Test the 503 for a request waiting on a transcode stopped at a deadline

A request that waited for another request's transcode, stopped at that
request's deadline, must get ErrProcessingTimedOut, which both image
routes answer 503. Declares that error so the tests build. Also tests
that Process, past its deadline before it starts, never reads its input.

Model: opus-5-5

README.md

@@ -26,6 +26,11 @@ make docker
 docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
 ```
 
+The example config allowlists no host (`allowlist_hosts`), so pixa serves an
+image only for a signed or encrypted URL. 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
 below for the list). Only `PIXA_SIGNING_KEY` is required; if it is unset the
 container exits at startup naming the variable. Everything else has a built-in
@@ -115,7 +120,9 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
     - `PIXA_SIGNING_KEY` (required): secret for signed and encrypted URLs and
       login, 32+ characters, for example from `openssl rand -base64 32`
     - `PIXA_ALLOWLIST_HOSTS`: upstream hosts served without a signature,
-      comma-separated
+      comma-separated; default none. Every image on a listed host is served to
+      anyone without a signature, so list only hosts you control, where nobody
+      else can put images.
     - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it;
       default 75% of (free space + what the cache holds)
     - the rest are in the table under Configuration below
@@ -306,9 +313,23 @@ cache at once, they share one upstream fetch (or one read of the cached source)
 and one transcode: the first request does the work, and the others wait for its
 image or its error, holding no upstream connection or processing slot of their
 own. A waiting request stops waiting when its own client goes away. The work
-goes on for the others even if the first request's client goes away, until that
-request's `downstream_timeout` ends. The shared fetch sends the first request's
-ID upstream, and the lines logged for the fetch and the transcode carry that ID.
+goes on for the others even if the first request's client goes away. The shared
+fetch sends the first request's ID upstream, and the lines logged for the fetch
+and the transcode carry that ID.
+
+A transcode whose request's `downstream_timeout` has ended (for a shared one,
+the first request's) stops when it gets its processing slot or before it begins
+encoding, and gives the slot back. libvips cannot stop an encode once it has
+begun, so an encode keeps its slot until it finishes, which can be after
+`downstream_timeout`. The limit of 8192 on each side of the source and of the
+output bounds how long that takes. With one libvips thread, as pixad runs it,
+encoding 8192x8192 random pixels takes up to about 1 second as JPEG, 15 as PNG,
+16 as GIF, 63 as WebP, 110 as AVIF and 96 as JPEG XL, the slowest of 8 or 16
+bits per sample, with or without an alpha channel. WebP, AVIF and JPEG XL are
+that slow only with an alpha channel: 8-bit RGB takes about 44, 53 and 28
+seconds. Decoding such a source adds up to about 11 seconds. A WebP, AVIF or
+JPEG XL encode can therefore hold its slot past the default `downstream_timeout`
+of 60 seconds.
 
 The login form (`POST /`) is limited to 5 attempts per minute per client
 address, counting an IPv6 client by its /64; an attempt over the limit is
@@ -412,8 +433,11 @@ HMAC-SHA256(secret, "host:path:query:width:height:format:expiration:quality:fit"
 Where:
 
 - `host` — source origin hostname (e.g. `cdn.example.com`)
-- `path` — source path (e.g. `/photos/cat.jpg`)
-- `query` — source query string, empty string if none
+- `path` — source path (e.g. `/photos/cat.jpg`), with each `%` written as `%25`
+  and each `:` as `%3A`, so `/photos/a:b.jpg` is signed as `/photos/a%3Ab.jpg`
+- `query` — source query string, empty string if none, with `%` and `:` written
+  as in `path`; neither then holds a `:`, so path `/a:b` with no query and path
+  `/a` with query `b:` are signed differently
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
 - `format` — output format, one of those listed under Routes, with `original`

TODO.md

@@ -30,6 +30,46 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 a `:` in the source path or query no longer moves where the path
+  ends and the query begins (closes #247): the signed data and the cache key
+  joined host, path and query with `:` and no escaping, so a URL signed for
+  `https://h/a:b` verified for `https://h/a?b:`; the two also shared a cache
+  key, and whichever was fetched first was served for both. In both, the path
+  and the query now have each `%` written as `%25` and each `:` as `%3A`; a path
+  and query holding neither are signed and keyed as before. Signature
+  Specification in `README.md` says so.
