incorrect use of pidfile(3)

[Dag-Erling Smørgrav]

Pawel Jakub Dawidek <pjd at FreeBSD.org> writes:
> Dag-Erling Smørgrav <des at 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.

DES
-- 
Dag-Erling Smørgrav - des at des.no