[oe] [RFD] increased dependency checking

[Frans Meulenbroeks]

Currently if a package in the DEPENDS list is modified, a package that
depends on it does not get updated even if explicitly build
E.g. when I just updated libexif and after that did a bitbake
mythplugins (which has a DEPENDS on libexif) libexif got build but
mythplugins did not get rebuild.

This is not really proper. E.g. if the lib changed a .h file the build
of the using package could fail, but it could easily get unnoticed as
one will only become aware of this when forcefully rebuilding that
using package.

I think we can do better by better exploiting the timestamp.
E.g. what about doing it this way:

If you build a package and the timestamp of e.g. do-install  (or
do-stage or another pass) of one of your DEPENDS packages is newer
than your do-install (or another task) then your dependency is most
likely newer, so you need a rebuild. (preceded with a clean to get rid
of the old stuff). This should be added to the task list.

Of course is is not as trivial as I sketch here.
This whole game needs to be played recursively bottom up.
So if A depends on B and B depends on C and C gets changed for
whatever reason, then bitbake A should lead to rebuilding B first.

A similar thing might also be needed for RDEPENDS.
If you run time depend on a package that is newer than you are, it
might also be that there are inconsistencies.

How does this sound? I assume it will increase the workload of
bitbake, but I feel this will lead to more stable packages.
Your feedback is appreciated!

Frans.