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.

store/retrylayer/retrylayer.go

@@ -6575,6 +6575,48 @@ func (s *RetryLayerPostStore) GetPostIdBeforeTime(channelID string, timestamp in
 
 }
 
+func (s *RetryLayerPostStore) GetPostReminderMetadata(postID string) (*store.PostReminderMetadata, error) {
+
+	tries := 0
+	for {
+		result, err := s.PostStore.GetPostReminderMetadata(postID)
+		if err == nil {
+			return result, nil
+		}
+		if !isRepeatableError(err) {
+			return result, err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return result, err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
+func (s *RetryLayerPostStore) GetPostReminders(now int64) ([]*model.PostReminder, error) {
+
+	tries := 0
+	for {
+		result, err := s.PostStore.GetPostReminders(now)
+		if err == nil {
+			return result, nil
+		}
+		if !isRepeatableError(err) {
+			return result, err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return result, err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerPostStore) GetPosts(options model.GetPostsOptions, allowFromCache bool, sanitizeOptions map[string]bool) (*model.PostList, error) {
 
 	tries := 0
@@ -7064,6 +7106,27 @@ func (s *RetryLayerPostStore) SearchPostsForUser(paramsList []*model.SearchParam
 
 }
 
+func (s *RetryLayerPostStore) SetPostReminder(reminder *model.PostReminder) error {
+
+	tries := 0
+	for {
+		err := s.PostStore.SetPostReminder(reminder)
+		if err == nil {
+			return nil
+		}
+		if !isRepeatableError(err) {
+			return err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerPostStore) Update(newPost *model.Post, oldPost *model.Post) (*model.Post, error) {
 
 	tries := 0