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Date: Mon, 20 Apr 2009 17:34:42 +0200
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: [1.7] flock change breaks autotools 'make -j2'
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On Apr 20 15:14, Eric Blake wrote:
> Corinna Vinschen <corinna-cygwin <at> cygwin.com> writes:
> 
> > > > I'd prefer a testcase in C.
> > > 
> > > So would I.  I'm not even sure whether perl was using flock or
> > > lockf/fcntl.  Do you still need me to try and write a STC, or at least
> > > test how perl behaves with your first patch?
> > 
> > I checked in a patch which hopefully solves both problems.  The lock.pl
> > testcase now works, at least.  Can you test if the original scenario
> > works now as well?  If not, I'd really need another testcase.
> 
> Nope; with snapshot 20090418 I'm still seeing the 'make -j2' failures.  I'll 

Too bad.

> see if I can come up with a C program that can be run as two processes to 
> simulate the failure as a simpler test case.  I did verify via strace that perl 
> is using flock(fd, LOCK_EX).

Yes, I saw this already while debugging the lock.pl testcase.

Whatever it is, it must be more complicated than this:

    #include <stdio.h>
    #include <sys/file.h>

    int
    main ()
    {
      int fd = open ("flock.c", O_RDWR);
      if (fd >= 0)
	{
	  printf ("About to call flock\n");
	  if (!flock (fd, LOCK_EX))
	    {
	      printf ("flock succeeded.  Waiting...\n");
	      getchar ();
	      close (fd);
	    }
	  else
	    perror ("flock");
	}
      return 0;
    }

Because this testcase works fine.  I hope it's not trying to do this:

  parent opens file
  fork
    child calls flock()
    exit
  fork
    second child relies on the lock.

because this is exactly the scenario which doesn't work with flock
right now.  I have it on my TODO list, but so far I'm at a loss as
to how to implement it.


Corinna

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

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