[web2py] important - need help with file locking

[Massimo Di Pierro]

I believe we have a problem with file locking. I managed to isolate
the problem in this non-web2py pure-python program:

==== begin file test.py ======
import fcntl
import multiprocessing
import unittest

N=10000

def lock(fp, mode):
    fcntl.flock(fp.fileno(), mode)

def unlock(fp):
    fcntl.flock(fp.fileno(), fcntl.LOCK_UN)

def read_write(args):
    (filename, iterations) = args
    for i in range(0, iterations):
        fp = open(filename,'r')
        lock(fp,fcntl.LOCK_SH) # shared
lock
        content = fp.read()
        unlock(fp)             #
unlock
        fp.close()
        if len(content)!=N:
            return False
        fp = open(filename,'w')
        lock(fp,fcntl.LOCK_EX) # exclusive
lock
        fp.write(content)
        unlock(fp)             #
unlock
        fp.close()
    return True

class TestParallelLocks(unittest.TestCase):

    def setUp(self):
        self.filename = 'test.txt'
        contents = 'x'*N
        fp = open(self.filename,'w')
        fp.write(contents)
        fp.close()

    def tearDown(self):
        try:
            os.remove(self.filename)
        except:
            pass

    def test_reads_and_writes(self):
        readwriters = 10
        pool = multiprocessing.Pool(processes = readwriters)
        results = pool.map(read_write, [[self.filename, 10]] *
readwriters)
        for result in results:
            self.assertTrue(result)

if __name__ == '__main__':
    unittest.main()
===== end file test.py ====

If I run it with python test.py it will often fail the test (tested on
Mac Leopard, Lion, Ubuntu, Python 2.5, 2.6, 2.7).

HELP! Can you reproduce the problem? Do you see anything wrong with
the code?

Massimo