Stop two special cache writes in read-only mode

Summary:
Ref T10769. The user availability cache write shouldn't happen in read-only mode, nor should the Differential parse cache write.

(We might want to turn off the availbility feature completely since it's potentially expensive if we can't cache it, but I think we're OK for now.)

Test Plan:
In read-only mode:

  - Browsed as a user with an out-of-date availability cache.
  - Loaded an older revision without cached parse data.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10769

Differential Revision: https://secure.phabricator.com/D15988

src/applications/differential/parser/DifferentialChangesetParser.php

@@ -458,6 +458,10 @@ final class DifferentialChangesetParser extends Phobject {
   }
 
   public function saveCache() {
+    if (PhabricatorEnv::isReadOnly()) {
+      return false;
+    }
+
     if ($this->highlightErrors) {
       return false;
     }

src/applications/people/storage/PhabricatorUser.php

@@ -968,6 +968,10 @@ final class PhabricatorUser
    * @task availability
    */
   public function writeAvailabilityCache(array $availability, $ttl) {
+    if (PhabricatorEnv::isReadOnly()) {
+      return $this;
+    }
+
     $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
     queryfx(
       $this->establishConnection('w'),