Re: flock in 2.20.1 can no longer lock the file it will execute

[Karel Zak <kzak@redhat.com>]

On Wed, Nov 16, 2011 at 10:25:46AM -0500, Mike Frysinger wrote:
> On Wednesday 16 November 2011 08:13:21 Karel Zak wrote:
> > On Mon, Nov 14, 2011 at 01:23:38AM -0500, Mike Frysinger wrote:
> > > i've been using a trick with flock to add locking to all of my shell
> > > scripts.
> > > 
> > > basically, i take a lock on the shell script itself:
> > > 	flock -eon ./test.sh ./test.sh
> > > 
> > > with <=util-linux-2.20, this has worked fine.  but starting with 2.20.1,
> > > i now
> > > 
> > > get -ETXTBSY (on ext4, but i doubt that matters):
> > > 	$ echo '#!/bin/sh' > test.sh
> > > 	$ chmod a+rx test.sh
> > > 	$ ./flock -eon ./test.sh ./test.sh
> > > 	./flock: ./test.sh: Text file busy
> > > 
> > > the only commit made to flock.c between 2.20 and 2.20.1 is this:
> > > 	commit 75aaee08f06b92d119ed827c53d1af5474eb16ff
> > > 	flock: make flock(1) work on NFSv4
> > > 
> > > and indeed, reverting that made my life happy again.  reading the small
> > > patch shows the obvious flaw: you can't open a file for O_RDWR and
> > > attempt to execute it at the same time.
> > 
> >  Hmm... maybe we can add a new --read-write command line option for
> >  NFS guys rather than be smart with access(). The regression is
> >  unacceptable.
> 
> i wonder what happens if you attempt to take a read-only lock on nfs.  do you 
> get back an error ?

 yes, EIO 

> if the former, one solution might be to open the file, attempt to grab the 
> lock, and if the flock() fails, retry the whole thing but with O_RDWR instead.
> 
> int rw_flags = O_RDONLY;
> ...
> retry:
> 	... current open logic which uses new rw_flags var ...
> 
> 	... if flock fails, and rw_flags == O_RDONLY, then set rw_flags to O_RDWR 
> and jump back to retry ...

 Good idea, implemented and tested (git pull to get the change). It
 seems it works with your example as well as with NFSv4.
 
    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com