VariantFile, error) {
 	format, err := readFormat(file)
 	if err != nil {
 		_ = file.Close()
 
-		return nil, 0, "", err
+		return VariantFile{}, err
 	}
 
 	c.touchVariant(cacheKey)
 
-	return file, info.Size(), format.MIMEType(), nil
+	return VariantFile{
+		Content:     file,
+		Size:        info.Size(),
+		ContentType: format.MIMEType(),
+		CheckedAt:   info.ModTime(),
+	}, nil
 }
 
-// readFormat reads the first bytes of a source or variant file, without
-// moving its offset, and returns the format they show. Sources and variants
-// are in the same six formats.
+// readFormat reads up to magic.MaxMagicBytes first bytes of a source or
+// variant file, without moving its offset, and returns the format they show.
+// Sources and variants are in the same seven formats; a variant is SVG only
+// as orig of an SVG source. A file shorter than magic.MinMagicBytes is
+// io.EOF.
 func readFormat(content io.ReaderAt) (imagerequest.Format, error) {
-	firstBytes := make([]byte, magic.MinMagicBytes)
+	firstBytes := make([]byte, magic.MaxMagicBytes)
 
-	_, err := content.ReadAt(firstBytes, 0)
-	if err != nil {
+	n, err := content.ReadAt(firstBytes, 0)
+	if err != nil && (!errors.Is(err, io.EOF) || n < magic.MinMagicBytes) {
 		return "", fmt.Errorf("failed to read the first bytes: %w", err)
 	}
 
-	return magic.DetectFormat(firstBytes)
+	return magic.DetectFormat(firstBytes[:n])
 }

internal/imgcache/cache_hit_internal_test.go

@@ -55,11 +55,13 @@ func TestVariant_ContentTypeFromFirstBytes(t *testing.T) {
 			t.Fatalf("PutVariant(%s) error = %v", format, err)
 		}
 
-		reader, size, contentType, err := cache.Variant(key)
+		variant, err := cache.Variant(key)
 		if err != nil {
 			t.Fatalf("Variant(%s) error = %v", format, err)
 		}
 
+		reader, size, contentType := variant.Content, variant.Size, variant.ContentType
+
 		served, err := io.ReadAll(reader)
 		_ = reader.Close()
 
@@ -96,8 +98,6 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 		wantErr error
 	}{
 		{"text", []byte("this is text, not an image"), magic.ErrUnknownFormat},
-		{"SVG", []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			magic.ErrUnknownFormat},
 		{"JPEG cut to 10 bytes", jpegImage[:10], io.EOF},
 		{"empty", nil, io.EOF},
 	}
@@ -110,16 +110,19 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 			t.Fatalf("%s: storing the variant file: %v", tt.name, err)
 		}
 
-		reader, _, contentType, err := cache.Variant(key)
+		variant, err := cache.Variant(key)
 		t.Logf("%s: Variant() error = %v", tt.name, err)
 
+		reader, contentType := variant.Content, variant.ContentType
 		if reader != nil || contentType != "" || !errors.Is(err, tt.wantErr) {
 			t.Errorf("%s: Variant() = reader %v, content type %q, error %v; "+
 				"want no reader and %v", tt.name, reader, contentType, err, tt.wantErr)
 		}
 	}
 
-	reader, _, _, err := cache.Variant(testVariantKeyFour)
+	variant, err := cache.Variant(testVariantKeyFour)
+
+	reader := variant.Content
 	if reader != nil || !errors.Is(err, ErrNotFound) {
 		t.Errorf("Variant() of a variant with no file = reader %v, error %v; "+
 			"want no reader and %v", reader, err, ErrNotFound)
@@ -127,7 +130,9 @@ func TestVariant_RefusesFileOfNoOutputFormat(t *testing.T) {
 
 	disabled, _ := newEvictionTestCache(t, 0)
 
-	reader, _, _, err = disabled.Variant(testVariantKeyOne)
+	variant, err = disabled.Variant(testVariantKeyOne)
+
+	reader = variant.Content
 	if reader != nil || !errors.Is(err, ErrNotFound) {
 		t.Errorf("Variant() on a disabled cache = reader %v, error %v; "+
 			"want no reader and %v", reader, err, ErrNotFound)

