Add a very forgiving GC for Differential viewstate information

Summary:
Ref T13455. Viewstates are fairly small and will probably grow less quickly than the changeset table, but the data is also not important to retain in the long term: if you revisit a change several months after hiding some files, it's fine if we've forgotten that you adjusted the view parameters.

Add a GC with a long default collection policy (180 days) so installs can manage the size of this table if it becomes necessary.

Test Plan: Ran via `bin/garbage` to adjust the GC policy and collect viewstates.

Maniphest Tasks: T13455

Differential Revision: https://secure.phabricator.com/D21164

src/__phutil_library_map__.php

@@ -715,6 +715,7 @@ phutil_register_library_map(array(
     'DifferentialUnitTestResult' => 'applications/differential/constants/DifferentialUnitTestResult.php',
     'DifferentialUpdateRevisionConduitAPIMethod' => 'applications/differential/conduit/DifferentialUpdateRevisionConduitAPIMethod.php',
     'DifferentialViewState' => 'applications/differential/storage/DifferentialViewState.php',
+    'DifferentialViewStateGarbageCollector' => 'applications/differential/garbagecollector/DifferentialViewStateGarbageCollector.php',
     'DifferentialViewStateQuery' => 'applications/differential/query/DifferentialViewStateQuery.php',
     'DiffusionAuditorDatasource' => 'applications/diffusion/typeahead/DiffusionAuditorDatasource.php',
     'DiffusionAuditorFunctionDatasource' => 'applications/diffusion/typeahead/DiffusionAuditorFunctionDatasource.php',
@@ -6803,6 +6804,7 @@ phutil_register_library_map(array(
       'DifferentialDAO',
       'PhabricatorPolicyInterface',
     ),
+    'DifferentialViewStateGarbageCollector' => 'PhabricatorGarbageCollector',
     'DifferentialViewStateQuery' => 'PhabricatorCursorPagedPolicyAwareQuery',
     'DiffusionAuditorDatasource' => 'PhabricatorTypeaheadCompositeDatasource',
     'DiffusionAuditorFunctionDatasource' => 'PhabricatorTypeaheadCompositeDatasource',

src/applications/differential/garbagecollector/DifferentialViewStateGarbageCollector.php

@@ -0,0 +1,29 @@
+<?php
+
+final class DifferentialViewStateGarbageCollector
+  extends PhabricatorGarbageCollector {
+
+  const COLLECTORCONST = 'differential.viewstate';
+
+  public function getCollectorName() {
+    return pht('Differential View States');
+  }
+
+  public function getDefaultRetentionPolicy() {
+    return phutil_units('180 days in seconds');
+  }
+
+  protected function collectGarbage() {
+    $table = new DifferentialViewState();
+    $conn = $table->establishConnection('w');
+
+    queryfx(
+      $conn,
+      'DELETE FROM %R WHERE dateModified < %d LIMIT 100',
+      $table,
+      $this->getGarbageEpoch());
+
+    return ($conn->getAffectedRows() == 100);
+  }
+
+}

src/applications/differential/storage/DifferentialViewState.php

@@ -23,6 +23,9 @@ final class DifferentialViewState
         'key_object' => array(
           'columns' => array('objectPHID'),
         ),
+        'key_modified' => array(
+          'columns' => array('dateModified'),
+        ),
       ),
     ) + parent::getConfiguration();
   }