Refactor backend SVG package and add tests (#26335)

Introduce a well-tested `svg.Normalize` function.
Make `RenderHTML` faster and more stable.

modules/svg/processor.go

@@ -0,0 +1,59 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package svg
+
+import (
+	"bytes"
+	"fmt"
+	"regexp"
+	"sync"
+)
+
+type normalizeVarsStruct struct {
+	reXMLDoc,
+	reComment,
+	reAttrXMLNs,
+	reAttrSize,
+	reAttrClassPrefix *regexp.Regexp
+}
+
+var (
+	normalizeVars     *normalizeVarsStruct
+	normalizeVarsOnce sync.Once
+)
+
+// Normalize normalizes the SVG content: set default width/height, remove unnecessary tags/attributes
+// It's designed to work with valid SVG content. For invalid SVG content, the returned content is not guaranteed.
+func Normalize(data []byte, size int) []byte {
+	normalizeVarsOnce.Do(func() {
+		normalizeVars = &normalizeVarsStruct{
+			reXMLDoc:  regexp.MustCompile(`(?s)<\?xml.*?>`),
+			reComment: regexp.MustCompile(`(?s)<!--.*?-->`),
+
+			reAttrXMLNs:       regexp.MustCompile(`(?s)\s+xmlns\s*=\s*"[^"]*"`),
+			reAttrSize:        regexp.MustCompile(`(?s)\s+(width|height)\s*=\s*"[^"]+"`),
+			reAttrClassPrefix: regexp.MustCompile(`(?s)\s+class\s*=\s*"`),
+		}
+	})
+	data = normalizeVars.reXMLDoc.ReplaceAll(data, nil)
+	data = normalizeVars.reComment.ReplaceAll(data, nil)
+
+	data = bytes.TrimSpace(data)
+	svgTag, svgRemaining, ok := bytes.Cut(data, []byte(">"))
+	if !ok || !bytes.HasPrefix(svgTag, []byte(`<svg`)) {
+		return data
+	}
+	normalized := bytes.Clone(svgTag)
+	normalized = normalizeVars.reAttrXMLNs.ReplaceAll(normalized, nil)
+	normalized = normalizeVars.reAttrSize.ReplaceAll(normalized, nil)
+	normalized = normalizeVars.reAttrClassPrefix.ReplaceAll(normalized, []byte(` class="`))
+	normalized = bytes.TrimSpace(normalized)
+	normalized = fmt.Appendf(normalized, ` width="%d" height="%d"`, size, size)
+	if !bytes.Contains(normalized, []byte(` class="`)) {
+		normalized = append(normalized, ` class="svg"`...)
+	}
+	normalized = append(normalized, '>')
+	normalized = append(normalized, svgRemaining...)
+	return normalized
+}