Make WaitForExistingTasksOnLoop from SingleClientDirectorySyncTest saner

This is an attempt to deflake the currently disabled
SingleClientDirectorySyncTest.SingleClientDirectorySyncTest. The test
was failing due to running into the 45s timeout in
WaitForExistingTasksOnLoop. The theory is that that's because that
function is based on WaitableEvent, and thus doesn't allow processing of
events on the main thread while waiting. This CL changes it to spin a
nested RunLoop instead.

I could never reproduce the flakes locally, so I don't know if this
helps at all. As such, this CL leaves the test disabled for now; I'll
send a separate one to try re-enabling.

Bug: 850980
Change-Id: I16823fe01574662586e578d5f601299c58004286
Reviewed-on: https://chromium-review.googlesource.com/1116549Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570710}

chrome/browser/sync/test/integration/single_client_directory_sync_test.cc

@@ -9,21 +9,17 @@
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
-#include "base/synchronization/waitable_event.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
-#include "base/time/time.h"
 #include "chrome/browser/sync/test/integration/bookmarks_helper.h"
 #include "chrome/browser/sync/test/integration/single_client_status_change_checker.h"
 #include "chrome/browser/sync/test/integration/sync_test.h"
 #include "chrome/browser/sync/test/integration/updated_progress_marker_checker.h"
 #include "components/browser_sync/profile_sync_service.h"
 #include "components/sync/syncable/directory.h"
-#include "content/public/browser/browser_thread.h"
 #include "sql/test/test_helpers.h"
 #include "url/gurl.h"
 
-using content::BrowserThread;
 using base::FileEnumerator;
 using base::FilePath;
 
@@ -51,16 +47,13 @@ class SingleClientDirectorySyncTest : public SyncTest {
   DISALLOW_COPY_AND_ASSIGN(SingleClientDirectorySyncTest);
 };
 
-void SignalEvent(base::WaitableEvent* e) {
-  e->Signal();
-}
-
-bool WaitForExistingTasksOnLoop(base::MessageLoop* loop) {
-  base::WaitableEvent e(base::WaitableEvent::ResetPolicy::MANUAL,
-                        base::WaitableEvent::InitialState::NOT_SIGNALED);
-  loop->task_runner()->PostTask(FROM_HERE, base::BindOnce(&SignalEvent, &e));
-  // Timeout stolen from StatusChangeChecker::GetTimeoutDuration().
-  return e.TimedWait(base::TimeDelta::FromSeconds(45));
+void WaitForExistingTasksOnLoop(base::MessageLoop* loop) {
+  base::RunLoop run_loop;
+  // Post a task to |loop| that will, in turn, post a task back to the current
+  // sequenced task runner to quit the nested loop.
+  loop->task_runner()->PostTaskAndReply(FROM_HERE, base::DoNothing(),
+                                        run_loop.QuitClosure());
+  run_loop.Run();
 }
 
 // A status change checker that waits for an unrecoverable sync error to occur.
@@ -94,7 +87,7 @@ IN_PROC_BROWSER_TEST_F(SingleClientDirectorySyncTest,
   run_loop.Run();
   // Wait for the directory deletion to finish.
   base::MessageLoop* sync_loop = sync_service->GetSyncLoopForTest();
-  ASSERT_TRUE(WaitForExistingTasksOnLoop(sync_loop));
+  WaitForExistingTasksOnLoop(sync_loop);
   ASSERT_FALSE(FolderContainsFiles(directory_path));
 }
 
@@ -114,7 +107,7 @@ IN_PROC_BROWSER_TEST_F(SingleClientDirectorySyncTest,
   browser_sync::ProfileSyncService* sync_service = GetSyncService(0);
   sync_service->FlushDirectory();
   base::MessageLoop* sync_loop = sync_service->GetSyncLoopForTest();
-  ASSERT_TRUE(WaitForExistingTasksOnLoop(sync_loop));
+  WaitForExistingTasksOnLoop(sync_loop);
 
   // Now corrupt the database.
   const FilePath directory_path(sync_service->GetDirectoryPathForTest());
@@ -147,7 +140,7 @@ IN_PROC_BROWSER_TEST_F(SingleClientDirectorySyncTest,
 
   // Wait until the sync loop has processed any existing tasks and see that the
   // directory no longer exists.
-  ASSERT_TRUE(WaitForExistingTasksOnLoop(sync_loop));
+  WaitForExistingTasksOnLoop(sync_loop);
   ASSERT_FALSE(FolderContainsFiles(directory_path));
 }