netty/netty — pull request #760 by trustin (2012-11-22 12:13:25+00:00)
Title: Reduce the chance of RejectedExecutionException

When a Netty application shuts down, a user often sees a REE
(RejectedExecutionException).

A REE is raised due to various reasons we don't have control over, such
as:

- A client connects to a server while the server is shutting down.

- An event is triggered for a closed Channel while its event loop is
also shutting down. Some of them are:
- channelDeregistered (triggered after a channel is closed)
- freeIn/OutboundBuffer (triggered after channelDeregistered)
- userEventTriggered (triggered anytime)

To address this issue, a new method called confirmShutdown() has been
added to SingleThreadEventExecutor. After a user calls shutdown(),
confirmShutdown() runs any remaining tasks in the task queue and ensures
no events are triggered for last 2 seconds. If any task are added to
the task queue before 2 seconds passes, confirmShutdown() prevents the
event loop from terminating by returning false.

Now that SingleThreadEventExecutor needs to accept tasks even after
shutdown(), its execute() method only rejects the task after the event
loop is terminated (i.e. isTerminated() returns true.) Except that,
there's no change in semantics.

SingleThreadEventExecutor also checks if its subclass called
confirmShutdown() in its run() implementation, so that Netty developers
can make sure they shut down their event loop impementation correctly.

It also fixes a bug in AioSocketChannel, revealed by delayed shutdown,
where an inboundBufferUpdated() event is triggered on a closed Channel
with deallocated buffers.

Caveats:

Because SingleThreadEventExecutor.takeTask() does not have a notion of
timeout, confirmShutdown() adds a dummy task (WAKEUP_TASK) to wake up
takeTask() immediately and instead sleeps hard-coded 100ms. I'll
address this issue later by modifying takeTask() times out dynamically.

Miscellaneous changes:

SingleThreadEventExecutor.wakeup() now has the default implementation.
Instead of interrupting the current thread, it simply adds a dummy task
(WAKEUP_TASK) to the task queue, which is more elegant and efficient.
NioEventLoop is the only implementation that overrides it. All other
implementations' wakeup()s were removed thanks to this change.