sync: Inject sync/'s dependency on invalidations

sync/internal_api/public/base/invalidation_interface.cc

+// Copyright 2014 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.
+
+#include "sync/internal_api/public/base/invalidation_interface.h"
+
+namespace syncer {
+
+bool InvalidationInterface::LessThan(const InvalidationInterface& a,

[tim (not reviewing), 2014/06/18 22:41:11]

nit - It's a bit weird to define a non-overrideable generic sounding comparator
like "less than" on a pure virtual interface as subclasses might introduce
dimensions unknown to InvalidationInterface that could impact it. Calling it
LessThanByVersion() or moving to a separate utility might help.  But we could
live with it.

[rlarocque, 2014/06/19 00:50:19]

Will rename.

I didn't like this either.  The problem is that this can't be defined on the
derived classes, otherwise it's useless to the code that's only allowed to
interact with the wrapper.  The comparator can only rely on
InvalidationInterface.

I suppose I could have put it in a _util.h file or otherwise defined it out of
this class, but I'm not sure that's much better.  I guess it depends on how you
feel about static methods.

[rlarocque, 2014/06/20 20:00:13]

Done.

+                                     const InvalidationInterface& b) {
+  if (a.IsUnknownVersion() && !b.IsUnknownVersion())
+    return true;
+
+  if (!a.IsUnknownVersion() && b.IsUnknownVersion())
+    return false;
+
+  if (a.IsUnknownVersion() && b.IsUnknownVersion())
+    return false;
+
+  return a.GetVersion() < b.GetVersion();
+}
+
+InvalidationInterface::InvalidationInterface() {
+}
+
+InvalidationInterface::~InvalidationInterface() {
+}
+
+}  // namespace syncer