Before executing svnserve, change the CWD to a readable directory

Summary: Fixes T10941. This avoids a confusing dead end when configuring Subversion hosting, where `svnserve` will fail to execute hooks if the CWD isn't readable by the vcs-user.

Test Plan:
  - Updated and committed in a hosted SVN repository.
  - Ran some git operations, too.
  - @dpotter confirmed this locally in T10941.

Reviewers: chad

Reviewed By: chad

Subscribers: dpotter

Maniphest Tasks: T10941

Differential Revision: https://secure.phabricator.com/D15879

src/applications/diffusion/protocol/DiffusionGitCommandEngine.php

@@ -24,8 +24,7 @@ final class DiffusionGitCommandEngine
     // really silly, but seems like the least damaging approach to
     // mitigating the issue.
 
-    $root = dirname(phutil_get_library_root('phabricator'));
-    $env['HOME'] = $root.'/support/empty/';
+    $env['HOME'] = PhabricatorEnv::getEmptyCWD();
 
     if ($this->isAnySSHProtocol()) {
       $env['GIT_SSH'] = $this->getSSHWrapper();

src/applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php

@@ -148,15 +148,25 @@ final class DiffusionSubversionServeSSHWorkflow
     if ($this->shouldProxy()) {
       $command = $this->getProxyCommand();
       $this->isProxying = true;
+      $cwd = null;
     } else {
       $command = csprintf(
         'svnserve -t --tunnel-user=%s',
         $this->getUser()->getUsername());
+      $cwd = PhabricatorEnv::getEmptyCWD();
     }
 
     $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
     $future = new ExecFuture('%C', $command);
 
+    // If we're receiving a commit, svnserve will fail to execute the commit
+    // hook with an unhelpful error if the CWD isn't readable by the user we
+    // are sudoing to. Switch to a readable, empty CWD before running
+    // svnserve. See T10941.
+    if ($cwd !== null) {
+      $future->setCWD($cwd);
+    }
+
     $this->inProtocol = new DiffusionSubversionWireProtocol();
     $this->outProtocol = new DiffusionSubversionWireProtocol();
 

src/infrastructure/env/PhabricatorEnv.php

@@ -877,4 +877,21 @@ final class PhabricatorEnv extends Phobject {
     umask(022);
   }
 
+
+  /**
+   * Get the path to an empty directory which is readable by all of the system
+   * user accounts that Phabricator acts as.
+   *
+   * In some cases, a binary needs some valid HOME or CWD to continue, but not
+   * all user accounts have valid home directories and even if they do they
+   * may not be readable after a `sudo` operation.
+   *
+   * @return string Path to an empty directory suitable for use as a CWD.
+   */
+  public static function getEmptyCWD() {
+    $root = dirname(phutil_get_library_root('phabricator'));
+    return $root.'/support/empty/';
+  }
+
+
 }