Subj : timEd message base locking (or lack thereof) in UNIX
To   : All
From : andrew clarke
Date : Sun Oct 28 2012 07:02:32

25 Oct 12 04:45, I wrote to all:

 ac> Or I could just not use Squish format, I suppose. On modern PCs with
 ac> new filesystems, having few thousand *.MSG files in a single directory
 ac> is not a big deal.

Actually in hindsight with *.MSG there's still a window where a tosser can 
overwrite a message editor's newly-created .MSG file. The problem is that both 
programs need to find the highest numbered .MSG file first before creating the 
new message, and the time this takes even on a modern PC that could be quite 
high - in the order of several seconds if the CPU is loaded, or the filesystem 
is on a remote machine. During this window it's possible for the other program 
to generate a new message, so the "highest message" counter can get out of sync 
between the two programs. The higher the number of messages the more likely it 
is to occur.

Just out of curiosity I hacked together two short Python programs. One to 
create the .msg files initially, and the other to look for the highest numbered 
..msg file. On my system both of these programs take less than a second to run, 
but this is on a fast CPU on a local filesystem.

#!/usr/bin/env python

# Create 5000 files named 1.msg to 5000.msg

for i in range(5000):
  fn = '%d.msg' % (i + 1, )
  fp = open(fn, 'w')
  fp.close()


#!/usr/bin/env python

# Find the highest numbered .msg file in a directory

import os, os.path

highest = 0

dp = os.listdir('.')

for fn in dp:
  filename, ext = os.path.splitext(fn)
  if ext == '.msg':
    n = 0
    try:
      n = int(filename)
    except ValueError:
      pass
    if n > highest:
      highest = n

print highest

--- GoldED+/BSD 1.1.5-b20110223-b20110223
 * Origin: Blizzard of Ozz, Melbourne, Victoria, Australia (3:633/267)

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