CPython Signal Handler Check for SIGKILL

[Scott McCarty]

All,  I just want to understand the C/Python piece better because I am
writing a tutorial on signals and I am using python to demonstrate. I
thought it would be fun to show that the SIGKILL is never processed, but
instead python errors out. There is something in Python checking the SIGKILL
signal handler, while not checking SIGTERM and I can't find the Python C
code that handles it. When I am writing something like this, I like to point
out the C code, not just cite the documentation (I did find this change in
behaviour noted in the Python change log).

I have searched everywhere (mostly the code and a little google) and I
cannot understand where the SIGKILL signal gets checked when it is set as a
handler. I have scoured the Modules/signalmodule.c only to find two
instances of the RuntimeError exception, but I cannot understand how python
knows when a handler is set for SIGKILL. I understand that this changed in
2.4 and I am not trying to change it, I just really want to understand where
this happens. I used grep to find SIGKILL and SIGTERM to see if I could
determine where the critical difference is, but I cannot figure it out.

I have about 2 hours of searching around and I can't figure it out, I assume
it has to rely on some default behaviour in Unix, but I have no idea. I
don't see a difference between SIGKILL and SIGTERM in the python code, but
obviously there is some difference. I understand what the difference is in
Unix/Linux, I just want to see it in the python code. Since python is
checking at run time to see what signals handlers are added, I know there
must be a difference. I am not asking about the signals, I understand them,
I am asking about the registration of the SIGNAL handler and how it knows
that you are trying to register SIGKILL, you get an error like this.

 ./signal-catcher.py
Traceback (most recent call last):
  File "./signal-catcher.py", line 22, in <module>
    signal.signal(signal.SIGKILL, signal_handler_kill)
RuntimeError: (22, 'Invalid argument')

And the code is very simple, this attempts to register a handler for
SIGKILL, but python knows and won't let you.

signal.signal(signal.SIGKILL, signal_handler_kill)

Please  can someone just point me in the right direction.

Thank You
Scott M
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