+- 2026-10-15 a transcode stops once its request's `downstream_timeout` has
+  ended, unless it has begun encoding (closes #243): the image processor looked
+  at its context only while it waited for a processing slot, so an image was
+  decoded and encoded, holding the slot, however long after its request's
+  deadline that took. `Process` now returns the context's error, and gives the
+  slot back, when the context has ended by the time it has the slot or before it
+  encodes. libvips cannot stop an encode once it has begun, so that encode keeps
+  its slot until it finishes; the limit of 8192 on each side of the source and
+  of the output bounds how long that takes. With one libvips thread, 8192x8192
+  random pixels take up to about 1 second to encode as JPEG, 15 as PNG, 16 as
+  GIF, 63 as WebP, 110 as AVIF and 96 as JPEG XL, the last three with an alpha
+  channel, plus up to about 11 seconds to decode the source; `README.md` gives
+  these times and no longer says the work stops when `downstream_timeout` ends.
+  The settings of the WebP, AVIF and JPEG XL encoders, which can hold a slot
+  past the default `downstream_timeout` of 60 seconds, are unchanged.
+- 2026-10-15 the example config allowlists no host (closes #249): it allowlisted
+  `github.com` and `user-images.githubusercontent.com`, where anyone can put an
+  image, so a deployment set up as Getting Started says served those images to
+  anyone without a signature, at any size and in any format.
+  `configs/config.example.yml` now leaves `allowlist_hosts` out, with one
+  commented entry under `example.com`. Getting Started and "Running under upaas"
+  in `README.md` say that every image on an allowlisted host is served without a
+  signature, so an allowlisted host must be one the operator controls.
+- 2026-10-15 a chunked upstream body of exactly `upstream_max_response_size`
+  bytes is accepted (closes #248): once the limit was used up, the fetcher's
+  next read refused the body without looking for its end. Go's HTTP client
+  returns the end of a body sent with a `Content-Length` together with its last
+  bytes, but the end of a chunked body can come in a later read, so such a body
+  was refused after it had been downloaded whole, and the image routes
+  answered 500. Once the limit is used up, the fetcher now reads one more byte:
+  if the body has ended it is accepted, and if there is more it is refused with
+  `ErrResponseTooLarge`, as before.
 - 2026-10-13 `fit=outside` no longer makes an output above 8192 on a side
   (closes #257): it scales the image to cover the requested size, and the side
   scaled past that size was held to no maximum, so `1x8192?fit=outside` of a
@@ -476,11 +516,11 @@ P2: security: per-origin rate limiting
   cancellation but keeps its deadline, and the others wait for its image or
   error holding no upstream connection or processing slot, and stop waiting when
   their own context ends; the request doing the processing waits for it even
-  then, up to its deadline; a request whose context has already ended starts
-  nothing; each request counts one miss, and the processing counts its fetch and
-  transcode once; a panic while processing is reported to Sentry when
-  `sentry_dsn` is set and becomes an error for every waiting request instead of
-  stopping pixad; documented in `README.md`.
+  then, until it ends, which can be after its deadline; a request whose context
+  has already ended starts nothing; each request counts one miss, and the
+  processing counts its fetch and transcode once; a panic while processing is
+  reported to Sentry when `sentry_dsn` is set and becomes an error for every
+  waiting request instead of stopping pixad; documented in `README.md`.
 - 2026-09-29 only the image routes send CORS headers (closes #98): the CORS
   middleware, with the `access_control_allow_origin` origin, moved from the
   router root onto a `/v1` subrouter holding `/v1/image/` and `/v1/e/`, where it

configs/config.example.yml

@@ -45,15 +45,14 @@ state_dir: ./data
 signing_key: "CHANGE_ME_generate_with_openssl_rand_base64_32"
 
 # Hosts that don't require signatures (default: none)
+# Every image on an allowlisted host is served to anyone without a
+# signature, at any size and in any format, so list only hosts you
+# control, where nobody else can put images.
 # Use "." prefix for wildcard subdomain matching (e.g., ".example.com" matches "cdn.example.com")
 # An entry that is neither a host name nor an IP address (IPv6 without
 # brackets), such as one with a port or a "*." wildcard, aborts startup.
-allowlist_hosts:
-  - s3.sneak.cloud
-  - static.sneak.cloud
-  - sneak.berlin
-  - github.com
-  - user-images.githubusercontent.com
+# allowlist_hosts:
+#   - images.example.com
 
 # Hosts whose pages may not show pixa's images, written as for
 # allowlist_hosts. A request to /v1/image/ or /v1/e/ whose Referer header

internal/config/config_internal_test.go

@@ -92,6 +92,32 @@ func TestGetStringSlice_Empty(t *testing.T) {
 	}
 }
 
+// TestExampleConfigAllowlistsNoHost loads configs/config.example.yml
+// with a real signing key, as Getting Started in README.md has it copied
+// and edited, and checks that it allowlists no host: every image on an
+// allowlisted host is served to anyone without a signature.