internal/imgcache/counters.go

@@ -1,31 +0,0 @@
-package imgcache
-
-import (
-	"fmt"
-
-	"github.com/prometheus/client_golang/prometheus"
-)
-
-// registerCounters makes the cache's counters and registers them on
-// metricsRegistry. The counters are kept in memory, so adding to one waits
-// for nothing and makes no database call. It fails when metricsRegistry
-// already holds counters of the same names.
-func (c *Cache) registerCounters(metricsRegistry prometheus.Registerer) error {
-	c.cacheHits = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "pixa_cache_hits_total",
-		Help: "Image requests served from the cache.",
-	})
-	c.cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "pixa_cache_misses_total",
-		Help: "Image requests not served from the cache.",
-	})
-
-	for _, counter := range []prometheus.Counter{c.cacheHits, c.cacheMisses} {
-		err := metricsRegistry.Register(counter)
-		if err != nil {
-			return fmt.Errorf("failed to register the cache's counters: %w", err)
-		}
-	}
-
-	return nil
-}

internal/imgcache/eviction.go

@@ -92,8 +92,9 @@ type evictionCandidate struct {
 }
 
 // EvictToLimit evicts least-recently-used cache entries until total
-// tracked usage is at or below the configured MaxBytes limit. It is a
-// no-op when the cache is disabled.
+// tracked usage is at or below the configured MaxBytes limit. It sets the
+// pixa_cache_usage_bytes gauge to each usage it reads. It is a no-op when
+// the cache is disabled.
 func (c *Cache) EvictToLimit(ctx context.Context) error {
 	if c.config.MaxBytes == 0 {
 		return nil
@@ -105,6 +106,8 @@ func (c *Cache) EvictToLimit(ctx context.Context) error {
 			return err
 		}
 
+		c.usageBytes.Set(float64(usage))
+
 		if usage <= c.config.MaxBytes {
 			return nil
 		}
@@ -345,37 +348,95 @@ func (c *Cache) sourceCandidates(ctx context.Context) ([]evictionCandidate, erro
 	return candidates, nil
 }
 
-// evictVariant removes one variant: accounting row first, then the
-// file, so the database never references a deleted file.
+// evictVariant removes one variant (see removeVariant) and counts the
+// eviction with the size its row recorded.
 func (c *Cache) evictVariant(
 	ctx context.Context, cacheKey imagerequest.VariantKey,
 ) error {
-	_, err := c.db.ExecContext(ctx,
-		`DELETE FROM variant_content WHERE cache_key = ?`, string(cacheKey))
-	if err != nil {
-		return fmt.Errorf("failed to delete variant accounting row: %w", err)
-	}
-
-	err = c.variants.Delete(string(cacheKey))
+	sizeBytes, err := c.removeVariant(ctx, cacheKey)
 	if err != nil {
 		return err
 	}
 
+	c.countEviction(sizeBytes)
+
 	return nil
 }
 
-// evictSourceBlob removes one source: its row first, then its file, as
-// evictVariant does for a variant. It holds no lock: a store of the same
-// source in between can leave the source's row with no file, which
-// Source treats as a miss.
-func (c *Cache) evictSourceBlob(ctx context.Context, key imagerequest.SourceKey) error {
-	_, err := c.db.ExecContext(ctx,
-		`DELETE FROM source_content WHERE source_key = ?`, string(key))
+// removeVariant removes one variant: accounting row first, then the
+// file, so the database never references a deleted file. It returns the
+// size the row recorded, or 0 when the variant had no row.
+func (c *Cache) removeVariant(
+	ctx context.Context, cacheKey imagerequest.VariantKey,
+) (int64, error) {
+	sizeBytes, err := c.deleteRowReturningSize(ctx,
+		`DELETE FROM variant_content WHERE cache_key = ? RETURNING size_bytes`,
+		string(cacheKey))
 	if err != nil {
-		return fmt.Errorf("failed to delete source content row: %w", err)
+		return 0, fmt.Errorf("failed to delete variant accounting row: %w", err)
 	}
 
