transitive builtin logic is busted

[David Holland <dholland-pkgtech%netbsd.org@localhost>]

Today because of another failure I happened to notice that I was
getting this:

   % make show-var VARNAME=USE_BUILTIN.fontconfig
   no
   % ls -l work/.buildlink/lib/libfontconfig.so
   lrwxr-xr-x  1 dholland  people  31 May 25 02:58 work/.buildlink/lib/libfontconfig.so -> /usr/X11R7/lib/libfontconfig.so

IOW, it was supposedly not using the builtin fontconfig, but was
buildlinking its files anyway instead of the pkgsrc version.

This then causes everything downstream of fontconfig to be linked
against two versions of freetype2, and things go downhill. (Though the
native and pkgsrc versions of freetype2 are not very different and
it's possible that there's no overt fallout or it's rare.)

This turns out to be caused by the following:

   - fontconfig's builtin.mk sets USE_BUILTIN.fontconfig to yes
     because there's no local reason not to;
   - this causes mk/x11.builtin.mk, when included by fontconfig's
     builtin.mk, to set BUILDLINK_PREFIX.fontconfig to $(X11BASE);
   - the transitivity logic in bsd.buildlink3.mk then sets
     USE_BUILTIN.fontconfig to no because of freetype2;
   - the logic further down in bsd.buildlink3.mk for setting
     BUILDLINK_PREFIX.fontconfig is skipped because it's already
     set;
   - later on the buildlink logic itself symlinks files from the
     wrong prefix, and then that's what's used to build.

I have no idea when this broke; it could have been some time ago. (I
do remember a few months back running into a report that something
seemed to be wrong with the transitivity logic, but I can't remember
whose report or what it was about, or where I saw it.)

x11.builtin.mk hasn't been changed in forever and the relevant
BUILDLINK_PREFIX logic in bsd.buildlink3.mk hasn't changed recently
either, and I doubt there have been major changes to the ordering of
things.

But, it did work once upon a time... anyway, the question is how to
fix it.

For the moment I've done this locally, which is enough to get my
builds to work, but it doesn't seem in any way like the right thing.
Ideas welcome...

Index: bsd.buildlink3.mk
===================================================================
RCS file: /cvsroot/pkgsrc/mk/buildlink3/bsd.buildlink3.mk,v
retrieving revision 1.263
diff -u -r1.263 bsd.buildlink3.mk
--- bsd.buildlink3.mk	13 Jan 2024 20:26:47 -0000	1.263
+++ bsd.buildlink3.mk	25 May 2024 23:34:15 -0000
@@ -135,6 +135,9 @@
 .      if ${_ok_} != yes
          # not ok for it to be builtin; force it to pkgsrc
          USE_BUILTIN.${_pkg_:S/^-//}:=no
+         # If the buildlink prefix is already set, clear it or we'll
+         # buildlink the wrong files. XXX: this is a hack
+.        undef BUILDLINK_PREFIX.${_pkg_:S/^-//}
          FORCED_PKGSRC+=${_pkg_:S/^-//}
          # using += here fails to evaluate ${_ok_} until after the loop
          FORCED_PKGSRC_REASONS:=${FORCED_PKGSRC_REASONS} ${_pkg_:S/^-//}:${_ok_:S/^no,//}


-- 
David A. Holland
dholland%netbsd.org@localhost