[oe] Checksums in Bitbake

[Frans Meulenbroeks]

Richard,

Interesting ideas.
I need to let this digest a little  bit.

Some initial thougths
The checksum should also depend on the checksum of the underlying
packages. E.g. if A depends on B and the checksum of B changes it
should trigger a rebuild of A.

A first crude approach would be to have a hash of the concatenation of
the unfolded recipe (so with all includes/requires expanded) and the
hashes of the recipes it depends on). Of course this is very rough as
even changing whitespace in a recipe will lead to a recompile.
A different approach would be to let it depend on PV + PR. That'll put
the developer in control (with all related issues, like the developer
not bumping PR).
And yet a different one would be to use variables and functions from the recipe.

I have mixed feelings on whether checksums also would depend on global
vars (e.g. code generated by the classes or variables in e.g.
local.conf).
On the one hand it seems pretty neat, on the other hand I worry about
performance (calculating the checksum).

Maybe a solution would be to have a hash or so per file and
concatenate all hashes on which a recipe depends (including class
files).

As said, just some initial thoughts.
Hope this helps.

Frans.