Whitelist blacklisting pcntl_ functions for setup checks so Debian installs don't fatal instantly

Summary: See IRC. This is dumb but I think we should try to work by default on Debian, and it doesn't cost us too much. See inline comment for more.

Test Plan:
  - No `disable_functions`, restarted, worked fine.
  - Set `disable_functions = pcntl_derp`, restarted, worked fine.
  - Set `disable_functions = derp`, restarted, setup fatal.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6741

src/applications/config/check/PhabricatorSetupCheckPHPConfig.php

@@ -34,7 +34,31 @@ final class PhabricatorSetupCheckPHPConfig extends PhabricatorSetupCheck {
 
     $disable_options = array('disable_functions', 'disable_classes');
     foreach ($disable_options as $disable_option) {
-      if (ini_get($disable_option)) {
+      $disable_value = ini_get($disable_option);
+      if ($disable_value) {
+
+        // By default Debian installs the pcntl extension but disables all of
+        // its functions using configuration. Whitelist disabling these
+        // functions so that Debian PHP works out of the box (we do not need to
+        // call these functions from the web UI). This is pretty ridiculous but
+        // it's not the users' fault and they haven't done anything crazy to
+        // get here, so don't make them pay for Debian's unusual choices.
+        // See: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=605571
+        $fatal = true;
+        if ($disable_option == 'disable_functions') {
+          $functions = preg_split('/[, ]+/', $disable_value);
+          $functions = array_filter($functions);
+          foreach ($functions as $k => $function) {
+            if (preg_match('/^pcntl_/', $function)) {
+              unset($functions[$k]);
+            }
+          }
+          if (!$functions) {
+            $fatal = false;
+          }
+        }
+
+        if ($fatal) {
           $message = pht(
             "You have '%s' enabled in your PHP configuration.\n\n".
             "This option is not compatible with Phabricator. Remove ".
@@ -49,6 +73,7 @@ final class PhabricatorSetupCheckPHPConfig extends PhabricatorSetupCheck {
             ->addPHPConfig($disable_option);
         }
       }
+    }
 
     $open_basedir = ini_get('open_basedir');
     if ($open_basedir) {