[Python-bugs-list] [ python-Bugs-756924 ] SIGSEGV causes hung threads (Linux)

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Bugs item #756924, was opened at 2003-06-18 16:28
Message generated for change (Tracker Item Submitted) made by Item Submitter
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Category: Threads
Group: Python 2.2.2
Status: Open
Resolution: None
Priority: 5
Submitted By: Greg Jones (morngnstar)
Assigned to: Nobody/Anonymous (nobody)
Summary: SIGSEGV causes hung threads (Linux)

Initial Comment:
When a segmentation fault happens on Linux in any 
thread but the main thread, the program exits, but 
zombie threads remain behind.

Steps to reproduce:
1. Download attached tar and extract files zombie.py 
and zombieCmodule.c.
2. Compile and link zombieCmodule.c as a shared library 
(or whatever other method you prefer for making a 
Python extension module).
3. Put the output from step 2 (zombieC.so) in your 
lib/python directory.
4. Run python2.2 zombie.py.
5. After the program exits, run ps.

zombie.py launches several threads that just loop 
forever, and one that calls a C function in zombieC. The 
latter prints "NULL!" then segfaults intentionally, 
printing "Segmentation fault". Then the program exits, 
returning control back to the shell.

Expected, and Python 2.1 behavior:
No Python threads appear in the output of ps.

Actual Python 2.2 behavior:
5 Python threads appear in the output of ps. To kill 
them, you have to apply kill -9 to each one individually.

  Not only does this bug leave around messy zombie 
threads, but the threads left behind hold on to program 
resources. For example, if the program binds a socket, 
that port cannot be bound again until someone kills the 
threads. Of course programs should not generate 
segfaults, but if they do they should fail gracefully.

  I have identified the cause of this bug. The old Python 
2.1 behavior can be restored by removing these lines of 
Python/thread_pthread.h:

sigfillset(&newmask);
SET_THREAD_SIGMASK(SIG_BLOCK, &newmask, 
&oldmask);

... and ...

SET_THREAD_SIGMASK(SIG_SETMASK, &oldmask, NULL);

  I guess even SIGSEGV gets blocked by this code, and 
somehow that prevents the default behavior of segfaults 
from working correctly.

  I'm not suggesting that removing this code is a good 
way to fix this bug. This is just an example to show that 
it seems to be the blocking of signals that causes this 
bug.

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