automagic pkg-config search path detection is bad, please remove it

[Ariadne Conill]

Hello,

I am the maintainer of pkgconf, one of the mainly used pkg-config 
implementations.  As a result of the problems with ncurses over the 
weekend in Alpine, I became aware of the new pkg-config path detection 
feature in ncurses' configure script.

I believe this feature to be philosophically and technically flawed, and 
plan to argue both points, as well as note a technical mitigation.

First, the philosophical concern: I don't think this feature is beneficial 
to end-users of ncurses.  The reason why is because user packages are 
almost always installed to /usr/local/lib/pkgconfig, which has been the 
defacto standard for the past 15 years.  Yes, there are some pkg-config 
implementations out in the wild which still check $datadir/pkg-config, but 
this has never been typical behavior of the pkg-config tool to search that 
path.

It is actively hostile toward distributions, as distributions build 
pkg-config with /usr/local paths first, so that user packages can be 
preferred over system ones.  If a distribution uses this automatic 
detection, it will detect the user path (since it comes first in the 
search list) instead of the system path.

Now, for the technical concern: the automatic detection is dependent on 
the debug output of pkg-config or pkgconf.  The debug output of either 
tool is not something that should be considered a stable interface, and is 
subject to change at the whim of the maintainer.  Related to this point, I 
plan on overhauling the debug output in pkgconf 2.0 anyway, as part of 
refactoring/rewriting that is already ongoing.  This new debug output will 
likely not be compatible with your autoconf macro.

A portable way of learning the pkg-config search path already exists:

$ pkgconf --variable=pc_path pkg-config
/usr/local/lib/pkgconfig:/usr/local/share/pkgconfig:/usr/lib/pkgconfig:/usr/share/pkgconfig

This portable way will work with the major implementations: we all agreed 
to implement it explicitly for users to query the search path.

But, really, please just install by default to $libdir/pkgconfig and get 
rid of all of this magic stuff.  That's all you need to do.  If some 
distribution is doing something else that's weird, by now, they surely 
have support for the /usr/local/lib/pkgconfig path.

Ariadne