diff --git a/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php b/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php
index 3458d27ed6..5eae19e030 100644
--- a/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php
+++ b/src/applications/diffusion/protocol/DiffusionGitCommandEngine.php
@@ -1,37 +1,36 @@
 <?php
 
 final class DiffusionGitCommandEngine
   extends DiffusionCommandEngine {
 
   protected function canBuildForRepository(
     PhabricatorRepository $repository) {
     return $repository->isGit();
   }
 
   protected function newFormattedCommand($pattern, array $argv) {
     $pattern = "git {$pattern}";
     return array($pattern, $argv);
   }
 
   protected function newCustomEnvironment() {
     $env = array();
 
     // NOTE: See T2965. Some time after Git 1.7.5.4, Git started fataling if
     // it can not read $HOME. For many users, $HOME points at /root (this
     // seems to be a default result of Apache setup). Instead, explicitly
     // point $HOME at a readable, empty directory so that Git looks for the
     // config file it's after, fails to locate it, and moves on. This is
     // 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();
     }
 
     return $env;
   }
 
 }
diff --git a/src/applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php b/src/applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php
index 820a380856..57a44d4707 100644
--- a/src/applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php
+++ b/src/applications/diffusion/ssh/DiffusionSubversionServeSSHWorkflow.php
@@ -1,440 +1,450 @@
 <?php
 
 /**
  * This protocol has a good spec here:
  *
  *   http://svn.apache.org/repos/asf/subversion/trunk/subversion/libsvn_ra_svn/protocol
  */
 final class DiffusionSubversionServeSSHWorkflow
   extends DiffusionSubversionSSHWorkflow {
 
   private $didSeeWrite;
 
   private $inProtocol;
   private $outProtocol;
 
   private $inSeenGreeting;
 
   private $outPhaseCount = 0;
 
   private $internalBaseURI;
   private $externalBaseURI;
   private $peekBuffer;
   private $command;
   private $isProxying;
 
   private function getCommand() {
     return $this->command;
   }
 
   protected function didConstruct() {
     $this->setName('svnserve');
     $this->setArguments(
       array(
         array(
           'name' => 'tunnel',
           'short' => 't',
         ),
       ));
   }
 
   protected function identifyRepository() {
     // NOTE: In SVN, we need to read the first few protocol frames before we
     // can determine which repository the user is trying to access. We're
     // going to peek at the data on the wire to identify the repository.
 
     $io_channel = $this->getIOChannel();
 
     // Before the client will send us the first protocol frame, we need to send
     // it a connection frame with server capabilities. To figure out the
     // correct frame we're going to start `svnserve`, read the frame from it,
     // send it to the client, then kill the subprocess.
 
     // TODO: This is pretty inelegant and the protocol frame will change very
     // rarely. We could cache it if we can find a reasonable way to dirty the
     // cache.
 
     $command = csprintf('svnserve -t');
     $command = PhabricatorDaemon::sudoCommandAsDaemonUser($command);
     $future = new ExecFuture('%C', $command);
     $exec_channel = new PhutilExecChannel($future);
     $exec_protocol = new DiffusionSubversionWireProtocol();
 
     while (true) {
       PhutilChannel::waitForAny(array($exec_channel));
       $exec_channel->update();
 
       $exec_message = $exec_channel->read();
       if ($exec_message !== null) {
         $messages = $exec_protocol->writeData($exec_message);
         if ($messages) {
           $message = head($messages);
           $raw = $message['raw'];
 
           // Write the greeting frame to the client.
           $io_channel->write($raw);
 
           // Kill the subprocess.
           $future->resolveKill();
           break;
         }
       }
 
       if (!$exec_channel->isOpenForReading()) {
         throw new Exception(
           pht(
             '%s subprocess exited before emitting a protocol frame.',
             'svnserve'));
       }
     }
 
     $io_protocol = new DiffusionSubversionWireProtocol();
     while (true) {
       PhutilChannel::waitForAny(array($io_channel));
       $io_channel->update();
 
       $in_message = $io_channel->read();
       if ($in_message !== null) {
         $this->peekBuffer .= $in_message;
         if (strlen($this->peekBuffer) > (1024 * 1024)) {
           throw new Exception(
             pht(
               'Client transmitted more than 1MB of data without transmitting '.
               'a recognizable protocol frame.'));
         }
 
         $messages = $io_protocol->writeData($in_message);
         if ($messages) {
           $message = head($messages);
           $struct = $message['structure'];
 
           // This is the:
           //
           //   ( version ( cap1 ... ) url ... )
           //
           // The `url` allows us to identify the repository.
 
