Support SVG sources (closes #68)

The fetcher accepts image/svg+xml and internal/magic detects SVG. orig
of an SVG at the orig size is the SVG unchanged, cached as a variant; at
another size it is ErrSVGNotOriginalSize (400). Any other format renders
it with libvips' thumbnail at the least size covering the output, at
most 8192 on a side, so it stays sharp. An SVG's own size is not
limited; one with a side over 8192 times the other is ErrSVGTooThin
(415). The cache reads up to 1024 first bytes to find a file's format.
Tests that pinned the SVG refusal now expect SVG to be read. Render time
and memory are unbounded: P1 in TODO.md until the owner decides on
#68.

Model: opus-5-5

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)
+	}
+}