Use strerror() or strerror_r() instead of sys_errlist and sys_nerr

[gnu-srs]

Hi,

Looks like your code is still using sys_errlist and sys_nerr for error messages on non-UCLIBC systems instead of strerror(). It has been specified by POSIX.1-2001, POSIX.1-2008, C89, and C99.
Why so, e.g. even GNU/Linux marks them as obsolete even if found:
/usr/include/x86_64-linux-gnu/bits/sys_errlist.h
/* sys_errlist and sys_nerr are deprecated. Use strerror instead. */

On GNU/Hurd they are not defined, therefore fish fails to build from source there. In case you want a thread-safe version of strerror(), why not use strerror_r(), as specified by POSIX.1-2001 and POSIX.1-2008.

Seems like the Debian maintainer is not comfortable with that change non-upstream.

Thanks!

[ridiculousfish]

strerror with glibc calls into gettext, which takes a lock. This can deadlock, if that lock was held during fork. See #472 for example of a hang under strerror_r.

[ridiculousfish]

To fix the error on GNU/Hurd, we should probably add a check for sys_errlist in the configure script.

[zanchey]

Thanks @gnu-srs. In case you are referring to my comment on Debian bug 771052, I'm not the Debian maintainer but one of the upstream committers. I see there is some confusion between async-safe and thread-safe on 771052 (see also #495 and fish's Fork and Pthread Manifesto).

On the downstream bug, Samuel Thibault notes that

It is always safe in the cases where one would be able to use sys_errlist directly, actualy: in that case the glibc source code boils down to return _sys_errlist_internal[errnum]).

although it is not clear that this is definitely async-safe.

There is lots of malloc in the glibc source for strerror, __strerror_r, and it calls gettext as well.

[zanchey]

What are the options if there is no sys_errlist? strerror and hope?

[zanchey]

The nginx project has also run into this problem, and have solved it by... setting up their own internal sys_errlist at the beginning of execution by calling strerror() on values from 0 to sys_nerr, the value of which is set in the configure script from sys_nerr. Wow.

[ridiculousfish]

That's wild

[zanchey]

To confirm, glibc is definitely not async-safe: __strerror_r definitely calls _ aka gettext.

Solaris doesn't have sys_nerr or sys_errlist either, and strerror calls gettext. I note the OpenSolaris maintainers have elected to proceed with the use of strerror on the grounds that the documentation does not specify either way but I don't think that is the right thing to do either.

The nginx implementation is starting to look alarmingly sane.

[zanchey]

Solaris uses _sys_errs[_sys_index[errnum]] internally.

[zanchey]

libiberty, which comes with GCC, does something fairly similar to ngnix:

https://github.com/gcc-mirror/gcc/blob/master/libiberty/strerror.c

[zanchey]

In 147078f there is a check for sys_errlist (and _sys_errs), and if neither are available, a generic message is printed with the errno. With that commit, fish now builds on Hurd so I'm going to close this issue.