Hackathon: Post Reminders (#20555)

This PR adds the post reminder backend work.

We add a new API endpoint via which a user can set a reminder for a post. An ephemeral message will be sent down the line to let the user know about the action. And then after the time is over, the system admin bot will send a DM message to the user about the reminder post.

app/server.go

@@ -681,6 +681,7 @@ func NewServer(options ...Option) (*Server, error) {
 			s.runLicenseExpirationCheckJob()
 			s.runInactivityCheckJob()
 			runDNDStatusExpireJob(appInstance)
+			runPostReminderJob(appInstance)
 		})
 		s.runJobs()
 	}
@@ -2066,31 +2067,53 @@ func (s *Server) ReadFile(path string) ([]byte, *model.AppError) {
 	return result, nil
 }
 
-func createDNDStatusExpirationRecurringTask(a *App) {
-	a.ch.dndTaskMut.Lock()
-	a.ch.dndTask = model.CreateRecurringTaskFromNextIntervalTime("Unset DND Statuses", a.UpdateDNDStatusOfUsers, 5*time.Minute)
-	a.ch.dndTaskMut.Unlock()
+func withMut(mut *sync.Mutex, f func()) {
+	mut.Lock()
+	defer mut.Unlock()
+	f()
 }
 
-func cancelDNDStatusExpirationRecurringTask(a *App) {
-	a.ch.dndTaskMut.Lock()
-	if a.ch.dndTask != nil {
-		a.ch.dndTask.Cancel()
-		a.ch.dndTask = nil
+func cancelTask(mut *sync.Mutex, task *model.ScheduledTask) {
+	mut.Lock()
+	defer mut.Unlock()
+	if task != nil {
+		task.Cancel()
+		task = nil
 	}
-	a.ch.dndTaskMut.Unlock()
 }
 
 func runDNDStatusExpireJob(a *App) {
 	if a.IsLeader() {
-		createDNDStatusExpirationRecurringTask(a)
+		withMut(&a.ch.dndTaskMut, func() {
+			a.ch.dndTask = model.CreateRecurringTaskFromNextIntervalTime("Unset DND Statuses", a.UpdateDNDStatusOfUsers, 5*time.Minute)
+		})
 	}
 	a.ch.srv.AddClusterLeaderChangedListener(func() {
 		mlog.Info("Cluster leader changed. Determining if unset DNS status task should be running", mlog.Bool("isLeader", a.IsLeader()))
 		if a.IsLeader() {
-			createDNDStatusExpirationRecurringTask(a)
+			withMut(&a.ch.dndTaskMut, func() {
+				a.ch.dndTask = model.CreateRecurringTaskFromNextIntervalTime("Unset DND Statuses", a.UpdateDNDStatusOfUsers, 5*time.Minute)
+			})
 		} else {
-			cancelDNDStatusExpirationRecurringTask(a)
+			cancelTask(&a.ch.dndTaskMut, a.ch.dndTask)
+		}
+	})
+}
+
+func runPostReminderJob(a *App) {
+	if a.IsLeader() {
+		withMut(&a.ch.postReminderMut, func() {
+			a.ch.postReminderTask = model.CreateRecurringTaskFromNextIntervalTime("Check Post reminders", a.CheckPostReminders, 5*time.Minute)
+		})
+	}
+	a.ch.srv.AddClusterLeaderChangedListener(func() {
+		mlog.Info("Cluster leader changed. Determining if post reminder task should be running", mlog.Bool("isLeader", a.IsLeader()))
+		if a.IsLeader() {
+			withMut(&a.ch.postReminderMut, func() {
+				a.ch.postReminderTask = model.CreateRecurringTaskFromNextIntervalTime("Check Post reminders", a.CheckPostReminders, 5*time.Minute)
+			})
+		} else {
+			cancelTask(&a.ch.postReminderMut, a.ch.postReminderTask)
 		}
 	})
 }