[CVS] pidfile support

[Rob Siemborski]

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.

> One other possile way to do it, is to use the _presence_ of the pidfile with
> a running process with that pid (specified in the pidfile) as the lock.  We
> remove the file on exit (clean or otherwise), and we ignore it at startup if
> there is no such a process running in the system.
[snip]

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,
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.

-Rob

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Rob Siemborski * Andrew Systems Group * Cyert Hall 207 * 412-268-7456
Research Systems Programmer * /usr/contributed Gatekeeper