Fix crash in ProfileSyncService::GetDisableReasons()

PeopleHandler somehow gets destroyed after ProfileSyncService shutdown,
so |auth_manager_| is nullptr during its destruction. This caused the
crash in GetDisableReasons().

Fix is to simply check |auth_manager_| before dereferencing. Avoiding
PSS usages after shutdown would be cleaner fix, but it's not clear why
they happen atm.

Bug: 1043642, 906995
Change-Id: Id6d54a2edee1210a579e36b2b771d360c8c40474
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2011220
Commit-Queue: Maksim Moskvitin <mmoskvitin@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733582}

components/sync/driver/profile_sync_service.cc

@@ -694,7 +694,6 @@ SyncService::DisableReasonSet ProfileSyncService::GetDisableReasons() const {
   // If Sync is disabled via command line flag, then ProfileSyncService
   // shouldn't even be instantiated.
   DCHECK(switches::IsSyncAllowedByFlag());
-
   DisableReasonSet result;
   if (!user_settings_->IsSyncAllowedByPlatform()) {
     result.Put(DISABLE_REASON_PLATFORM_OVERRIDE);
@@ -715,7 +714,11 @@ SyncService::DisableReasonSet ProfileSyncService::GetDisableReasons() const {
     result.Put(DISABLE_REASON_UNRECOVERABLE_ERROR);
   }
   if (base::FeatureList::IsEnabled(switches::kStopSyncInPausedState)) {
-    if (auth_manager_->IsSyncPaused()) {
+    // Some crashes on Chrome OS (crbug.com/1043642) suggest that
+    // ProfileSyncService gets called after its shutdown. It's not clear why
+    // this actually happens. To avoid crashes check that |auth_manager_| isn't
+    // null.
+    if (auth_manager_ && auth_manager_->IsSyncPaused()) {
       result.Put(DISABLE_REASON_PAUSED);
     }
   }

components/sync/driver/profile_sync_service_unittest.cc

@@ -323,6 +323,19 @@ class ProfileSyncServiceTest : public ::testing::Test {
   std::unique_ptr<ProfileSyncService> service_;
 };
 
+class ProfileSyncServiceTestWithStopSyncInPausedState
+    : public ProfileSyncServiceTest {
+ public:
+  ProfileSyncServiceTestWithStopSyncInPausedState() {
+    override_features_.InitAndEnableFeature(switches::kStopSyncInPausedState);
+  }
+
+  ~ProfileSyncServiceTestWithStopSyncInPausedState() override = default;
+
+ private:
+  base::test::ScopedFeatureList override_features_;
+};
+
 // Gets the now time used for testing user demographics.
 base::Time GetNowTime() {
   base::Time now;
@@ -1525,5 +1538,16 @@ TEST_F(ProfileSyncServiceTest, GetExperimentalAuthenticationKeyLocalSync) {
   EXPECT_FALSE(service()->GetExperimentalAuthenticationKey());
 }
 
+// Regression test for crbug.com/1043642, can be removed once
+// ProfileSyncService usages after shutdown are addressed.
+TEST_F(ProfileSyncServiceTestWithStopSyncInPausedState,
+       ShouldProvideDisableReasonsAfterShutdown) {
+  CreateService(ProfileSyncService::AUTO_START);
+  InitializeForFirstSync();
+  SignIn();
+  service()->Shutdown();
+  EXPECT_FALSE(service()->GetDisableReasons().Empty());
+}
+
 }  // namespace
 }  // namespace syncer