[Pacemaker] How to deal with unix signals in a glib mainloop (was: [Problem] The attrd does not sometimes stop.)

[Lars Ellenberg]

On Fri, Jan 20, 2012 at 09:21:58AM +1100, Angus Salkeld wrote:
> On 19/01/12 22:23 +0100, Lars Ellenberg wrote:
> >On Tue, Jan 17, 2012 at 12:13:37AM +0100, Lars Ellenberg wrote:
> >>On Tue, Jan 17, 2012 at 09:52:35AM +1100, Andrew Beekhof wrote:
> >>>
> >>> Ok, done:
> >>>
> >>> https://github.com/beekhof/pacemaker/commit/2a6b296
> >>>
> >>> If I'm adding voodoo, I at least want the reason well documented so it
> >>> can be removed again if the reason goes away.
> >>
> >>That about sums it up, then ;-)
> >
> >But as having to do this was just "too ugly to be true",
> >I dug a little deeper...
> >
> >The way to do this is obviously to use the glib api ;-)
> >http://developer.gnome.org/glib/2.30/glib-UNIX-specific-utilities-and-integration.html#g-unix-signal-add-full
> >
> >(Since glib 2.30, yay; if you don't have that yet, read on anyways)
> >
> >What it does internally, and what other people have also done for a long
> >time  to solve this and similar problems, is:
> >
> >Add to the main context a "wakeup pipe",
> >which is an eventfd if available,
> >or an actual pipe if not.
> >If it is a pipe, set those file descriptors non-blocking.
> >And, of course, add the evenfd (or the read end of the pipe)
> >to the poll loop (with default priority, btw,
> >which is good enough to have the poll terminate).
> >
> >That is done internally when creating the main context.
> >http://git.gnome.org/browse/glib/tree/glib/gmain.c#n548
> >http://git.gnome.org/browse/glib/tree/glib/gwakeup.c#n138
> >
> >(the line numbers are correct for glib master as of today,
> >which should correspond to 41fbf42)
> >
> >The g_unix_signal_handler then sets the triggers variables,
> >and calls g_wakeup_signal(that internal wakeup source),
> >which simply posts and event to the eventfd,
> >or does a short (1 byte) write to the write end of the pipe.
> >http://git.gnome.org/browse/glib/tree/glib/gmain.c#n4442
> >http://git.gnome.org/browse/glib/tree/glib/gwakeup.c#n230
> >
> >Problem solved, without having to do a full check() everything,
> >prepare() everything, and poll() again cycle every 500ms.
> >
> >"back in those days", when this mechanism was not really there,
> >you could do all that "by hand".
> >And people did. Very common idiom in glib and other mainloop
> >applications, also frequently used to "signal" availability of work
> >or completion of tasks between threads.
> >
> >static int my_wakeup_fds[2] = { -1, -1 };
> >
> >Then just pipe2(my_wakeup_fds, O_NONBLOCK), add my_wakeup_fds[0] as
> >normal read fd source, and add a write(my_wakeup_fds[1], "", 1); to the
> >signal handlers.
> 
> signalfd makes this much easier too "man 2 signalfd"

See https://bugzilla.gnome.org/show_bug.cgi?id=652072#c32
following (or the whole bug, if you like).

Also, pipes are portable.

	Lars