Restructure markup & markdown to prepare for multiple markup language… (#2411)

* restructure markup & markdown to prepare for multiple markup languages support

* adjust some functions between markdown and markup

* fix tests

* improve the comments

modules/markup/sanitizer.go

@@ -0,0 +1,58 @@
+// Copyright 2017 The Gitea Authors. All rights reserved.
+// Copyright 2017 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package markup
+
+import (
+	"regexp"
+	"sync"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/microcosm-cc/bluemonday"
+)
+
+// Sanitizer is a protection wrapper of *bluemonday.Policy which does not allow
+// any modification to the underlying policies once it's been created.
+type Sanitizer struct {
+	policy *bluemonday.Policy
+	init   sync.Once
+}
+
+var sanitizer = &Sanitizer{}
+
+// NewSanitizer initializes sanitizer with allowed attributes based on settings.
+// Multiple calls to this function will only create one instance of Sanitizer during
+// entire application lifecycle.
+func NewSanitizer() {
+	sanitizer.init.Do(func() {
+		sanitizer.policy = bluemonday.UGCPolicy()
+		// We only want to allow HighlightJS specific classes for code blocks
+		sanitizer.policy.AllowAttrs("class").Matching(regexp.MustCompile(`^language-\w+$`)).OnElements("code")
+
+		// Checkboxes
+		sanitizer.policy.AllowAttrs("type").Matching(regexp.MustCompile(`^checkbox$`)).OnElements("input")
+		sanitizer.policy.AllowAttrs("checked", "disabled").OnElements("input")
+
+		// Custom URL-Schemes
+		sanitizer.policy.AllowURLSchemes(setting.Markdown.CustomURLSchemes...)
+	})
+}
+
+// Sanitize takes a string that contains a HTML fragment or document and applies policy whitelist.
+func Sanitize(s string) string {
+	NewSanitizer()
+	return sanitizer.policy.Sanitize(s)
+}
+
+// SanitizeBytes takes a []byte slice that contains a HTML fragment or document and applies policy whitelist.
+func SanitizeBytes(b []byte) []byte {
+	if len(b) == 0 {
+		// nothing to sanitize
+		return b
+	}
+	NewSanitizer()
+	return sanitizer.policy.SanitizeBytes(b)
+}