Version: ToT
OS: ChromeOS

This happens only when ArcAuthService::reenable_arc_ is true without data removal, AFAIK. Also, AFAIK, there should be no use case.

How it would happen:
- ARC container is terminated.
- DBus signal is sent.
- SessionManagerClient calls ArcSession::ArcInstanceStopped(), as a part of ObserverList call.
- ArcSessionImpl::OnStopped()
- ArcBridgeServiceImpl::OnStopped()
  Old ArcSessionImpl instance is deleted here.
- ArcAuthService::OnBridgeStopped()
  In this function, if data removal is not run then OnArcDataRemoved(true) is called.
- ArcAuthService::OnArcDataRemoved(true):
  If reenable_arc_ is true, EnableArc() is called, then in enable case,
- ArcAuthService::EnableArc()
- ArcAuthService::OnOptInPreferenceChanged()
- ArcAuthService::StartArc()
- ArcBridgeServiceImpl::RequestStart()
- ArcBridgeServiceImpl::PrerequisitesChanged()
- new ArcSessionImpl is created.
- In ctor of ArcSessionImpl, the instance is registered to SessionManagerClient.

The above procedure happens on one stack, so after returning from ArcSessionImpl::OnStopped(),
SessionManagerClient tries to call the remaining observers, which contains new ArcSessionImpl instance.

So, new ArcSessionImpl instance's ArcInstanceStopped() is called, then the ARC restarting is stopped here.


* Possible solutions:
There could be various approaches for the fix.
1) Set NOTIFY_EXISTING_ONLY to the SessionManagerClient's observer_list.
  pros: simplest. cons: large side effect.
2) Inject intermediate class between SessionManagerClient and ArcSessionImpl, which support ObserverList with NOTIFY_EXISTING_ONLY.
  pros: less risky. cons: yet another layer is needed.
3) Return some ID to indicate individual ARC container instance (e.g. PID) via DBus, and route the message to the ArcInstanceStopped() properly.
  pros: in future, we may want to do this for multi-container support. cons: DBus API change is needed.
4) (Short time workaround) PostTask() onto the same thread in this case.
  pros: no worry about regression in unrelated place. cons: looks a bit tricky. Not a general solution.
5) Others.

Ryo, Luis, WDYT?

Note: For short-term workaround, I'll use 4).