           $uri = $struct[2]['value'];
           $path = $this->getPathFromSubversionURI($uri);
 
           return $this->loadRepositoryWithPath($path);
         }
       }
 
       if (!$io_channel->isOpenForReading()) {
         throw new Exception(
           pht(
             'Client closed connection before sending a complete protocol '.
             'frame.'));
       }
 
       // If the client has disconnected, kill the subprocess and bail.
       if (!$io_channel->isOpenForWriting()) {
         throw new Exception(
           pht(
             'Client closed connection before receiving response.'));
       }
     }
   }
 
   protected function executeRepositoryOperations() {
     $repository = $this->getRepository();
 
     $args = $this->getArgs();
     if (!$args->getArg('tunnel')) {
       throw new Exception(pht('Expected `%s`!', 'svnserve -t'));
     }
 
     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();
 
     $this->command = id($this->newPassthruCommand())
       ->setIOChannel($this->getIOChannel())
       ->setCommandChannelFromExecFuture($future)
       ->setWillWriteCallback(array($this, 'willWriteMessageCallback'))
       ->setWillReadCallback(array($this, 'willReadMessageCallback'));
 
     $this->command->setPauseIOReads(true);
 
     $err = $this->command->execute();
 
     if (!$err && $this->didSeeWrite) {
       $this->getRepository()->writeStatusMessage(
         PhabricatorRepositoryStatusMessage::TYPE_NEEDS_UPDATE,
         PhabricatorRepositoryStatusMessage::CODE_OKAY);
     }
 
     return $err;
   }
 
   public function willWriteMessageCallback(
     PhabricatorSSHPassthruCommand $command,
     $message) {
 
     $proto = $this->inProtocol;
     $messages = $proto->writeData($message);
 
     $result = array();
     foreach ($messages as $message) {
       $message_raw = $message['raw'];
       $struct = $message['structure'];
 
       if (!$this->inSeenGreeting) {
         $this->inSeenGreeting = true;
 
         // The first message the client sends looks like:
         //
         //   ( version ( cap1 ... ) url ... )
         //
         // We want to grab the URL, load the repository, make sure it exists and
         // is accessible, and then replace it with the location of the
         // repository on disk.
 
         $uri = $struct[2]['value'];
         $struct[2]['value'] = $this->makeInternalURI($uri);
 
         $message_raw = $proto->serializeStruct($struct);
       } else if (isset($struct[0]) && $struct[0]['type'] == 'word') {
 
         if (!$proto->isReadOnlyCommand($struct)) {
           $this->didSeeWrite = true;
           $this->requireWriteAccess($struct[0]['value']);
         }
 
         // Several other commands also pass in URLs. We need to translate
         // all of these into the internal representation; this also makes sure
         // they're valid and accessible.
 
         switch ($struct[0]['value']) {
           case 'reparent':
             // ( reparent ( url ) )
             $struct[1]['value'][0]['value'] = $this->makeInternalURI(
               $struct[1]['value'][0]['value']);
             $message_raw = $proto->serializeStruct($struct);
             break;
           case 'switch':
             // ( switch ( ( rev ) target recurse url ... ) )
             $struct[1]['value'][3]['value'] = $this->makeInternalURI(
               $struct[1]['value'][3]['value']);
             $message_raw = $proto->serializeStruct($struct);
             break;
           case 'diff':
             // ( diff ( ( rev ) target recurse ignore-ancestry url ... ) )
             $struct[1]['value'][4]['value'] = $this->makeInternalURI(
               $struct[1]['value'][4]['value']);
             $message_raw = $proto->serializeStruct($struct);
             break;
           case 'add-file':
           case 'add-dir':
             // ( add-file ( path dir-token file-token [ copy-path copy-rev ] ) )
             // ( add-dir ( path parent child [ copy-path copy-rev ] ) )
             if (isset($struct[1]['value'][3]['value'][0]['value'])) {
               $copy_from = $struct[1]['value'][3]['value'][0]['value'];
               $copy_from = $this->makeInternalURI($copy_from);
               $struct[1]['value'][3]['value'][0]['value'] = $copy_from;
             }
             $message_raw = $proto->serializeStruct($struct);
             break;
         }
       }
 
       $result[] = $message_raw;
     }
 
     if (!$result) {
       return null;
     }
 
     return implode('', $result);
   }
 
   public function willReadMessageCallback(
     PhabricatorSSHPassthruCommand $command,
     $message) {
 
     $proto = $this->outProtocol;
     $messages = $proto->writeData($message);
 
     $result = array();
     foreach ($messages as $message) {
       $message_raw = $message['raw'];
       $struct = $message['structure'];
 
       if (isset($struct[0]) && ($struct[0]['type'] == 'word')) {
 
         if ($struct[0]['value'] == 'success') {
           switch ($this->outPhaseCount) {
             case 0:
               // This is the "greeting", which announces capabilities.
 
               // We already sent this when we were figuring out which
               // repository this request is for, so we aren't going to send
               // it again.
 
               // Instead, we're going to replay the client's response (which
               // we also already read).
 
