Exit Sage gracefully upon SIGHUP

[jdemeyer]

Change the signal handling code to allow a graceful forced exit of Sage. "graceful" meaning that code is interrupted and the normal exit handlers are run. This graceful exit is not guaranteed to work, code which is currently not interruptible would not be exited.

This might be used by the doctesting framework to handle timeouts.

This patch also improves interrupt debugging:

1. Different debug levels, set in a C file such that not everything needs to be recompiled when changing the debug level.
2. Show time between signal received and exception raised.

Apply: 13908_terminate.patch​

[vbraun]

Sounds good to me.

[jdemeyer]

See sage-devel.

[jdemeyer]

Since the main motivation is to have some way of gracefully terminating Sage, I'm handling only SIGHUP now, not SIGTERM.

[vbraun]

Hmm I didn't see any discussion on sage-devel about SIGTERM but getting started with just catching SIGHUP is fine, too.

[jdemeyer]

For the SIGTERM problem, see ​https://groups.google.com/forum/?fromgroups#!search/sage-devel$2013908/sage-devel/5BfDSSdN5qw/AcWANg5v2KwJ

[stumpc5]

I was running a moinmoin wiki with sage. With this patch applied, that doesn't work anymore because of the new interruption handling in sage.ext._init_csage:

When running the wiki using

$ sage wikiserver.py

and importing sage.all.* therein, I get the following error:

    from sage.all import *
  File "/home/stumpc5/progs/sage-5.7/devel/sage-main/build/sage/all.py", line 59, in <module>
    _init_csage()
  File "c_lib.pyx", line 41, in sage.ext.c_lib._init_csage (sage/ext/c_lib.c:569)
    
ValueError: signal only works in main thread

The issue seems to be solved when I comment out the signal in _init_csage.

Question: Is there a (reasonable and finite) way to fix this issue properly? Can something serious go wrong if I comment these lines out?

Thanks, Christian

[jdemeyer]

Replying to stumpc5:

Can something serious go wrong if I comment these lines out?

Not very serious. Interrupt handling might be a bit messed up, in particular some interrupts (CTRL-C) might be seen twice: once by the default Python handler (raise KeyboardInterrupt) and once by the special Sage handler (written in C).

The line

signal.signal(signal.SIGINT, sage_python_check_interrupt)

is mainly used to ensure that only the Sage signal handler is used.

But this will certainly have no consequences beyond interrupt handling.