On Tue, Jan 15, 2008 at 10:50:34AM -0500, Chris Shoemaker wrote:
> On Tue, Jan 15, 2008 at 05:06:27AM +0100, Marc Lehmann wrote:
> > On Mon, Jan 14, 2008 at 10:56:54AM -0500, Chris Shoemaker <[EMAIL
> > PROTECTED]> wrote:
> > > I believe the attached program demonstrates some bug related to signal
> > > delivery after a fork.
> >
> > Oh, what you see is that ev_default_fork only sets a flag for the next run
> > of ev_loop. You will have to run ev_loop to reinitialise the kernel state
> > after a fork (e.g. ev_loop (EVLOOP_NONBLOCK) will do).
> >
> > The documentation will point this out in the next release (and it will
> > contain other things required by kqueue, which makes it less flexible).
>
> Adding:
>
> ev_loop(loop, EVLOOP_NONBLOCK);
>
> immediately after the call to ev_default_fork() did not noticeably
> change the behavior of the program. It still hangs about about 10% of
> the time.
I've narrowed this down considerably by tracing both good and bad
executions and comparing. I'll comment the differences in the code:
******
if ((pid = fork()) == 0) {
close(pdes[0]);
ev_default_fork();
ev_loop(loop, EVLOOP_NONBLOCK);
ev_signal_init (&signal_watcher, signal_cb, SIGHUP);
ev_signal_start (loop, &signal_watcher);
write(pdes[1], "baz", 4);
close(pdes[1]);
/* In a good run, the child will enter the ev_loop and block before
receiving the SIGHUP. Then it notifies the watcher correctly.
In a bad run, the child receives the SIGHUP before this ev_loop
blocks, notices the SIGHUP, but never wakes the watcher. */
ev_loop(loop, 0);
exit(99);
} else {
close(pdes[1]);
read(pdes[0], buf, 4); /* After this point, the ev_signal has started */
close(pdes[0]);
kill(pid, SIGHUP);
res = waitpid(pid, &status, 0);
printf("Result = %d, ExitStatus = %d\n", res, WEXITSTATUS(status));
}
********
Looking at ev.c, it seems there is a race between the first active
watcher and the delayed fork-fixup function. Ah, yes, there it is.
The following patch fixes it, and my test program no longer hangs.
-chris
----
diff --git a/shotgun/external_libs/libev/ev.c b/shotgun/external_libs/libev/ev.c
index b21b1a4..f8357b8 100644
--- a/shotgun/external_libs/libev/ev.c
+++ b/shotgun/external_libs/libev/ev.c
@@ -1534,13 +1534,13 @@ ev_loop (EV_P_ int flags)
call_pending (EV_A);
}
- if (expect_false (!activecnt))
- break;
-
/* we might have forked, so reify kernel state if necessary */
if (expect_false (postfork))
loop_fork (EV_A);
+ if (expect_false (!activecnt))
+ break;
+
/* update fd-related kernel structures */
fd_reify (EV_A);
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