File locking is impossible in Windows?

[Pekka Niiranen]

Hi,

I have used the following example from win32 extensions:

-----SCRIPT STARTS----

import win32file
import win32con
import win32security
import pywintypes

class Flock:
     	def __init__(self,file):
		self.file=file
		secur_att = win32security.SECURITY_ATTRIBUTES()
		secur_att.Initialize()
		self.highbits=-0x7fff0000
		self.hfile=win32file.CreateFile( self.file,\
		win32con.GENERIC_READ|win32con.GENERIC_WRITE,\
		win32con.FILE_SHARE_READ|win32con.FILE_SHARE_WRITE,\
		secur_att, win32con.OPEN_ALWAYS,\
		win32con.FILE_ATTRIBUTE_NORMAL , 0 )
	def lock(self):
		lock_flags=win32con.LOCKFILE_EXCLUSIVE_LOCK|\
		win32con.LOCKFILE_FAIL_IMMEDIATELY
		self.ov=pywintypes.OVERLAPPED()
		win32file.LockFileEx(self.hfile,lock_flags,0,\
		self.highbits,self.ov)
	def unlock(self):
		win32file.UnlockFileEx(self.hfile,0,\
		self.highbits,self.ov)
		self.hfile.Close()

if __name__ == '__main__':
	from time import time, strftime, localtime
	import sys
	
	l=Flock("e:\\\\log.txt")
	print 'calling lock'
	l.lock()
	print "Now locked.  Hit enter to release lock."
	dummy = sys.stdin.readline()

	l.unlock()
	print 'now unlocked'

-----SCRIPT ENDS----

If I start one python process from dos window I get message:

E:\>python lockker.py
calling lock
Now locked.  Hit enter to release lock.

All well, now if

1)	I start another Dos -shell and run the same command I get:

	E:\>python lockker.py
	calling lock
	Traceback (most recent call last):
	  File "lockker.py", line 35, in ?
	    l.lock()
	  File "lockker.py", line 23, in lock
	   win32file.LockFileEx(self.hfile,lock_flags,0,\
	   self.highbits,self.ov)
	pywintypes.error: (33, 'LockFileEx',\
	'The process cannot access the file because\
	 another process has locked a portion of  the file.')

	Which is correct.

2)	I try to read the contents of the file from Dos -shell, I get:
	E:\>type log.txt
	The process cannot access the file because another\
	process has locked a portion of the file.
	
	This is correct.

3)	When I open the file into notepad.exe I can edit the screen
	but not write changes to disk. Correct again!

4)	I cannot delete the file from Dos shell or from W2K explorer
	which is correct.

5)	However, I can overwrite the file over with:
	E:\>copy d:\log.txt log.txt
         1 file(s) copied.

	Which is WRONG as is me being able to copy another file over it
	with W2K explorer too.


Is there a way around this? How can I stop file being COPIED OVER while
it is being open? Is this window's feature? Is readlines() operation
"atomic" enough for me not to worry about these issues?

My python script modifies set of files from a directory one by one.
I try to lock them all exclusively for the script
until all are modified. If one of the files gets overwritten
by another version (by another process) the script may fail.

-pekka-