Accept a chunked upstream body of exactly the maximum size (closes #248)

Once MaxResponseSize bytes were read, limitedReader.Read refused the
next read without looking for the end of the body. A body sent with a
Content-Length gets io.EOF with its last bytes, but the end of a
chunked body can come in a later read, so a chunked body of exactly
the limit was refused. Once the limit is used up, Read now reads one
more byte: at the end of the body it returns io.EOF, and with more
data it returns ErrResponseTooLarge, as does every later Read.

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
@@ -263,6 +270,12 @@ once, is refused with 400.
   both optional (values under Signature Specification). Both are part of what is
   cached, so each value of either is a separate cached image.
 
+A size with one side `0` has that side scaled to keep the source's proportions,
+and is refused with 400 when that side would come out above 8192, the most
+either side may be; so is a size with `fit=outside`, which scales the image to
+cover the size and so past it on one side, when that side would come out
+above 8192.
+
 The source image may be JPEG, PNG, GIF, WebP, AVIF or JPEG XL. The upstream's
 `Content-Type` must name its format, and the image's first bytes must match it.
 An upstream answering with any other `Content-Type`, SVG included, is refused
@@ -345,7 +358,8 @@ nor change what it asks for.
    (`POST /generate`). The format is JPEG XL unless another is chosen; a form
    sent with an empty format, or none, also makes a JPEG XL URL. Width and
    height both empty or `0` keep the original size; if only one of them is empty
-   or `0`, that side is scaled to keep the image's proportions.
+   or `0`, that side is scaled to keep the image's proportions. A URL whose
+   scaled side would come out above 8192 answers 400.
 3. The page shows the URL, `https://<host>/v1/e/<token>/img.<format>`, and when
    it expires. `<host>` is the host the page was opened on, and the URL starts
    with `http` instead while `debug` is on. The name after the token is ignored

TODO.md

@@ -30,6 +30,39 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 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
+  3000x600 source was resized to 40960x8192 and encoded. The image processor now
+  refuses such a size with `ErrOutputTooLarge` before resizing, and both image
+  routes answer it 400, as for a size with one side `0`. The other fit modes
+  make the requested size or one within it, so none can go above 8192.
+- 2026-10-13 a size with one side `0` no longer makes an output above 8192 on a
+  side (closes #241): that side is computed from the source's proportions, and
+  nothing held it to the 8192 a requested side is held to, so `0x8192` of a
+  3000x600 source was resized to 40960x8192 and encoded. The image processor now
+  refuses such a size with `ErrOutputTooLarge` once it knows the source's
+  dimensions, before resizing, and both image routes answer it 400 with the JSON
+  error body. The refusal is not cached as a failed fetch, so the same source is
+  still served at other sizes. `imgcache.MaxDimension` is now the image
+  processor's `MaxOutputDimension`, so the two cannot differ.
 - 2026-10-13 the unused cache lifetime is removed (closes #69):
   `CacheConfig.CacheTTL`, which pixad set to `DefaultCacheTTL` (24 hours), was
   never read, so cached images never expired. The field and the constant are

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/image.go

@@ -255,6 +255,14 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, imageprocessor.ErrOutputTooLarge) {
+		s.respondError(w, fmt.Sprintf("size too large for the source: the side "+
+			"computed from its proportions is above %d",
+			imageprocessor.MaxOutputDimension), http.StatusBadRequest)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 

internal/handlers/imageenc.go

@@ -165,6 +165,10 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat):
 		s.respondError(w, "unsupported source format",
 			http.StatusUnsupportedMediaType)
+	case errors.Is(err, imageprocessor.ErrOutputTooLarge):
+		s.respondError(w, fmt.Sprintf("size too large for the source: the side "+
+			"computed from its proportions is above %d",
+			imageprocessor.MaxOutputDimension), http.StatusBadRequest)
 	default:
 		s.log.Error("image request failed", "error", err)
 		s.respondError(w, "internal error", http.StatusInternalServerError)

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/computed_side_internal_test.go

