Add a robots.txt file to disallow /diffusion/

Summary:
Fixes T4610. Open to suggestions, etc., if there's anything I'm missing.

Also:

  - Moves these "system" endpoints into a real application.
  - Makes `isUnlisted()` work a little more consistently.

Test Plan: Accessed `/robots.txt`, `/status/` and `/debug/`.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4610

Differential Revision: https://secure.phabricator.com/D8532

src/applications/system/controller/PhabricatorRobotsController.php

@@ -0,0 +1,37 @@
+<?php
+
+final class PhabricatorRobotsController extends PhabricatorController {
+
+  public function shouldRequireLogin() {
+    return false;
+  }
+
+  public function processRequest() {
+    $out = array();
+
+    // Prevent indexing of '/diffusion/', since the content is not generally
+    // useful to index, web spiders get stuck scraping the history of every
+    // file, and much of the content is Ajaxed in anyway so spiders won't even
+    // see it. These pages are also relatively expensive to generate.
+
+    // Note that this still allows commits (at '/rPxxxxx') to be indexed.
+    // They're probably not hugely useful, but suffer fewer of the problems
+    // Diffusion suffers and are hard to omit with 'robots.txt'.
+
+    $out[] = 'User-Agent: *';
+    $out[] = 'Disallow: /diffusion/';
+
+    // Add a small crawl delay (number of seconds between requests) for spiders
+    // which respect it. The intent here is to prevent spiders from affecting
+    // performance for users. The possible cost is slower indexing, but that
+    // seems like a reasonable tradeoff, since most Phabricator installs are
+    // probably not hugely concerned about cutting-edge SEO.
+    $out[] = 'Crawl-delay: 1';
+
+    $content = implode("\n", $out)."\n";
+
+    return id(new AphrontPlainTextResponse())
+      ->setContent($content)
+      ->setCacheDurationInSeconds(phutil_units('2 hours in seconds'));
+  }
+}