+func TestExampleConfigAllowlistsNoHost(t *testing.T) {
+	t.Setenv("PIXA_SIGNING_KEY", validTestSigningKey)
+
+	examplePath := filepath.Join("..", "..", "configs", "config.example.yml")
+
+	sc, err := loadTestConfig(examplePath)
+	if err != nil {
+		t.Fatalf("failed to load %s: %v", examplePath, err)
+	}
+
+	c, err := newFromSmartConfig(sc)
+	if err != nil {
+		t.Fatalf("%s is not a valid config: %v", examplePath, err)
+	}
+
+	t.Logf("allowlist_hosts in %s: %v", examplePath, c.AllowlistHosts)
+
+	if len(c.AllowlistHosts) != 0 {
+		t.Errorf("%s allowlists %v, want no host", examplePath, c.AllowlistHosts)
+	}
+}
+
 // loadTestConfig is a helper to load a config file for testing.
 func loadTestConfig(path string) (*smartconfig.Config, error) {
 	return smartconfig.NewFromConfigPath(path)

internal/handlers/server_busy_internal_test.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"net/http"
@@ -47,3 +48,39 @@ func TestServerBusyAnswers503(t *testing.T) {
 		}
 	}
 }
+
+// TestProcessingTimedOutAnswers503 checks that both image routes answer 503
+// with the server busy error when the image service reports that processing
+// stopped at the deadline of the request doing it, wrapped as the service
+// wraps it. A request that waited for that processing gets this error while
+// its own deadline may have time left.
+func TestProcessingTimedOutAnswers503(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	req := &imgcache.ImageRequest{SourceHost: "img.example.com", SourcePath: "/a.jpg"}
+	err := fmt.Errorf("%w: failed to encode: %w",
+		imgcache.ErrProcessingTimedOut, context.DeadlineExceeded)
+
+	plain := httptest.NewRecorder()
+	h.respondImageError(plain, req, err)
+
+	encrypted := httptest.NewRecorder()
+	h.handleImageError(encrypted, err)
+
+	for route, rec := range map[string]*httptest.ResponseRecorder{
+		"/v1/image/": plain, "/v1/e/": encrypted,
+	} {
+		t.Logf("%s: status = %d, body = %q", route, rec.Code, rec.Body.String())
+
+		if rec.Code != http.StatusServiceUnavailable {
+			t.Errorf("%s: status = %d, want %d",
+				route, rec.Code, http.StatusServiceUnavailable)
+		}
+
+		if !strings.Contains(rec.Body.String(), "server busy, try again later") {
+			t.Errorf("%s: body = %q, want the server busy error",
+				route, rec.Body.String())
+		}
+	}
+}

internal/httpfetcher/fetch_internal_test.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 	"testing"
@@ -297,6 +298,139 @@ func TestFetchMaxResponseSizeEnforced(t *testing.T) {
 	}
 }
 
+// fetchWithMaxResponseSize serves handler as the fake upstream and fetches
+// its /image route with MaxResponseSize set to maxResponseSize.
+func fetchWithMaxResponseSize(
+	t *testing.T,
+	maxResponseSize int64,
+	handler http.HandlerFunc,
+) *FetchResult {
+	t.Helper()
+
+	srv := httptest.NewServer(handler)
+	t.Cleanup(srv.Close)
+
+	cfg := DefaultConfig()
+	cfg.MaxResponseSize = maxResponseSize
+
+	f, _ := newServerFetcher(t, srv, cfg)
+
+	res := fetchImage(t, f, "/image")
+	t.Cleanup(func() { _ = res.Content.Close() })
+
+	return res
+}
+
+// TestFetchContentLengthBodyAtMaxResponseSize checks that a body sent with a
+// Content-Length is read whole at exactly MaxResponseSize bytes and refused
+// at one byte more.
+func TestFetchContentLengthBodyAtMaxResponseSize(t *testing.T) {
+	t.Parallel()
+
+	const maxResponseSize = 8
+
+	for _, tc := range []struct {
+		size    int
+		wantErr error
+	}{
+		{maxResponseSize, nil},
+		{maxResponseSize + 1, ErrResponseTooLarge},
+	} {
+		t.Run(strconv.Itoa(tc.size)+" bytes", func(t *testing.T) {
+			t.Parallel()
+
+			body := strings.Repeat("x", tc.size)
+
+			res := fetchWithMaxResponseSize(t, maxResponseSize,
+				func(w http.ResponseWriter, _ *http.Request) {
+					w.Header().Set("Content-Type", contentTypeJPEG)
+					w.Header().Set("Content-Length", strconv.Itoa(tc.size))
+					_, _ = io.WriteString(w, body)
+				})
+
+			if res.ContentLength != int64(tc.size) {
+				t.Fatalf("ContentLength = %d, want %d", res.ContentLength, tc.size)
+			}
+
+			data, err := io.ReadAll(res.Content)
+			t.Logf("body of %d bytes: read %d, error %v", tc.size, len(data), err)
+
+			if !errors.Is(err, tc.wantErr) {
+				t.Fatalf("read error = %v, want %v", err, tc.wantErr)
+			}
+
+			if err == nil && string(data) != body {
+				t.Errorf("read %q, want %q", data, body)
+			}
+		})
+	}
+}
+
+// TestFetchChunkedBodyAtMaxResponseSize checks that a chunked body is read
+// whole at exactly MaxResponseSize bytes and refused at one byte more. The
+// upstream sends the end of the body only once the test has read
+// MaxResponseSize bytes, so the read after them comes before the end of the
+// body has arrived, as it can from a real upstream.
