Re: undefined behavior in closeout, aggravated by libsigsegv

[Eric Blake]

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According to Bruno Haible on 11/21/2009 10:11 AM:
>> Here's three patches that add test cases, then respectively fixes cygwin
>> 1.7 and 1.5 to pass the new tests.  I've also tested that on platforms
>> like Linux that the new tests pass, as well as testing that mingw skips
>> the tests but otherwise still compiles fine.
>>
>> $ git pull git://repo.or.cz/libsigsegv/ericb.git master
> 
> Thanks for this. You find a new libsigsegv tarball with your patches included
> (with small modifications) in
>     http://www.haible.de/bruno/gnu/libsigsegv-2.8-pre1.tar.gz

I have confirmed that, with --enable-EFAULT, the latest libsigsegv.git
passes on both cygwin 1.5 and 1.7.  Without --enable-EFAULT, then a build
of m4 1.4.12 on cygwin 1.5 with the latest libsigsegv still has the same
bug that sparked this thread.  I'm still not sure why anyone would ever
use libsigsegv on cygwin without --enable-EFAULT, because this entire bug
report was due to a fault that was both caused and handled internally by
cygwin.  I.e. there was no invalid pointer passed in by the application;
rather, cygwin1.dll took a shortcut that the implicit fclose() during
exit() could try to lock stdout, even though that attempt will fault if
stdout has already been closed, and was gracefully handling the resulting
fault until the point that libsigsegv interfered.

EFAULT was one manifestation of how libsigsegv interfering with cygwin's
fault handling caused observable behavior, but I won't go so far as to
guarantee that EFAULT handling was the only observable behavior being
changed by libsigsegv.  In other words, I think it is wrong for libsigsegv
to ever be compiled without --enable-EFAULT on cygwin, on the grounds that
libsigsegv should not be second-guessing cygwin's fault handling.
Instead, relying on POSIX signal handling for the cases where cygwin1.dll
wants the fault exposed to the application is the only portable way for
libsigsegv to fit in with cygwin's goal of being a POSIX emulation.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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