MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/app/post.go

@@ -130,7 +130,7 @@ func (a *App) deduplicateCreatePost(rctx request.CTX, post *model.Post) (foundPo
 
 	// If the other thread finished creating the post, return the created post back to the
 	// client, making the API call feel idempotent.
-	actualPost, err := a.GetSinglePost(postID, false)
+	actualPost, err := a.GetSinglePost(rctx, postID, false)
 	if err != nil {
 		return nil, model.NewAppError("deduplicateCreatePost", "api.post.deduplicate_create_post.failed_to_get", nil, "", http.StatusInternalServerError).Wrap(err)
 	}
@@ -795,7 +795,7 @@ func (a *App) publishWebsocketEventForPermalinkPost(c request.CTX, post *model.P
 		return false, nil
 	}
 
-	previewedPost, err := a.GetSinglePost(previewedPostID, false)
+	previewedPost, err := a.GetSinglePost(c, previewedPostID, false)
 	if err != nil {
 		if err.StatusCode == http.StatusNotFound {
 			a.CountNotificationReason(model.NotificationStatusError, model.NotificationTypeAll, model.NotificationReasonFetchError)
@@ -880,7 +880,7 @@ func (a *App) publishWebsocketEventForPermalinkPost(c request.CTX, post *model.P
 }
 
 func (a *App) PatchPost(c request.CTX, postID string, patch *model.PostPatch) (*model.Post, *model.AppError) {
-	post, err := a.GetSinglePost(postID, false)
+	post, err := a.GetSinglePost(c, postID, false)
 	if err != nil {
 		return nil, err
 	}
@@ -971,8 +971,8 @@ func (a *App) GetPostsSince(options model.GetPostsSinceOptions) (*model.PostList
 	return postList, nil
 }
 
-func (a *App) GetSinglePost(postID string, includeDeleted bool) (*model.Post, *model.AppError) {
-	post, err := a.Srv().Store().Post().GetSingle(postID, includeDeleted)
+func (a *App) GetSinglePost(rctx request.CTX, postID string, includeDeleted bool) (*model.Post, *model.AppError) {
+	post, err := a.Srv().Store().Post().GetSingle(rctx, postID, includeDeleted)
 	if err != nil {
 		var nfErr *store.ErrNotFound
 		switch {
@@ -1362,7 +1362,7 @@ func (a *App) GetPostsForChannelAroundLastUnread(c request.CTX, channelID, userI
 }
 
 func (a *App) DeletePost(c request.CTX, postID, deleteByID string) (*model.Post, *model.AppError) {
-	post, err := a.Srv().Store().Post().GetSingle(postID, false)
+	post, err := a.Srv().Store().Post().GetSingle(sqlstore.RequestContextWithMaster(c), postID, false)
 	if err != nil {
 		return nil, model.NewAppError("DeletePost", "app.post.get.app_error", nil, "", http.StatusBadRequest).Wrap(err)
 	}
@@ -1730,7 +1730,7 @@ func (a *App) SearchPostsForUser(c request.CTX, terms string, userID string, tea
 func (a *App) GetFileInfosForPostWithMigration(rctx request.CTX, postID string, includeDeleted bool) ([]*model.FileInfo, *model.AppError) {
 	pchan := make(chan store.StoreResult[*model.Post], 1)
 	go func() {
-		post, err := a.Srv().Store().Post().GetSingle(postID, includeDeleted)
+		post, err := a.Srv().Store().Post().GetSingle(rctx, postID, includeDeleted)
 		pchan <- store.StoreResult[*model.Post]{Data: post, NErr: err}
 		close(pchan)
 	}()
@@ -2077,7 +2077,7 @@ func (a *App) GetThreadMembershipsForUser(userID, teamID string) ([]*model.Threa
 }
 
 func (a *App) GetPostIfAuthorized(c request.CTX, postID string, session *model.Session, includeDeleted bool) (*model.Post, *model.AppError) {
-	post, err := a.GetSinglePost(postID, includeDeleted)
+	post, err := a.GetSinglePost(c, postID, includeDeleted)
 	if err != nil {
 		return nil, err
 	}
@@ -2269,7 +2269,7 @@ func (a *App) CheckPostReminders(rctx request.CTX) {
 
 func (a *App) GetPostInfo(c request.CTX, postID string) (*model.PostInfo, *model.AppError) {
 	userID := c.Session().UserId
-	post, appErr := a.GetSinglePost(postID, false)
+	post, appErr := a.GetSinglePost(c, postID, false)
 	if appErr != nil {
 		return nil, appErr
 	}