Cache README content for repositories

Summary:
Ref T11954. Especially with higher-latency file stores like S3, we can spend a lot of time reading README data and then pulling it out of file storage.

Instead, cache it.

Test Plan: Browsed a repostory with a README, saw faster pages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11954

Differential Revision: https://secure.phabricator.com/D17002

src/applications/diffusion/controller/DiffusionController.php

@@ -340,7 +340,18 @@ abstract class DiffusionController extends PhabricatorController {
 
     $drequest = $this->getDiffusionRequest();
     $viewer = $this->getViewer();
+    $repository = $drequest->getRepository();
+    $repository_phid = $repository->getPHID();
+    $stable_commit = $drequest->getStableCommit();
 
+    $cache = PhabricatorCaches::getMutableStructureCache();
+    $cache_key = "diffusion".
+      ".repository({$repository_phid})".
+      ".commit({$stable_commit})".
+      ".readme({$readme_path})";
+
+    $readme_cache = $cache->getKey($cache_key);
+    if (!$readme_cache) {
       try {
         $result = $this->callConduitWithDiffusionRequest(
           'diffusion.filecontentquery',
@@ -367,14 +378,22 @@ abstract class DiffusionController extends PhabricatorController {
 
       $corpus = $file->loadFileData();
 
-    if (!strlen($corpus)) {
+      $readme_cache = array(
+        'corpus' => $corpus,
+      );
+
+      $cache->setKey($cache_key, $readme_cache);
+    }
+
+    $readme_corpus = $readme_cache['corpus'];
+    if (!strlen($readme_corpus)) {
       return null;
     }
 
     return id(new DiffusionReadmeView())
       ->setUser($this->getViewer())
       ->setPath($readme_path)
-      ->setContent($corpus);
+      ->setContent($readme_corpus);
   }
 
 }