[Sync] Add context proto and directory support

sync/syncable/directory.cc

Index: sync/syncable/directory.cc
diff --git a/sync/syncable/directory.cc b/sync/syncable/directory.cc
index 97e7071e591da8a084cc5971f642bb8cdb23c5be..85b0a172b1fe9100af246927d45f83031914ca68 100644
--- a/sync/syncable/directory.cc
+++ b/sync/syncable/directory.cc
@@ -792,6 +792,19 @@ void Directory::IncrementTransactionVersion(ModelType type) {
   kernel_->persisted_info.transaction_version[type]++;
 }
 
+void Directory::GetDataTypeContext(ModelType type,
+                                   sync_pb::DataTypeContext* context) const {
+  kernel_->transaction_mutex.AssertAcquired();

[maniscalco, 2014/04/03 16:59:37]

I noticed that in some methods we acquire a lock (ScopedKernelLock(this);), but
in others (like here), we simply assert we've acquired the transaction_mutex. 
Can you help me understand when we do one vs. the other?

[Nicolas Zea, 2014/04/03 21:01:11]

It has to do with whether it's expected that a transaction is being held at the
time the change is made. Previously, when I was doing things through a
ProcessSyncChanges call, that would hold true. Now though I realize this is
incorrect. I've updated it to acquire the lock (here and below)

+  context->CopyFrom(kernel_->persisted_info.datatype_context[type]);
+}
+
+void Directory::SetDataTypeContext(
+    ModelType type,
+    const sync_pb::DataTypeContext& context) {
+  kernel_->transaction_mutex.AssertAcquired();

[rlarocque, 2014/04/03 17:36:55]

What's up with this pattern?  I thought the traditional approach was to require
a pointer to the current transaction.  That way you would be able to verify if
it was a read or write transaction.

(Not that it really matters, since we're not using a real read-write mutex, but
still it seems like a good idea to maintain the abstraction.)

[Nicolas Zea, 2014/04/03 21:01:11]

This is actually wrong, see above comment. I've updated to acquire the lock
there.

+  kernel_->persisted_info.datatype_context[type].CopyFrom(context);
+}
+
 ModelTypeSet Directory::InitialSyncEndedTypes() {
   syncable::ReadTransaction trans(FROM_HERE, this);
   ModelTypeSet protocol_types = ProtocolTypes();