-	return c.srcContent.Delete(string(key))
+	err = c.variants.Delete(string(cacheKey))
+	if err != nil {
+		return 0, err
+	}
+
+	return sizeBytes, nil
+}
+
+// evictSourceBlob removes one source (see removeSourceBlob) and counts the
+// eviction with the size its row recorded.
+func (c *Cache) evictSourceBlob(ctx context.Context, key imagerequest.SourceKey) error {
+	sizeBytes, err := c.removeSourceBlob(ctx, key)
+	if err != nil {
+		return err
+	}
+
+	c.countEviction(sizeBytes)
+
+	return nil
+}
+
+// removeSourceBlob removes one source: its row first, then its file, as
+// removeVariant does for a variant, and returns the size the row recorded,
+// or 0 when the source had no row. It holds no lock: a store of the same
+// source in between can leave the source's row with no file, which Source
+// treats as a miss.
+func (c *Cache) removeSourceBlob(
+	ctx context.Context, key imagerequest.SourceKey,
+) (int64, error) {
+	sizeBytes, err := c.deleteRowReturningSize(ctx,
+		`DELETE FROM source_content WHERE source_key = ? RETURNING size_bytes`,
+		string(key))
+	if err != nil {
+		return 0, fmt.Errorf("failed to delete source content row: %w", err)
+	}
+
+	err = c.srcContent.Delete(string(key))
+	if err != nil {
+		return 0, err
+	}
+
+	return sizeBytes, nil
+}
+
+// deleteRowReturningSize runs deleteQuery, which deletes the row of key and
+// returns its size_bytes, and returns that size, or 0 when there was no row.
+func (c *Cache) deleteRowReturningSize(
+	ctx context.Context, deleteQuery string, key string,
+) (int64, error) {
+	var sizeBytes int64
+
+	err := c.db.QueryRowContext(ctx, deleteQuery, key).Scan(&sizeBytes)
+	if errors.Is(err, sql.ErrNoRows) {
+		return 0, nil
+	}
+
+	if err != nil {
+		return 0, err
+	}
+
+	return sizeBytes, nil
 }
 
 // notifyWritePressure wakes the background evictor after a store, so
@@ -673,7 +734,9 @@ func (c *Cache) reconcileVariantFile(
 	c.log.Warn("removing damaged cached variant file", "path", path,
 		"file_bytes", info.Size(), "recorded_bytes", recordedSize)
 
-	return c.evictVariant(ctx, cacheKey)
+	_, err = c.removeVariant(ctx, cacheKey)
+
+	return err
 }
 
 // adoptVariantFile inserts an accounting row for a variant file that
@@ -835,7 +898,9 @@ func (c *Cache) reconcileSourceFile(
 	c.log.Warn("removing damaged cached source file", "path", path,
 		"file_bytes", info.Size(), "recorded_bytes", recordedSize)
 
-	return c.evictSourceBlob(ctx, key)
+	_, err = c.removeSourceBlob(ctx, key)
+
+	return err
 }
 
 // removeUntrackedSourceFile deletes a source blob file that has no

internal/imgcache/filestore.go

@@ -3,6 +3,7 @@ package imgcache
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"time"
@@ -55,26 +56,26 @@ func (s *FileStore) Write(key string, data []byte) error {
 	return writeFileSynced(path, data)
 }
 
-// Open opens the file for key and returns it with its size in bytes. A
-// missing file is ErrNotFound.
-func (s *FileStore) Open(key string) (*os.File, int64, error) {
+// Open opens the file for key and returns it with its information, which
+// gives its size and its modification time. A missing file is ErrNotFound.
+func (s *FileStore) Open(key string) (*os.File, fs.FileInfo, error) {
 	f, err := os.Open(s.keyToPath(key))
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, 0, ErrNotFound
+			return nil, nil, ErrNotFound
 		}
 
-		return nil, 0, fmt.Errorf("failed to open content: %w", err)
+		return nil, nil, fmt.Errorf("failed to open content: %w", err)
 	}
 
-	stat, err := f.Stat()
+	info, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
 
-		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
+		return nil, nil, fmt.Errorf("failed to stat content: %w", err)
 	}
 
-	return f, stat.Size(), nil
+	return f, info, nil
 }
 
 // Size returns the size in bytes of the file for key. A missing file is

internal/imgcache/filestore_internal_test.go

@@ -38,15 +38,15 @@ func TestFileStore_WriteAndOpen(t *testing.T) {
 	}
 
 	// Open and verify size and content
-	r, size, err := storage.Open(key)
+	r, info, err := storage.Open(key)
 	if err != nil {
 		t.Fatalf("Open() error = %v", err)
 	}
 
 	defer func() { _ = r.Close() }()
 
-	if size != int64(len(content)) {
-		t.Errorf("Open() size = %d, want %d", size, len(content))
+	if info.Size() != int64(len(content)) {
+		t.Errorf("Open() size = %d, want %d", info.Size(), len(content))
 	}
 
 	loaded, err := io.ReadAll(r)

