diff --git a/scripts/repository/commit_hook.php b/scripts/repository/commit_hook.php
index 2c7fe4289b..4132df941e 100755
--- a/scripts/repository/commit_hook.php
+++ b/scripts/repository/commit_hook.php
@@ -1,6 +1,25 @@
 #!/usr/bin/env php
 <?php
 
+// Commit hooks execute in an unusual context where the environment may be
+// unavailable, particularly in SVN. The first parameter to this script is
+// either a bare repository identifier ("X"), or a repository identifier
+// followed by an instance identifier ("X:instance"). If we have an instance
+// identifier, unpack it into the environment before we start up. This allows
+// subclasses of PhabricatorConfigSiteSource to read it and build an instance
+// environment.
+
+if ($argc > 1) {
+  $context = $argv[1];
+  $context = explode(':', $context, 2);
+  $argv[1] = $context[0];
+
+  if (count($context) > 1) {
+    $_ENV['PHABRICATOR_INSTANCE'] = $context[1];
+    putenv('PHABRICATOR_INSTANCE='.$context[1]);
+  }
+}
+
 $root = dirname(dirname(dirname(__FILE__)));
 require_once $root.'/scripts/__init_script__.php';
 
diff --git a/src/applications/config/option/PhabricatorClusterConfigOptions.php b/src/applications/config/option/PhabricatorClusterConfigOptions.php
index 830208823d..23d0827c18 100644
--- a/src/applications/config/option/PhabricatorClusterConfigOptions.php
+++ b/src/applications/config/option/PhabricatorClusterConfigOptions.php
@@ -52,6 +52,18 @@ final class PhabricatorClusterConfigOptions
             '0.0.0.0/0',
           ),
           pht('Allow Any Host (Insecure!)')),
+      $this->newOption('cluster.instance', 'string', null)
+        ->setLocked(true)
+        ->setSummary(pht('Instance identifier for multi-tenant clusters.'))
+        ->setDescription(
+          pht(
+            'WARNING: This is a very advanced option, and only useful for '.
+            'hosting providers running multi-tenant clusters.'.
+            "\n\n".
+            'If you provide an instance identifier here (normally by '.
+            'injecting it with a `PhabricatorConfigSiteSource`), Phabricator '.
+            'will pass it to subprocesses and commit hooks in the '.
+            '`PHABRICATOR_INSTANCE` environmental variable.')),
     );
   }
 
diff --git a/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php b/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
index 8ac193fad7..6acc21fe05 100644
--- a/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
+++ b/src/applications/repository/engine/PhabricatorRepositoryPullEngine.php
@@ -161,7 +161,7 @@ final class PhabricatorRepositoryPullEngine
     $this->log('%s', pht('Installing commit hook to "%s"...', $path));
 
     $repository = $this->getRepository();
-    $callsign = $repository->getCallsign();
+    $identifier = $this->getHookContextIdentifier($repository);
 
     $root = dirname(phutil_get_library_root('phabricator'));
     $bin = $root.'/bin/commit-hook';
@@ -171,7 +171,7 @@ final class PhabricatorRepositoryPullEngine
       'exec %s -f %s -- %s "$@"',
       $full_php_path,
       $bin,
-      $callsign);
+      $identifier);
 
     $hook = "#!/bin/sh\nexport TERM=dumb\n{$cmd}\n";
 
@@ -190,6 +190,17 @@ final class PhabricatorRepositoryPullEngine
     Filesystem::writeFile($path.'/README', $readme);
   }
 
+  private function getHookContextIdentifier(PhabricatorRepository $repository) {
+    $identifier = $repository->getCallsign();
+
+    $instance = PhabricatorEnv::getEnvConfig('cluster.instance');
+    if (strlen($instance)) {
+      $identifier = "{$identifier}:{$instance}";
+    }
+
+    return $identifier;
+  }
+
 
 /* -(  Pulling Git Working Copies  )----------------------------------------- */
 
@@ -412,6 +423,8 @@ final class PhabricatorRepositoryPullEngine
     $repository = $this->getRepository();
     $path = $repository->getLocalPath().'/.hg/hgrc';
 
+    $identifier = $this->getHookContextIdentifier($repository);
+
     $root = dirname(phutil_get_library_root('phabricator'));
     $bin = $root.'/bin/commit-hook';
 
@@ -422,14 +435,14 @@ final class PhabricatorRepositoryPullEngine
     $data[] = csprintf(
       'pretxnchangegroup.phabricator = %s %s %s',
       $bin,
-      $repository->getCallsign(),
+      $identifier,
       'pretxnchangegroup');
 
     // This one handles creating bookmarks.
     $data[] = csprintf(
       'prepushkey.phabricator = %s %s %s',
       $bin,
-      $repository->getCallsign(),
+      $identifier,
       'prepushkey');
 
     $data[] = null;
diff --git a/src/infrastructure/env/PhabricatorEnv.php b/src/infrastructure/env/PhabricatorEnv.php
index ae40ba4a5c..7e266ef469 100644
--- a/src/infrastructure/env/PhabricatorEnv.php
+++ b/src/infrastructure/env/PhabricatorEnv.php
@@ -112,6 +112,15 @@ final class PhabricatorEnv {
     // subprocess environments.
     $_ENV['PATH'] = $env_path;
 
+
+    // If an instance identifier is defined, write it into the environment so
+    // it's available to subprocesses.
+    $instance = PhabricatorEnv::getEnvConfig('cluster.instance');
+    if (strlen($instance)) {
+      putenv('PHABRICATOR_INSTANCE='.$instance);
+      $_ENV['PHABRICATOR_INSTANCE'] = $instance;
+    }
+
     PhabricatorEventEngine::initialize();
 
     $translation = PhabricatorEnv::newObjectFromConfig('translation.provider');