On Sat, 2014-06-14 at 16:41 +0200, Michał Górny wrote:
> Considering the libtool versioning, there are two kinds of library
> bumps relevant to us:
> 
> 1) when ABI is altered in backwards-compatible way (so old stuff is not
> touched),
> 
> 2) when ABI is altered in backwards-incompatible way.

The situation is more nuanced. I have also seen the following cases:

3) a package provides multiple libraries or entry points, and only some
of them have their ABI altered in a backwards-incompatible way.
Examples: xorg-server changes ABI for video drivers, but not for input
drivers; poppler changes ABI for libpoppler, but not libpoppler-glib.

4) a package alters a "private" ABI which matters for a small number of
closely-tied packages, but is transparent to normal users of the
library. Example: glib and gobject-introspection.

A solution to unnecessary rebuilds in these situations, as well as for
case (1), might be in the form of subslots as a key:value list, with
different users subscribing to be rebuilt for specific keys.