[devel] RFC: girar: optimize rebuild

[Vladimir D. Seleznev]

Hi!

The first part of rebuilt packages optimization for girar. It introduces
pkg_identity() and simple optimization of the rebuilt sourcerpm.

pkg_identity() takes RPM package and returns a value called package identity,
a hash of subset of RPM package header. That subset is the entire header
without some nonessential artifacts like buildhost, buildtime, header hashsum,
etc.

The two package builds of the same NEVR might have equal or different
package identities. The equal identities mean that build results of these
packages are equal too, that allows build optimization. The practical
example of simple rebuilt sourcerpm optimization also introduced.

The future work can be about optimization of "copied" to another branch
sourcerpm with retrieved from archive sourcerpm, and binary packages
optimization (this case has an issue when binary subpackages are mixed
archs, i.e. arch and noarch, this probably could work only with single-arch
builds).

Please review and discuss.

--
   WBR,
   Vladimir D. Seleznev