[Patches] [ python-Patches-1564547 ] Py_signal_pipe

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Patches item #1564547, was opened at 2006-09-24 15:13
Message generated for change (Comment added) made by gustavo
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Category: Core (C code)
Group: Python 2.6
Status: Open
Resolution: None
Priority: 5
Submitted By: Gustavo J. A. M. Carneiro (gustavo)
Assigned to: Nobody/Anonymous (nobody)
Summary: Py_signal_pipe

Initial Comment:
Problem: how to wakeup extension modules running poll()
so that they can let python check for signals.

Solution: use a pipe to communicate between signal
handlers and main thread.  The read end of the pipe can
then be monitored by poll/select for input events and
wake up poll().  As a side benefit, it avoids the usage
of Py_AddPendingCall / Py_MakePendingCalls, which are
patently not "async safe".

All explained in this thread:

http://mail.python.org/pipermail/python-dev/2006-September/068569.html


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>Comment By: Gustavo J. A. M. Carneiro (gustavo)
Date: 2006-09-27 14:42

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> * Needs documentation ...

  True, I'll try to add more documentation...

> * I think we should be more paranoid about the range of
possible signals.  NSIG does not appear to be defined by
SUSv2 (no clue about Posix).  We should size the Handlers
array to UCHAR_MAX and set any signals outside the range of
0..UCHAR_MAX to either 0 (null signal) or UCHAR_MAX.  I'm
not sure we should ever use NSIG.

I disagree.  Creating an array of size UCHAR_MAX is just
wasting memory.  If you check the original python code,
there's already fallback code to define NSIG if it's not
already defined (if not defined, it could end up being
defines as 64).

> * In signal_hander() sizeof(signum_c) is inherently 1. ;)

  And? I occasionally hear horror stories of platforms where
 sizeof(char) != 1, I'm not taking any chances :)

> * PyOS_InterruptOccurred() should probably still check
that it's called from the main thread.

check_signals already bails out if that is the case.  But in
fact it bails out without setting the interrupt_occurred
output parameter, so I fixed that.

fcntl error checking... will work on it.

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Comment By: Adam Olsen (rhamphoryncus)
Date: 2006-09-27 00:53

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I've looked over the patch, although I haven't tested it.  I
have the following suggestions:

* Needs documentation explaining the signal weirdness (may
drop signals, may delay indefinitely, new handlers may get
signals intended for old, etc)
* Needs to be explicit that users must only poll/select to
check for readability of the pipe, NOT read from it
* The comment for is_tripped refers to sigcheck(), which
doesn't exist
* I think we should be more paranoid about the range of
possible signals.  NSIG does not appear to be defined by
SUSv2 (no clue about Posix).  We should size the Handlers
array to UCHAR_MAX and set any signals outside the range of
0..UCHAR_MAX to either 0 (null signal) or UCHAR_MAX.  I'm
not sure we should ever use NSIG.
* In signal_hander() sizeof(signum_c) is inherently 1. ;)
* The set_nonblock macro doesn't check for errors from
fcntl().  I'm not sure it's worth having a macro for that
anyway.
* Needs some documentation of the assumptions about
read()/write() being memory barriers.
* In check_signals() sizeof(signum) is inherently 1.
* There's a blank line with tabs near the end of
check_signals() ;)
* PyErr_SetInterrupt() should use a compile-time check for
SIGINT being within 0..UCHAR_MAX, assuming NSIG is ripped
out entierly.
* PyErr_SetInterrupt() needs to set is_tripped after the
call to write(), not before.
* PyOS_InterruptOccurred() should probably still check that
it's called from the main thread.

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