Shuffle up the way we figure out which image to rebuild next so that

multiple concurrent tasks actually work in parallel and don't stomp
on each other.  Fixes #1498.

1 files changed, 6 insertions, 1 deletions

diff --git a/modules/gallery/helpers/gallery_task.php b/modules/gallery/helpers/gallery_task.php
index d56edabb..e69ff91a 100644
--- a/modules/gallery/helpers/gallery_task.php
+++ b/modules/gallery/helpers/gallery_task.php
@@ -71,7 +71,12 @@ class gallery_task_Core {
   static function rebuild_dirty_images($task) {
     $errors = array();
     try {
-      $result = graphics::find_dirty_images_query()->select("id")->execute();
+      // Choose the dirty images in a random order so that if we run this task multiple times
+      // concurrently each task is rebuilding different images simultaneously.
+      $result = graphics::find_dirty_images_query()->select("id")
+        ->select(new Database_Expression("RAND() as r"))
+        ->order_by("r", "ASC")
+        ->execute();
       $total_count = $task->get("total_count", $result->count());
       $mode = $task->get("mode", "init");
       if ($mode == "init") {