[CVS] pidfile support
Henrique de Moraes Holschuh
hmh at debian.org
Tue Dec 17 10:34:36 EST 2002
On Tue, 17 Dec 2002, Rob Siemborski wrote:
> On Tue, 17 Dec 2002, Henrique de Moraes Holschuh wrote:
> > It won't. We will have to transfer the lock to B without a
> > race-condition. That means IPC, or signals, or something like that...
> Yeah, I was discussing this with someone in the office yesterday, and
> there's a half-decent way to do this, but it does requrie signals (well, a
> pipe), and two locks.
Looks good to me... pipes quite sane in most unixes, AFAIK.
> I'm not sure I like using the presence of the pidfile, and this method
> feels a bit kudgy to me (using kill to detect the presence of the process,
Kludgy it is :P but using kill to detect the presence of a process is
actually reasonably portable AFAIK, and safe :-)
> for example). Like you mention, there's also a minor race. I think we
> can do better:
> [A] Create/lock pidfile.lock. If locked, exit(failure).
> [A] Create a pipe
> [A] Fork [B]
> [A] Block on reading exit code from pipe
> [B] Create/lock pidfile If locked, write failure code to pipe and exit(failure)
> [B] write pid to pidfile
> [B] write success code to pipe & finish starting up
> [A] exit(code read from pipe)
> I think this works no matter how the things are interleaved after forking.
I like it. It also allows us to keep [A] around for a little while if we
wish to do so, to detect startup errors of other sorts...
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