@@ -0,0 +1,52 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+// TestImageProcessor_RefusesComputedSideAboveMaximum verifies that a size
+// with one side 0, which Process computes from the input's proportions, is
+// served when that side comes out at MaxOutputDimension and refused with
+// ErrOutputTooLarge when it would be above. A 4096x1 or 1x4096 input given
+// 2 for its short side has 8192 for its long side.
+func TestImageProcessor_RefusesComputedSideAboveMaximum(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+		size                    Size
+		wantErr                 error
+	}{
+		{"width at the maximum", 4096, 1, Size{Height: 2}, nil},
+		{"width above the maximum", 4096, 1, Size{Height: 3}, ErrOutputTooLarge},
+		{"height at the maximum", 1, 4096, Size{Width: 2}, nil},
+		{"height above the maximum", 1, 4096, Size{Width: 3}, ErrOutputTooLarge},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createTestJPEG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{Size: tt.size, Format: FormatJPEG, FitMode: FitCover})
+			if result != nil {
+				t.Logf("Process() made %dx%d", result.Width, result.Height)
+
+				if max(result.Width, result.Height) != MaxOutputDimension {
+					t.Errorf("Process() made %dx%d, want a side of %d",
+						result.Width, result.Height, MaxOutputDimension)
+				}
+			}
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Errorf("Process() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}

internal/imageprocessor/fit_outside_internal_test.go

@@ -0,0 +1,62 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+)
+
+// TestImageProcessor_RefusesFitOutsideAboveMaximum verifies that FitOutside,
+// which scales the input to cover the requested size and so past it on one
+// side, is refused with ErrOutputTooLarge when that side would come out above
+// MaxOutputDimension, and otherwise makes the scaled size. A 1024x256 or
+// 256x1024 input scaled by 8 has 8192 for its long side.
+func TestImageProcessor_RefusesFitOutsideAboveMaximum(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name                    string
+		inputWidth, inputHeight int
+		size                    Size
+		// wantWidth and wantHeight are the output's, 0 for a refusal
+		wantWidth, wantHeight int
+	}{
+		{"width above the maximum", 3000, 600, Size{Width: 1, Height: 8192}, 0, 0},
+		{"height above the maximum", 600, 3000, Size{Width: 8192, Height: 1}, 0, 0},
+		{"width at the maximum", 1024, 256, Size{Width: 1, Height: 2048}, 8192, 2048},
+		{"height at the maximum", 256, 1024, Size{Width: 2048, Height: 1}, 2048, 8192},
+		{"within the maximum", 3000, 600, Size{Width: 1, Height: 1200}, 6000, 1200},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			input := createTestJPEG(t, tt.inputWidth, tt.inputHeight)
+
+			result, err := New(Params{}).Process(t.Context(),
+				bytes.NewReader(input),
+				&Request{Size: tt.size, Format: FormatJPEG, FitMode: FitOutside})
+			if result != nil {
+				t.Logf("Process() made %dx%d", result.Width, result.Height)
+			}
+
+			if tt.wantWidth == 0 {
+				if !errors.Is(err, ErrOutputTooLarge) {
+					t.Errorf("Process() error = %v, want %v", err, ErrOutputTooLarge)
+				}
+
+				return
+			}
+
+			if err != nil {
+				t.Fatalf("Process() error = %v", err)
+			}
+
+			if result.Width != tt.wantWidth || result.Height != tt.wantHeight {
+				t.Errorf("Process() made %dx%d, want %dx%d",
+					result.Width, result.Height, tt.wantWidth, tt.wantHeight)
+			}
+		})
+	}
+}

internal/imageprocessor/imageprocessor.go

@@ -133,6 +133,16 @@ var ErrInputTooLarge = errors.New("input image dimensions exceed maximum")
 // configured byte limit.
 var ErrInputDataTooLarge = errors.New("input data exceeds maximum allowed size")
 