               $command = $this->getCommand();
               $command->writeIORead($this->peekBuffer);
               $command->setPauseIOReads(false);
 
               $message_raw = null;
               break;
             case 1:
               // This responds to the client greeting, and announces auth.
               break;
             case 2:
               // This responds to auth, which should be trivial over SSH.
               break;
             case 3:
               // This contains the URI of the repository. We need to edit it;
               // if it does not match what the client requested it will reject
               // the response.
               $struct[1]['value'][1]['value'] = $this->makeExternalURI(
                 $struct[1]['value'][1]['value']);
               $message_raw = $proto->serializeStruct($struct);
               break;
             default:
               // We don't care about other protocol frames.
               break;
           }
 
           $this->outPhaseCount++;
         } else if ($struct[0]['value'] == 'failure') {
           // Find any error messages which include the internal URI, and
           // replace the text with the external URI.
           foreach ($struct[1]['value'] as $key => $error) {
             $code = $error['value'][0]['value'];
             $message = $error['value'][1]['value'];
 
             $message = str_replace(
               $this->internalBaseURI,
               $this->externalBaseURI,
               $message);
 
             // Derp derp derp derp derp. The structure looks like this:
             //   ( failure ( ( code message ... ) ... ) )
             $struct[1]['value'][$key]['value'][1]['value'] = $message;
           }
           $message_raw = $proto->serializeStruct($struct);
         }
 
       }
 
       if ($message_raw !== null) {
         $result[] = $message_raw;
       }
     }
 
     if (!$result) {
       return null;
     }
 
     return implode('', $result);
   }
 
   private function getPathFromSubversionURI($uri_string) {
     $uri = new PhutilURI($uri_string);
 
     $proto = $uri->getProtocol();
     if ($proto !== 'svn+ssh') {
       throw new Exception(
         pht(
           'Protocol for URI "%s" MUST be "%s".',
           $uri_string,
           'svn+ssh'));
     }
     $path = $uri->getPath();
 
     // Subversion presumably deals with this, but make sure there's nothing
     // sketchy going on with the URI.
     if (preg_match('(/\\.\\./)', $path)) {
       throw new Exception(
         pht(
           'String "%s" is invalid in path specification "%s".',
           '/../',
           $uri_string));
     }
 
     $path = $this->normalizeSVNPath($path);
 
     return $path;
   }
 
   private function makeInternalURI($uri_string) {
     if ($this->isProxying) {
       return $uri_string;
     }
 
     $uri = new PhutilURI($uri_string);
 
     $repository = $this->getRepository();
 
     $path = $this->getPathFromSubversionURI($uri_string);
     $external_base = $this->getBaseRequestPath();
 
     // Replace "/diffusion/X" in the request with the repository local path,
     // so "/diffusion/X/master/" becomes "/path/to/repository/X/master/".
     $local_path = rtrim($repository->getLocalPath(), '/');
     $path = $local_path.substr($path, strlen($external_base));
 
     // NOTE: We are intentionally NOT removing username information from the
     // URI. Subversion retains it over the course of the request and considers
     // two repositories with different username identifiers to be distinct and
     // incompatible.
 
     $uri->setPath($path);
 
     // If this is happening during the handshake, these are the base URIs for
     // the request.
     if ($this->externalBaseURI === null) {
       $pre = (string)id(clone $uri)->setPath('');
 
       $external_path = $external_base;
       $external_path = $this->normalizeSVNPath($external_path);
       $this->externalBaseURI = $pre.$external_path;
 
       $internal_path = rtrim($repository->getLocalPath(), '/');
       $internal_path = $this->normalizeSVNPath($internal_path);
       $this->internalBaseURI = $pre.$internal_path;
     }
 
     return (string)$uri;
   }
 
   private function makeExternalURI($uri) {
     if ($this->isProxying) {
       return $uri;
     }
 
     $internal = $this->internalBaseURI;
     $external = $this->externalBaseURI;
 
     if (strncmp($uri, $internal, strlen($internal)) === 0) {
       $uri = $external.substr($uri, strlen($internal));
     }
 
     return $uri;
   }
 
   private function normalizeSVNPath($path) {
     // Subversion normalizes redundant slashes internally, so normalize them
     // here as well to make sure things match up.
     $path = preg_replace('(/+)', '/', $path);
 
     return $path;
   }
 
 }
diff --git a/src/infrastructure/env/PhabricatorEnv.php b/src/infrastructure/env/PhabricatorEnv.php
index 60bd9e4595..dd8c11e629 100644
--- a/src/infrastructure/env/PhabricatorEnv.php
+++ b/src/infrastructure/env/PhabricatorEnv.php
@@ -1,880 +1,897 @@
 <?php
 