internal/imgcache/metrics.go

@@ -0,0 +1,88 @@
+package imgcache
+
+import (
+	"fmt"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// registerMetrics makes the cache's metrics and registers them on
+// metricsRegistry. The metrics are kept in memory, so updating one waits
+// for nothing and makes no database call. It fails when metricsRegistry
+// already holds metrics of the same names.
+func (c *Cache) registerMetrics(metricsRegistry prometheus.Registerer) error {
+	c.cacheHits = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_hits_total",
+		Help: "Image requests served from the cache.",
+	})
+	c.cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_misses_total",
+		Help: "Image requests not served from the cache.",
+	})
+	c.sourceCacheHits = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_source_cache_hits_total",
+		Help: "Variants made or checked whose source was in the cache.",
+	})
+	c.sourceCacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_source_cache_misses_total",
+		Help: "Variants made or checked whose source was not in the cache.",
+	})
+	c.usageBytes = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "pixa_cache_usage_bytes",
+		Help: "Bytes of sources and variants in the cache, as eviction last read them.",
+	})
+	c.maxBytes = prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "pixa_cache_max_bytes",
+		Help: "The disk cache limit in bytes, cache_max_bytes.",
+	})
+	c.evictions = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_evictions_total",
+		Help: "Sources and variants removed by eviction.",
+	})
+	c.evictedBytes = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "pixa_cache_evicted_bytes_total",
+		Help: "Bytes of the sources and variants removed by eviction.",
+	})
+
+	c.maxBytes.Set(float64(c.config.MaxBytes))
+
+	for _, metric := range []prometheus.Collector{
+		c.cacheHits, c.cacheMisses, c.sourceCacheHits, c.sourceCacheMisses,
+		c.usageBytes, c.maxBytes, c.evictions, c.evictedBytes,
+	} {
+		err := metricsRegistry.Register(metric)
+		if err != nil {
+			return fmt.Errorf("failed to register the cache's metrics: %w", err)
+		}
+	}
+
+	return nil
+}
+
+// IncrementStats counts a cache hit or miss of an image request. It only
+// adds to the cache's counters in memory, so it never waits for the
+// database or fails.
+func (c *Cache) IncrementStats(hit bool) {
+	if hit {
+		c.cacheHits.Inc()
+	} else {
+		c.cacheMisses.Inc()
+	}
+}
+
+// IncrementSourceStats counts whether a variant being made or checked found
+// its source in the cache. Like IncrementStats, it never waits or fails.
+func (c *Cache) IncrementSourceStats(hit bool) {
+	if hit {
+		c.sourceCacheHits.Inc()
+	} else {
+		c.sourceCacheMisses.Inc()
+	}
+}
+
+// countEviction counts one source or variant of sizeBytes bytes removed by
+// eviction.
+func (c *Cache) countEviction(sizeBytes int64) {
+	c.evictions.Inc()
+	c.evictedBytes.Add(float64(sizeBytes))
+}

