Bug 1105468 - OpenFileAndSendFDRunnable::OpenFile needs to ensure we don't release off main thread. r=bent

dom/ipc/TabParent.cpp

@@ -171,7 +171,8 @@ private:
         }
     }
 
-    void OpenFile()
+    // Helper method to avoid gnarly control flow for failures.
+    void OpenFileImpl()
     {
         MOZ_ASSERT(!NS_IsMainThread());
         MOZ_ASSERT(!mFD);
@@ -185,10 +186,21 @@ private:
         NS_ENSURE_SUCCESS_VOID(rv);
 
         mFD = fd;
+    }
+
+    void OpenFile()
+    {
+        MOZ_ASSERT(!NS_IsMainThread());
+
+        OpenFileImpl();
 
         if (NS_FAILED(NS_DispatchToMainThread(this))) {
             NS_WARNING("Failed to dispatch to main thread!");
 
+            // Intentionally leak the runnable (but not the fd) rather
+            // than crash when trying to release a main thread object
+            // off the main thread.
+            mTabParent.forget();
             CloseFile();
         }
     }