 /**
  * Manages the execution environment configuration, exposing APIs to read
  * configuration settings and other similar values that are derived directly
  * from configuration settings.
  *
  *
  * = Reading Configuration =
  *
  * The primary role of this class is to provide an API for reading
  * Phabricator configuration, @{method:getEnvConfig}:
  *
  *   $value = PhabricatorEnv::getEnvConfig('some.key', $default);
  *
  * The class also handles some URI construction based on configuration, via
  * the methods @{method:getURI}, @{method:getProductionURI},
  * @{method:getCDNURI}, and @{method:getDoclink}.
  *
  * For configuration which allows you to choose a class to be responsible for
  * some functionality (e.g., which mail adapter to use to deliver email),
  * @{method:newObjectFromConfig} provides a simple interface that validates
  * the configured value.
  *
  *
  * = Unit Test Support =
  *
  * In unit tests, you can use @{method:beginScopedEnv} to create a temporary,
  * mutable environment. The method returns a scope guard object which restores
  * the environment when it is destroyed. For example:
  *
  *   public function testExample() {
  *     $env = PhabricatorEnv::beginScopedEnv();
  *     $env->overrideEnv('some.key', 'new-value-for-this-test');
  *
  *     // Some test which depends on the value of 'some.key'.
  *
  *   }
  *
  * Your changes will persist until the `$env` object leaves scope or is
  * destroyed.
  *
  * You should //not// use this in normal code.
  *
  *
  * @task read     Reading Configuration
  * @task uri      URI Validation
  * @task test     Unit Test Support
  * @task internal Internals
  */
 final class PhabricatorEnv extends Phobject {
 
   private static $sourceStack;
   private static $repairSource;
   private static $overrideSource;
   private static $requestBaseURI;
   private static $cache;
   private static $localeCode;
   private static $readOnly;
   private static $readOnlyReason;
 
   const READONLY_CONFIG = 'config';
   const READONLY_UNREACHABLE = 'unreachable';
   const READONLY_SEVERED = 'severed';
   const READONLY_MASTERLESS = 'masterless';
 
   /**
    * @phutil-external-symbol class PhabricatorStartup
    */
   public static function initializeWebEnvironment() {
     self::initializeCommonEnvironment();
   }
 
   public static function initializeScriptEnvironment() {
     self::initializeCommonEnvironment();
 
     // NOTE: This is dangerous in general, but we know we're in a script context
     // and are not vulnerable to CSRF.
     AphrontWriteGuard::allowDangerousUnguardedWrites(true);
 
     // There are several places where we log information (about errors, events,
     // service calls, etc.) for analysis via DarkConsole or similar. These are
     // useful for web requests, but grow unboundedly in long-running scripts and
     // daemons. Discard data as it arrives in these cases.
     PhutilServiceProfiler::getInstance()->enableDiscardMode();
     DarkConsoleErrorLogPluginAPI::enableDiscardMode();
     DarkConsoleEventPluginAPI::enableDiscardMode();
   }
 
 
   private static function initializeCommonEnvironment() {
     PhutilErrorHandler::initialize();
 
     self::resetUmask();
     self::buildConfigurationSourceStack();
 
     // Force a valid timezone. If both PHP and Phabricator configuration are
     // invalid, use UTC.
     $tz = self::getEnvConfig('phabricator.timezone');
     if ($tz) {
       @date_default_timezone_set($tz);
     }
     $ok = @date_default_timezone_set(date_default_timezone_get());
     if (!$ok) {
       date_default_timezone_set('UTC');
     }
 
     // Prepend '/support/bin' and append any paths to $PATH if we need to.
     $env_path = getenv('PATH');
     $phabricator_path = dirname(phutil_get_library_root('phabricator'));
     $support_path = $phabricator_path.'/support/bin';
     $env_path = $support_path.PATH_SEPARATOR.$env_path;
     $append_dirs = self::getEnvConfig('environment.append-paths');
     if (!empty($append_dirs)) {
       $append_path = implode(PATH_SEPARATOR, $append_dirs);
       $env_path = $env_path.PATH_SEPARATOR.$append_path;
     }
     putenv('PATH='.$env_path);
 
     // Write this back into $_ENV, too, so ExecFuture picks it up when creating
     // subprocess environments.
     $_ENV['PATH'] = $env_path;
 
 
     // If an instance identifier is defined, write it into the environment so
     // it's available to subprocesses.
     $instance = self::getEnvConfig('cluster.instance');
     if (strlen($instance)) {
       putenv('PHABRICATOR_INSTANCE='.$instance);
       $_ENV['PHABRICATOR_INSTANCE'] = $instance;
     }
 
     PhabricatorEventEngine::initialize();
 
     // TODO: Add a "locale.default" config option once we have some reasonable
     // defaults which aren't silly nonsense.
     self::setLocaleCode('en_US');
   }
 
   public static function beginScopedLocale($locale_code) {
     return new PhabricatorLocaleScopeGuard($locale_code);
   }
 
   public static function getLocaleCode() {
     return self::$localeCode;
   }
 
   public static function setLocaleCode($locale_code) {
     if (!$locale_code) {
       return;
     }
 
     if ($locale_code == self::$localeCode) {
       return;
     }
 
     try {
       $locale = PhutilLocale::loadLocale($locale_code);
       $translations = PhutilTranslation::getTranslationMapForLocale(
         $locale_code);
 
