Re: maintainer mode parallel make issue

[Ralf Wildenhues]

Hi Eric,

* Eric Blake wrote on Mon, Apr 20, 2009 at 06:44:17PM CEST:
> Ralf Wildenhues <Ralf.Wildenhues <at> gmx.de> writes:
> 
> > You can try this; test against git next of Automake.
> > However, it also causes a hang on Cygwin, so there may be an independent
> > bug (in the test or in perl); the lock() called from the thread seems to
> > never complete.
> 
> Hmm.  On some systems flock(2) is documented as being per-process only (that 
> is, you can't lock a file from yourself in a multi-threaded app[1]).

Oh.  I didn't know that.  My test was a rather quick hack anyway, good
to have a good reason now not to commit it.

> I'm not 
> sure if flockfile(3) (at the FILE*) level, or fcntl(2)/lockf() at the fd 
> level, 
> do any better, but that doesn't matter since perl's flock states that whether 
> it was implemented with flock or fcntl depends on how perl was configured.

It doesn't really matter: nothing within Automake requires file locking
at the perl threads level.  Well, it may matter for the implementation
issues that the Cygwin developers work on.

> Therefore, is this test even valid, since you did it via perl threads as 
> opposed to independent processes?

Likely not.

> But the point remains that we DO want perl's 
> flock to work for cross-process locking (automake and autoconf both need a 
> lock 
> on autom4te.cache/requests, to block whichever task starts late).

Yes, definitely.  And a testsuite addition to Autoconf or Automake to
ensure this would be good, I guess.

Thanks for doing the research!

Cheers,
Ralf