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Subject: Re: [boost] [Boost-users] Brainstorming [WAS: Subject: Formal Review of Proposed Boost.Process library starts tomorrow]
From: Jeremy Maitin-Shepard (jeremy_at_[hidden])
Date: 2011-02-14 15:14:32

On 02/14/2011 11:43 AM, Oliver Kowalke wrote:
> Am 14.02.2011 19:56, schrieb Jeremy Maitin-Shepard:
>> I do not think that a general signal handling library, while potentially
>> useful, is required for Boost.Process to do asynchronous waiting. The
>> reason is that simply letting two independent pieces of code be notified
>> of SIGCHLD is not sufficient, because (in order to be efficient) the
>> SIGCHLD handler needs to then invoke waitpid(-1, ...), which affects the
>> global state of the program. Instead, what is needed is basically a
>> SIGCHLD library, which allows registering a callback to be invoked when
>> a given PID changes state (exited, suspended, or continued). There
>> should be an option to invoke the callback directly in the signal
>> handler, and there should also be a way (possibly built on top of the
>> first invocation method) to invoke a callback asynchronously using ASIO.
>> Ideally, Boost.Process could use this SIGCHLD library by default, but
>> also be able to work with an alternate SIGCHLD multiplexing library.
> This would require that the thread executing async. waiting of
> boost.process has to block all signals except SIGCHLD and the other
> thread handling the other signals must block SIGCHLD and unblock signals
> of interest (SIGTERM, SIGINT, ...). Other worker-threads have to block
> all signals.
> Wouldn't be the async. waiting of boost.process be a subset of the
> signalling library?

I don't think a dedicated thread is necessarily needed at all for
SIGCHLD. Instead, at any point in the program that needs to fork(), use
this sequence:

block SIGCHLD in current thread
wait until a lock is obtained on a signal-safe mutex
(the SIGCHLD handler will also wait to lock the safe mutex; this cannot
deadlock, since the mutex is only locked by threads that have blocked
add child pid to data structure containing the notification list for the
SIGCHLD handler
unlock mutex
reset SIGCHLD blocked status

SIGCHLD handler will repeatedly invoke waitpid(-1, &status, WNOHANG |
WUNTRACED | WCONTINUED), and then invoke any registered handlers. These
handlers need to be designed to be able to run from a signal handler in
an arbitrary thread. If a handler needs to run outside of the signal
handler context, something like ASIO can be notified to invoke the
handler later.

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