+func TestFetchChunkedBodyAtMaxResponseSize(t *testing.T) {
+	t.Parallel()
+
+	const maxResponseSize = 8
+
+	for _, tc := range []struct {
+		size    int
+		wantErr error
+	}{
+		{maxResponseSize, nil},
+		{maxResponseSize + 1, ErrResponseTooLarge},
+	} {
+		t.Run(strconv.Itoa(tc.size)+" bytes", func(t *testing.T) {
+			t.Parallel()
+
+			body := strings.Repeat("x", tc.size)
+			endBody := make(chan struct{})
+
+			res := fetchWithMaxResponseSize(t, maxResponseSize,
+				func(w http.ResponseWriter, r *http.Request) {
+					w.Header().Set("Content-Type", contentTypeJPEG)
+					_, _ = io.WriteString(w, body)
+					// With no Content-Length, flushing sends the body
+					// chunked; its end is sent when the handler returns.
+					_ = http.NewResponseController(w).Flush()
+
+					select {
+					case <-endBody:
+					case <-r.Context().Done():
+					}
+				})
+
+			if res.ContentLength != -1 {
+				t.Fatalf("ContentLength = %d, want -1 (chunked)", res.ContentLength)
+			}
+
+			head := make([]byte, maxResponseSize)
+
+			_, err := io.ReadFull(res.Content, head)
+			if err != nil {
+				t.Fatalf("read the first %d bytes: %v", maxResponseSize, err)
+			}
+
+			close(endBody)
+
+			rest, err := io.ReadAll(res.Content)
+			data := string(head) + string(rest)
+			t.Logf("body of %d bytes: read %d, error %v", tc.size, len(data), err)
+
+			if !errors.Is(err, tc.wantErr) {
+				t.Fatalf("read error = %v, want %v", err, tc.wantErr)
+			}
+
+			if err == nil && data != body {
+				t.Errorf("read %q, want %q", data, body)
+			}
+		})
+	}
+}
+
 func TestFetchSemaphoreReleasedOnError(t *testing.T) {
 	t.Parallel()
 

internal/httpfetcher/httpfetcher.go

@@ -660,14 +660,33 @@ func dialSSRFSafe(
 	return nil, fmt.Errorf("%w to %s", errConnectFailed, host)
 }
 
-// limitedReader wraps a reader and limits the number of bytes read.
+// limitedReader wraps a reader and refuses a body longer than remaining
+// bytes with ErrResponseTooLarge. remaining goes below 0 once the body is
+// found to be longer.
 type limitedReader struct {
 	reader    io.Reader
 	remaining int64
 }
 
+// Read reads at most the remaining bytes. Once they are read, it reads one
+// more byte to tell the end of the body from more data: at the end it
+// returns the wrapped reader's error, io.EOF; with more data it returns
+// ErrResponseTooLarge, as does every later Read.
 func (r *limitedReader) Read(p []byte) (int, error) {
-	if r.remaining <= 0 {
+	if r.remaining < 0 {
+		return 0, ErrResponseTooLarge
+	}
+
+	if r.remaining == 0 {
+		var extra [1]byte
+
+		n, err := r.reader.Read(extra[:])
+		r.remaining -= int64(n)
+
+		if n == 0 {
+			return 0, err
+		}
+
 		return 0, ErrResponseTooLarge
 	}
 

internal/imageprocessor/deadline_internal_test.go

@@ -0,0 +1,118 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"testing"
+	"time"
+)
+
+// readerAfterDeadline is a Process input whose first Read waits until done
+// is closed, so the deadline passes after Process has taken its slot and
+// before it encodes.
+type readerAfterDeadline struct {
+	data   *bytes.Reader
+	done   <-chan struct{}
+	waited bool
+}
+
+func (r *readerAfterDeadline) Read(p []byte) (int, error) {
+	if !r.waited {
+		r.waited = true
+
+		<-r.done
+	}
+
+	return r.data.Read(p)
+}
+
+// TestProcessStopsWhenDeadlinePassed checks that Process returns
+// context.DeadlineExceeded, and holds no slot afterwards, when its context's
+// deadline has passed before it starts or while it reads its input. libvips
+// cannot stop an encode once it has begun, so Process must not begin one for
+// a request that has ended.
