fcntl cooperation with server side processes

Chris Green cmg at dok.org
Tue Jul 20 21:35:14 GMT 2004

Good day,

This is a repost/recap of a question and asked on
samba at lists.samba.org.  I'm using Samba 2.2.8a [1] on Slackware 9.1
with 2000/XP clients.  I'm using the Linux 2.4.25 kernel.

I want to have a "drop box" that I have NT/2000 clients writing to
share and then have a unix process read and delete the file.

My problem is that samba isn't doing a lock that I can see from the
Linux side. My usage pattern is:

1) Copy cygwin.tar (250M) to share using Windows Explorer on W2K.
2) from the server's command line while the file is still copying, run

   ./lockview cygwin.tar

http://cmg.dok.org/code/lockview.c.  This code uses fcntl(fd,F_GETLK,
&flockstruct) to see what kind of locks are on the file.

 output/Testing# ~/lockview cygwin.tar
   this file has no lock!

If I use ``strict locking = yes'', and I have a process use fcntl to
perform a fcntl(fd, F_SETLK, {F_WRLCK, ....}), I get a message on the
windows side instantly saying

    ``Cannot copy cygwin: The process cannot access the file because
      another process has locked a portion of the file''

If I don't use strict locking, rather than aborting the copy, it appears
to unlink when the copy is complete.      
As far as I've been able to trace it with debug level = 10, it seems
that there is no lock associated with file creation.  I'm not able to
dig out what the values of share mode means. My lock fails even if I
turn oplocks = off.    

[2004/07/20 13:21:37, 5] smbd/files.c:file_new(123)
  allocated file structure 5634, fnum = 9730 (2 used)
[2004/07/20 13:21:37, 10] smbd/open.c:open_file_shared1(807)
  open_file_shared: fname = output/Testing/cygwin.tar, share_mode = 11, ofun = 10, mode = 660, oplock request
= 3

I just noticed:

    if(fcntl(fd, 1024, F_WRLCK) < 0)
       perror("Unable to get F_WRLCK\n");

Will not work unless samba is done writing to the file.  I think this
might be my linux specific answer.

Is there a better way to know when the copy is done from the linux

Is there an option I can give samba at run time to force it to use
fcntl locks?  

[1]  Samba is compiled:
 --with-fhs \
  --prefix=/usr \
  --localstatedir=/var \
  --bindir=/usr/bin \
  --sbindir=/usr/sbin \
  --with-lockdir=/var/cache/samba \
  --sysconfdir=/etc \
  --with-configdir=/etc/samba \
  --with-privatedir=/etc/samba/private \
  --with-codepagedir=/etc/codepages \
  --with-smbmount \
  --with-quotas \
  --with-syslog \
  --with-utmp \
  --with-sambabook=/usr/share/swat/using_samba \
  --with-ssl \
  --with-sslinc=/usr/include/openssl \
  --with-ssllib=/usr \
  --with-msdfs \
  --with-vfs \
  --with-libsmbclient \
Chris Green <cmg at dok.org>
Chicken's thinkin'

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