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

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
@@ -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.
@@ -464,10 +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.
@@ -487,9 +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
@@ -498,6 +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:
 
@@ -708,8 +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;
@@ -718,14 +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
@@ -760,11 +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;
@@ -777,7 +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

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)
 }

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_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

@@ -69,6 +69,10 @@ var (
 	ErrInvalidContentType = errors.New("invalid or unsupported content type")
 	ErrUpstreamError      = errors.New("upstream server error")
 	ErrUpstreamTimeout    = errors.New("upstream request timeout")
+	// 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")
 )
 
@@ -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/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

@@ -735,16 +735,18 @@ func (c *Cache) finishOpeningVariant(
 	}, 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

@@ -98,8 +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},
 	}

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

@@ -49,7 +49,7 @@ func TestDetectFormat(t *testing.T) {
 		{
 			name:       "SVG",
 			data:       []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`),
-			wantErr: ErrUnknownFormat,
+			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/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)
 
-	s, _, _ := startImageProxy(t, upstream)
+	return upstream, &requests
+}
 
-	for _, sizeFormat := range []string{"64x48.orig", "32x24.png"} {
-		target := "/v1/image/" + upstreamHost + "/drawing.svg/" + sizeFormat
+// 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, target, nil))
-		t.Logf("GET %s: %d, Content-Type %s, body %s",
-			target, rec.Code, rec.Header().Get("Content-Type"), rec.Body.String())
+		t.Context(), http.MethodGet, resized, nil))
+	t.Logf("GET %s: %d %s", resized, rec.Code, rec.Body.String())
 
-		if rec.Code != http.StatusUnsupportedMediaType {
-			t.Errorf("GET %s status = %d, want %d",
-				target, rec.Code, http.StatusUnsupportedMediaType)
+	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 _, 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 := 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.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)
 		}
 	}
 }