+// MaxOutputDimension is the maximum width or height of an output image, and
+// so of a requested size (imgcache.MaxDimension).
+const MaxOutputDimension = 8192
+
+// ErrOutputTooLarge is returned, before resizing, when the output would have a
+// side above MaxOutputDimension: the side of a size with one side 0, computed
+// from the input's proportions, or with FitOutside, the side scaled past the
+// requested size.
+var ErrOutputTooLarge = errors.New("output image dimensions exceed maximum")
+
 // ErrUnsupportedOutputFormat is returned when the requested output format is
 // not supported.
 var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
@@ -259,8 +269,11 @@ func (p *ImageProcessor) Process(
 		return nil, ErrInputTooLarge
 	}
 
-	// Determine target dimensions
-	targetWidth, targetHeight := targetDimensions(req.Size, origWidth, origHeight)
+	// Determine target dimensions; too large ones are refused before resizing
+	targetWidth, targetHeight, err := targetDimensions(req.Size, origWidth, origHeight)
+	if err != nil {
+		return nil, err
+	}
 
 	// Resize if needed
 	if targetWidth != origWidth || targetHeight != origHeight {
@@ -296,21 +309,29 @@ func (p *ImageProcessor) Process(
 
 // targetDimensions calculates the output dimensions for a requested size,
 // scaling proportionally when only one dimension is given and keeping the
-// original dimensions when both are zero.
-func targetDimensions(size Size, origWidth, origHeight int) (int, int) {
+// original dimensions when both are zero. A side scaled proportionally can
+// come out above MaxOutputDimension, which is ErrOutputTooLarge.
+func targetDimensions(size Size, origWidth, origHeight int) (int, int, error) {
+	width, height := size.Width, size.Height
+
 	switch {
-	case size.Width == 0 && size.Height == 0:
+	case width == 0 && height == 0:
 		// Both are 0: keep original size
-		return origWidth, origHeight
-	case size.Width == 0:
+		return origWidth, origHeight, nil
+	case width == 0:
 		// Only height specified: calculate width proportionally
-		return origWidth * size.Height / origHeight, size.Height
-	case size.Height == 0:
+		width = origWidth * height / origHeight
+	case height == 0:
 		// Only width specified: calculate height proportionally
-		return size.Width, origHeight * size.Width / origWidth
-	default:
-		return size.Width, size.Height
+		height = origHeight * width / origWidth
 	}
+
+	if width > MaxOutputDimension || height > MaxOutputDimension {
+		return 0, 0, fmt.Errorf("%w: %dx%d is above %d", ErrOutputTooLarge,
+			width, height, MaxOutputDimension)
+	}
+
+	return width, height, nil
 }
 
 // MIME types for the supported image formats.
@@ -441,7 +462,9 @@ func (p *ImageProcessor) detectFormat(imageType vips.ImageType) string {
 	}
 }
 
-// resize resizes the image according to the fit mode.
+// resize resizes the image according to the fit mode. With FitOutside one
+// side can come out larger than requested; when it would be above
+// MaxOutputDimension, resize returns ErrOutputTooLarge without resizing.
 func (p *ImageProcessor) resize(
 	img *vips.ImageRef, width, height int, fit FitMode,
 ) error {
@@ -489,6 +512,12 @@ func (p *ImageProcessor) resize(
 		newW := int(float64(imgW) * scale)
 		newH := int(float64(imgH) * scale)
 
+		// The dimension that exceeds the target is held to the maximum too
+		if newW > MaxOutputDimension || newH > MaxOutputDimension {
+			return fmt.Errorf("%w: %dx%d is above %d", ErrOutputTooLarge,
+				newW, newH, MaxOutputDimension)
+		}
+
 		return img.Thumbnail(newW, newH, vips.InterestingNone)
 
 	default:

internal/imgcache/urlparser.go

@@ -7,6 +7,8 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+
+	"sneak.berlin/go/pixa/internal/imageprocessor"
 )
 
 // URL parsing errors.
@@ -20,8 +22,9 @@ var (
 	ErrPathTraversal     = errors.New("path traversal detected")
 )
 