+func TestProcessStopsWhenDeadlinePassed(t *testing.T) {
+	t.Parallel()
+
+	input := createTestJPEG(t, 50, 50)
+
+	t.Run("before Process starts", func(t *testing.T) {
+		t.Parallel()
+
+		ctx, cancel := context.WithDeadline(t.Context(),
+			time.Now().Add(-time.Second))
+		defer cancel()
+
+		checkProcessStopsAtDeadline(ctx, t, bytes.NewReader(input))
+	})
+
+	t.Run("while Process reads its input", func(t *testing.T) {
+		t.Parallel()
+
+		ctx, cancel := context.WithTimeout(t.Context(), 100*time.Millisecond)
+		defer cancel()
+
+		checkProcessStopsAtDeadline(ctx, t, &readerAfterDeadline{
+			data: bytes.NewReader(input), done: ctx.Done(),
+		})
+	})
+}
+
+// checkProcessStopsAtDeadline runs Process on input with ctx, whose deadline
+// has passed by the time Process could encode, and fails the test unless
+// Process returns context.DeadlineExceeded and gives its slot back.
+func checkProcessStopsAtDeadline(
+	ctx context.Context, t *testing.T, input io.Reader,
+) {
+	t.Helper()
+
+	proc := New(Params{MaxConcurrentProcessing: 1})
+
+	result, err := proc.Process(ctx, input, smallJPEGRequest())
+	if result != nil {
+		t.Logf("Process() made a %dx%d image after its deadline passed",
+			result.Width, result.Height)
+
+		_ = result.Content.Close()
+	}
+
+	t.Logf("Process() error = %v", err)
+
+	if !errors.Is(err, context.DeadlineExceeded) {
+		t.Errorf("Process() error = %v, want context.DeadlineExceeded", err)
+	}
+
+	if held := len(proc.processingSemaphore); held != 0 {
+		t.Errorf("%d slots still held after Process() returned", held)
+	}
+}
+
+// inputNotToBeRead is a Process input that fails the test when it is read.
+type inputNotToBeRead struct {
+	t *testing.T
+}
+
+func (r inputNotToBeRead) Read([]byte) (int, error) {
+	r.t.Error("Process read its input although its deadline had passed")
+
+	return 0, io.EOF
+}
+
+// TestProcessReadsNoInputOnceDeadlinePassed checks that Process, called with
+// a context whose deadline passed before it starts, gives its slot back
+// without reading its input. With a slot free, the select in acquireSlot
+// takes the slot or sees the ended context at random, so Process is called
+// many times.
+func TestProcessReadsNoInputOnceDeadlinePassed(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithDeadline(t.Context(),
+		time.Now().Add(-time.Second))
+	defer cancel()
+
+	for range 100 {
+		checkProcessStopsAtDeadline(ctx, t, inputNotToBeRead{t: t})
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -208,7 +208,9 @@ func New(params Params) *ImageProcessor {
 // Process transforms an image according to the request. When
 // MaxConcurrentProcessing images are already being processed, it waits up
 // to ProcessingWaitTimeout for one to finish, then fails with
-// ErrTooManyImages.
+// ErrTooManyImages. When ctx has ended once Process has a slot, or before it
+// encodes, it returns ctx's error and gives the slot back. libvips cannot stop
+// an encode once it has begun, so an encode keeps its slot until it finishes.
 func (p *ImageProcessor) Process(
 	ctx context.Context,
 	input io.Reader,
@@ -290,7 +292,7 @@ func (p *ImageProcessor) Process(
 	}
 
 	// Encode to target format
-	output, err := p.encode(img, outputFormat, req.Quality)
+	output, err := p.encode(ctx, img, outputFormat, req.Quality)
 	if err != nil {
 		return nil, fmt.Errorf("failed to encode: %w", err)
 	}
@@ -417,25 +419,28 @@ func (p *ImageProcessor) WaitForProcessing(ctx context.Context) int {
 
 // acquireSlot takes a slot in processingSemaphore, waiting at most
 // processingWaitTimeout for one to free up, and returns the func that gives
-// it back. A free slot is taken even when ctx has ended; only the wait for
-// one stops when ctx ends, as the rest of Process does not check ctx.
