Re: [patch] Fix BZ 11420 -- configure uses incorrect link order when testing libpython

[Simon Marchi <simon dot marchi at polymtl dot ca>]

On 2018-04-17 11:52, Paul Pluzhnikov via gdb-patches wrote:
> Greetings,
>
> https://stackoverflow.com/a/49868387
> https://sourceware.org/bugzilla/show_bug.cgi?id=11420
>
> Configure uses "gcc -o conftest -g ... conftest.c -ldl -lncurses -lm 
> -ldl
> ... -lpthread ... -lpython2.7" when deciding whether give libpython is
> usable.
>
> That of course is the wrong link order, and only works for shared 
> libraries
> (mostly by accident), and only on some systems.
>
> Attached patch fixes this.

Hi Paul,

Sorry for letting this slip through the cracks.

It looks like our python-config.py is a (modified) copy of an old 
python-config from CPython.  A similar bug was reported and fixed 
upstream a while ago:

https://bugs.python.org/issue18096

and your fix in python-config.py makes our version closer to the 
upstream version, so this part LGTM.

> I am not sure about this part of the patch:
>
> -  LIBS="$LIBS $new_LIBS"
> +  LIBS="$new_LIBS $LIBS"
>
> I think it's always better to prepend new libraries.
>
> Thanks,
>
> 2018-04-17  Paul Pluzhnikov  <ppluzhnikov@google.com>
>
>              PR gdb/11420
>              * gdb/configure.ac: Prepend libpython.
>              * gdb/python/python-config.py: Likewise.
>              * gdb/configure: Regenerate.

The configure changes seem good as well, or at worst noops.  For 
example, in this context, I am wondering if the python_libs value can be 
other than empty:

  elif test "${have_python_config}" != failed; then
    if test "${have_libpython}" = no; then
      AC_TRY_LIBPYTHON(python2.7, have_libpython,
                       ${python_includes}, "-lpython2.7 ${python_libs}")
    fi

If we are in that context, it's because have_python_config is "no".  And 
it looks like the only code path that sets have_python_config to "no" 
also sets python_libs to empty.  If you want to investigate that further 
and possibly simplify the code, you are welcome.

I've pushed your patch with these changes:

- Fix pre-existing formatting issue in the python-config.py line
- Re-generate configure (those changes were not included in the patch)
- Remove the gdb/ prefixes in the ChangeLog filenames (they should be 
relative to the ChangeLog file itself, which is in gdb/).

Thanks!

Simon