[debian-mysql] Bug#1140296: libmariadb-dev multi-arch — proposed approach before I open an MR

[Dick Hollenbeck]

Thanks — happy to drive this. Before I push an MR, here's the approach I have
   in mind; since you've hit conflicts on past attempts, I'd rather confirm the
   direction (and hear what previously went wrong) than re-tread it.

   First, the complete scope. I diffed every shared-path (non-triplet) file in
   the dependency closure across amd64 and arm64. The entire co-install blocker
   is just three things:

     * libmariadb-dev-compat: 0 files differ between arches. It only lacks
       Multi-Arch: same.
     * libmariadb-dev: exactly 2 files differ, nothing else (148 shared files
       checked; my_config.h and all other headers are byte-identical):
         - /usr/bin/mariadb_config            (compiled ELF, differs entirely)
         - /usr/include/mariadb/mariadb_version.h  (differs only in
           MARIADB_MACHINE_TYPE and MARIADB_PLUGINDIR)

   If past attempts "ran into conflicts," my guess is an incomplete inventory —
   a missed differing file. I believe the list above is exhaustive for 11.8.8-1.

   Proposed fix:

   1. libmariadb-dev-compat — add Multi-Arch: same. Nothing else differs.
      Open question for you: does its self-Provides + Conflicts/Breaks of
      libmariadbclient-dev-compat / libmariadb-client-lgpl-dev-compat need
      adjusting once it's M-A: same, or is two instances of the same package
      providing the same virtual fine? I'll verify empirically either way.

   2. libmariadb-dev — add Multi-Arch: same and resolve the 2 files:
      a) mariadb_config: replace the compiled binary with an
         architecture-independent POSIX shell script that derives the multiarch
         triplet at runtime (cc -print-multiarch, falling back to
         dpkg-architecture -qDEB_HOST_MULTIARCH). This is the historical
         mysql_config form and makes the file byte-identical across arches.
      b) mariadb_version.h: relocate it to the multiarch include dir
         /usr/include/<triplet>/mariadb/ (auto-searched by the compiler), so
         MARIADB_PLUGINDIR / MARIADB_MACHINE_TYPE may legitimately differ. I'd
         prefer this over neutralizing the defines, which could break consumers
         that read MARIADB_PLUGINDIR — but if you'd rather keep the header in the
         common path and drop/derive those defines, I'm fine with that too.

   Two questions:
     - Does (2a) the shell-script mariadb_config conflict with anything you tried
       before, or with a reason it was made a compiled program?
     - For (2b), do you prefer relocation or neutralizing the defines?

   Once you confirm the direction I'll build it, verify amd64+arm64 actually
   co-install in a clean trixie/sid container, and open the MR at
   salsa.debian.org/mariadb-team/mariadb-server.

   Thanks,
   Dick