+// it back. When ctx has ended by the time it has a slot, it gives the slot
+// back and returns ctx's error.
 func (p *ImageProcessor) acquireSlot(ctx context.Context) (func(), error) {
-	release := func() { <-p.processingSemaphore }
-
 	select {
 	case p.processingSemaphore <- struct{}{}:
-		return release, nil
-	default:
-	}
-
-	select {
-	case p.processingSemaphore <- struct{}{}:
-		return release, nil
 	case <-time.After(p.processingWaitTimeout):
 		return nil, ErrTooManyImages
 	case <-ctx.Done():
 		return nil, ctx.Err()
 	}
+
+	release := func() { <-p.processingSemaphore }
+
+	// select takes the slot even when ctx has ended at the same moment
+	err := ctx.Err()
+	if err != nil {
+		release()
+
+		return nil, err
+	}
+
+	return release, nil
 }
 
 // detectFormat returns the format string for a govips image type.
@@ -527,10 +532,17 @@ func (p *ImageProcessor) resize(
 
 const defaultQuality = 85
 
-// encode encodes an image to the specified format.
+// encode encodes an image to the specified format. libvips cannot stop an
+// encode once it has begun, so when ctx has ended encode returns ctx's error
+// without beginning one.
 func (p *ImageProcessor) encode(
-	img *vips.ImageRef, format Format, quality int,
+	ctx context.Context, img *vips.ImageRef, format Format, quality int,
 ) ([]byte, error) {
+	err := ctx.Err()
+	if err != nil {
+		return nil, err
+	}
+
 	if quality <= 0 {
 		quality = defaultQuality
 	}
@@ -540,7 +552,7 @@ func (p *ImageProcessor) encode(
 	// libvips' built-in profile; govips' own sRGB path variable is set on
 	// first use but read without a lock, so concurrent requests race on it.
 	if img.HasICCProfile() {
-		err := img.TransformICCProfileWithFallback("srgb", "srgb")
+		err = img.TransformICCProfileWithFallback("srgb", "srgb")
 		if err != nil {
 			return nil, fmt.Errorf("failed to convert to sRGB: %w", err)
 		}

internal/imgcache/concurrent_misses_internal_test.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/getsentry/sentry-go"
 	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/magic"
 )
 
@@ -435,6 +436,47 @@ func TestService_Get_ReturnsByItsDeadline(t *testing.T) {
 	}
 }
 
+// TestService_Get_WaiterGetsProcessingTimedOut has a request with no deadline
+// join the held fetch of a first request with a short one. With the only
+// processing slot held, the first request's transcode waits for it until the
+// first request's deadline ends, and stops then. The waiting request, whose
+// own context has not ended, must get ErrProcessingTimedOut, which the image
+// routes answer 503 as when pixa is busy.
+func TestService_Get_WaiterGetsProcessingTimedOut(t *testing.T) {
+	t.Parallel()
+
+	svc, fixtures, fetcher := setupHeldFetchService(t)
+	svc.processor = imageprocessor.New(
+		imageprocessor.Params{MaxConcurrentProcessing: 1},
+	)
+
+	holdProcessingSlot(t, svc.processor)
+
+	const firstTimeout = 500 * time.Millisecond
+
+	firstCtx, cancelFirst := context.WithTimeout(t.Context(), firstTimeout)
+	defer cancelFirst()
+
+	first := startGet(firstCtx, svc, photoVariant(fixtures, 85, FitCover))
+
+	<-fetcher.started
+
+	waiter := startGet(t.Context(), svc, photoVariant(fixtures, 85, FitCover))
+
+	time.Sleep(arrivalWait)
+	fetcher.releaseFetches()
+
+	t.Logf("first request: Get() error = %v", (<-first).err)
+
+	got := <-waiter
+	t.Logf("waiting request: Get() error = %v", got.err)
+
+	if !errors.Is(got.err, ErrProcessingTimedOut) {
+		t.Errorf("waiting request: Get() error = %v, want %v",
+			got.err, ErrProcessingTimedOut)
+	}
+}
+
 // panickingFetcher panics on every fetch.
 type panickingFetcher struct{}
 

internal/imgcache/service.go

@@ -128,6 +128,12 @@ func NewService(cfg *ServiceConfig) (*Service, error) {
 // ErrNegativeCached is returned when a URL is in the negative cache (recently failed).
 var ErrNegativeCached = errors.New("request is in negative cache (recently failed)")
 
+// ErrProcessingTimedOut is returned when processing an image stopped because
+// the downstream_timeout of the request doing it ended, to that request and to
+// every request that waited for its image.
+var ErrProcessingTimedOut = errors.New(
+	"image processing stopped at the deadline of the request doing it")
+
 // Get retrieves a processed image, fetching and processing if necessary.
 func (s *Service) Get(ctx context.Context, req *ImageRequest) (*ImageResponse, error) {
 	// Propagate AllowHTTP setting to the request

internal/imgcache/storage.go

@@ -10,6 +10,7 @@ import (
 	"log/slog"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 )
 
@@ -371,11 +372,13 @@ func HashPath(path string) PathHash {
 
 // CacheKey generates a unique key for a request variant.
