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

[Angus Salkeld]

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"

-Angus

>
>I may even write the patch.
>But not before May.
>
>And I did not mention the year ;-)
>
>Cheers,
>
>-- 
>: Lars Ellenberg
>: LINBIT | Your Way to High Availability
>: DRBD/HA support and consulting http://www.linbit.com
>
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