Re: [Boost-users] asio: transporting exceptions across threads

Subject: Re: [Boost-users] asio: transporting exceptions across threads
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-07-21 17:16:47

On Tue, Jul 21, 2009 at 3:38 PM, Emil
Dotchevski<emildotchevski_at_[hidden]> wrote:
> I have never used boost::asio but a complete example of how to use
> boost::exception_ptr can be found here (see Cloning and Re-Throwing an
> Exception) :
>
> http://www.boost.org/doc/libs/release/libs/exception/doc/tutorial_exception_ptr.html
>
> Let me know if this works for you.
>
> Emil Dotchevski
> Reverge Studios, Inc.
> http://www.revergestudios.com/reblog/index.php?n=ReCode

Yea that was the code I was referring to when I was saying I'd looked
over the tutorial code in the docs.  The problem is that it's
difficult to explain what the problem is.

Essentially I have N threads.  1 of these we can consider the "master"
thread, which is also the main application thread.  The other threads
are all worker threads in some form or another.  In order to "start"
asio, you at some point run this function which is essentially an
event loop that doesn't return until it runs out of work to do, or
until an exception propagates outside of the event loop.

The model in place is that you run this event loop in each thread that
is supposed to participate in asio.  So, with the exception of the
main application thread, I've created all of these worker threads with
a very simple thread start routine function that all it does is run
the event loop.

In the example in the documentation, it relies on the fact that there
is a well-defined sequence point at which the main thread can call
thread::join on the child thread.  But this doesn't really happen in
asio.  Nothing in the parent thread's handlers or worker code has any
idea that other threads are running at all.  I can let the exception
propagate all the way out of my worker thread's event loop and have
the thread function just return, but then the parent thread hangs
because it posts a message to this service and the service just
doesn't respond.

It's also not clear how such error handling will play well with the
fact that there's many different threads running.  For example, I
encounter the error in worker thread B, and thread A is trying to post
a message to thread B at the same time.  If I simply shut thread B's
service down when I encounter the error, is this goign to exhibit
well-defined behavior?

I've thought of a couple of possible ideas, but all of them seem
hackish and I'm not sure how well they'll scale.  Some of them I've
tried, but then asio encounters weird issues in the destructor of one
of its internal classes called service_registry.

Was hoping someone had encountered this before and come up with something.