 // Format: sha256(host:path:query:width:height:format:quality:fit_mode)
+// The path and query go through escapePercentAndColon, so that a ":" in
+// either cannot be taken for the one between them.
 func CacheKey(req *ImageRequest) VariantKey {
 	data := fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%s",
 		req.SourceHost,
-		req.SourcePath,
-		req.SourceQuery,
+		escapePercentAndColon(req.SourcePath),
+		escapePercentAndColon(req.SourceQuery),
 		req.Size.Width,
 		req.Size.Height,
 		req.Format,
@@ -387,6 +390,14 @@ func CacheKey(req *ImageRequest) VariantKey {
 	return VariantKey(hex.EncodeToString(h[:]))
 }
 
+// escapePercentAndColon returns s with each "%" written as "%25" and each
+// ":" as "%3A", so that the result holds no ":".
+func escapePercentAndColon(s string) string {
+	s = strings.ReplaceAll(s, "%", "%25")
+
+	return strings.ReplaceAll(s, ":", "%3A")
+}
+
 // VariantStorage handles key-based file storage for processed image variants.
 // Files are stored at: <basedir>/<ab>/<cd>/<cache_key>
 // Metadata is stored at: <basedir>/<ab>/<cd>/<cache_key>.meta

internal/imgcache/storage_internal_test.go

@@ -407,6 +407,62 @@ func TestCacheKey(t *testing.T) {
 	}
 }
 
+// TestCacheKey_ColonOrPercentInPathOrQuery checks that two different source
+// URLs whose path or query holds ":" or "%" get different keys.
+func TestCacheKey_ColonOrPercentInPathOrQuery(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name          string
+		path1, query1 string
+		path2, query2 string
+	}{
+		{
+			name:  "colon moved from path to query",
+			path1: "/a:b", query1: "",
+			path2: "/a", query2: "b:",
+		},
+		{
+			name:  "colon in path and its percent-encoding",
+			path1: "/a:b", query1: "",
+			path2: "/a%3Ab", query2: "",
+		},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			key1 := CacheKey(&ImageRequest{
+				SourceHost:  testHostCDN,
+				SourcePath:  tt.path1,
+				SourceQuery: tt.query1,
+				Size:        Size{Width: 800, Height: 600},
+				Format:      FormatWebP,
+				Quality:     85,
+				FitMode:     FitCover,
+			})
+			key2 := CacheKey(&ImageRequest{
+				SourceHost:  testHostCDN,
+				SourcePath:  tt.path2,
+				SourceQuery: tt.query2,
+				Size:        Size{Width: 800, Height: 600},
+				Format:      FormatWebP,
+				Quality:     85,
+				FitMode:     FitCover,
+			})
+
+			t.Logf("CacheKey() of path %q query %q = %s", tt.path1, tt.query1, key1)
+			t.Logf("CacheKey() of path %q query %q = %s", tt.path2, tt.query2, key2)
+
+			if key1 == key2 {
+				t.Errorf("CacheKey() gave path %q query %q and path %q query %q "+
+					"the same key", tt.path1, tt.query1, tt.path2, tt.query2)
+			}
+		})
+	}
+}
+
 // TestVariantStorage_StoreLogsFailedMetaWrite verifies that a .meta write
 // that fails is logged at warn and the store still succeeds.
 func TestVariantStorage_StoreLogsFailedMetaWrite(t *testing.T) {

internal/signature/escaping_test.go

@@ -0,0 +1,113 @@
+package signature_test
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/signature"
+)
+
+// TestSigner_Verify_ColonCannotMovePathQueryBoundary checks that a URL
+// signed for path "/a:b" with no query does not verify for path "/a" with
+// query "b:", a different resource on the same host.
+func TestSigner_Verify_ColonCannotMovePathQueryBoundary(t *testing.T) {
+	t.Parallel()
+
+	signer := signature.New(goldenSigningKey)
+
+	signed := &signature.Request{
+		SourceHost: testHost,
+		SourcePath: "/a:b",
+		Width:      800,
+		Height:     600,
+		Format:     testFormatWebP,
+		Quality:    85,
+		FitMode:    testFitCover,
+		Expires:    time.Now().Add(1 * time.Hour),
+	}
+	signed.Signature = signer.Sign(signed)
+
+	replayed := *signed
+	replayed.SourcePath = "/a"
+	replayed.SourceQuery = "b:"
+
+	t.Logf("signature for path %q query %q: %s",
+		signed.SourcePath, signed.SourceQuery, signed.Signature)
+	t.Logf("signature for path %q query %q: %s",
+		replayed.SourcePath, replayed.SourceQuery, signer.Sign(&replayed))
+
+	err := signer.Verify(&replayed)
+	if !errors.Is(err, signature.ErrInvalid) {
+		t.Errorf("Verify() of path %q query %q with the signature for "+
+			"path %q = %v, want %v",
+			replayed.SourcePath, replayed.SourceQuery, signed.SourcePath,
+			err, signature.ErrInvalid)
+	}
+}
+
+// TestSigner_EscapedGoldenVectors pins the signature of requests whose path
+// or query holds ":" or "%", which are signed with "%" written as "%25" and
+// ":" as "%3A". The expected values were computed once, outside pixa, from
+// the signed data in each comment.
