Keep track of which attachments are referenced by which sync entries.

sync/internal_api/public/base_transaction.h

 // Copyright 2012 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.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_BASE_TRANSACTION_H_
 #define SYNC_INTERNAL_API_PUBLIC_BASE_TRANSACTION_H_
 
 #include "sync/base/sync_export.h"
 #include "sync/internal_api/public/base/model_type.h"
 #include "sync/internal_api/public/user_share.h"
 #include "sync/util/cryptographer.h"
 
 namespace syncer {
 
 namespace syncable {
 class BaseTransaction;
 class Directory;
 }
 
 // Sync API's BaseTransaction, ReadTransaction, and WriteTransaction allow for
 // batching of several read and/or write operations.  The read and write
 // operations are performed by creating ReadNode and WriteNode instances using
 // the transaction. These transaction classes wrap identically named classes in
 // syncable, and are used in a similar way. Unlike syncable::BaseTransaction,
 // whose construction requires an explicit syncable::Directory, a sync
 // API BaseTransaction is created from a UserShare object.
+//
+// Note, these transactions are not atomic. Individual operations can
+// fail. There is no built-in rollback or undo mechanism.
 class SYNC_EXPORT BaseTransaction {
  public:
   // Provide access to the underlying syncable objects from BaseNode.
   virtual syncable::BaseTransaction* GetWrappedTrans() const = 0;
   Cryptographer* GetCryptographer() const;
   ModelTypeSet GetEncryptedTypes() const;
 
   syncable::Directory* GetDirectory() const {
     if (!user_share_) {
       return NULL;
@@ skipping 14 matching lines @@
 
  private:
   UserShare* user_share_;
 
   DISALLOW_COPY_AND_ASSIGN(BaseTransaction);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_BASE_TRANSACTION_H_