Shutdown hook should grab the lock if there is one

[duke8253]

This was causing some crashes during shutdown.

[SolidWallOfCode]

Talk to @shinrich - she had the same problem and we tweaked a call (I think the mutex locking) to transparently deal with null mutexes.

Also, please work with @dyrock on plugin coordination - one of the points of that is to unify this kind of code so bugs like this don't have to stepped on one place at a time.

[shinrich]

Yes, we tidied up the mutex try lock to gracefully handle the null mutex case. Need to remember where we made that change.
Functionally, this fix does address the missing lock acquisition problem.

[SolidWallOfCode]

See #4926.

[SolidWallOfCode]

Why not

while (hook) {
  SCOPED_MUTEX_LOCK(lock, hook->m_cont->mutex, this_ethread());
  hook->invoke(TS_EVENT_LIFECYCLE_SHUTDOWN, nullptr);
  hook = hook->next;
}

?

[ywkaras]

So the lock object will be a harmless do-nothing if the mutex is null?

[SolidWallOfCode]

Yes. It was designed that way precisely to get this effect. Better to check the null in one place, and not every callsite.

[ywkaras]

To be OCD about not holding mutexes longer than necessary, I would prefer:

while (hook) {
  {
    SCOPED_MUTEX_LOCK(lock, hook->m_cont->mutex, this_ethread());
    hook->invoke(TS_EVENT_LIFECYCLE_SHUTDOWN, nullptr);
  }
  hook = hook->next;
}

[duke8253]

It's during shutdown, I think I want it to hold it for longer, might expose other problems in the code.