[Sync] Add support for enabling session sync remotely.

chrome/browser/sync/glue/sync_backend_host.cc

Index: chrome/browser/sync/glue/sync_backend_host.cc
diff --git a/chrome/browser/sync/glue/sync_backend_host.cc b/chrome/browser/sync/glue/sync_backend_host.cc
index efa717b1d97cdac7119bb3f4691b24a8436caecf..7807dfa8c460e79d508dbda2f5c63cbb2665f0bf 100644
--- a/chrome/browser/sync/glue/sync_backend_host.cc
+++ b/chrome/browser/sync/glue/sync_backend_host.cc
@@ -15,6 +15,8 @@
 #include "base/threading/thread_restrictions.h"
 #include "base/tracked.h"
 #include "base/utf_string_conversions.h"
+#include "chrome/browser/about_flags.h"
+#include "chrome/browser/browser_process.h"
 #include "chrome/browser/net/gaia/token_service.h"
 #include "chrome/browser/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile.h"
@@ -25,6 +27,7 @@
 #include "chrome/browser/sync/glue/history_model_worker.h"
 #include "chrome/browser/sync/glue/http_bridge.h"
 #include "chrome/browser/sync/glue/password_model_worker.h"
+#include "chrome/browser/sync/glue/session_data_type_controller.h"
 #include "chrome/browser/sync/glue/sync_backend_host.h"
 #include "chrome/browser/sync/internal_api/base_transaction.h"
 #include "chrome/browser/sync/internal_api/sync_manager.h"
@@ -94,6 +97,7 @@ SyncBackendHost::~SyncBackendHost() {
 
 void SyncBackendHost::Initialize(
     SyncFrontend* frontend,
+    ProfileSyncFactory* factory,
     const WeakHandle<JsEventHandler>& event_handler,
     const GURL& sync_service_url,
     const syncable::ModelTypeSet& initial_types,
@@ -103,6 +107,7 @@ void SyncBackendHost::Initialize(
     return;
 
   frontend_ = frontend;
+  profile_sync_factory_ = factory;
   DCHECK(frontend);
 
   registrar_.workers[GROUP_DB] = new DatabaseModelWorker();
@@ -862,9 +867,46 @@ void SyncBackendHost::Core::HandleSyncCycleCompletedOnFrontendLoop(
 
   host_->last_snapshot_.reset(snapshot);
 
-  const syncable::ModelTypeSet& to_migrate =
+  syncable::ModelTypeSet to_migrate =
       snapshot->syncer_status.types_needing_local_migration;
-  if (!to_migrate.empty())
+  if (sync_manager()->ShouldEnableSessions() && host_->initialized()) {
+    // Have to grab these before we register the sessions DTC.
+    syncable::ModelTypeSet enabled_types;
+    host_->frontend_->GetPreferredDataTypes(&enabled_types);
+    syncable::ModelTypeSet registered_types;
+    host_->frontend_->GetRegisteredDataTypes(&registered_types);
+
+    // Received notice to enable tab (sessions) sync. Check if sessions are
+    // registered, and if not register a new datatype controller.
+    if (registered_types.count(syncable::SESSIONS) == 0) {

[tim (not reviewing), 2011/08/22 15:42:08]

Something about this is circuitous.  First, 'Get'ing things from the
SyncFrontend observer is strange.  Especially since you add a pointer to the
factory here but still need to have a Register callback. Seems like
toe-stepping...  I also think we should avoid having so much sessions-specific
(or any type specific) code here.  All of this if (ShouldEnableSessions) logic
doesn't belong in SyncBackendHost.  Can we send a notification out instead?

[Nicolas Zea, 2011/08/22 22:50:15]

Done.

+      ProfileSyncService* service = profile_->GetProfileSyncService();
+      ProfileSyncFactory* factory = host_->profile_sync_factory_;
+      host_->frontend_->RegisterDataTypeController(
+          new SessionDataTypeController(factory, profile_, service));
+
+      // Check if the user has "Keep Everything Synced" enabled. If so, we want
+      // to turn on sessions if it's not already on. Otherwise we leave it off.
+      // Note: if sessions are already registered, we don't turn it on. This
+      // covers the case where we're already in the process of reconfiguring
+      // to turn sessions on.
+      if (profile_->GetPrefs()->GetBoolean(prefs::kKeepEverythingSynced) &&
+          enabled_types.count(syncable::SESSIONS) == 0){
+        // Enable the about:flags switch for sessions so we don't have to always
+        // perform this reconfiguration. Once we set this, sessions will remain
+        // registered, so we will no longer go down this code path.
+        about_flags::SetExperimentEnabled(g_browser_process->local_state(),
+                                          "sync-sessions",
+                                          true);
+        profile_->GetPrefs()->SetBoolean(prefs::kSyncSessions, true);
+        to_migrate.insert(syncable::SESSIONS);
+      }
+    }
+  }
+
+  // We can only start migration if we have finished initialization. Any
+  // datatypes that need migration currently will still require it after we
+  // finish initializing.
+  if (!to_migrate.empty() && host_->initialized())
     host_->frontend_->OnMigrationNeededForTypes(to_migrate);
 
   // If we are waiting for a configuration change, check here to see