internal/imgcache/metrics_internal_test.go

@@ -0,0 +1,137 @@
+package imgcache
+
+import (
+	"bytes"
+	"testing"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// gatheredValue returns the value of the metric name on registry: a
+// counter's or a gauge's value.
+func gatheredValue(t *testing.T, registry *prometheus.Registry, name string) float64 {
+	t.Helper()
+
+	families, err := registry.Gather()
+	if err != nil {
+		t.Fatalf("failed to gather the metrics: %v", err)
+	}
+
+	for _, family := range families {
+		if family.GetName() != name {
+			continue
+		}
+
+		metric := family.GetMetric()[0]
+		if metric.GetGauge() != nil {
+			return metric.GetGauge().GetValue()
+		}
+
+		return metric.GetCounter().GetValue()
+	}
+
+	t.Fatalf("the metric %s is not on the registry", name)
+
+	return 0
+}
+
+// TestEvictToLimit_CountsEvictionsAndUsage stores four variants of 1000
+// bytes in a cache limited to 3000 bytes. EvictToLimit must count the one
+// eviction and its bytes, and set the usage and limit gauges.
+func TestEvictToLimit_CountsEvictionsAndUsage(t *testing.T) {
+	t.Parallel()
+
+	const limit = 3000
+
+	registry := prometheus.NewRegistry()
+
+	cache, err := NewCache(evictionTestDB(t),
+		Config{StateDir: t.TempDir(), MaxBytes: limit}, registry)
+	if err != nil {
+		t.Fatalf("NewCache() error = %v", err)
+	}
+
+	keys := []imagerequest.VariantKey{
+		testVariantKeyOne, testVariantKeyTwo,
+		testVariantKeyThree, testVariantKeyFour,
+	}
+
+	for i, key := range keys {
+		storeEvictionTestVariant(t, cache, key, bytes.Repeat([]byte{0x01}, 1000))
+		setVariantLastAccessed(t, cache, key,
+			time.Now().Add(-time.Duration(len(keys)-i)*time.Hour))
+	}
+
+	err = cache.EvictToLimit(t.Context())
+	if err != nil {
+		t.Fatalf("EvictToLimit() error = %v", err)
+	}
+
+	want := map[string]float64{
+		"pixa_cache_evictions_total":     1,
+		"pixa_cache_evicted_bytes_total": 1000,
+		"pixa_cache_usage_bytes":         3000,
+		"pixa_cache_max_bytes":           limit,
+	}
+
+	for name, value := range want {
+		got := gatheredValue(t, registry, name)
+		t.Logf("%s = %v", name, got)
+
+		if got != value {
+			t.Errorf("%s = %v, want %v", name, got, value)
+		}
+	}
+}
+
+// TestEvictNotCheckedRecently_CountsEvictions stores a source of 6 bytes and
+// a variant of 7 bytes last checked with upstream 31 days ago.
+// EvictNotCheckedRecently must count both evictions and their 13 bytes.
+func TestEvictNotCheckedRecently_CountsEvictions(t *testing.T) {
+	t.Parallel()
+
+	registry := prometheus.NewRegistry()
+
+	cache, err := NewCache(evictionTestDB(t),
+		Config{StateDir: t.TempDir(), MaxBytes: 1 << 30}, registry)
+	if err != nil {
+		t.Fatalf("NewCache() error = %v", err)
+	}
+
+	old := time.Now().Add(-31 * 24 * time.Hour)
+
+	_, err = cache.PutSource(t.Context(),
+		imagerequest.Source{Host: testHostCDN, Path: "/old.jpg"},
+		bytes.NewReader([]byte("source")),
+		SourceInfo{FetchedAt: old, CheckedAt: old})
+	if err != nil {
+		t.Fatalf("PutSource() error = %v", err)
+	}
+
+	err = cache.PutVariant(t.Context(), testVariantKeyOne,
+		SourceInfo{CheckedAt: old}, bytes.NewReader([]byte("variant")))
+	if err != nil {
+		t.Fatalf("PutVariant() error = %v", err)
+	}
+
+	err = cache.EvictNotCheckedRecently(t.Context())
+	if err != nil {
+		t.Fatalf("EvictNotCheckedRecently() error = %v", err)
+	}
+
+	want := map[string]float64{
+		"pixa_cache_evictions_total":     2,
+		"pixa_cache_evicted_bytes_total": 13,
+	}
+
+	for name, value := range want {
+		got := gatheredValue(t, registry, name)
+		t.Logf("%s = %v", name, got)
+
+		if got != value {
+			t.Errorf("%s = %v, want %v", name, got, value)
+		}
+	}
+}

internal/imgcache/source_content_internal_test.go

@@ -216,8 +216,6 @@ func TestSource_RefusesFileOfNoSourceFormat(t *testing.T) {
 		wantErr error
 	}{
 		{"text", []byte("this is text, not an image"), magic.ErrUnknownFormat},
-		{"SVG", []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			magic.ErrUnknownFormat},
 		{"JPEG cut to 10 bytes", jpegImage[:10], io.EOF},
 		{"empty file", nil, io.EOF},
 	}

internal/imgcache/svg_internal_test.go

@@ -0,0 +1,54 @@
+package imgcache
+
+import (
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// testSVG is an SVG with a comment before its root element, so that its
+// first magic.MinMagicBytes bytes do not show its format.
+const testSVG = `<!-- a drawing --><svg xmlns="http://www.w3.org/2000/svg" ` +
+	`width="4" height="4"/>`
+
+// TestSourceAndVariant_ReadSVG checks that a cached SVG source is read as
+// SVG, and a cached SVG variant (orig of an SVG source) as image/svg+xml.
+func TestSourceAndVariant_ReadSVG(t *testing.T) {
+	t.Parallel()
+
+	cache, _ := newEvictionTestCache(t, 1<<30)
+
+	storeEvictionTestSource(t, cache, testHostCDN, "/drawing.svg", []byte(testSVG))
+
+	reader, _, format, err := cache.Source(t.Context(),
+		imagerequest.Source{Host: testHostCDN, Path: "/drawing.svg"})
+	t.Logf("Source() format = %q, error = %v", format, err)
+
+	if err != nil || format != imagerequest.FormatSVG {
+		t.Errorf("Source() = format %q, error %v; want %q", format, err,
+			imagerequest.FormatSVG)
+	}
+
+	if reader != nil {
+		_ = reader.Close()
+	}
+
+	key := imagerequest.VariantKey("aabbccdd3001")
+
+	err = cache.variants.Write(string(key), []byte(testSVG))
+	if err != nil {
+		t.Fatalf("storing the variant file: %v", err)
+	}
+
+	variant, err := cache.Variant(key)
+	t.Logf("Variant() content type = %q, error = %v", variant.ContentType, err)
+
+	if err != nil || variant.ContentType != "image/svg+xml" {
+		t.Errorf("Variant() = content type %q, error %v; want image/svg+xml",
+			variant.ContentType, err)
+	}
+
+	if variant.Content != nil {
+		_ = variant.Content.Close()
+	}
+}

internal/magic/magic.go

@@ -20,6 +20,10 @@ var (
 // MinMagicBytes is the minimum number of bytes needed to detect format.
 const MinMagicBytes = 12
 
