[PSS] Fix a broken invariant aroung !is_stopping_and_clearing_

This CL fixes a broken invariant that is_stopping_and_clearing is false
whenever StopAndClear() gets called. This does not have to be true and
this CL treats this special case without talking to UserSettings.

Bug: 928216
Change-Id: I4d6d9cf471d4327035f29d06635f340bfb707861
Reviewed-on: https://chromium-review.googlesource.com/c/1452002
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629155}

components/browser_sync/profile_sync_service.cc

@@ -1908,12 +1908,25 @@ bool ProfileSyncService::IsSyncAllowedByFlag() {
 
 void ProfileSyncService::StopAndClear() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  DCHECK(!is_stopping_and_clearing_);
+
+  // This can happen if the user had disabled sync before and is now setting up
+  // sync again but hits the "Cancel" button on the confirmation dialog.
+  // TODO(crbug.com/906034): Maybe we can streamline the defaults and the
+  // behavior on setting up sync so that either this whole early return goes
+  // away or it treats all "Cancel the confirmation" cases?
+  if (!user_settings_->IsSyncRequested()) {
+    StopImpl(CLEAR_DATA);
+    return;
+  }
+
   // We need to remember that clearing of data is needed when sync will be
   // stopped. This flag is cleared in OnSyncRequestedPrefChange() where sync
-  // gets stopped.
+  // gets stopped. This happens synchronously when |user_settings_| get changed
+  // below.
+  DCHECK(!is_stopping_and_clearing_);
   is_stopping_and_clearing_ = true;
   user_settings_->SetSyncRequested(false);
+  DCHECK(!is_stopping_and_clearing_);
 }
 
 void ProfileSyncService::ReconfigureDatatypeManager(

components/browser_sync/profile_sync_service_unittest.cc

@@ -951,6 +951,28 @@ TEST_F(ProfileSyncServiceWithStandaloneTransportTest, ClearDataOnSignOut) {
   EXPECT_TRUE(local_device_info_provider()->GetLocalDeviceInfo());
 }
 
+TEST_F(ProfileSyncServiceWithStandaloneTransportTest, CancelSyncAfterSignOut) {
+  SignIn();
+  CreateService(ProfileSyncService::AUTO_START);
+  InitializeForNthSync();
+  ASSERT_EQ(syncer::SyncService::TransportState::ACTIVE,
+            service()->GetTransportState());
+  base::Time last_synced_time = service()->GetLastSyncedTime();
+  ASSERT_LT(base::Time::Now() - last_synced_time,
+            base::TimeDelta::FromMinutes(1));
+
+  // Disable sync.
+  service()->StopAndClear();
+  EXPECT_FALSE(service()->IsSyncFeatureEnabled());
+
+  // Calling StopAndClear while already stopped should not crash. This may
+  // (under some circumstances) happen when the user enables sync again but hits
+  // the cancel button at the end of the process.
+  ASSERT_FALSE(service()->GetUserSettings()->IsSyncRequested());
+  service()->StopAndClear();
+  EXPECT_FALSE(service()->IsSyncFeatureEnabled());
+}
+
 // Verify that credential errors get returned from GetAuthError().
 TEST_F(ProfileSyncServiceTest, CredentialErrorReturned) {
   // This test needs to manually send access tokens (or errors), so disable