sync: Support non-blocking initial sync in proto

sync/protocol/sync.proto

 // Copyright (c) 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.
 //
 // Sync protocol for communication between sync client and server.
 
 // Update proto_value_conversions{.h,.cc,_unittest.cc} if you change
 // any fields in this file.
 
 syntax = "proto2";
@@ skipping 459 matching lines @@
   // associated with user actions.  For example, at the time of this writing
   // the most common (only?) source of refresh requests is when a user opens
   // the new tab page on a platform that does not support sessions
   // invalidations.
   // Introduced in M29.
   optional int64 datatype_refresh_nudges = 5;
 
   // This flag is set if the invalidation server reports that it may have
   // dropped some invalidations at some point.  Introduced in M33.
   optional bool server_dropped_hints = 6;
+
+  // This flag is set if this GetUpdate request is due at least in part due
+  // to the fact that this type has not finished initial sync yet, and the
+  // client would like to initialize itself with the server data.
+  //
+  // Only some types support performing an initial sync as part of a normal
+  // GetUpdate request.  Many types must be in configure mode when fetching
+  // initial sync data.
+  //
+  // Introduced in M38.
+  optional bool initial_sync_in_progress = 7;

[Nicolas Zea, 2014/07/17 16:58:08]

I wonder, should we specify which types are requesting the initial sync? It
might be useful for the backend to e.g. know which backing stores it actually
needs to hit.

[rlarocque, 2014/07/17 18:58:06]

The triggers are per-type.  I think they're co-located with the progress
markers.

All the data required to gather per-type stats are here.  Whether or not it's
easy to turn it into useful graphs on the server may be another matter, though.

 }
 
 message GarbageCollectionDirective {
   enum Type {
     UNKNOWN = 0;
     VERSION_WATERMARK = 1;
     AGE_WATERMARK = 2;
     MAX_ITEM_COUNT = 3;
   }
 
@@ skipping 457 matching lines @@
     // Currently only meaningful if |error_type| is throttled. If this field
     // is absent then the whole client (all datatypes) is throttled.
     repeated int32 error_data_type_ids = 5;
   }
   optional Error error = 13;
 
   // The new per-client state for this client. If set, should be persisted and
   // sent with any subsequent ClientToServerMessages.
   optional ChipBag new_bag_of_chips = 14;
 };