Binary packages not rebuilding against updated libraries

[Jan Stary]

On Apr 26 03:16:37, raimue at macports.org wrote:
> On 2018-04-26 00:11, Jan Stary wrote:
> > On Apr 24 09:31:04, ken.cunningham.webuse at gmail.com wrote:
> >> Portfile authors need to manually "revbump"
> >> the library's dependent ports when
> >> supporting libraries change significantly.
> >> It's not automatically figured out by MacPorts.
> > 
> > Wait, what? I'm still relatively new to MP
> > and this seems a bit weird.
> > 
> > "Port A depends on library B. The port of library B has
> > just been rebuilt/upgraded. So port A needs to be rebuilt
> > as well now, if only to record the new dependency."
> 
> An update to port B cannot cause a new dependency in port A.

I mean dependency on the new lib.
Say a port produces a package containing 'bin/prog'
that depends (as in otool -L) on libz.1.2.11.dylib.
If zlib is updated, and provides libz.1.2.12.dylib now,
bin/prog ca no longer depend on libz.1.2.11.dylib
(which no longer exists), but on libz.1.2.12.dylib.
That's what I meant by "new dependency", in the
installed package of the port.

> > MP does not have that?
> Port A only has to be rebuilt if the compatibility version of port B
> changes in a way that makes it incompatible with the previous version
> (basically increasing soname on other systems). This is quite seldom.

Aren't most library port updates like that?

For comparison, the OpenBSD port system has resigned on upstreams'
library versioning, and versions the libraries itself. For example,
in audio/libsndfile (1.0.28):

	SHARED_LIBS +=  sndfile   6.0   # .1.28

and it installs /usr/local/lib/libsndfile.so.6.0.
Is that incompatible with the previous? That was libsndfile.so.5.0,
so yes, it's incompatible; that's why the maintainer named it so.
Hence, rebuild everything that depends on libsndfile
(without bumping _their_ revision).

In an ideal world, the decision could be based on the libfoo.X.Y.Z,
but often it cannot.

> If port B was updated and the compatibility version changed, the
> automatic run of 'port rev-upgrade' after the installation will detect
> that a binary/library of port A has broken linkage. It will then rebuild
> port A from source, ensuring you have a working installation locally.

Yes.

> To produce new packages, the revision of port A has to be increased.

That's what I don't get: new packages of what?
The packages that were installed before get rebuilt
with port rev-upgrade, automatically,
without anyone touching their Portfile.
Which ports need to get a new revision in the Portfile, and why?

	Jan