Date: Thu, 13 Oct 2011 09:21:11 -0300 From: "Carlos A. M. dos Santos" <unixmania@gmail.com> To: =?ISO-8859-1?Q?Dag=2DErling_Sm=F8rgrav?= <des@des.no> Cc: Pawel Jakub Dawidek <pjd@freebsd.org>, current@freebsd.org Subject: Re: incorrect use of pidfile(3) Message-ID: <CAJ4jsadYrZuJ5fMz_YkphVhX%2B5gE0gFOm5ZY_kZ_SRbSz7PkDQ@mail.gmail.com> In-Reply-To: <86botlaz41.fsf@ds4.des.no> References: <86pqi1b1qp.fsf@ds4.des.no> <20111013113024.GE1667@garage.freebsd.pl> <86botlaz41.fsf@ds4.des.no>
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2011/10/13 Dag-Erling Smørgrav <des@des.no>:
> Pawel Jakub Dawidek <pjd@FreeBSD.org> writes:
>> Dag-Erling Smørgrav <des@des.no> writes:
>> > How do we fix this? My suggestion is to loop until pidfile_open()
>> > succeeds or errno != EAGAIN. Does anyone have any objections to that
>> > approach?
>> I think we already do that internally in pidfile_open(). Can you take a look at
>> the source and confirm that this is what you mean?
>
> No, it doesn't; pidfile_open(3) returns NULL with errno == EAGAIN if the
> pidfile is locked but empty, as is the case in the window between a
> successful pidfile_open(3) and the first pidfile_write(3). This is
> documented in the man page:
>
> [EAGAIN] Some process already holds the lock on the given pid‐
> file, but the file is truncated. Most likely, the
> existing daemon is writing new PID into the file.
>
> I have a patch that adds a pidfile to dhclient(8), where I do this:
>
> for (;;) {
> pidfile = pidfile_open(path_dhclient_pidfile, 0600, &otherpid);
> if (pidfile != NULL || errno != EAGAIN)
> break;
> sleep(1);
> }
> if (pidfile == NULL) {
> if (errno == EEXIST)
> error("dhclient already running, pid: %d.", otherpid);
> warning("Cannot open or create pidfile: %m");
> }
>
> I'm not sure I agree with the common idiom (which I copied here) of
> ignoring all other errors than EEXIST, but that's a different story.
You are also ignoring the return value of sleep(1), which would tell
you if the call was interrupted by a signal handler. This can be fine
for dhclient(8) but other utilities might require some guards against
such interruptions.
--
"The flames are all long gone, but the pain lingers on"
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