[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>

app/notification_push.go

@@ -16,6 +16,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/shared/i18n"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+	"github.com/mattermost/mattermost-server/v5/utils"
 )
 
 type notificationType string
@@ -55,7 +56,13 @@ type PushNotification struct {
 func (a *App) sendPushNotificationSync(post *model.Post, user *model.User, channel *model.Channel, channelName string, senderName string,
 	explicitMention bool, channelWideMention bool, replyToThreadType string) *model.AppError {
 	cfg := a.Config()
-	msg, err := a.BuildPushNotificationMessage(
+	message, err := utils.StripMarkdown(post.Message)
+	if err != nil {
+		mlog.Warn("Failed parse to markdown", mlog.String("post_id", post.Id), mlog.Err(err))
+	} else {
+		post.Message = message
+	}
+	msg, appErr := a.BuildPushNotificationMessage(
 		*cfg.EmailSettings.PushNotificationContents,
 		post,
 		user,
@@ -66,8 +73,8 @@ func (a *App) sendPushNotificationSync(post *model.Post, user *model.User, chann
 		channelWideMention,
 		replyToThreadType,
 	)
-	if err != nil {
-		return err
+	if appErr != nil {
+		return appErr
 	}
 
 	return a.sendPushNotificationToAllSessions(msg, user.Id, "")