[MM-11143] Strip markdown formatting characters from push notifications (#17775)

* WIP: trim markdown for push notification

* fix: golangci

* fix: table regex

* fix: regex code block

* doc: add license

* WIP: custom renderer with goldmark

* WIP: update goldmark version to 1.38

* fix: use goldmark as parser

* fix: remove table extension

* fix: change buf to `strings.Builder`

* fix: return original string, log warning if error

* refactor: change `WriteString` to `WriteByte`

* refactor: change if condition

* refactor: use assertion

* refactor: move to inline

* fix: remove handle multiline

Already handled by mobile

* refactor: wrap same function

* refactor: move strip markdown to `sendPushNotificationSync`

* refactor: renaming variable aren't used

* fix: move log to func `sendPushNotificationSync`

* docs: add comment to `StripMarkdown`

* fix: move assign message to else

* appErr to err rename

Co-authored-by: Agniva De Sarker <agnivade@yahoo.co.in>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

utils/markdown.go

@@ -0,0 +1,142 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package utils
+
+import (
+	"strings"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/extension"
+	astExt "github.com/yuin/goldmark/extension/ast"
+	"github.com/yuin/goldmark/renderer"
+	"github.com/yuin/goldmark/util"
+)
+
+// StripMarkdown remove some markdown syntax
+func StripMarkdown(markdown string) (string, error) {
+	md := goldmark.New(
+		goldmark.WithExtensions(extension.Strikethrough),
+		goldmark.WithRenderer(
+			renderer.NewRenderer(renderer.WithNodeRenderers(
+				util.Prioritized(newNotificationRenderer(), 500),
+			)),
+		),
+	)
+
+	var buf strings.Builder
+	if err := md.Convert([]byte(markdown), &buf); err != nil {
+		return "", err
+	}
+
+	return strings.TrimSpace(buf.String()), nil
+}
+
+type notificationRenderer struct {
+}
+
+func newNotificationRenderer() *notificationRenderer {
+	return &notificationRenderer{}
+}
+
+func (r *notificationRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	// block
+	reg.Register(ast.KindDocument, r.renderDefault)
+	reg.Register(ast.KindHeading, r.renderItem)
+	reg.Register(ast.KindBlockquote, r.renderDefault)
+	reg.Register(ast.KindCodeBlock, r.renderCodeBlock)
+	reg.Register(ast.KindFencedCodeBlock, r.renderFencedCodeBlock)
+	reg.Register(ast.KindHTMLBlock, r.renderDefault)
+	reg.Register(ast.KindList, r.renderDefault)
+	reg.Register(ast.KindListItem, r.renderItem)
+	reg.Register(ast.KindParagraph, r.renderItem)
+	reg.Register(ast.KindTextBlock, r.renderTextBlock)
+	reg.Register(ast.KindThematicBreak, r.renderDefault)
+
+	// inlines
+	reg.Register(ast.KindAutoLink, r.renderDefault)
+	reg.Register(ast.KindCodeSpan, r.renderDefault)
+	reg.Register(ast.KindEmphasis, r.renderDefault)
+	reg.Register(ast.KindImage, r.renderDefault)
+	reg.Register(ast.KindLink, r.renderDefault)
+	reg.Register(ast.KindRawHTML, r.renderDefault)
+	reg.Register(ast.KindText, r.renderText)
+	reg.Register(ast.KindString, r.renderString)
+
+	// strikethrough
+	reg.Register(astExt.KindStrikethrough, r.renderDefault)
+}
+
+// renderDefault renderer function to renderDefault without changes
+func (r *notificationRenderer) renderDefault(_ util.BufWriter, _ []byte, _ ast.Node, _ bool) (ast.WalkStatus, error) {
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) renderItem(w util.BufWriter, _ []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		if node.NextSibling() != nil {
+			_ = w.WriteByte(' ')
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) renderCodeBlock(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.CodeBlock)
+	if entering {
+		r.writeLines(w, source, n)
+	}
+
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) renderFencedCodeBlock(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.FencedCodeBlock)
+	if entering {
+		r.writeLines(w, source, n)
+	}
+
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) renderText(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	n := node.(*ast.Text)
+	segment := n.Segment
+	_, _ = w.Write(segment.Value(source))
+	if !n.IsRaw() {
+		if n.HardLineBreak() || n.SoftLineBreak() {
+			_ = w.WriteByte('\n')
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) renderTextBlock(w util.BufWriter, _ []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		if _, ok := node.NextSibling().(ast.Node); ok && node.FirstChild() != nil {
+			_ = w.WriteByte(' ')
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) renderString(w util.BufWriter, _ []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	n := node.(*ast.String)
+	_, _ = w.Write(n.Value)
+	return ast.WalkContinue, nil
+}
+
+func (r *notificationRenderer) writeLines(w util.BufWriter, source []byte, n ast.Node) {
+	for i := 0; i < n.Lines().Len(); i++ {
+		line := n.Lines().At(i)
+		value := line.Value(source)
+		_, _ = w.Write(value)
+	}
+}