It seems like a simple before/after comparison of active useflags and the text 
of the src_* functions (skipping build and install if they are completely 
identical) should catch the majority of unnecessary rebuilds.

On 25 July 2014 13:36 Ian Stakenvicius wrote:
> On 25/07/14 01:15 PM, Andreas K. Huettel wrote:
> >
> >>> Maybe this could be solved by having two kinds of revisions: -
> >>> One would rebuild all as usually (for example, -r1...) - The
> >>> other one would only regenerate VDB and wouldn't change the
> >>> installed files (for example, -r1.1)
> >>>
> >>> But I am not sure if it could be viable from a "technical"
> >>> point of view :(
> >>
> >> I'm afraid it couldn't. The major problem is not knowing *when*
> >> to migrate metadata, portage usually gets that right. The problem
> >> is in getting the correct output which is often near to
> >> impossible.
> >
> > I think we'd appreciate some more information here:
> >
> > * What exactly is the metadata that we're talking about? * How much
> > of it can be generated by sourcing the ebuild, without running
> > phases? * When does that exactly break? Example?
> >
> 
> It may help this overall discussion to step back even further here:
> 
> * What do we want to accomplish, exactly?
> 
>  - allow portage to emerge updated packages without rebuilding them
> (ie running src_* and pkg_*inst phases), when it can be determined
> from metadata changes (or for -rX.Y method perhaps it is assumed
> -just- on filename changes) that this is not necessary.
> 
> * What are all of the cases that would validly trigger this
> dont-bother-to-run-phases shortcut?
> 
> - addition of missing dependency [1]
> - new useflag not enabled by default
> - removed useflag that was not previously enabled
> - changing of dependency atom(s) -- ie swapping a specific atom to
> virtual one
> - EAPI bump (maybe??)
> - ....  <-- add more please!
> 
> 
> * For each of the above, in what cases is a full rebuild probably
> necessary anyhow??
> 
> - dependency atom minimum version is increased, and the in-VDB version
> of this dep is too old (??)
> - new dependency atom is missing from VDB (??)
> - EAPI bump adds new entries to VDB that need to be calculated from
> say, merge phase
> - ....
> 
> 
> 
> ..i think from there we can perhaps determine what metadata could be
> updated and/or needs to be updated to maintain consistency in the
> dont-rebuild cases we want.
>