       $override = self::getEnvConfig('translation.override');
       if (!is_array($override)) {
         $override = array();
       }
 
       PhutilTranslator::getInstance()
         ->setLocale($locale)
         ->setTranslations($override + $translations);
 
       self::$localeCode = $locale_code;
     } catch (Exception $ex) {
       // Just ignore this; the user likely has an out-of-date locale code.
     }
   }
 
   private static function buildConfigurationSourceStack() {
     self::dropConfigCache();
 
     $stack = new PhabricatorConfigStackSource();
     self::$sourceStack = $stack;
 
     $default_source = id(new PhabricatorConfigDefaultSource())
       ->setName(pht('Global Default'));
     $stack->pushSource($default_source);
 
     $env = self::getSelectedEnvironmentName();
     if ($env) {
       $stack->pushSource(
         id(new PhabricatorConfigFileSource($env))
           ->setName(pht("File '%s'", $env)));
     }
 
     $stack->pushSource(
       id(new PhabricatorConfigLocalSource())
         ->setName(pht('Local Config')));
 
     // If the install overrides the database adapter, we might need to load
     // the database adapter class before we can push on the database config.
     // This config is locked and can't be edited from the web UI anyway.
     foreach (self::getEnvConfig('load-libraries') as $library) {
       phutil_load_library($library);
     }
 
     // If custom libraries specify config options, they won't get default
     // values as the Default source has already been loaded, so we get it to
     // pull in all options from non-phabricator libraries now they are loaded.
     $default_source->loadExternalOptions();
 
     // If this install has site config sources, load them now.
     $site_sources = id(new PhutilClassMapQuery())
       ->setAncestorClass('PhabricatorConfigSiteSource')
       ->setSortMethod('getPriority')
       ->execute();
 
     foreach ($site_sources as $site_source) {
       $stack->pushSource($site_source);
     }
 
     $master = PhabricatorDatabaseRef::getMasterDatabaseRef();
     if (!$master) {
       self::setReadOnly(true, self::READONLY_MASTERLESS);
     } else if ($master->isSevered()) {
       $master->checkHealth();
       if ($master->isSevered()) {
         self::setReadOnly(true, self::READONLY_SEVERED);
       }
     }
 
     try {
       $stack->pushSource(
         id(new PhabricatorConfigDatabaseSource('default'))
           ->setName(pht('Database')));
     } catch (AphrontQueryException $exception) {
       // If the database is not available, just skip this configuration
       // source. This happens during `bin/storage upgrade`, `bin/conf` before
       // schema setup, etc.
     }
   }
 
   public static function repairConfig($key, $value) {
     if (!self::$repairSource) {
       self::$repairSource = id(new PhabricatorConfigDictionarySource(array()))
         ->setName(pht('Repaired Config'));
       self::$sourceStack->pushSource(self::$repairSource);
     }
     self::$repairSource->setKeys(array($key => $value));
     self::dropConfigCache();
   }
 
   public static function overrideConfig($key, $value) {
     if (!self::$overrideSource) {
       self::$overrideSource = id(new PhabricatorConfigDictionarySource(array()))
         ->setName(pht('Overridden Config'));
       self::$sourceStack->pushSource(self::$overrideSource);
     }
     self::$overrideSource->setKeys(array($key => $value));
     self::dropConfigCache();
   }
 
   public static function getUnrepairedEnvConfig($key, $default = null) {
     foreach (self::$sourceStack->getStack() as $source) {
       if ($source === self::$repairSource) {
         continue;
       }
       $result = $source->getKeys(array($key));
       if ($result) {
         return $result[$key];
       }
     }
     return $default;
   }
 
   public static function getSelectedEnvironmentName() {
     $env_var = 'PHABRICATOR_ENV';
 
     $env = idx($_SERVER, $env_var);
 
     if (!$env) {
       $env = getenv($env_var);
     }
 
     if (!$env) {
       $env = idx($_ENV, $env_var);
     }
 
     if (!$env) {
       $root = dirname(phutil_get_library_root('phabricator'));
       $path = $root.'/conf/local/ENVIRONMENT';
       if (Filesystem::pathExists($path)) {
         $env = trim(Filesystem::readFile($path));
       }
     }
 
     return $env;
   }
 
 
 /* -(  Reading Configuration  )---------------------------------------------- */
 
 
   /**
    * Get the current configuration setting for a given key.
    *
    * If the key is not found, then throw an Exception.
    *
    * @task read
    */
   public static function getEnvConfig($key) {
     if (isset(self::$cache[$key])) {
       return self::$cache[$key];
     }
 
     if (array_key_exists($key, self::$cache)) {
       return self::$cache[$key];
     }
 
     $result = self::$sourceStack->getKeys(array($key));
     if (array_key_exists($key, $result)) {
       self::$cache[$key] = $result[$key];
       return $result[$key];
     } else {
       throw new Exception(
         pht(
           "No config value specified for key '%s'.",
           $key));
     }
   }
 
