[MM-42194] Get file information from a deleted post (#20279)

* Introduced inlcude_deleted query parameter to allow admins to retrieve contents of post regardless of deletion status

* Introduced new client route and tests for getting file info of deleted posts

* Fixed tests due to caching of posts

* gofmt

* Small formatting updates

* Invalidating file infos cache on delete of post if post includes files

* Including deleted in migration flow

* Moved invalidating of cache

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/post.go

@@ -1269,6 +1269,8 @@ func (a *App) DeletePost(c request.CTX, postID, deleteByID string) (*model.Post,
 		a.Srv().Go(func() {
 			a.deletePostFiles(post.Id)
 		})
+		a.Srv().Store.FileInfo().InvalidateFileInfosForPostCache(postID, true)
+		a.Srv().Store.FileInfo().InvalidateFileInfosForPostCache(postID, false)
 	}
 	a.Srv().Go(func() {
 		a.deleteFlaggedPosts(post.Id)
@@ -1541,16 +1543,16 @@ func (a *App) GetRecentSearchesForUser(userID string) ([]*model.SearchParams, *m
 	return searchParams, nil
 }
 
-func (a *App) GetFileInfosForPostWithMigration(postID string) ([]*model.FileInfo, *model.AppError) {
+func (a *App) GetFileInfosForPostWithMigration(postID string, includeDeleted bool) ([]*model.FileInfo, *model.AppError) {
 
 	pchan := make(chan store.StoreResult, 1)
 	go func() {
-		post, err := a.Srv().Store.Post().GetSingle(postID, false)
+		post, err := a.Srv().Store.Post().GetSingle(postID, includeDeleted)
 		pchan <- store.StoreResult{Data: post, NErr: err}
 		close(pchan)
 	}()
 
-	infos, err := a.GetFileInfosForPost(postID, false)
+	infos, err := a.GetFileInfosForPost(postID, false, includeDeleted)
 	if err != nil {
 		return nil, err
 	}
@@ -1580,8 +1582,8 @@ func (a *App) GetFileInfosForPostWithMigration(postID string) ([]*model.FileInfo
 	return infos, nil
 }
 
-func (a *App) GetFileInfosForPost(postID string, fromMaster bool) ([]*model.FileInfo, *model.AppError) {
-	fileInfos, err := a.Srv().Store.FileInfo().GetForPost(postID, fromMaster, false, true)
+func (a *App) GetFileInfosForPost(postID string, fromMaster bool, includeDeleted bool) ([]*model.FileInfo, *model.AppError) {
+	fileInfos, err := a.Srv().Store.FileInfo().GetForPost(postID, fromMaster, includeDeleted, true)
 	if err != nil {
 		return nil, model.NewAppError("GetFileInfosForPost", "app.file_info.get_for_post.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}