[Python-Dev] Segfault

[Neal Norwitz]

On 8/21/07, Hrvoje Nikšić <hrvoje.niksic at avl.com> wrote:
> On Mon, 2007-08-20 at 20:27 +0200, Maciej Fijalkowski wrote:
> > import thread
> >
> > while 1:
> >     f = open("/tmp/dupa", "w")
> >     thread.start_new_thread(f.close, ())
> >     f.close()
>
> file_close inadvertently allows fclose to be called twice on the same
> stdio file.  This patch should fix the problem:

The patch is insufficient to prevent all types of crashes that occur
when accessing a file from 2 threads (closing in one and doing
whatever in another).  Almost every place that accesses f_fp is a
problem.  For example, try changing one of the f.close to f.tell.  Or
in the main thread do a write.  With all of these changes to the test
above, I was able to crash python with the patch (until I fixed the
uses of f_fp).

For example:

import thread

while 1:
    f = open("/tmp/dupa", "w")
    thread.start_new_thread(f.close, ())
    try:
      f.write('a')
      f.close()
    except: pass

I've attached a patch (against 2.5) that fixes most of the problems,
but there are still a few more.  (Search for Py_BEGIN_ALLOW_THREADS
and check for usage of f_fp within the block.)  I'm not convinced the
attached patch is good enough though.

I think there might already be a bug report about file not being
thread-safe.  (It could have also been socket, but I think Martin
fixed a problem in socket a while ago.)

n