Refactor markup render system (#32533)

Remove unmaintainable sanitizer rules. No need to add special "class"
regexp rules anymore, use RenderInternal.SafeAttr instead, more details
(and examples) are in the tests

modules/markup/internal/renderinternal.go

@@ -0,0 +1,82 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package internal
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"html/template"
+	"io"
+	"regexp"
+	"strings"
+	"sync"
+
+	"code.gitea.io/gitea/modules/htmlutil"
+
+	"golang.org/x/net/html"
+)
+
+var reAttrClass = sync.OnceValue[*regexp.Regexp](func() *regexp.Regexp {
+	// TODO: it isn't a problem at the moment because our HTML contents are always well constructed
+	return regexp.MustCompile(`(<[^>]+)\s+class="([^"]+)"([^>]*>)`)
+})
+
+// RenderInternal also works without initialization
+// If no initialization (no secureID), it will not protect any attributes and return the original name&value
+type RenderInternal struct {
+	secureID       string
+	secureIDPrefix string
+}
+
+func (r *RenderInternal) Init(output io.Writer) io.WriteCloser {
+	buf := make([]byte, 12)
+	_, err := rand.Read(buf)
+	if err != nil {
+		panic("unable to generate secure id")
+	}
+	return r.init(base64.URLEncoding.EncodeToString(buf), output)
+}
+
+func (r *RenderInternal) init(secID string, output io.Writer) io.WriteCloser {
+	r.secureID = secID
+	r.secureIDPrefix = r.secureID + ":"
+	return &finalProcessor{renderInternal: r, output: output}
+}
+
+func (r *RenderInternal) RecoverProtectedValue(v string) (string, bool) {
+	if !strings.HasPrefix(v, r.secureIDPrefix) {
+		return "", false
+	}
+	return v[len(r.secureIDPrefix):], true
+}
+
+func (r *RenderInternal) SafeAttr(name string) string {
+	if r.secureID == "" {
+		return name
+	}
+	return "data-attr-" + name
+}
+
+func (r *RenderInternal) SafeValue(val string) string {
+	if r.secureID == "" {
+		return val
+	}
+	return r.secureID + ":" + val
+}
+
+func (r *RenderInternal) NodeSafeAttr(attr, val string) html.Attribute {
+	return html.Attribute{Key: r.SafeAttr(attr), Val: r.SafeValue(val)}
+}
+
+func (r *RenderInternal) ProtectSafeAttrs(content template.HTML) template.HTML {
+	if r.secureID == "" {
+		return content
+	}
+	return template.HTML(reAttrClass().ReplaceAllString(string(content), `$1 data-attr-class="`+r.secureIDPrefix+`$2"$3`))
+}
+
+func (r *RenderInternal) FormatWithSafeAttrs(w io.Writer, fmt string, a ...any) error {
+	_, err := w.Write([]byte(r.ProtectSafeAttrs(htmlutil.HTMLFormat(fmt, a...))))
+	return err
+}