[Sync] Make syncer commands avoid posting tasks on threads with no work to do

chrome/browser/sync/engine/apply_updates_command.cc

Index: chrome/browser/sync/engine/apply_updates_command.cc
diff --git a/chrome/browser/sync/engine/apply_updates_command.cc b/chrome/browser/sync/engine/apply_updates_command.cc
index 8b5a6b8db8d28e10eca8fe405b2cb0b2b03df8d0..b7716ff97c511f61adeb5c5680972e9b645c2d52 100644
--- a/chrome/browser/sync/engine/apply_updates_command.cc
+++ b/chrome/browser/sync/engine/apply_updates_command.cc
@@ -17,6 +17,30 @@ using sessions::SyncSession;
 ApplyUpdatesCommand::ApplyUpdatesCommand() {}
 ApplyUpdatesCommand::~ApplyUpdatesCommand() {}
 
+std::set<ModelSafeGroup> ApplyUpdatesCommand::GetGroupsToChange(
+    const sessions::SyncSession& session) const {
+  std::set<ModelSafeGroup> groups_with_unapplied_updates;
+  syncable::ScopedDirLookup dir(session.context()->directory_manager(),
+                                session.context()->account_name());
+  if (!dir.good()) {
+    LOG(ERROR) << "Scoped dir lookup failed!";
+    return groups_with_unapplied_updates;
+  }
+
+  syncable::ReadTransaction trans(FROM_HERE, dir);
+  syncable::Directory::UnappliedUpdateMetaHandles handles;
+  dir->GetUnappliedUpdateMetaHandles(&trans, &handles);
+  for (syncable::Directory::UnappliedUpdateMetaHandles::const_iterator it =

[tim (not reviewing), 2011/11/29 00:01:59]

Might it make sense to keep an index on this in Directory?  Seems like it'd be
pretty cheap / easy.

[akalin, 2011/11/30 01:33:03]

Done.  Not quite trivial, but doable.  I basically split out the
unapplied_updates index by type and used that.

+           handles.begin(); it != handles.end(); ++it) {
+    const syncable::Entry entry(&trans, syncable::GET_BY_HANDLE, *it);
+    groups_with_unapplied_updates.insert(
+        GetGroupForModelType(entry.GetServerModelType(),
+                             session.routing_info()));
+  }
+
+  return groups_with_unapplied_updates;
+}
+
 void ApplyUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
   syncable::ScopedDirLookup dir(session->context()->directory_manager(),
                                 session->context()->account_name());