(Originally reported in https://bugs.chromium.org/p/chromium/issues/detail?id=879503#c4)

The following DCHECK can fire if data type configuration fails:

[9616:9616:0831/170337.544315:FATAL:profile_sync_service.cc(854)] Check failed: !CanConfigureDataTypes(). 
#0 0x7fb378ce94ec base::debug::StackTrace::StackTrace()
#1 0x7fb378c151ab logging::LogMessage::~LogMessage()
#2 0x55bd2bd66926 browser_sync::ProfileSyncService::GetTransportState()
#3 0x55bd2b40c93d syncer::SyncService::IsSyncActive()
#4 0x55bd2c29cb86 RecentTabsSubMenuModel::BuildTabsFromOtherDevices()
#5 0x55bd2c29dd06 RecentTabsSubMenuModel::OnForeignSessionUpdated()
#6 0x55bd2bd69596 browser_sync::ProfileSyncService::OnConfigureDone()
#7 0x55bd2bd3625b syncer::DataTypeManagerImpl::NotifyDone()
#8 0x55bd2bd377ee syncer::DataTypeManagerImpl::Abort()
#9 0x55bd2bd374e9 syncer::DataTypeManagerImpl::OnModelAssociationDone()
#10 0x55bd2bd3c8ce syncer::ModelAssociationManager::ModelAssociationDone()
#11 0x55bd2bd3ce11 syncer::ModelAssociationManager::TypeStartCallback()

The problem is that ProfileSyncService::OnConfigureDone calls OnSyncConfigurationCompleted on its observers, a) even if the configuration failed (which is questionable), and b) before updating its own state, i.e. handling the error. If one of those observer callbacks triggers a call to GetTransportState, then this DCHECK will trigger due to the (temporary) inconsistent state.

Possible solutions:
1) Don't call OnSyncConfigurationCompleted if it didn't succeed.
2) Call OnSyncConfigurationCompleted only after handling the error.
3) Remove the DCHECK.

The intended semantics of OnSyncConfigurationCompleted aren't really clear to me, so I don't know yet whether 1) makes sense. 2) should definitely be possible. I wouldn't like to remove the DCHECK because of this corner case.