Reverse meaning of task priority column

Summary:
Ref T6615. Mixing ASC and DESC ordering on a multipart key makes it dramatically less effective (or perhaps totally ineffective).

Reverse the meaning of the `priority` column so it goes in the same direction as the `id` column (both ascending, lower values execute sooner).

Test Plan:
  - Queued 1.2M tasks with `bin/worker flood`.
  - Processed ~1 task/second with `bin/phd debug taskmaster` before patch.
  - Applied patch, took ~5 seconds for ~1.2M rows.
  - Processed ~100-200 tasks/second with `bin/phd debug taskmaster` after patch.
  - "Next in Queue" query on daemon page dropped from 1.5s to <1ms.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aklapper, 20after4, epriestley

Maniphest Tasks: T6615

Differential Revision: https://secure.phabricator.com/D10895

resources/sql/autopatches/20141123.taskpriority.1.sql

@@ -0,0 +1,2 @@
+UPDATE {$NAMESPACE}_worker.worker_activetask
+  SET priority = 5000 - priority;

resources/sql/autopatches/20141123.taskpriority.2.sql

@@ -0,0 +1,2 @@
+UPDATE {$NAMESPACE}_worker.worker_archivetask
+  SET priority = 5000 - priority;

src/infrastructure/daemon/workers/PhabricatorWorker.php

@@ -9,10 +9,14 @@ abstract class PhabricatorWorker {
   private static $runAllTasksInProcess = false;
   private $queuedTasks = array();
 
-  const PRIORITY_ALERTS  = 4000;
+  // NOTE: Lower priority numbers execute first. The priority numbers have to
-  const PRIORITY_DEFAULT = 3000;
+  // have the same ordering that IDs do (lowest first) so MySQL can use a
-  const PRIORITY_BULK    = 2000;
+  // multipart key across both of them efficiently.
-  const PRIORITY_IMPORT  = 1000;
+
+  const PRIORITY_ALERTS  = 1000;
+  const PRIORITY_DEFAULT = 2000;
+  const PRIORITY_BULK    = 3000;
+  const PRIORITY_IMPORT  = 4000;
 
 
 /* -(  Configuring Retries and Failures  )----------------------------------- */

src/infrastructure/daemon/workers/__tests__/PhabricatorWorkerTestCase.php

@@ -160,14 +160,14 @@ final class PhabricatorWorkerTestCase extends PhabricatorTestCase {
     $this->expectNextLease($task1);
   }
 
-  public function testLeasedIsHighestPriority() {
+  public function testLeasedIsLowestPriority() {
-    $task1 = $this->scheduleTask(array(), 1);
+    $task1 = $this->scheduleTask(array(), 2);
-    $task2 = $this->scheduleTask(array(), 1);
+    $task2 = $this->scheduleTask(array(), 2);
-    $task3 = $this->scheduleTask(array(), 2);
+    $task3 = $this->scheduleTask(array(), 1);
 
     $this->expectNextLease(
       $task3,
-      'Tasks with a higher priority should be scheduled first.');
+      'Tasks with a lower priority should be scheduled first.');
     $this->expectNextLease(
       $task1,
       'Tasks with the same priority should be FIFO.');

src/infrastructure/daemon/workers/query/PhabricatorWorkerLeaseQuery.php

@@ -215,8 +215,8 @@ final class PhabricatorWorkerLeaseQuery extends PhabricatorQuery {
     switch ($phase) {
       case self::PHASE_UNLEASED:
         // When selecting new tasks, we want to consume them in order of
-        // decreasing priority (and then FIFO).
+        // increasing priority (and then FIFO).
-        return qsprintf($conn_w, 'ORDER BY priority DESC, id ASC');
+        return qsprintf($conn_w, 'ORDER BY priority ASC, id ASC');
       case self::PHASE_EXPIRED:
         // When selecting failed tasks, we want to consume them in roughly
         // FIFO order of their failures, which is not necessarily their original