 
   /**
    * Get the current configuration setting for a given key. If the key
    * does not exist, return a default value instead of throwing. This is
    * primarily useful for migrations involving keys which are slated for
    * removal.
    *
    * @task read
    */
   public static function getEnvConfigIfExists($key, $default = null) {
     try {
       return self::getEnvConfig($key);
     } catch (Exception $ex) {
       return $default;
     }
   }
 
 
   /**
    * Get the fully-qualified URI for a path.
    *
    * @task read
    */
   public static function getURI($path) {
     return rtrim(self::getAnyBaseURI(), '/').$path;
   }
 
 
   /**
    * Get the fully-qualified production URI for a path.
    *
    * @task read
    */
   public static function getProductionURI($path) {
     // If we're passed a URI which already has a domain, simply return it
     // unmodified. In particular, files may have URIs which point to a CDN
     // domain.
     $uri = new PhutilURI($path);
     if ($uri->getDomain()) {
       return $path;
     }
 
     $production_domain = self::getEnvConfig('phabricator.production-uri');
     if (!$production_domain) {
       $production_domain = self::getAnyBaseURI();
     }
     return rtrim($production_domain, '/').$path;
   }
 
   public static function getAllowedURIs($path) {
     $uri = new PhutilURI($path);
     if ($uri->getDomain()) {
       return $path;
     }
 
     $allowed_uris = self::getEnvConfig('phabricator.allowed-uris');
     $return = array();
     foreach ($allowed_uris as $allowed_uri) {
       $return[] = rtrim($allowed_uri, '/').$path;
     }
 
     return $return;
   }
 
 
   /**
    * Get the fully-qualified production URI for a static resource path.
    *
    * @task read
    */
   public static function getCDNURI($path) {
     $alt = self::getEnvConfig('security.alternate-file-domain');
     if (!$alt) {
       $alt = self::getAnyBaseURI();
     }
     $uri = new PhutilURI($alt);
     $uri->setPath($path);
     return (string)$uri;
   }
 
 
   /**
    * Get the fully-qualified production URI for a documentation resource.
    *
    * @task read
    */
   public static function getDoclink($resource, $type = 'article') {
     $uri = new PhutilURI('https://secure.phabricator.com/diviner/find/');
     $uri->setQueryParam('name', $resource);
     $uri->setQueryParam('type', $type);
     $uri->setQueryParam('jump', true);
     return (string)$uri;
   }
 
 
   /**
    * Build a concrete object from a configuration key.
    *
    * @task read
    */
   public static function newObjectFromConfig($key, $args = array()) {
     $class = self::getEnvConfig($key);
     return newv($class, $args);
   }
 
   public static function getAnyBaseURI() {
     $base_uri = self::getEnvConfig('phabricator.base-uri');
 
     if (!$base_uri) {
       $base_uri = self::getRequestBaseURI();
     }
 
     if (!$base_uri) {
       throw new Exception(
         pht(
           "Define '%s' in your configuration to continue.",
           'phabricator.base-uri'));
     }
 
     return $base_uri;
   }
 
   public static function getRequestBaseURI() {
     return self::$requestBaseURI;
   }
 
   public static function setRequestBaseURI($uri) {
     self::$requestBaseURI = $uri;
   }
 
   public static function isReadOnly() {
     if (self::$readOnly !== null) {
       return self::$readOnly;
     }
     return self::getEnvConfig('cluster.read-only');
   }
 
   public static function setReadOnly($read_only, $reason) {
     self::$readOnly = $read_only;
     self::$readOnlyReason = $reason;
   }
 
   public static function getReadOnlyMessage() {
     $reason = self::getReadOnlyReason();
     switch ($reason) {
       case self::READONLY_MASTERLESS:
         return pht(
           'Phabricator is in read-only mode (no writable database '.
           'is configured).');
       case self::READONLY_UNREACHABLE:
         return pht(
           'Phabricator is in read-only mode (unreachable master).');
       case self::READONLY_SEVERED:
         return pht(
           'Phabricator is in read-only mode (major interruption).');
     }
 
     return pht('Phabricator is in read-only mode.');
   }
 
   public static function getReadOnlyURI() {
     return urisprintf(
       '/readonly/%s/',
       self::getReadOnlyReason());
   }
 
   public static function getReadOnlyReason() {
     if (!self::isReadOnly()) {
       return null;
     }
 
     if (self::$readOnlyReason !== null) {
       return self::$readOnlyReason;
     }
 
     return self::READONLY_CONFIG;
   }
 
 
 /* -(  Unit Test Support  )-------------------------------------------------- */
 
 
   /**
    * @task test
    */
   public static function beginScopedEnv() {
     return new PhabricatorScopedEnv(self::pushTestEnvironment());
   }
 