+func TestSigner_EscapedGoldenVectors(t *testing.T) {
+	t.Parallel()
+
+	signer := signature.New(goldenSigningKey)
+
+	vectors := []struct {
+		name          string
+		path          string
+		query         string
+		wantSignature string
+	}{
+		{
+			name: "colon in path",
+			path: "/photos/a:b.jpg",
+			// Signed data:
+			// "cdn.example.com:/photos/a%3Ab.jpg::800:600:webp:1704067200:85:cover"
+			wantSignature: "WnEpZ7bQp1Zvw8WDatlj2vu-4wYKWE3BFBizV8m__6k=",
+		},
+		{
+			name:  "colon in query",
+			path:  testPath,
+			query: "t=1:2",
+			// Signed data:
+			// "cdn.example.com:/photos/cat.jpg:t=1%3A2:800:600:webp:1704067200:85:cover"
+			wantSignature: "85kRoN8hhr4cCUkEUjNvUsvVYKjYYfWdeffx5TjJjqA=",
+		},
+		{
+			name:  "percent in path and query",
+			path:  "/photos/a%20b.jpg",
+			query: "q=50%",
+			// Signed data:
+			// "cdn.example.com:/photos/a%2520b.jpg:q=50%25:800:600:webp:1704067200:85:cover"
+			wantSignature: "MjDiiRdgXMrsjqPMsENtD4aG1hp1e-a6zy67vKTX4CQ=",
+		},
+	}
+
+	for _, tt := range vectors {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := &signature.Request{
+				SourceHost:  testHost,
+				SourcePath:  tt.path,
+				SourceQuery: tt.query,
+				Width:       800,
+				Height:      600,
+				Format:      testFormatWebP,
+				Quality:     85,
+				FitMode:     testFitCover,
+				Expires:     time.Unix(goldenExpiresUnix, 0),
+			}
+
+			gotSignature := signer.Sign(req)
+			t.Logf("Sign() of path %q query %q = %s", tt.path, tt.query, gotSignature)
+
+			if gotSignature != tt.wantSignature {
+				t.Errorf("Sign() = %q, want %q", gotSignature, tt.wantSignature)
+			}
+		})
+	}
+}

internal/signature/signature.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"net/url"
 	"strconv"
+	"strings"
 	"time"
 )
 
@@ -154,15 +155,17 @@ func (s *Signer) GenerateSignedURL(
 
 // buildSignatureData creates the string to be signed.
 // Format: "host:path:query:width:height:format:expiration:quality:fit"
-// All components are used verbatim (exact match). No normalization,
-// suffix matching, or wildcard expansion is performed. Quality and fit
-// are the effective transform values, so replaying a signed URL with a
-// different quality or fit mode fails verification.
+// The path and query go through escapePercentAndColon, so that a ":" in
+// either cannot be taken for the one between them; every other component
+// is used verbatim. No normalization, suffix matching, or wildcard
+// expansion is performed. Quality and fit are the effective transform
+// values, so replaying a signed URL with a different quality or fit mode
+// fails verification.
 func (s *Signer) buildSignatureData(req *Request) string {
 	return fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%d:%s",
 		req.SourceHost,
-		req.SourcePath,
-		req.SourceQuery,
+		escapePercentAndColon(req.SourcePath),
+		escapePercentAndColon(req.SourceQuery),
 		req.Width,
 		req.Height,
 		req.Format,
@@ -172,6 +175,14 @@ func (s *Signer) buildSignatureData(req *Request) string {
 	)
 }
 
+// escapePercentAndColon returns s with each "%" written as "%25" and each
+// ":" as "%3A", so that the result holds no ":".
+func escapePercentAndColon(s string) string {
+	s = strings.ReplaceAll(s, "%", "%25")
+
+	return strings.ReplaceAll(s, ":", "%3A")
+}
+
 // ParseParams extracts signature and expiration from query parameters.
 func ParseParams(sig, expStr string) (string, time.Time, error) {
 	if expStr == "" {