+// MaxMagicBytes is the most bytes DetectFormat looks at. An SVG's root
+// element must start within them, as libvips requires.
+const MaxMagicBytes = 1024
+
 // Magic byte signatures for supported formats.
 // These are effectively constants but Go doesn't support const slices.
 //
@@ -39,6 +43,12 @@ var (
 	}
 )
 
+// utf8BOM is the UTF-8 byte order mark, which a text file such as an SVG may
+// start with.
+//
+//nolint:gochecknoglobals // immutable lookup data
+var utf8BOM = []byte{0xEF, 0xBB, 0xBF}
+
 // WebP identifier appears at offset 8 after RIFF header.
 //
 //nolint:gochecknoglobals // immutable lookup data
@@ -53,7 +63,7 @@ var (
 )
 
 // DetectFormat detects the image format from magic bytes. Data in none of
-// the formats pixa reads, SVG included, is ErrUnknownFormat.
+// the formats pixa reads is ErrUnknownFormat.
 func DetectFormat(data []byte) (imagerequest.Format, error) {
 	if len(data) < MinMagicBytes {
 		return "", ErrNotEnoughData
@@ -96,9 +106,25 @@ func DetectFormat(data []byte) (imagerequest.Format, error) {
 		return imagerequest.FormatJXL, nil
 	}
 
+	if isSVG(data) {
+		return imagerequest.FormatSVG, nil
+	}
+
 	return "", ErrUnknownFormat
 }
 
+// isSVG reports whether data starts as an SVG does: with a "<", after an
+// optional UTF-8 byte order mark and white space, and with "<svg" within the
+// first MaxMagicBytes bytes. Before the root element an SVG may have an XML
+// declaration, comments and a DOCTYPE.
+func isSVG(data []byte) bool {
+	data = data[:min(len(data), MaxMagicBytes)]
+	text := bytes.TrimLeft(bytes.TrimPrefix(data, utf8BOM), " \t\r\n")
+
+	return bytes.HasPrefix(text, []byte("<")) &&
+		bytes.Contains(text, []byte("<svg"))
+}
+
 // ValidateMagicBytes detects the format of data from its magic bytes, and
 // returns it when it is the format the declared MIME type names. Data in
 // another format is ErrMagicByteMismatch.

internal/magic/magic_internal_test.go

@@ -47,9 +47,9 @@ func TestDetectFormat(t *testing.T) {
 		{name: "AVIF", data: avif, wantFormat: imagerequest.FormatAVIF},
 		{name: "AVIF sequence", data: avis, wantFormat: imagerequest.FormatAVIF},
 		{
-			name:    "SVG",
-			data:    []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			wantErr: ErrUnknownFormat,
+			name:       "SVG",
+			data:       []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
+			wantFormat: imagerequest.FormatSVG,
 		},
 		{
 			name:    "unknown format",

