[Python-bugs-list] [ python-Bugs-815648 ] thread unsafe file objects cause crash

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Bugs item #815648, was opened at 2003-09-30 23:32
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Category: Python Interpreter Core
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Jan Olderdissen (janixia)
Assigned to: Nobody/Anonymous (nobody)
Summary: thread unsafe file objects cause crash

Initial Comment:
Note: This may be a dupe or a generalization of 595601.









Running below code snippet on 2.3.1 release and debug 

build on Windows 2000/XP a few times will inevitably lead 

to a crash. 2.2.2 also exhibits this behavior.









The crashing code:









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





import thread









f=open("tmp1", "w")









def worker():





    global f





    while 1:





        f.close()





        f=open("tmp1", "w")





        f.seek (0, 0)









thread.start_new_thread(worker, ())





thread.start_new_thread(worker, ())





while 1: pass





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









The issue appears to be this (and similar) code sections 

from fileobject.c:









	Py_BEGIN_ALLOW_THREADS





	errno = 0;





	ret = _portable_fseek(f->f_fp, offset, whence);





	Py_END_ALLOW_THREADS









Note that due to 

Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS, 

f->f_fp can be set to NULL by file_close prior to entering 

_portable_fseek. Similar crashes can be observed with 

read, write and close itself when they are mixed with a 

concurrent close.









Obviously, the offending python code is buggy and a lock 

should be used to prevent concurrent access to f. 

However, it seems unreasonable for Python to crash 

because of it.









A mutex preventing concurrent access to the file object 

seems like a reasonable way to fix this.



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