[sync] Purge directory data when migrating to USS

Make use of the existing PurgeEntriesWithTypeIn to purge the directory
of entries of the type we just migrated. Add a test that ensures this
happens.

R=skym@chromium.org

BUG=716156

Review-Url: https://codereview.chromium.org/2842373004
Cr-Commit-Position: refs/heads/master@{#468170}

components/sync/engine_impl/model_type_registry.cc

@@ -82,8 +82,6 @@ void ModelTypeRegistry::ConnectNonBlockingType(
       activation_context->model_type_state.initial_sync_done();
   // Attempt migration if the USS initial sync hasn't been done, there is a
   // migrator function, and directory has data for this type.
-  // Note: The injected migrator function is currently null outside of testing
-  // until issues with triggering initial sync correctly are addressed.
   bool do_migration = !initial_sync_done && !uss_migrator_.is_null() &&
                       directory()->InitialSyncEndedForType(type);
   bool trigger_initial_sync = !initial_sync_done && !do_migration;
@@ -124,17 +122,24 @@ void ModelTypeRegistry::ConnectNonBlockingType(
     // TODO(crbug.com/658002): Store a pref before attempting migration
     // indicating that it was attempted so we can avoid failure loops.
     if (uss_migrator_.Run(type, user_share_, worker_ptr)) {
-      UMA_HISTOGRAM_ENUMERATION("Sync.USSMigrationSuccess", type,
+      UMA_HISTOGRAM_ENUMERATION("Sync.USSMigrationSuccess",
+                                ModelTypeToHistogramInt(type),
                                 MODEL_TYPE_COUNT);
+      // If we succesfully migrated, purge the directory of data for the type.
+      // Purging removes the directory's local copy of the data only.
+      directory()->PurgeEntriesWithTypeIn(ModelTypeSet(type), ModelTypeSet(),
+                                          ModelTypeSet());
     } else {
-      UMA_HISTOGRAM_ENUMERATION("Sync.USSMigrationFailure", type,
+      UMA_HISTOGRAM_ENUMERATION("Sync.USSMigrationFailure",
+                                ModelTypeToHistogramInt(type),
                                 MODEL_TYPE_COUNT);
     }
   }
 
-  // TODO(crbug.com/658002): Delete directory data here if initial_sync_done and
-  // has_directory_data are both true.
-
+  // We want to check that we haven't accidentally enabled both the non-blocking
+  // and directory implementations for a given model type. This is true even if
+  // migration fails; our fallback in this case is to do an initial GetUpdates,
+  // not to use the directory implementation.
   DCHECK(Intersection(GetEnabledDirectoryTypes(), GetEnabledNonBlockingTypes())
              .Empty());
 }

components/sync/engine_impl/model_type_registry_unittest.cc

@@ -75,8 +75,18 @@ class ModelTypeRegistryTest : public ::testing::Test {
     directory()->MarkInitialSyncEndedForType(&trans, type);
   }
 
+  void SetDummyProgressMarkerForType(ModelType type) {
+    sync_pb::DataTypeProgressMarker progress_marker;
+    progress_marker.set_token("dummy");
+    directory()->SetDownloadProgress(type, progress_marker);
+  }
+
   bool migration_attempted() { return migration_attempted_; }
 
+  syncable::MetahandleSet metahandles_to_purge() {
+    return directory()->kernel()->metahandles_to_purge;
+  }
+
  private:
   bool MigrateDirectory(ModelType type,
                         UserShare* user_share,
@@ -208,15 +218,20 @@ TEST_F(ModelTypeRegistryTest, GetInitialSyncEndedTypes) {
 }
 
 // Tests that when directory data is present for type ConnectNonBlockingType
-// triggers USS migration.
-TEST_F(ModelTypeRegistryTest, UssMigrationAttempted) {
+// triggers USS migration and purges old directory data.
+TEST_F(ModelTypeRegistryTest, UssMigration) {
   EXPECT_FALSE(migration_attempted());
 
   MarkInitialSyncEndedForDirectoryType(THEMES);
+  // Purge only proceeds in the presence of a progress marker for the type(s)
+  // being purged.
+  SetDummyProgressMarkerForType(THEMES);
+  EXPECT_EQ(0u, metahandles_to_purge().size());
   registry()->ConnectNonBlockingType(
       THEMES, MakeActivationContext(MakeInitialModelTypeState(THEMES)));
 
   EXPECT_TRUE(migration_attempted());
+  EXPECT_NE(0u, metahandles_to_purge().size());
 }
 
 }  // namespace syncer