internal/magic/svg_internal_test.go

@@ -0,0 +1,85 @@
+package magic
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/imagerequest"
+)
+
+// TestDetectFormat_SVG checks that DetectFormat names SVG text with what may
+// come before the root element, and does not name as SVG text that does not
+// start with "<" or has no "<svg" within its first MaxMagicBytes bytes.
+func TestDetectFormat_SVG(t *testing.T) {
+	t.Parallel()
+
+	const root = `<svg xmlns="http://www.w3.org/2000/svg" width="4" height="4"/>`
+
+	tests := []struct {
+		name    string
+		data    string
+		wantErr error
+	}{
+		{name: "root element only", data: root},
+		{name: "XML declaration", data: `<?xml version="1.0"?>` + "\n" + root},
+		{name: "byte order mark and white space", data: "\xEF\xBB\xBF \n\t" + root},
+		{name: "comment", data: "<!-- a drawing -->" + root},
+		{
+			name: "DOCTYPE",
+			data: `<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" ` +
+				`"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">` + root,
+		},
+		{
+			name:    "root element after the first bytes",
+			data:    "<!--" + strings.Repeat(" ", MaxMagicBytes) + "-->" + root,
+			wantErr: ErrUnknownFormat,
+		},
+		{name: "text naming <svg", data: "draw it with " + root, wantErr: ErrUnknownFormat},
+		{name: "HTML", data: "<!DOCTYPE html><html></html>", wantErr: ErrUnknownFormat},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := DetectFormat([]byte(tt.data))
+			t.Logf("DetectFormat() = %q, %v", got, err)
+
+			if tt.wantErr != nil {
+				if !errors.Is(err, tt.wantErr) {
+					t.Errorf("DetectFormat() error = %v, want %v", err, tt.wantErr)
+				}
+
+				return
+			}
+
+			if err != nil || got != imagerequest.FormatSVG {
+				t.Errorf("DetectFormat() = %q, %v; want %q", got, err,
+					imagerequest.FormatSVG)
+			}
+		})
+	}
+}
+
+// TestValidateMagicBytes_SVG checks that SVG data is SVG for the
+// Content-Type image/svg+xml, with a charset too, and a mismatch for another.
+func TestValidateMagicBytes_SVG(t *testing.T) {
+	t.Parallel()
+
+	data := []byte(`<svg xmlns="http://www.w3.org/2000/svg" width="4" height="4"/>`)
+
+	for _, declared := range []string{"image/svg+xml", "image/svg+xml; charset=utf-8"} {
+		got, err := ValidateMagicBytes(data, declared)
+		if err != nil || got != imagerequest.FormatSVG {
+			t.Errorf("ValidateMagicBytes(%q) = %q, %v; want %q", declared, got,
+				err, imagerequest.FormatSVG)
+		}
+	}
+
+	_, err := ValidateMagicBytes(data, "image/png")
+	if !errors.Is(err, ErrMagicByteMismatch) {
+		t.Errorf("ValidateMagicBytes(image/png) error = %v, want %v", err,
+			ErrMagicByteMismatch)
+	}
+}

internal/providers/providers.go

@@ -35,7 +35,7 @@ import (
 
 // Constructors returns the constructors of the parts this package builds,
 // for fx.Provide. The fx graph must also provide the prometheus.Registerer
-// the cache and the image service register their counters on.
+// the cache and the image service register their metrics on.
 func Constructors() []any {
 	return []any{
 		newKeys,

internal/server/svg_internal_test.go

@@ -4,38 +4,134 @@ import (
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"os"
+	"sync/atomic"
 	"testing"
+
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imageproxy"
+	"sneak.berlin/go/pixa/internal/imagerequest"
+	"sneak.berlin/go/pixa/internal/magic"
+	"sneak.berlin/go/pixa/internal/middleware"
 )
 
-// TestImageProxyRefusesSVG requests an image the upstream answers as SVG,
-// in its own format (orig) and resized as PNG, through pixa's router,
-// upstream fetcher and image processor. pixa does not accept SVG as a
-// source, so each request is answered 415, never with an image.
-func TestImageProxyRefusesSVG(t *testing.T) {
-	t.Parallel()
+// TestMain runs this test binary as an SVG child process when the image
+// processor starts it again as one, as the SVG tests below make it do.
+func TestMain(m *testing.M) {
+	if imageprocessor.IsSVGChildProcess(os.Args[1:]) {
+		os.Exit(imageprocessor.RunAsSVGChildProcess(os.Args[1:]))
+	}
+
+	os.Exit(m.Run())
+}
+
+// testSVG is the SVG the upstream serves in the SVG tests below.
+const testSVG = `<svg xmlns="http://www.w3.org/2000/svg" width="64" height="48">` +
+	`<rect width="64" height="48"/></svg>`
+
+// startSVGUpstream starts an upstream that answers every request with
+// testSVG as image/svg+xml, and returns it with its count of requests.
+func startSVGUpstream(t *testing.T) (*httptest.Server, *atomic.Int64) {
+	t.Helper()
+
+	var requests atomic.Int64
 
 	upstream := httptest.NewServer(http.HandlerFunc(
 		func(w http.ResponseWriter, _ *http.Request) {
+			requests.Add(1)
 			w.Header().Set("Content-Type", "image/svg+xml")
-			_, _ = io.WriteString(w, `<svg xmlns="http://www.w3.org/2000/svg" `+
-				`width="64" height="48"><rect width="64" height="48"/></svg>`)
+			_, _ = io.WriteString(w, testSVG)
 		}))
 	t.Cleanup(upstream.Close)
 
