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

chrome/browser/sync/engine/model_changing_syncer_command.h

Index: chrome/browser/sync/engine/model_changing_syncer_command.h
diff --git a/chrome/browser/sync/engine/model_changing_syncer_command.h b/chrome/browser/sync/engine/model_changing_syncer_command.h
index 9a2311e96d907365d7f61a6cd519aa431f962cc3..16c7c4f9afe1c80955846a2dd6e9274222582bf2 100644
--- a/chrome/browser/sync/engine/model_changing_syncer_command.h
+++ b/chrome/browser/sync/engine/model_changing_syncer_command.h
@@ -6,7 +6,10 @@
 #define CHROME_BROWSER_SYNC_ENGINE_MODEL_CHANGING_SYNCER_COMMAND_H_
 #pragma once
 
+#include <set>
+
 #include "base/compiler_specific.h"
+#include "chrome/browser/sync/engine/model_safe_worker.h"
 #include "chrome/browser/sync/engine/syncer_command.h"
 #include "chrome/browser/sync/util/unrecoverable_error_info.h"
 
@@ -41,6 +44,22 @@ class ModelChangingSyncerCommand : public SyncerCommand {
     return UnrecoverableErrorInfo();
   }
 
+  std::set<ModelSafeGroup> GetGroupsToChangeForTest(
+      const sessions::SyncSession& session) const {
+    return GetGroupsToChange(session);
+  }
+
+ protected:
+  // This should return the set of groups in |session| that need to be
+  // changed.  The returned set should be a subset of
+  // GetActiveGroups().  Subclasses can guarantee this either by
+  // calling GetActiveGroups() or GetActiveGroupsWithConflicts() and
+  // filtering that, or using GetGroupForModelType() (which handles
+  // top-level/unspecified nodes) to project from model types to
+  // groups.
+  virtual std::set<ModelSafeGroup> GetGroupsToChange(
+      const sessions::SyncSession& session) const = 0;
+
   // Sometimes, a command has work to do that needs to touch global state
   // belonging to multiple ModelSafeGroups, but in a way that is known to be
   // safe.  This will be called once, prior to ModelChangingExecuteImpl,
@@ -55,6 +74,16 @@ class ModelChangingSyncerCommand : public SyncerCommand {
   // running on an unsafe thread are siloed away.
   virtual void ModelChangingExecuteImpl(sessions::SyncSession* session) = 0;
 
+  // Returns the set of groups that have active (i.e., enabled) types.

[tim (not reviewing), 2011/11/22 18:40:05]

Can we avoid introducing a new adjective if we can avoid it :)  If it's "i.e.
enabled", lets use enabled?

[akalin, 2011/11/23 03:25:18]

Done.

+  // Useful for implementing GetGroupsToChange().
+  static std::set<ModelSafeGroup> GetActiveGroups(
+      const sessions::SyncSession& session);
+
+  // Returns the set of active groups that have conflicts.  Useful for
+  // implementing GetGroupsToChange().
+  static std::set<ModelSafeGroup> GetActiveGroupsWithConflicts(
+      const sessions::SyncSession& session);
+
  private:
   // ExecuteImpl is expected to be run by SyncerCommand to set work_session.
   // StartChangingModel is called to start this command running.