Blows up under released Windows 2.0 and CVS Pythons (so it's not due to anything new):

import thread

def read(f):
    import time
    time.sleep(.01)
    n = 0
    while n < 1000000:
        x = f.readline()
        n += len(x)
        print "r",
    print "read " + `n`
    m.release()

m = thread.allocate_lock()
f = open("ga", "w+")
print "opened"
m.acquire()
thread.start_new_thread(read, (f,))
n = 0
x = "x" * 113 + "\n"
while n < 1000000:
    f.write(x)
    print "w",
    n += len(x)
m.acquire()
print "done"

Typical run:

C:\Python20>\code\python\dist\src\pcbuild\python temp.py
opened
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w r w r
w r w r w r w r w r w r w r w r w r w r w r w r w r w r w r w r w
r r w r w r w r w r w r

and then it dies in msvcrt.dll with a bad pointer.  Also dies under the debugger (yay!) ... always dies like so:

+ We (Python) call the MS fwrite, from fileobject.c file_write.
+ MS fwrite succeeds with its _lock_str(stream) call.
+ MS fwrite then calls MS _fwrite_lk.
+ MS _fwrite_lk calls memcpy, which blows up for a non-obvious reason.

Looks like the stream's _cnt member has gone mildly negative, which _fwrite_lk casts to unsigned and so treats like a giant positive count, and so memcpy eventually runs off the end of the process address space.

Only thing I can conclude from this is that MS's internal stream-locking implementation is buggy.  At least on W98SE.  Other flavors of Windows?  Other platforms?

Note that I don't claim the program above is *sensible*, just that it shouldn't blow up.  Alas, short of adding a separate mutex in Python file objects-- or writing our own stdio --I don't believe I can fix this.