Re: [PATCH 1/1] gnu: eudev: Update home-page URL.

[Leo Famulari <leo@famulari.name>]

On Sun, Dec 27, 2015 at 10:46:12AM -0500, Mark H Weaver wrote:
> Leo Famulari <leo@famulari.name> writes:
> 
> > On Fri, Dec 25, 2015 at 09:11:04AM -0500, Mark H Weaver wrote:
> >> Leo Famulari <leo@famulari.name> writes:
> >> > * gnu/packages/linux.scm (eudev)[home-page]: Add up-to-date URL.
> >> 
> >> Looks good, please push!
> >> 
> >> This won't require any rebuilds, because the 'home-page' field is not
> >> included in the generated derivations.
> >
> > Ah, right, of course! Because it doesn't affect the sha256 hash of the
> > source.
> 
> It's true that it doesn't affect the hash of the source, but that would
> not be sufficient to ensure no rebuilds.  Adding a new phase would not
> change the hash of the source, but it would affect the _derivation_ (as
> serialized in the *.drv file) and thus would require rebuilds.  The
> 'home-page' field is not included in the derivation.
> 
> > And the same for the source URI?
> 
> Changing the source URI affects the derivation that downloads the
> source, however, that derivation is of a special kind called a "fixed
> output derivation", which means that the sha256 hash of the output is
> known in advance.  Such derivations do not need to be rebuilt even if
> the derivation changes, as long as the hash stays the same, because it
> is presumed that the output would not change.

Thank you for taking the time to explain this. For everyone reading
along, there is some more information about fixed-output derivations in
the manual here:
https://www.gnu.org/software/guix/manual/html_node/Derivations.html