[Sync] Add context proto and directory support

sync/protocol/sync.proto

Index: sync/protocol/sync.proto
diff --git a/sync/protocol/sync.proto b/sync/protocol/sync.proto
index b9f0de538db0c40628daa4f99612a3be5c014f1d..e69f51015d78579d795093cb55a4545065595210 100644
--- a/sync/protocol/sync.proto
+++ b/sync/protocol/sync.proto
@@ -408,6 +408,9 @@ message CommitMessage {
   // syncing a particular data type regardless of whether a commit for that
   // datatype is currently being sent up.
   optional ClientConfigParams config_params = 4;
+
+  // Set of optional per-client datatype contexts.
+  repeated DataTypeContext client_contexts = 5;
 };
 
 // This message communicates additional per-type information related to
@@ -626,6 +629,9 @@ message GetUpdatesMessage {
   // Whether this GU also serves as a retry GU. Any GU that happens after
   // retry timer timeout is a retry GU effectively.
   optional bool is_retry = 10 [default = false];
+
+  // Set of optional per-client datatype contexts.

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

Just for my own benefit... I assume that you're using the term set here
intentionally.  I.e. the client promises not to send a list of contexts with
duplicate types.  Or does it make sense to ever send (or receive) two contexts
with the same type?

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

No, there should only be one per type.

+  repeated DataTypeContext client_contexts = 11;

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

I don't think we've been consistent, but going forward what's our naming
convention for repeated fields, singular or plural?

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

I don't think we have one. I try to use plural though.

 };
 
 message AuthenticateMessage {
@@ -669,6 +675,17 @@ message ClientStatus {
   optional bool hierarchy_conflict_detected = 1;
 }
 
+// A single datatype's sync context. Allows the datatype to pass along
+// datatype specific information with its own server backend.
+message DataTypeContext {
+  // The type this context is associated with.
+  optional int32 type = 1;

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

Elsewhere in this file I see "data_type_id".  Is this the same thing?  If so,
consider renaming to data_type_id so it's clear that it's the same thing.

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

Done.

+  // The context for the datatype.
+  optional bytes context = 2;
+  // The version of the context.
+  optional int64 version = 3;
+}
+
 message ClientToServerMessage {
   required string share = 1;
   optional int32 protocol_version = 2 [default = 31];
@@ -832,6 +849,9 @@ message GetUpdatesResponse {
   // the server has updated the set of encryption keys (e.g. due to a key
   // rotation).
   repeated bytes encryption_keys = 6;
+
+  // Set of optional datatype contexts server mutations.
+  repeated DataTypeContext context_mutations = 7;
 };
 
 // The metadata response for GetUpdatesMessage.  This response is sent when