Pref service: add support for tracked prefs.

services/preferences/tracked/tracked_persistent_pref_store_factory.h

Index: services/preferences/tracked/tracked_persistent_pref_store_factory.h
diff --git a/services/preferences/tracked/tracked_persistent_pref_store_factory.h b/services/preferences/tracked/tracked_persistent_pref_store_factory.h
new file mode 100644
index 0000000000000000000000000000000000000000..fb426bab47db4e8453b68801ba6aa2b84dc6834e
--- /dev/null
+++ b/services/preferences/tracked/tracked_persistent_pref_store_factory.h
@@ -0,0 +1,27 @@
+// Copyright 2017 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.
+
+#ifndef SERVICES_PREFERENCES_TRACKED_TRACKED_PERSISTENT_PREF_STORE_FACTORY_H_
+#define SERVICES_PREFERENCES_TRACKED_TRACKED_PERSISTENT_PREF_STORE_FACTORY_H_
+
+#include <utility>
+#include "services/preferences/public/interfaces/preferences_configuration.mojom.h"
+
+namespace base {
+class DictionaryValue;
+class SequencedWorkerPool;
+}
+
+class PersistentPrefStore;
+
+PersistentPrefStore* CreateTrackedPersistentPrefStore(
+    prefs::mojom::TrackedPersistentPrefStoreConfigurationPtr config,
+    base::SequencedWorkerPool* worker_pool);

[gab, 2017/04/03 16:11:17]

This is running service side, right? (i.e. not a /public interface)

Is it really appropriate to take a base::SequencedWorkerPool* here? Seems this
ties this service to the calling process and also to this executing while the
refcount on |worker_pool| is still up? Does it always execute synchronously and
I'm just not seeing it?

[Sam McNally, 2017/04/04 03:24:49]

Yes.
When the service is used, |worker_pool| is provided by the service. In the short
term, the service runs in-process and the end-session shutdown procedure needs
to be able to find the task runner used to write prefs so it can wait for that
to complete.

[gab, 2017/04/04 20:59:51]

Ok, that works for now then, medium term SequencedWorkerPool will be replaced by
base/task_scheduler anyways which has an implicit instance in every process and
won't need to be passed around :)

Where is the code where "|worker_pool| is provided by the service"? (trying to
understand the mojo paradigms as much as possible :))

[Sam McNally, 2017/04/05 00:44:30]

The main class of the service PrefStoreManagerImpl has a ref to a
SequencedWorkerPool, which it passes into the common PersistentPrefStore factory
function, which forwards it to this function if tracked prefs are enabled.
PrefStoreManagerImpl receives its SequencedWorkerPool at construction time
(which chrome provides since it's running in the browser process).

+
+// TODO(sammc): This should move somewhere more appropriate in the longer term.
+void InitializeMasterPrefsTracking(
+    prefs::mojom::TrackedPersistentPrefStoreConfigurationPtr configuration,
+    base::DictionaryValue* master_prefs);
+
+#endif  // SERVICES_PREFERENCES_TRACKED_TRACKED_PERSISTENT_PREF_STORE_FACTORY_H_