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Date:	Fri, 17 Apr 2009 13:47:05 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: [1.7] flock change breaks autotools 'make -j2'
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On Apr 17 12:01, Corinna Vinschen wrote:
> On Apr 16 20:39, Eric Blake wrote:
> > This change in cygwin 1.7:
> > 
> > - File locking is now advisory, not mandatory anymore.  The fcntl(2) and
> >   the new lockf(2) APIs create and maintain locks with POSIX semantics,
> >   the flock(2) API creates and maintains locks with BSD semantics.
> >   POSIX and BSD locks are independent of each other.
> > 
> > is causing parallel make bugs in autotooled projects:
> > 
> > http://lists.gnu.org/archive/html/bug-automake/2009-04/msg00048.html
> > 
> > The problem is that perl's flock is no longer locking files the way it
> > used to, so parallel instances of autom4te (the engine behind both
> > autoconf and automake) end up stomping on each others' output.  I
> > don't know if it something that needs to be fixed in perl, in
> > automake's XFile::lock wrapper around perl's flock, or even in cygwin
> > itself.  But it seems like if two instances of the same perl program
> > are using the same advisory locking mechanism, that they should keep
> > each other out of the file properly (advisory means that processes not
> > using flock can still stomp on the file, but not other processes using
> > the same locking mechanism).
> 
> Default question:  Do you have a simple testcase to reproduce this
> problem?  I'm going to play with the lock.pl perl script from
> http://lists.gnu.org/archive/html/bug-automake/2009-04/msg00053.html but
> I don't know if I can get it working.  Perl isn't exactly my domain.
> I'd prefer a testcase in C.

I found two different bugs in the flock implementation, one of them
seems to be easily fixed (as far as synchronization is easy at all),
the other one is triggered when locking files in threads of the same
process.  This one is a bit more tricky.  I have to re-think parts
of the implementation.  Stay tuned.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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