Refuse a size whose computed side exceeds the maximum (closes #241)

A side given as 0 is computed from the source's proportions, and
nothing held it to 8192: 0x8192 of a 3000x600 source was resized to
40960x8192 and encoded. Process now refuses such a size with
ErrOutputTooLarge once it knows the source's dimensions, before the
resize and the encode, and both image routes answer it 400 with the
JSON error body. It is a processing error, not a failed fetch, so it
is never negative-cached. imgcache.MaxDimension is now
imageprocessor.MaxOutputDimension, so the request limit and the output
limit are one value. README.md says it under Routes and Encrypted URLs.

Model: opus-5-5

internal/imageprocessor/imageprocessor.go

@@ -267,14 +267,10 @@ func (p *ImageProcessor) Process(
 		return nil, ErrInputTooLarge
 	}
 
-	// Determine target dimensions
+	// Determine target dimensions; too large ones are refused before resizing
-	targetWidth, targetHeight := targetDimensions(req.Size, origWidth, origHeight)
+	targetWidth, targetHeight, err := targetDimensions(req.Size, origWidth, origHeight)
-
+	if err != nil {
-	// A side given as 0 is computed from the input's proportions, so it can
+		return nil, err
-	// come out above MaxOutputDimension; refuse it before resizing
-	if targetWidth > MaxOutputDimension || targetHeight > MaxOutputDimension {
-		return nil, fmt.Errorf("%w: %dx%d is above %d", ErrOutputTooLarge,
-			targetWidth, targetHeight, MaxOutputDimension)
 	}
 
 	// Resize if needed
@@ -311,21 +307,29 @@ 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.
+// original dimensions when both are zero. A side scaled proportionally can
-func targetDimensions(size Size, origWidth, origHeight int) (int, int) {
+// 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
+		return origWidth, origHeight, nil
-	case size.Width == 0:
+	case width == 0:
 		// Only height specified: calculate width proportionally
-		return origWidth * size.Height / origHeight, size.Height
+		width = origWidth * height / origHeight
-	case size.Height == 0:
+	case height == 0:
 		// Only width specified: calculate height proportionally
-		return size.Width, origHeight * size.Width / origWidth
+		height = origHeight * width / origWidth
-	default:
-		return size.Width, size.Height
 	}
+
+	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.

internal/server/computed_side_internal_test.go

@@ -22,6 +22,12 @@ import (
 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)
 
@@ -29,7 +35,7 @@ func TestImageProxyRefusesComputedSideAboveMaximum(t *testing.T) {
 		func(w http.ResponseWriter, r *http.Request) {
 			w.Header().Set("Content-Type", "image/png")
 
-			if r.URL.Path == "/tall.png" {
+			if r.URL.Path == tallPath {
 				_, _ = w.Write(tall)
 
 				return
@@ -55,12 +61,12 @@ func TestImageProxyRefusesComputedSideAboveMaximum(t *testing.T) {
 		// wantWidth and wantHeight are the served image's, 0 for a refusal
 		wantWidth, wantHeight int
 	}{
-		{"/wide.png", "0x8192", 0, 8192, 0, 0},
+		{widePath, "0x8192", 0, 8192, 0, 0},
-		{"/tall.png", "8192x0", 8192, 0, 0, 0},
+		{tallPath, "8192x0", 8192, 0, 0, 0},
-		{"/wide.png", "orig", 0, 0, 3000, 600},
+		{widePath, "orig", 0, 0, 3000, 600},
-		{"/wide.png", "300x200", 300, 200, 300, 200},
+		{widePath, "300x200", 300, 200, 300, 200},
-		{"/wide.png", "0x300", 0, 300, 1500, 300},
+		{widePath, "0x300", 0, 300, 1500, 300},
-		{"/tall.png", "300x0", 300, 0, 300, 1500},
+		{tallPath, "300x0", 300, 0, 300, 1500},
 	}
 
 	for _, tt := range tests {