Sync: don't start up again (in transport mode) after StopAndClear

This is essentially a revert of crrev.com/c/1082267 (undoing the actual
1-line change from that CL, but keeping the comment improvements).
This is a temporary measure to investigate crbug.com/1120899.

Bug: 1120899, 1082267
Change-Id: I7761418c038824218a7fc36b325f1b364f7bbbe5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401460
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805489}

components/sync/driver/profile_sync_service.cc

@@ -59,6 +59,9 @@ namespace syncer {
 
 namespace {
 
+constexpr base::Feature kSyncRestartAfterStopAndClear{
+    "SyncRestartAfterStopAndClear", base::FEATURE_DISABLED_BY_DEFAULT};
+
 // The initial state of sync, for the Sync.InitialState histogram. Even if
 // this value is CAN_START, sync startup might fail for reasons that we may
 // want to consider logging in the future, such as a passphrase needed for
@@ -1855,8 +1858,12 @@ void ProfileSyncService::StopAndClear() {
   // away or it treats all "Cancel the confirmation" cases?
   if (!user_settings_->IsSyncRequested()) {
     StopImpl(CLEAR_DATA);
-    // Try to start up again (in transport-only mode).
-    startup_controller_->TryStart(/*force_immediate=*/true);
+    // TODO(crbug.com/1120899): Doing this might cause some clients to enter a
+    // persistent error state.
+    if (base::FeatureList::IsEnabled(kSyncRestartAfterStopAndClear)) {
+      // Try to start up again (in transport-only mode).
+      startup_controller_->TryStart(/*force_immediate=*/true);
+    }
     return;
   }
 

components/sync/driver/profile_sync_service_startup_unittest.cc

@@ -413,7 +413,8 @@ TEST_F(ProfileSyncServiceStartupTest, DisableSync) {
   // mode. It should immediately start up again in transport mode.
   SetUpFakeSyncEngine();
   data_type_manager = SetUpDataTypeManagerMock();
-  EXPECT_CALL(*data_type_manager, Configure(_, _));
+  // TODO(crbug.com/1082267, crbug.com/1120899): Reinstate this expectation.
+  // EXPECT_CALL(*data_type_manager, Configure(_, _));
   sync_service()->StopAndClear();
 }