Signal handler calling Py_Finalize() when child processes being killed.

         Key: MODPYTHON-109
     Project: mod_python
        Type: Bug
  Components: core  
    Versions: 3.2    
    Reporter: Graham Dumpleton

When Apache is killing off child processes as part of actions taken when the 
"apachectl restart" or "apachectl graceful" command is run, it sends a SIGTERM 
signal to the child processes. This causes a signal handler registered by 
Apache to be run. That signal handler destroys the main child memory pool. That 
memory pool has though a cleanup handler associated with it which was 
registered by mod_python. That cleanup handler ultimately calls Py_Finalize().

The problem with this is that Py_Finalize() isn't safe to be called from a 
signal handler and if a handler is still executing or there is a separate 
thread running in the context of Python, a deadlock will likely ensue. This 
will prevent the child process exiting due to the SIGTERM causing the Apache 
parent process to send it a SIGKILL to really kill it.

For a more detailed assessment of the problem and what lead to this conclusion 

To avoid the problem, the only choice seems to be avoid calling Py_Finalize() 
from the signal handler. The simplistic way of doing this seems to be to add:

     if (child_init_pool)
         return APR_SUCCESS;

at the start of python_finalize(). This will mean that Py_Finalize() is never 
called in child processes. The full consequences of this is unknown, but on 
face value it would seem that it might be a reasonable thing to do. More 
research may be required.

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