[MM-41185]: new API to mark thread as unread (#19951)

* [MM-41185]: new API to mark thread as unread

Until now we mark a thread as read/unread by sending a timestamp to the
server. This created some issues when marking a thread as unread from a
post. We needed to send the post.create_at - 1 as a timestamp but we
didn't do so consistently.

This commit adds a new API to set a thread as unread by post id.
Making all clients agnostic of the timestamp, and thus solving consistency issues.

Endpoint: /api/v4/users/{user_id}/teams/{team_id}/threads/{thread_id}/set_unread/{post_id}

* Updates client.go adding SetThreadUnreadByPostId

* Guards endpoint behind read channel permission

* Returns status 400 if post_id not belong in thread

* Root post as post_id should be permitted

api4/user.go

@@ -102,6 +102,7 @@ func (api *API) InitUser() {
 	api.BaseRoutes.UserThread.Handle("/following", api.APISessionRequired(followThreadByUser)).Methods("PUT")
 	api.BaseRoutes.UserThread.Handle("/following", api.APISessionRequired(unfollowThreadByUser)).Methods("DELETE")
 	api.BaseRoutes.UserThread.Handle("/read/{timestamp:[0-9]+}", api.APISessionRequired(updateReadStateThreadByUser)).Methods("PUT")
+	api.BaseRoutes.UserThread.Handle("/set_unread/{post_id:[A-Za-z0-9]+}", api.APISessionRequired(setUnreadThreadByPostId)).Methods("POST")
 }
 
 func createUser(c *Context, w http.ResponseWriter, r *http.Request) {
@@ -3097,6 +3098,42 @@ func updateReadStateThreadByUser(c *Context, w http.ResponseWriter, r *http.Requ
 	auditRec.Success()
 }
 
+func setUnreadThreadByPostId(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequireUserId().RequireThreadId().RequirePostId().RequireTeamId()
+	if c.Err != nil {
+		return
+	}
+
+	auditRec := c.MakeAuditRecord("setUnreadThreadByPostId", audit.Fail)
+	defer c.LogAuditRec(auditRec)
+	auditRec.AddMeta("user_id", c.Params.UserId)
+	auditRec.AddMeta("thread_id", c.Params.ThreadId)
+	auditRec.AddMeta("team_id", c.Params.TeamId)
+	auditRec.AddMeta("post_id", c.Params.PostId)
+
+	if !c.App.SessionHasPermissionToUser(*c.AppContext.Session(), c.Params.UserId) {
+		c.SetPermissionError(model.PermissionEditOtherUsers)
+		return
+	}
+
+	if !c.App.SessionHasPermissionToChannelByPost(*c.AppContext.Session(), c.Params.ThreadId, model.PermissionReadChannel) {
+		c.SetPermissionError(model.PermissionReadChannel)
+		return
+	}
+
+	thread, err := c.App.UpdateThreadReadForUserByPost(c.AppContext.Session().Id, c.Params.UserId, c.Params.TeamId, c.Params.ThreadId, c.Params.PostId)
+	if err != nil {
+		c.Err = err
+		return
+	}
+
+	if err := json.NewEncoder(w).Encode(thread); err != nil {
+		mlog.Warn("Error while writing response", mlog.Err(err))
+	}
+
+	auditRec.Success()
+}
+
 func unfollowThreadByUser(c *Context, w http.ResponseWriter, r *http.Request) {
 	c.RequireUserId().RequireThreadId().RequireTeamId()
 	if c.Err != nil {