Fix deadlock by introducing a new transaction event.

chrome/browser/sync/engine/syncapi.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines the "sync API", an interface to the syncer
 // backend that exposes (1) the core functionality of maintaining a consistent
 // local snapshot of a hierarchical object set; (2) a means to transactionally
 // access and modify those objects; (3) a means to control client/server
 // synchronization tasks, namely: pushing local object modifications to a
 // server, pulling nonlocal object modifications from a server to this client,
@@ skipping 456 matching lines @@
 
   DISALLOW_COPY_AND_ASSIGN(BaseTransaction);
 };
 
 // Sync API's ReadTransaction is a read-only BaseTransaction.  It wraps
 // a syncable::ReadTransaction.
 class ReadTransaction : public BaseTransaction {
  public:
   // Start a new read-only transaction on the specified repository.
   explicit ReadTransaction(UserShare* share);
+
+  // Resume the middle of a transaction. Will not close transaction.
+  ReadTransaction(UserShare* share, syncable::BaseTransaction* trans);
+
   virtual ~ReadTransaction();
 
   // BaseTransaction override.
   virtual syncable::BaseTransaction* GetWrappedTrans() const;
  private:
   void* operator new(size_t size);  // Transaction is meant for stack use only.
 
   // The underlying syncable object which this class wraps.
-  syncable::ReadTransaction* transaction_;
+  syncable::BaseTransaction* transaction_;
+  bool close_transaction_;
 
   DISALLOW_COPY_AND_ASSIGN(ReadTransaction);
 };
 
 // Sync API's WriteTransaction is a read/write BaseTransaction.  It wraps
 // a syncable::WriteTransaction.
 class WriteTransaction : public BaseTransaction {
  public:
   // Start a new read/write transaction.
   explicit WriteTransaction(UserShare* share);
@@ skipping 385 matching lines @@
   // This allows actual HttpPostProvider subclass implementations to be
   // reference counted, which is useful if a particular implementation uses
   // multiple threads to serve network requests.
   virtual void Destroy(HttpPostProviderInterface* http) = 0;
   virtual ~HttpPostProviderFactory() { }
 };
 
 }  // namespace sync_api
 
 #endif  // CHROME_BROWSER_SYNC_ENGINE_SYNCAPI_H_