 
   /**
    * @task test
    */
   private static function pushTestEnvironment() {
     self::dropConfigCache();
     $source = new PhabricatorConfigDictionarySource(array());
     self::$sourceStack->pushSource($source);
     return spl_object_hash($source);
   }
 
 
   /**
    * @task test
    */
   public static function popTestEnvironment($key) {
     self::dropConfigCache();
     $source = self::$sourceStack->popSource();
     $stack_key = spl_object_hash($source);
     if ($stack_key !== $key) {
       self::$sourceStack->pushSource($source);
       throw new Exception(
         pht(
           'Scoped environments were destroyed in a different order than they '.
           'were initialized.'));
     }
   }
 
 
 /* -(  URI Validation  )----------------------------------------------------- */
 
 
   /**
    * Detect if a URI satisfies either @{method:isValidLocalURIForLink} or
    * @{method:isValidRemoteURIForLink}, i.e. is a page on this server or the
    * URI of some other resource which has a valid protocol. This rejects
    * garbage URIs and URIs with protocols which do not appear in the
    * `uri.allowed-protocols` configuration, notably 'javascript:' URIs.
    *
    * NOTE: This method is generally intended to reject URIs which it may be
    * unsafe to put in an "href" link attribute.
    *
    * @param string URI to test.
    * @return bool True if the URI identifies a web resource.
    * @task uri
    */
   public static function isValidURIForLink($uri) {
     return self::isValidLocalURIForLink($uri) ||
            self::isValidRemoteURIForLink($uri);
   }
 
 
   /**
    * Detect if a URI identifies some page on this server.
    *
    * NOTE: This method is generally intended to reject URIs which it may be
    * unsafe to issue a "Location:" redirect to.
    *
    * @param string URI to test.
    * @return bool True if the URI identifies a local page.
    * @task uri
    */
   public static function isValidLocalURIForLink($uri) {
     $uri = (string)$uri;
 
     if (!strlen($uri)) {
       return false;
     }
 
     if (preg_match('/\s/', $uri)) {
       // PHP hasn't been vulnerable to header injection attacks for a bunch of
       // years, but we can safely reject these anyway since they're never valid.
       return false;
     }
 
     // Chrome (at a minimum) interprets backslashes in Location headers and the
     // URL bar as forward slashes. This is probably intended to reduce user
     // error caused by confusion over which key is "forward slash" vs "back
     // slash".
     //
     // However, it means a URI like "/\evil.com" is interpreted like
     // "//evil.com", which is a protocol relative remote URI.
     //
     // Since we currently never generate URIs with backslashes in them, reject
     // these unconditionally rather than trying to figure out how browsers will
     // interpret them.
     if (preg_match('/\\\\/', $uri)) {
       return false;
     }
 
     // Valid URIs must begin with '/', followed by the end of the string or some
     // other non-'/' character. This rejects protocol-relative URIs like
     // "//evil.com/evil_stuff/".
     return (bool)preg_match('@^/([^/]|$)@', $uri);
   }
 
 
   /**
    * Detect if a URI identifies some valid linkable remote resource.
    *
    * @param string URI to test.
    * @return bool True if a URI idenfies a remote resource with an allowed
    *              protocol.
    * @task uri
    */
   public static function isValidRemoteURIForLink($uri) {
     try {
       self::requireValidRemoteURIForLink($uri);
       return true;
     } catch (Exception $ex) {
       return false;
     }
   }
 
 
   /**
    * Detect if a URI identifies a valid linkable remote resource, throwing a
    * detailed message if it does not.
    *
    * A valid linkable remote resource can be safely linked or redirected to.
    * This is primarily a protocol whitelist check.
    *
    * @param string URI to test.
    * @return void
    * @task uri
    */
   public static function requireValidRemoteURIForLink($raw_uri) {
     $uri = new PhutilURI($raw_uri);
 
     $proto = $uri->getProtocol();
     if (!strlen($proto)) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid linkable resource. A valid linkable '.
           'resource URI must specify a protocol.',
           $raw_uri));
     }
 
     $protocols = self::getEnvConfig('uri.allowed-protocols');
     if (!isset($protocols[$proto])) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid linkable resource. A valid linkable '.
           'resource URI must use one of these protocols: %s.',
           $raw_uri,
           implode(', ', array_keys($protocols))));
     }
 
     $domain = $uri->getDomain();
     if (!strlen($domain)) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid linkable resource. A valid linkable '.
           'resource URI must specify a domain.',
           $raw_uri));
     }
   }
 
 
   /**
    * Detect if a URI identifies a valid fetchable remote resource.
    *
    * @param string URI to test.
    * @param list<string> Allowed protocols.
    * @return bool True if the URI is a valid fetchable remote resource.
    * @task uri
    */
   public static function isValidRemoteURIForFetch($uri, array $protocols) {
     try {
       self::requireValidRemoteURIForFetch($uri, $protocols);
       return true;
     } catch (Exception $ex) {
       return false;
     }
   }
 
