From: Brian Candler <B.Candler@...>
Date: 2003-07-26T23:57:47+09:00
Subject: Re: Confused about locking a file via file.flock(File::LOCK_EX)

On Sat, Jul 26, 2003 at 08:57:00PM +0900, Ludwigi Beethoven wrote:
> I guess I would have to grab the file and update it
> rather quickly, and hope that nobody else is updating
> it at the same time via some other way.

The requirement was to prevent another user editing the same file with 'vi',
right?

On my system (FreeBSD), if I edit a file with vi, then try to edit the same
file in vi in another window, I get:

/etc/motd already locked, session is read-only.
/etc/motd: unmodified, readonly: line 1

This implies that there is some sort of locking going on.

Now, if I run this test program:

def testlock(filename)
  f = File.open(filename)
  f.flock(File::LOCK_EX)
  sleep 30
  f.close
end
testlock("/etc/motd")

then vi /etc/motd also reports that the file is locked. Is that not the case
for you?

If not, then the "update it rather quickly and hope" is most safely done by:
- creating a copy of the file with a different name and modified contents
- doing a 'rename' operation to replace the original file with it

This is an atomic operation - it will either succeed or fail. Of course
someone else who has it open in a buffer could overwrite it again, but at
least it will always be a complete and valid file, not corrupted by two
people writing to it simultaneously.

Regards,

Brian.