[Sync] Do not deadlock when joining sync thread with a pending HistoryModelWorker task.

components/history/core/browser/history_model_worker.h

Index: components/history/core/browser/history_model_worker.h
diff --git a/components/history/core/browser/history_model_worker.h b/components/history/core/browser/history_model_worker.h
index 6421c64d3a198d1080cc5ccb00a04d219dbcea03..b945afc25b41d5a5b498154d3511b73f27c4263d 100644
--- a/components/history/core/browser/history_model_worker.h
+++ b/components/history/core/browser/history_model_worker.h
@@ -11,6 +11,8 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
+#include "base/synchronization/lock.h"
+#include "base/synchronization/waitable_event.h"
 #include "base/task/cancelable_task_tracker.h"
 #include "components/sync/engine/model_safe_worker.h"
 
@@ -29,6 +31,7 @@ class HistoryModelWorker : public syncer::ModelSafeWorker {
       const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread);
 
   // syncer::ModelSafeWorker implementation.
+  void RequestStop() override;
   syncer::ModelSafeGroup GetModelSafeGroup() override;
   bool IsOnModelThread() override;
 
@@ -37,17 +40,45 @@ class HistoryModelWorker : public syncer::ModelSafeWorker {
       const syncer::WorkCallback& work) override;
 
  private:
+  class WorkerTask;
+
   ~HistoryModelWorker() override;
 
+  // Called before a WorkCallback starts running. If this returns false, the

[maxbogue, 2017/03/20 16:52:37]

nit: mention the side effect of this function somewhere in the comment

[fdoray, 2017/03/20 18:54:54]

n/a

+  // WorkCallback must not run.
+  bool WillRunWorkCallback();
+
+  // Called after a WorkCallback has run.
+  void DidRunWorkCallback();
+
   const base::WeakPtr<history::HistoryService> history_service_;
 
   // A reference to the UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
 
+  // Signaled when a task posted by DoWorkAndWaitUntilDoneImpl() is deleted,
+  // i.e. after it runs or when it is abandoned. Reset at the beginning of every
+  // DoWorkAndWaitUntilDoneImpl() call.
+  base::WaitableEvent work_done_or_abandoned_;
+
+  // Signaled to unblock DoWorkAndWaitUntilDoneImpl() and prevent any
+  // WorkCallback from being scheduled when RequestStop() is called and there is
+  // no pending task from this worker on the history DB thread.
+  base::WaitableEvent stop_requested_;
+
+  // True if a WorkCallback posted by this worker is currently running.
+  bool work_callback_running_ = false;
+
+  // Synchronizes access to |stop_requested_| and |work_callback_running_|.
+  base::Lock lock_;
+
   // Helper object to make sure we don't leave tasks running on the history
   // thread.
   std::unique_ptr<base::CancelableTaskTracker> cancelable_tracker_;
 
+  // Verifies that calls to DoWorkAndWaitUntilDoneImpl() are sequenced.
+  base::SequenceChecker sequence_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(HistoryModelWorker);
 };