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Date:	Mon, 10 Feb 2014 11:48:40 +0100
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: spawnv() unlocks files in the calling program
Message-ID:	<20140210104840.GA31131@calimero.vinschen.de>
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On Feb  9 21:38, Corinna Vinschen wrote:
> On Feb  9 15:06, Steven Bardwell wrote:
> > > How do you test that?  You're calling fcntl(F_SETLKW) exactly once at
> > > the start of your test application, but never again later.  We're
> > > talking advisory file locking here, so, where's the next fcntl call
> > > waiting for the lock?
> > >=20
> > > I debugged your test app and the lock still exists after the spawn ca=
ll.
> > >=20
> > >=20
> > >=20
> > > Corinna
> >=20
> > To test this, I start this program to check for the lock:
>=20
> Ok, I can reproduce it, but it's too late to debug this today.
>=20
> I have to say, though, that fcntl advisory locking is POSIX
> functionality, while the spawn functions are not.  In fact these dreaded
> spawn entry points are rather old stuff, which hasn't been tested for a
> long time.  FWIW, advisory file locking has never been tested with them,
> and the fact that it doesn't work as expected doesn't exactly disturb me.

I found the reason.  On UNIX, an exec'ed process substitutes the calling
process.  When that happens, fcntl locks are inherited by the child.
The internal implementation of exec and spawn share a lot of code.  One
of the code bits is the function which hands over the fcntl locks from
parent to child.  So after spawn, the owner of the lock is not the
parent anymore, but the child process.  After the child process died,
the lock is abandoned and your 2nd test application rightfully claims
that the file region is unlocked.

I applied a patch which now differs between exec and spawn in the
function handling the inheritance of locks in the child.  If it has been
spawned, it does not take over POSIX locks anymore, but rather it just
gives them up in favor of the parent process.

The current snapshot at http://cygwin.com/snapshots/ does not have this
patch, but we will certainly produce another snapshot in the next few
days.


HTH,
Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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