 
   /**
    * Detect if a URI identifies a valid fetchable remote resource, throwing
    * a detailed message if it does not.
    *
    * A valid fetchable remote resource can be safely fetched using a request
    * originating on this server. This is a primarily an address check against
    * the outbound address blacklist.
    *
    * @param string URI to test.
    * @param list<string> Allowed protocols.
    * @return pair<string, string> Pre-resolved URI and domain.
    * @task uri
    */
   public static function requireValidRemoteURIForFetch(
     $uri,
     array $protocols) {
 
     $uri = new PhutilURI($uri);
 
     $proto = $uri->getProtocol();
     if (!strlen($proto)) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid fetchable resource. A valid fetchable '.
           'resource URI must specify a protocol.',
           $uri));
     }
 
     $protocols = array_fuse($protocols);
     if (!isset($protocols[$proto])) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid fetchable resource. A valid fetchable '.
           'resource URI must use one of these protocols: %s.',
           $uri,
           implode(', ', array_keys($protocols))));
     }
 
     $domain = $uri->getDomain();
     if (!strlen($domain)) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid fetchable resource. A valid fetchable '.
           'resource URI must specify a domain.',
           $uri));
     }
 
     $addresses = gethostbynamel($domain);
     if (!$addresses) {
       throw new Exception(
         pht(
           'URI "%s" is not a valid fetchable resource. The domain "%s" could '.
           'not be resolved.',
           $uri,
           $domain));
     }
 
     foreach ($addresses as $address) {
       if (self::isBlacklistedOutboundAddress($address)) {
         throw new Exception(
           pht(
             'URI "%s" is not a valid fetchable resource. The domain "%s" '.
             'resolves to the address "%s", which is blacklisted for '.
             'outbound requests.',
             $uri,
             $domain,
             $address));
       }
     }
 
     $resolved_uri = clone $uri;
     $resolved_uri->setDomain(head($addresses));
 
     return array($resolved_uri, $domain);
   }
 
 
   /**
    * Determine if an IP address is in the outbound address blacklist.
    *
    * @param string IP address.
    * @return bool True if the address is blacklisted.
    */
   public static function isBlacklistedOutboundAddress($address) {
     $blacklist = self::getEnvConfig('security.outbound-blacklist');
 
     return PhutilCIDRList::newList($blacklist)->containsAddress($address);
   }
 
   public static function isClusterRemoteAddress() {
     $cluster_addresses = self::getEnvConfig('cluster.addresses');
     if (!$cluster_addresses) {
       return false;
     }
 
     $address = idx($_SERVER, 'REMOTE_ADDR');
     if (!$address) {
       throw new Exception(
         pht(
           'Unable to test remote address against cluster whitelist: '.
           'REMOTE_ADDR is not defined.'));
     }
 
     return self::isClusterAddress($address);
   }
 
   public static function isClusterAddress($address) {
     $cluster_addresses = self::getEnvConfig('cluster.addresses');
     if (!$cluster_addresses) {
       throw new Exception(
         pht(
           'Phabricator is not configured to serve cluster requests. '.
           'Set `cluster.addresses` in the configuration to whitelist '.
           'cluster hosts before sending requests that use a cluster '.
           'authentication mechanism.'));
     }
 
     return PhutilCIDRList::newList($cluster_addresses)
       ->containsAddress($address);
   }
 
 /* -(  Internals  )---------------------------------------------------------- */
 
 
   /**
    * @task internal
    */
   public static function envConfigExists($key) {
     return array_key_exists($key, self::$sourceStack->getKeys(array($key)));
   }
 
 
   /**
    * @task internal
    */
   public static function getAllConfigKeys() {
     return self::$sourceStack->getAllKeys();
   }
 
   public static function getConfigSourceStack() {
     return self::$sourceStack;
   }
 
   /**
    * @task internal
    */
   public static function overrideTestEnvConfig($stack_key, $key, $value) {
     $tmp = array();
 
     // If we don't have the right key, we'll throw when popping the last
     // source off the stack.
     do {
       $source = self::$sourceStack->popSource();
       array_unshift($tmp, $source);
       if (spl_object_hash($source) == $stack_key) {
         $source->setKeys(array($key => $value));
         break;
       }
     } while (true);
 
     foreach ($tmp as $source) {
       self::$sourceStack->pushSource($source);
     }
 
     self::dropConfigCache();
   }
 
   private static function dropConfigCache() {
     self::$cache = array();
   }
 
   private static function resetUmask() {
     // Reset the umask to the common standard umask. The umask controls default
     // permissions when files are created and propagates to subprocesses.
 
     // "022" is the most common umask, but sometimes it is set to something
     // unusual by the calling environment.
 
     // Since various things rely on this umask to work properly and we are
     // not aware of any legitimate reasons to adjust it, unconditionally
     // normalize it until such reasons arise. See T7475 for discussion.
     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/';
+  }
+
+
 }