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, so it
stays sharp. 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). The cache reads up to 1024 first
bytes to find a file's format. Tests pinning the SVG refusal now expect
SVG. Render time and memory are unbounded: P1 in TODO.md until the
owner decides on #68.

Model: opus-5-5

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()
+	}
+}