-// MaxDimension is the maximum allowed width or height.
-const MaxDimension = 8192
+// MaxDimension is the maximum allowed width or height, the same as the image
+// processor's maximum for an output image.
+const MaxDimension = imageprocessor.MaxOutputDimension
 
 // ValidateDimension checks one requested width or height; name ("width" or
 // "height") appears in the error. 0 means "original size" and is valid.

internal/server/computed_side_internal_test.go

@@ -0,0 +1,153 @@
+package server
+
+import (
+	"bytes"
+	"encoding/json"
+	"image/jpeg"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// TestImageProxyRefusesComputedSideAboveMaximum requests sizes of a 3000x600
+// and a 600x3000 PNG through both image routes. A size with one side 0, whose
+// side computed from the source's proportions would be above 8192, is refused
+// with 400 and the JSON error body. The sizes after those refusals are served
+// as before, which also shows that a refusal is not cached as a failed fetch of
+// the source: orig, a size with both sides given, and a size with one side 0
+// whose computed side stays within 8192.
+func TestImageProxyRefusesComputedSideAboveMaximum(t *testing.T) {
+	t.Parallel()
+
+	// The upstream paths of the two sources
+	const (
+		widePath = "/wide.png"
+		tallPath = "/tall.png"
+	)
+
+	wide := encodeTestPNG(t, 3000, 600)
+	tall := encodeTestPNG(t, 600, 3000)
+
+	upstream := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "image/png")
+
+			if r.URL.Path == tallPath {
+				_, _ = w.Write(tall)
+
+				return
+			}
+
+			_, _ = w.Write(wide)
+		}))
+	t.Cleanup(upstream.Close)
+
+	s, _, _ := startImageProxy(t, upstream)
+
+	generator, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator: %v", err)
+	}
+
+	tests := []struct {
+		path string
+		// size is the size of the /v1/image/ URL, and width and height are
+		// those of the encrypted URL
+		size          string
+		width, height int
+		// wantWidth and wantHeight are the served image's, 0 for a refusal
+		wantWidth, wantHeight int
+	}{
+		{widePath, "0x8192", 0, 8192, 0, 0},
+		{tallPath, "8192x0", 8192, 0, 0, 0},
+		{widePath, "orig", 0, 0, 3000, 600},
+		{widePath, "300x200", 300, 200, 300, 200},
+		{widePath, "0x300", 0, 300, 1500, 300},
+		{tallPath, "300x0", 300, 0, 300, 1500},
+	}
+
+	for _, tt := range tests {
+		token, err := generator.Generate(&encurl.Payload{
+			SourceHost: upstreamHost, SourcePath: tt.path,
+			Width: tt.width, Height: tt.height, Format: imgcache.FormatJPEG,
+		})
+		if err != nil {
+			t.Fatalf("encurl Generate: %v", err)
+		}
+
+		for _, target := range []string{
+			"/v1/image/" + upstreamHost + tt.path + "/" + tt.size + ".jpeg",
+			"/v1/e/" + token + "/img.jpg",
+		} {
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, target, nil))
+			t.Logf("GET %s: %d, Content-Type %s",
+				target, rec.Code, rec.Header().Get("Content-Type"))
+
+			if tt.wantWidth == 0 {
+				checkSizeRefused(t, rec)
+			} else {
+				checkServedSize(t, rec, tt.wantWidth, tt.wantHeight)
+			}
+		}
+	}
+}
+
+// checkSizeRefused checks that rec answers 400 with the JSON error body that
+// refuses a size whose side computed from the source's proportions is above
+// 8192.
+func checkSizeRefused(t *testing.T, rec *httptest.ResponseRecorder) {
+	t.Helper()
+
+	const wantError = "size too large for the source: " +
+		"the side computed from its proportions is above 8192"
+
+	var body struct {
+		Error  string `json:"error"`
+		Status int    `json:"status"`
+	}
+
+	err := json.Unmarshal(rec.Body.Bytes(), &body)
+	if err != nil {
+		t.Errorf("status %d, body is not JSON: %v", rec.Code, err)
+
+		return
+	}
+
+	if rec.Code != http.StatusBadRequest || body.Status != http.StatusBadRequest ||
+		body.Error != wantError {
+		t.Errorf("status %d, body %+v, want %d and the error %q",
+			rec.Code, body, http.StatusBadRequest, wantError)
+	}
+}
+
+// checkServedSize checks that rec answers 200 with a JPEG of wantWidth by
+// wantHeight.
+func checkServedSize(
+	t *testing.T, rec *httptest.ResponseRecorder, wantWidth, wantHeight int,
+) {
+	t.Helper()
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("status %d, want %d; body %s",
+			rec.Code, http.StatusOK, rec.Body.String())
+
+		return
+	}
+
+	served, err := jpeg.DecodeConfig(bytes.NewReader(rec.Body.Bytes()))
+	if err != nil {
+		t.Errorf("decoding the served JPEG: %v", err)
+
+		return
+	}
+
+	if served.Width != wantWidth || served.Height != wantHeight {
+		t.Errorf("served %dx%d, want %dx%d",
+			served.Width, served.Height, wantWidth, wantHeight)
+	}
+}

