When already running as the daemon user, don't "sudo" daemon commands

Summary:
The cluster synchronization code runs either actively (before returning a response to `git clone`, for example) or passively (routinely, as the daemons update reposiories).

The active sync runs as the web user (if running `git clone http://...`) or the VCS user (if running `git clone ssh://...`). But the passive sync runs as the daemon user.

All of these sync processes need to run actual commands as the daemon user (`git fetch ...`).

For the active ones, we must `sudo`.

For the passive ones, we're already the right user. We run the same code, and end up trying to sudo to ourselves, which `sudo` isn't happy about by default.

Depending on how `sudo` is configured and which users things are running as this might work anyway, but it's silly and if it doesn't work it requires you to go make non-obvious, weird config changes that are unintuitive and somewhat nonsensical. This is probably worse on the balance than adding a bit of complexity to the code.

Instead, test which user we're running as. If it's already the right user, don't sudo.

Test Plan:
  - Ran `bin/repository update --trace` as daemon user, saw no more `sudo`.
  - Ran a `git clone` to make sure that didn't break.

Reviewers: chad, avivey

Reviewed By: avivey

Differential Revision: https://secure.phabricator.com/D16391

src/infrastructure/daemon/PhabricatorDaemon.php

@@ -34,6 +34,24 @@ abstract class PhabricatorDaemon extends PhutilDaemon {
       return $command;
     }
 
+    // We may reach this method while already running as the daemon user: for
+    // example, active and passive synchronization of clustered repositories
+    // run the same commands through the same code, but as different users.
+
+    // By default, `sudo` won't let you sudo to yourself, so we can get into
+    // trouble if we're already running as the daemon user unless the host has
+    // been configured to let the daemon user run commands as itself.
+
+    // Since this is silly and more complicated than doing this check, don't
+    // use `sudo` if we're already running as the correct user.
+    if (function_exists('posix_getuid')) {
+      $uid = posix_getuid();
+      $info = posix_getpwuid($uid);
+      if ($info && $info['name'] == $user) {
+        return $command;
+      }
+    }
+
     // Get the absolute path so we're safe against the caller wiping out
     // PATH.
     $sudo = Filesystem::resolveBinary('sudo');