[Python-bugs-list] [Bug #128210] Threads using same stream blow up (Windows)

[noreply@sourceforge.net]

Bug #128210, was updated on 2001-Jan-09 13:10
Here is a current snapshot of the bug.

Project: Python
Category: Windows
Status: Open
Resolution: None
Bug Group: 3rd Party
Priority: 1
Submitted by: tim_one
Assigned to : nobody
Summary: Threads using same stream blow up (Windows)

Details: 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.


Follow-Ups:

Date: 2001-Jan-09 14:41
By: tim_one

Comment:
Adding info communicated via email:

+ Also blows up under Windows 2000.

+ Yes, C doesn't define what happens here even if it weren't threaded (you
can't mix reads and writes willy-nilly (without e.g. seeking before
reading) and expect something sensible).  The program is not sensible.  But
even silly programs shouldn't *blow up* in Python.

-------------------------------------------------------

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http://sourceforge.net/bugs/?func=detailbug&bug_id=128210&group_id=5470