alarm broken on cygwin

[gouezel]

On cygwin (32 and 64), the alarm mechanism is broken:

sage: alarm(0.1); sum(xrange(100000000))
4999999950000000

instead of the expected interrupt.

I traced the bug by dichotomy to a ppl component named watchdog. Its role is similar to alarm, involving signals and interrupts. I guess that, due to the peculiarities of cygwin's implementation of signals, watchdog somehow intercepts the python signals and does dot send them back.

watchdog is not an optional component of ppl, so it can not be disabled by a configure flag. However, it is disabled on systems that don't have setitimer. Cygwin has setitimer, but a simple hack of ppl's makefile can hide it, thereby disabling watchdog and fixing the sage alarm mechanism. This is done in the proposed patch.

A better fix would be to dig into the ppl and cygwin signal mechanism, of course...

[gouezel]

Should it be considered as a PPL bug that the methods are not defined in the header? Or a cython bug that this suffices to break the interrupts mechanism? Or a cygwin bug since it does not seem to be a problem on other platforms?

[gouezel]

The PPL code seems to be correct, I am stuck on this one...

[gouezel]

New commits:

​1a8ecd2

disable ppl watchdog on cygwin

[jdemeyer]

What I don't like about this ticket is that it's not clear that the "ppl watchdog" is not needed. What does it do and why does Sage not need it?

[gouezel]

Excellent question. There is essentially no documentation on this ppl watchdog that I could find. It seems to be a classical watchdog mechanism, i.e., it detects if some computation loops forever, and in this case it interrupts it automatically and resets things to a nicer state. In particular, it should not play any role in non-buggy situations.

It is not necessary to PPL (since it is disabled on some platforms, those which don't have setitimer), and I do not see how it could be relevant to sage. So, my impression is that it is safe to disable it (note that the patch only disables it on cygwin, so it will not break anything on officially supported platforms). On cygwin, with the patch, sage -t all does not show any ppl-related failure. By the way, without the patch, sage -t all hangs forever and eats all available memory, since some tests rely on alarm to be interrupted! So, in my opinion, the patch gives a definitive improvement on cygwin.

[jdemeyer]

Replying to gouezel:

note that the patch only disables it on cygwin, so it will not break anything on officially supported platforms

For me, that's actually a reason to be against this patch. Either we need it and we shouldn't disable it on Cygwin, or we don't need it and we can just disable it everywhere.

[gouezel]

I get your point. Unfortunately, I can't say for sure that watchdog is not helpful to ppl in rare situations (for instance, maybe it uses some algorithms whose termination is not guaranteed, counting on watchdog to exit bad situations), so I would rather keep watchdog wherever possible (i.e., not on cygwin, where setitimer is not fully functional).

I leave the ticket as needs_work until someone more knowledgeable on ppl steps up (or someone who really needs the patch on cygwin pushes for it)

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​457bfa5

#17650: touch files in ppl spkg_install to avoid reconfiguration

[jpflori]

I'm just posting random links potentially related:

• ​http://www.cs.unipr.it/pipermail/ppl-devel/2008-October/013156.html

According to ​http://trac.sagemath.org/ticket/10039#comment:49 there used to be a --disable-watchdog configure flag, too bad it disappeared.

I guess we should open a bug on ppl bugtracker (if one exists).

Unfortunately it seems cygwin does not ship ppl anymore, so we cannot look at what they would have done. It could still be useful to post on the cygwin ml's.

@sebastien: can you provide a minimal C file reproducing the issue? Maybe something inspired in the aforementioned 2008 post can help.

[jdemeyer]

The post does indeed look related. I don't understand why it was posted on PPL-devel, since it seems to have nothing to do with PPL really.

@sebastien: can you provide a minimal C file reproducing the issue?

That would be great, especially to send to the upstream bug trackers (PPL and/or Cygwin).

[gouezel]

Replying to jpflori:

@sebastien: can you provide a minimal C file reproducing the issue? Maybe something inspired in the aforementioned 2008 post can help.

I just tried, but unfortunately I was not able to reproduce the issue with plain C or C++ files (for instance, taking the file in the link you give and adding ppl headers and initialization does not break the itimer mechanism).

For the record, steps to reproduce the issue using python:

File alarm.pyx:

import signal

def essai(n):
  signal.setitimer(signal.ITIMER_REAL, 0.5 , 0)
  return sum(xrange(n))

File alarme_c.cpp

#include <ppl.hh>

File setup.py

from distutils.core import setup
from Cython.Build import cythonize
from distutils.extension import Extension

setup(
    ext_modules = cythonize(Extension("alarme", ['alarme.pyx', 'alarme_c.cpp'], libraries = ['ppl']))
)

File essai.py

import alarme

print alarme.essai(100000000)

Compile the extension with

python setup.py build_ext --inplace

Then python essai.py results in 4999999950000000, no interruption.

Commenting out the header inclusion in alarme_c.cpp, then one gets Alarm clock instead.

[gouezel]

duplicate of #21190

[embray]

Closing tickets in the sage-duplicate/invalid/wontfix module with positive_review (i.e. someone has confirmed they should be closed).