Re: [Proposed Fix] daemon.c: not initializing revents
From: Junio C Hamano <hidden>
Date: 2019-02-11 20:57:00
"Randall S. Becker" [off-list ref] writes:
Hi All, I found this while trying to track down a hang in t5562 - this isn't the fix, but here it is something that could be considered a code-inspection. If there have been random unexplained hangs when git runs as a daemon, this might be the cause. According to many systems (other than Linux), the revents field is supposed to be 0 on return to poll(). This was the cause of some heart-ache a while back in compat/poll/poll.c.
I am having a hard time grokking "supposed to be 0 on return to", but do you mean "the caller must clear .revents field before calling poll()"? http://pubs.opengroup.org/onlinepubs/9699919799/functions/poll.html has this In each pollfd structure, poll() shall clear the revents member, except that where the application requested a report on a condition by setting one of the bits of events listed above, poll() shall set the corresponding bit in revents if the requested condition is true. In addition, poll() shall set the POLLHUP, POLLERR, and POLLNVAL flag in revents if the condition is true, even if the application did not set the corresponding bit in events. and I am also having a hard time interpreting the "except that". If we asked to report (e.g. we set POLLIN in the .events field), poll() does not have to clear .revents but just set whatever bits it needs to set to report the condition in the field? If that is the case, it makes it a bug not to clear .revents before calling poll; the sample code snippet on the same page in EXAMPLES section does not, though, so I am puzzled. In any case, no matter what POSIX says, if clearing .revents before calling poll() helps on platforms in the real world, the patch is worth taking as a fix, I would think.
quoted hunk
I am not certain whether that copy of poll() is used in daemon, but I wanted to point out that the value is being returned to poll, outside of compat/poll/poll.c and may present a potential for poll returning an error on that FD due to random values that might be in revents. Please see 61b2a1acaae for a related change/justification.diff --git a/daemon.c b/daemon.c index 9d2e0d20ef..1e275fc8b3 100644 --- a/daemon.c +++ b/daemon.c@@ -1194,6 +1194,7 @@ static int service_loop(struct socketlist *socklist) } handle(incoming, &ss.sa, sslen); } + pfd[i].revents = 0; } } }Regards, Randall