internal/server/fit_outside_internal_test.go

@@ -0,0 +1,91 @@
+package server
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// TestImageProxyRefusesFitOutsideAboveMaximum requests sizes with fit=outside
+// through both image routes. fit=outside scales the source to cover the size,
+// and so past it on one side: a size for which that side would be above 8192
+// is refused with 400 and the JSON error body, whether it is the width or the
+// height, and a size for which it stays within 8192 is served at the scaled
+// size.
+func TestImageProxyRefusesFitOutsideAboveMaximum(t *testing.T) {
+	t.Parallel()
+
+	// The upstream paths of the sources
+	const (
+		widePath   = "/wide.png"
+		rowPath    = "/row.png"
+		columnPath = "/column.png"
+	)
+
+	sources := map[string][]byte{
+		widePath:   encodeTestPNG(t, 3000, 600),
+		rowPath:    encodeTestPNG(t, 8192, 1),
+		columnPath: encodeTestPNG(t, 1, 8192),
+	}
+
+	upstream := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "image/png")
+			_, _ = w.Write(sources[r.URL.Path])
+		}))
+	t.Cleanup(upstream.Close)
+
+	s, _, _ := startImageProxy(t, upstream)
+
+	generator, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator: %v", err)
+	}
+
+	tests := []struct {
+		path          string
+		width, height int
+		// wantWidth and wantHeight are the served image's, 0 for a refusal
+		wantWidth, wantHeight int
+	}{
+		{widePath, 1, 8192, 0, 0},
+		{rowPath, 1, 8192, 0, 0},
+		{columnPath, 8192, 1, 0, 0},
+		{widePath, 1, 1200, 6000, 1200},
+	}
+
+	for _, tt := range tests {
+		token, err := generator.Generate(&encurl.Payload{
+			SourceHost: upstreamHost, SourcePath: tt.path,
+			Width: tt.width, Height: tt.height, Format: imgcache.FormatJPEG,
+			FitMode: imgcache.FitOutside,
+		})
+		if err != nil {
+			t.Fatalf("encurl Generate: %v", err)
+		}
+
+		size := fmt.Sprintf("%dx%d", tt.width, tt.height)
+
+		for _, target := range []string{
+			"/v1/image/" + upstreamHost + tt.path + "/" + size +
+				".jpeg?fit=outside",
+			"/v1/e/" + token + "/img.jpg",
+		} {
+			rec := httptest.NewRecorder()
+			s.ServeHTTP(rec, httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, target, nil))
+			t.Logf("GET %s: %d, Content-Type %s",
+				target, rec.Code, rec.Header().Get("Content-Type"))
+
+			if tt.wantWidth == 0 {
+				checkSizeRefused(t, rec)
+			} else {
+				checkServedSize(t, rec, tt.wantWidth, tt.wantHeight)
+			}
+		}
+	}
+}