Can't get exclusive file lock when safely renaming a file

[Steven D'Aprano]

I'm trying to safely rename a file without over-writing any existing 
files, and I've run into a problem with file locks. Here's a naive way of 
renaming without over-writing:

import os
oldname = "ham.txt"
newname = "spam.txt"
if not os.path.exists(newname):
    os.rename(oldname, newname)

It's naive because there's a race-condition: if file newname is created 
by another process after the call to exists(), but before the call to 
rename(), then it will be over-written.

Here's my current solution, based on advice given by people on this 
thread:
http://mail.python.org/pipermail/python-list/2006-October/411432.html

and this recipe:
http://code.activestate.com/recipes/65203/

but it isn't working for me.


import os, fcntl
oldname = "ham.txt"
newname = "spam.txt"

def lock_destination(name):
    fileno = os.open(name, os.O_CREAT | os.O_EXCL)
    fcntl.flock(fileno, fcntl.LOCK_EX)  # POSIX systems only
    return fileno

# Create a test file to be renamed.
f = open(oldname, 'w')
f.write('this is my file\n')
f.close()
fileno = lock_destination(newname)

# At this point, I can see "ham.txt" plus an empty file 
# "spam.txt" in my file browser

os.rename(oldname, newname)



The rename works, but here is my problem: after getting what I thought 
was an exclusive lock on the new file, but before calling os.rename(), I 
can still over-write it from another process:

$ echo "this comes from another process" > spam.txt
$ cat spam.txt
this comes from another process


This is despite running lock_destination("spam.txt"). What am I doing 
wrong?

Before anyone asks, yes I have checked that the Python process and the 
shell process are in the same working directory, and therefore are 
writing to the same file:

>>> os.read(fileno, 200)
'this comes from another process\n'


I'm using Linux (Fedora).



-- 
Steven