openssh unix dev - Apr 2011 - advisory file locks in linux - do they work?

Hi - I'm trying to verify if OpenSSH/SFTP will in fact lock files with 
advisory file locking in Linux.  I can test locking with the linux 
"flock" command to verify that file locking does work - but when I 
upload or download a file with SFTP it will not detect a lock.  I'm 
asking about Linux specifically because about a year ago I was doing a 
similar process in Solaris 9 and it seemed to work.

OS'n I've tried this on-
RHEL5.5 with OpenSSH 5.6p1 (compiled)
Ubuntu 10.10 with OpenSSH 5.5p1

Test
# get 100MB testfile from remote box via sftp
(sftp to remote host, get testfile)
# While that's running use the following command to check for the lock:
flock -xn testfile -c "echo test > testfile"
#flock command returns an exit status of 0.  Expected result is 1 
(failure).  Test "test" is in testfile

Control Test
# Open a stream of data to testfile and set exclusive advisory lock
flock -xn testfile -c "cat /dev/random > testfile"
# Test for lock by trying to set another exclusive lock
flock -xn testfile -c "echo test > testfile"
Result is 1 (failure because the file is locked.  Text "test" is not
in
testfile.)

USENET - sorry for the double post, apparently you don't forward to the 
email list?

On Tue, 12 Apr 2011, Jeff Gibson wrote:
> Hi - I'm trying to verify if OpenSSH/SFTP will in fact lock files with
> advisory file locking in Linux.
no

Jeff Gibson wrote:
> Hi - I'm trying to verify if OpenSSH/SFTP will in fact lock files
> with advisory file locking in Linux.
You seem to think that all programs do file locking?  Or perhaps you
are aware that they do not but you think they /should/ use file
locking and this is your way of lobbying to add it?  By the way ...
Most other programs such as 'cp', 'mv', 'cat',
'rsync', do not do file
locking either.  File locking is actually a rather trouble prone
technique and tends to have problems in practice on networked
filesystems such as NFS.  It is almost always better to avoid it and
use techniques that do not require it.  For one random example the
Maildir format often works better than mbox format since there is no
need for file locking with Maildirs.
> I can test locking with the linux "flock" command to verify that
> file locking does work - but when I upload or download a file with
> SFTP it will not detect a lock.
Correct.  But as you are aware the Unix philosophy is to use small
programs that each do one thing well and then to integrate them.  Your
use of the flock command from util-linux-ng is an example.  There is
no need to add file locking to sftp.  Instead use the flock program
(or other similar programs) to semaphore between processes.
> # get 100MB testfile from remote box via sftp
> (sftp to remote host, get testfile)
> # While that's running use the following command to check for the lock:
> flock -xn testfile -c "echo test > testfile"
> #flock command returns an exit status of 0.  Expected result is 1
> (failure).  Test "test" is in testfile
Since you already know about the 'flock -c cmd' then you already know
how to add this to sftp in your process.

  # Use 'sleep 20' here to simulate a large file transfer that takes a
while.
  flock testfile -c "sleep 20 ; echo get testfile | sftp
remote.example.com" &
  sleep 2  # give time for the above to get going and lock the file
  flock testfile -c "echo testfile is not locked now"

As such there is no need for file locking to be added to any
particular program.  You have the control to semaphore between any
process that you wish to coordinate.  This is much more flexible and
powerful than creating an infinite number of infinitely large
monolithic programs that contain all possible functionality.

Bob