Here's a brainstorm towards improving our handling of pkg upgrades.  It
would probably require additions to both pkgsrc and the pkg_* tools.

- Upgrading a pkg with the same version number (usually a bugfix or build
  fix):  pkg will be deleted and replaced with the new pkg, preserving the
  +REQUIRED_BY file.

- Upgrading a pkg to a new version number:
  * If no '.*/lib\*\.so\.[0-9]+\.[0-9]+$' library files exist in +CONTENTS:
    = Install as with the same version number above.
    = Update dependent pkgs' @pkgdep lines to the new version.
  * If shared libraries exist:
    = Delete all files except these libraries, and ignore @dirrm errors.
    = Strip the +CONTENTS down to only the shared objects (and if on ELF,
      also the major-version-only symlink).
    = Keep the old pkg version installed, replacing the +CONTENTS file
      with one containing only the shared objects
    = Install the new version as well, and copy the +REQUIRED_BY file from
      the old version to the new version (both will contain the same
      dependencies).
    = Update dependent pkgs' @pkgdep lines to have dependencies on _both_
      the old and new pkg versions.
    = Note: could also care for updating where some or all libraries have
      the same major version number - there's a bit of trickiness there.

- If a new pkg version has changes (moved files, dropped binaries) that
  could be used directly by a dependent pkg, add a CONFLICTS entry.  This
  can be hard to see in some programs and easier in others.  Example: moving
  the Tcl "script library" directory.

The schema for this took the following factors into account:

- pkgs can depend on other pkgs for shared libraries
  (and should not break if a dependent pkg updates)
- pkgs can depend on other pkgs for user programs run via system()/exec()
  (such as scripts needing gm4, gmake, ....)
- pkgs can depend on other pkgs for data and config files
  (such as Tcl and Perl library scripts)

-- 
-- Todd Vierling (Personal tv@pobox.com; Bus. todd_vierling@xn.xerox.com)