Add a daemon overseer module to restart daemons when config changes

Summary: Fixes T7053. Depends on D14452.

Test Plan:
Created a custom daemon which dumps out the config hash (by querying `PhabricatorEnv::calculateEnvironmentHash()`). Ran this daemon with `./bin/phd debug PhabricatorDebugDaemon` and saw the config hash update within 30 seconds.

{P1886}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T7053

Differential Revision: https://secure.phabricator.com/D14458

src/__phutil_library_map__.php

@@ -2014,6 +2014,7 @@ phutil_register_library_map(array(
     'PhabricatorDaemonManagementStatusWorkflow' => 'applications/daemon/management/PhabricatorDaemonManagementStatusWorkflow.php',
     'PhabricatorDaemonManagementStopWorkflow' => 'applications/daemon/management/PhabricatorDaemonManagementStopWorkflow.php',
     'PhabricatorDaemonManagementWorkflow' => 'applications/daemon/management/PhabricatorDaemonManagementWorkflow.php',
+    'PhabricatorDaemonOverseerModule' => 'infrastructure/daemon/overseer/PhabricatorDaemonOverseerModule.php',
     'PhabricatorDaemonReference' => 'infrastructure/daemon/control/PhabricatorDaemonReference.php',
     'PhabricatorDaemonTaskGarbageCollector' => 'applications/daemon/garbagecollector/PhabricatorDaemonTaskGarbageCollector.php',
     'PhabricatorDaemonTasksTableView' => 'applications/daemon/view/PhabricatorDaemonTasksTableView.php',
@@ -6076,6 +6077,7 @@ phutil_register_library_map(array(
     'PhabricatorDaemonManagementStatusWorkflow' => 'PhabricatorDaemonManagementWorkflow',
     'PhabricatorDaemonManagementStopWorkflow' => 'PhabricatorDaemonManagementWorkflow',
     'PhabricatorDaemonManagementWorkflow' => 'PhabricatorManagementWorkflow',
+    'PhabricatorDaemonOverseerModule' => 'PhutilDaemonOverseerModule',
     'PhabricatorDaemonReference' => 'Phobject',
     'PhabricatorDaemonTaskGarbageCollector' => 'PhabricatorGarbageCollector',
     'PhabricatorDaemonTasksTableView' => 'AphrontView',

src/infrastructure/daemon/overseer/PhabricatorDaemonOverseerModule.php

@@ -0,0 +1,67 @@
+<?php
+
+/**
+ * Overseer module.
+ *
+ * The primary purpose of this overseer module is to poll for configuration
+ * changes and reload daemons when the configuration changes.
+ */
+final class PhabricatorDaemonOverseerModule
+  extends PhutilDaemonOverseerModule {
+
+  private $configVersion;
+  private $timestamp;
+
+  public function __construct() {
+    $this->timestamp = PhabricatorTime::getNow();
+  }
+
+  public function shouldReloadDaemons() {
+    if ($this->timestamp < PhabricatorTime::getNow() - 10) {
+      return false;
+    }
+
+    return $this->updateConfigVersion();
+  }
+
+  /**
+   * Calculate a version number for the current Phabricator configuration.
+   *
+   * The version number has no real meaning and does not provide any real
+   * indication of whether a configuration entry has been changed. The config
+   * version is intended to be a rough indicator that "something has changed",
+   * which indicates to the overseer that the daemons should be reloaded.
+   *
+   * @return int
+   */
+  private function loadConfigVersion() {
+    $conn_r = id(new PhabricatorConfigEntry())->establishConnection('r');
+    return head(queryfx_one(
+      $conn_r,
+      'SELECT MAX(id) FROM %T',
+      id(new PhabricatorConfigTransaction())->getTableName()));
+  }
+
+  /**
+   * Update the configuration version and timestamp.
+   *
+   * @return bool  True if the daemons should restart, otherwise false.
+   */
+  private function updateConfigVersion() {
+    $config_version = $this->loadConfigVersion();
+    $this->timestamp = PhabricatorTime::getNow();
+
+    if (!$this->configVersion) {
+      $this->configVersion = $config_version;
+      return false;
+    }
+
+    if ($this->configVersion != $config_version) {
+      $this->configVersion = $config_version;
+      return true;
+    }
+
+    return false;
+  }
+
+}