Allow Celerity to return "304 Not Modified" responses

Summary:
We always return HTTP 200 right now and don't send a "Last-Modified" header, so
browsers download more data then necessary if you sit on a page mashing reload
(for example).

Test Plan:
Used Charles to verify HTTP response codes from 400, 404 and 304 responses.

Mashed reload a bunch and saw that the server sent back 304s.

Changed the resource hash seed and saw 200s, then 304s on reload.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: bmaurer, aran, tuomaspelkonen
Differential Revision: 253

Author: epriestley

conf/default.conf.php

@@ -337,4 +337,12 @@
   // track running daemons.
   'phd.pid-directory' => '/var/tmp/phd',
 
+  // This value is an input to the hash function when building resource hashes.
+  // It has no security value, but if you accidentally poison user caches (by
+  // pushing a bad patch or having something go wrong with a CDN, e.g.) you can
+  // change this to something else and rebuild the Celerity map to break user
+  // caches. Unless you are doing Celerity development, it is exceptionally
+  // unlikely that you need to modify this.
+  'celerity.resource-hash' => 'd9455ea150622ee044f7931dabfa52aa',
+
 );

scripts/celerity_mapper.php

@@ -71,6 +71,7 @@
 
 
 require_once dirname(__FILE__).'/__init_script__.php';
+require_once dirname(__FILE__).'/__init_env__.php';
 
 if ($argc != 2) {
   $self = basename($argv[0]);
@@ -96,12 +97,14 @@
 
 echo "Processing ".count($files)." files";
 
+$resource_hash = PhabricatorEnv::getEnvConfig('celerity.resource-hash');
+
 $file_map = array();
 foreach ($files as $path => $hash) {
   echo ".";
   $name = '/'.Filesystem::readablePath($path, $root);
   $file_map[$name] = array(
-    'hash' => $hash,
+    'hash' => md5($hash.$name.$resource_hash),
     'disk' => $path,
   );
 }