[4.0] Fix possible segfault switching between read/write windows

[heifner]

• Always execute the lambda after pop()ing it from the queue. This is needed as the read_write queue is used to post the lambda that switches to the read window. The read window can then start and begin pushing future tasks to the read_write queue. This allows multiple threads to access the read_write at one time as the original call to switch to the read window is running unlocked, once it finishes it pops from the queue while other threads can access it. Backport of Fix: segfault switching between read/write windows #1718
• Fix for producer shutdown to prevent segfault and deadlock on shutdown. Backport of 1368ecd

Backport of #1718

Resolves #1490

[greg7mdp]

No need to change, but just to chat I would have written it like that.

Suggested change

	bool execute_highest()
	{
	if( !handlers_.empty() ) {
	auto t = pop();
	bool empty = handlers_.empty();
	t->execute();
	return !empty;
	}
	return false;
	}
	bool execute_highest()
	{
	bool empty = handlers_.empty();
	if( !empty ) {
	auto t = pop();
	empty = handlers_.empty();
	t->execute();
	}
	return !empty;
	}

[spoonincode]

there doesn't appear to be a need for the second ro_ec here

[spoonincode]

Can you explain this line some more? In what cases would we make it here and the appbase executor would still be running?

[heifner]

The appbase executor is not running here. However, the read-only threads can still be running and some can be paused on the condition variable in exec_pri_queue, trying to then join their threads cause them to deadlock.