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-

 

[Fredrik Lundh]

Is there a way around this? How can I stop file being COPIED OVER while
it is being open?

you could rename the file before you start updating it, and rename it back when
done.

</F>

 

[Roger Upole]

I get

The process cannot access the file because another process has locked a
portion of the file.
0 file(s) copied.

on both Win2k and WinXP. (Python 2.4, Pywin32 build 203)
Are you sure the log.txt you're copying to is actually the right one ? You
should at least get a
prompt to confirm overwrite on the command line.

Roger

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-

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[elbertlev]

I just have written the program in "C", which does the same. It behaves
almost the way you described.

Tthe copy command gives such diagnostic:

The process cannot access the file because
another process has locked a portion of the file.
0 file(s) copied.

BUT THE FILE IS ACTUALLY OVERWRITTEN..

I'm posting this question on MS newsgroup
microsoft.public.win32.programmer.kernel

 

[Pekka Niiranen]

Hi everybody:

I played with the class Flock and changed the line
win32con.FILE_SHARE_READ|win32con.FILE_SHARE_WRITE,\
to
win32con.FILE_SHARE_READ,\
and now I cannot copy the file over which suits me.

When file is NOT locked I get:
E:\>copy d:\log.txt .
Overwrite .\log.txt? (Yes/No/All): y
1 file(s) copied.

When file IS locked I get:

E:\>copy d:\log.txt .
The process cannot access the file because it is being used by another
process.
0 file(s) copied.

Below is the new script completely. Note that when
upgrading to Python v2.4 I had to change self.highbits
from 0xffff0000 to -0x7fff0000.

-----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,\ 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__':
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----

-pekka-

 

[elbertlev]

Sure it will do if one of the processes needs read access only.

Scenario when you need shared rw acces with locking:
In the file you have "records" say 30 bytes long, 2 processes are
reading/writing these records by: lock-read-unlock or lock-write-unlock
.. Both processes have to open the file with rw access and the third
process can overwrite the locked file. Actually the common method to
prevent file from been overwritten is: lock the region outside the
file. Such feacure is added to lock/unlock namely for this purpose.

You found the bug in WIN32.

In fact the file locked and overwritten hav ALL BYTES ZERO and the
length of the copyed file.

 

[elbertlev]

Sure it will do if one of the processes needs read access only.

Scenario when you need shared rw acces with locking:
In the file you have "records" say 30 bytes long, 2 processes are
reading/writing these records by: lock-read-unlock or lock-write-unlock
.. Both processes have to open the file with rw access. But the third
process can overwrite the locked file! Actually the common method to
prevent file from been overwritten is: lock the region outside the
file. Such feacure is added to lock/unlock namely for this purpose.

You found the bug in WIN32.

PS.

When you copy the file (say c:\b) over file with locked region say
(c:\a), the length of the file c:\b becomes the length of c:\a, and the
content of c:\a is all zeroes. By the way, I would understand the logic
if copy copyes bytes outside locked regions but preserves all locks.