sync: Add non-blocking type encryption (retry)

sync/engine/entity_tracker.cc

Index: sync/engine/entity_tracker.cc
diff --git a/sync/engine/entity_tracker.cc b/sync/engine/entity_tracker.cc
index 0f10906ab15c67fc253b0e24c2115d650c0ef4b0..bad76f9932575f43300e7ea74cea38e9e3d4b6df 100644
--- a/sync/engine/entity_tracker.cc
+++ b/sync/engine/entity_tracker.cc
@@ -193,8 +193,14 @@ void EntityTracker::ReceiveCommitResponse(const std::string& response_id,
 }
 
 void EntityTracker::ReceiveUpdate(int64 version) {
-  highest_gu_response_version_ =
-      std::max(highest_gu_response_version_, version);
+  if (version <= highest_gu_response_version_)
+    return;
+
+  highest_gu_response_version_ = version;
+
+  // Got an applicable update newer than any pending updates.  It must be safe
+  // to discard the old pending update, if there was one.
+  ClearPendingUpdate();
 
   if (IsInConflict()) {
     // Incoming update clobbers the pending commit on the sync thread.
@@ -203,10 +209,35 @@ void EntityTracker::ReceiveUpdate(int64 version) {
   }
 }
 
+bool EntityTracker::ReceivePendingUpdate(const UpdateResponseData& data) {
+  if (data.response_version < highest_gu_response_version_)
+    return false;
+
+  highest_gu_response_version_ = data.response_version;
+  pending_update_.reset(new UpdateResponseData(data));
+  ClearPendingCommit();
+  return true;
+}
+
+bool EntityTracker::HasPendingUpdate() const {
+  return !!pending_update_;
+}
+
+UpdateResponseData EntityTracker::GetPendingUpdate() const {
+  return *pending_update_;
+}
+
+void EntityTracker::ClearPendingUpdate() {
+  pending_update_.reset();
+}
+
 bool EntityTracker::IsInConflict() const {
   if (!is_commit_pending_)
     return false;
 
+  if (HasPendingUpdate())
+    return true;
+
   if (highest_gu_response_version_ <= highest_commit_response_version_) {
     // The most recent server state was created in a commit made by this
     // client.  We're fully up to date, and therefore not in conflict.