Create AphrontWriteGuard, a backup mechanism for CSRF validation

Summary:
Provide a catchall mechanism to find unprotected writes.

  - Depends on D758.
  - Similar to WriteOnHTTPGet stuff from Facebook's stack.
  - Since we have a small number of storage mechanisms and highly structured
read/write pathways, we can explicitly answer the question "is this page
performing a write?".
  - Never allow writes without CSRF checks.
  - This will probably break some things. That's fine: they're CSRF
vulnerabilities or weird edge cases that we can fix. But don't push to Facebook
for a few days unless you're prepared to deal with this.
  - **>>> MEGADERP: All Conduit write APIs are currently vulnerable to CSRF!
<<<**

Test Plan:
  - Ran some scripts that perform writes (scripts/search indexers), no issues.
  - Performed normal CSRF submits.
  - Added writes to an un-CSRF'd page, got an exception.
  - Executed conduit methods.
  - Did login/logout (this works because the logged-out user validates the
logged-out csrf "token").
  - Did OAuth login.
  - Did OAuth registration.

Reviewers: pedram, andrewjcg, erling, jungejason, tuomaspelkonen, aran,
codeblock
Commenters: pedram
CC: aran, epriestley, pedram
Differential Revision: 777

src/applications/auth/controller/oauth/PhabricatorOAuthLoginController.php

@@ -129,7 +129,8 @@ class PhabricatorOAuthLoginController extends PhabricatorAuthController {
       }
 
       $oauth_info->setUserID($current_user->getID());
-      $oauth_info->save();
+
+      $this->saveOAuthInfo($oauth_info);
 
       return id(new AphrontRedirectResponse())
         ->setURI('/settings/page/'.$provider_key.'/');
@@ -149,7 +150,7 @@ class PhabricatorOAuthLoginController extends PhabricatorAuthController {
 
       $session_key = $known_user->establishSession('web');
 
-      $oauth_info->save();
+      $this->saveOAuthInfo($oauth_info);
 
       $request->setCookie('phusr', $known_user->getUsername());
       $request->setCookie('phsid', $session_key);
@@ -317,4 +318,12 @@ class PhabricatorOAuthLoginController extends PhabricatorAuthController {
     return $oauth_info;
   }
 
+  private function saveOAuthInfo(PhabricatorUserOAuthInfo $info) {
+    // UNGUARDED WRITES: Logging-in users don't have their CSRF set up yet.
+    $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
+    $info->save();
+  }
+
+
+
 }