+	return upstream, &requests
+}
+
+// TestImageProxyServesSVGAsItIs requests an SVG source in its own format
+// (orig) at the orig size through pixa's router, upstream fetcher, image
+// processor and cache, twice. Both answers are the SVG itself, never another
+// format, the second from the cache; the upstream is asked once. Both carry
+// the Content-Security-Policy of an SVG response in place of the
+// middleware's. orig at another size is refused with 400.
+func TestImageProxyServesSVGAsItIs(t *testing.T) {
+	t.Parallel()
+
+	const wantPolicy = "default-src 'none'; style-src 'unsafe-inline'; " +
+		"img-src data:; frame-ancestors 'none'; sandbox"
+
+	upstream, requests := startSVGUpstream(t)
+	s, _, _ := startImageProxy(t, upstream)
+	target := "/v1/image/" + upstreamHost + "/drawing.svg/orig.orig"
+
+	for _, wantCache := range []imageproxy.CacheStatus{
+		imageproxy.CacheMiss, imageproxy.CacheHit,
+	} {
+		rec := getImage(t, s, target)
+
+		if rec.Header().Get("Content-Type") != "image/svg+xml" ||
+			rec.Body.String() != testSVG {
+			t.Errorf("GET %s: %s %q, want image/svg+xml %q", target,
+				rec.Header().Get("Content-Type"), rec.Body.String(), testSVG)
+		}
+
+		if got := rec.Header().Get("X-Pixa-Cache"); got != string(wantCache) {
+			t.Errorf("GET %s: X-Pixa-Cache = %q, want %q", target, got, wantCache)
+		}
+
+		policy := rec.Header().Get("Content-Security-Policy")
+		if policy != wantPolicy {
+			t.Errorf("GET %s: Content-Security-Policy = %q, want %q", target,
+				policy, wantPolicy)
+		}
+	}
+
+	if requests.Load() != 1 {
+		t.Errorf("upstream requests = %d, want 1", requests.Load())
+	}
+
+	resized := "/v1/image/" + upstreamHost + "/drawing.svg/32x24.orig"
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, resized, nil))
+	t.Logf("GET %s: %d %s", resized, rec.Code, rec.Body.String())
+
+	if rec.Code != http.StatusBadRequest {
+		t.Errorf("GET %s: %d %s, want 400", resized, rec.Code, rec.Body.String())
+	}
+}
+
+// TestImageProxyRendersSVG requests an SVG source as each raster format
+// through pixa's router, upstream fetcher and image processor. Each answer
+// is an image in that format, with the middleware's Content-Security-Policy.
+func TestImageProxyRendersSVG(t *testing.T) {
+	t.Parallel()
+
+	upstream, _ := startSVGUpstream(t)
 	s, _, _ := startImageProxy(t, upstream)
 
-	for _, sizeFormat := range []string{"64x48.orig", "32x24.png"} {
-		target := "/v1/image/" + upstreamHost + "/drawing.svg/" + sizeFormat
+	for _, format := range []imagerequest.Format{
+		imagerequest.FormatJPEG, imagerequest.FormatPNG, imagerequest.FormatGIF,
+		imagerequest.FormatWebP, imagerequest.FormatAVIF, imagerequest.FormatJXL,
+	} {
+		target := "/v1/image/" + upstreamHost + "/drawing.svg/32x24." +
+			string(format)
 
-		rec := httptest.NewRecorder()
-		s.ServeHTTP(rec, httptest.NewRequestWithContext(
-			t.Context(), http.MethodGet, target, nil))
-		t.Logf("GET %s: %d, Content-Type %s, body %s",
-			target, rec.Code, rec.Header().Get("Content-Type"), rec.Body.String())
+		rec := getImage(t, s, target)
+		detected, err := magic.DetectFormat(rec.Body.Bytes())
+		t.Logf("GET %s: %s, body in %q (%v)", target,
+			rec.Header().Get("Content-Type"), detected, err)
 
-		if rec.Code != http.StatusUnsupportedMediaType {
-			t.Errorf("GET %s status = %d, want %d",
-				target, rec.Code, http.StatusUnsupportedMediaType)
+		if rec.Header().Get("Content-Type") != format.MIMEType() ||
+			err != nil || detected != format {
+			t.Errorf("GET %s: %s, body in %q (%v); want %s", target,
+				rec.Header().Get("Content-Type"), detected, err, format.MIMEType())
+		}
+
+		policy := rec.Header().Get("Content-Security-Policy")
+		if policy != middleware.ContentSecurityPolicyValue {
+			t.Errorf("GET %s: Content-Security-Policy = %q, want %q", target,
+				policy, middleware.ContentSecurityPolicyValue)
 		}
 	}
 }

internal/templates/form_refused.html

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Pixa - Form Refused</title>
+    <link rel="stylesheet" href="/static/style.css">
+</head>
+<body class="login">
+    <div class="card">
+        <h1>Form Refused</h1>
+
+        <div class="error">
+            {{.Cause}}
+        </div>
+
+        <p>
+            <a href="/">Back to the start page</a>
+        </p>
+    </div>
+</body>
+</html>