Re: inputs vs. native-inputs vs. propagated-inputs

[Lukas Gradl <lgradl@openmailbox.org>]

ludo@gnu.org (Ludovic Courtès) writes:

> Leo Famulari <leo@famulari.name> skribis:
>
>> I _think_ that `gc --references` works by querying the database for a
>> list of references in the store item you give it as an argument. I think
>> that the information in the database is created after building, by
>> scanning the files of the package's output in /gnu/store, looking for
>> strings that appear to be paths in /gnu/store. I say "I think" because I
>> am not sure. I don't understand that part of the code very well yet.
>
> That’s 100% correct!  :-)  Namely, ‘guix gc --references’ makes an RPC to
> the daemon, which then looks things up in the database (see the ‘Refs’
> table in nix/libstore/schema.sql.)
>
> Scanning for references indeed happens at the end of a successful build,
> in ‘scanForReferences’ in libstore/references.cc.  Since scanning is
> expensive (I/O-intensive), the result is stored in the database.

Sorry, this might be a dumb question, I don't quite understand the
concept of these references.  Why are references important?  Shouldn't
every store item know which other store-items are related to it from the
"inputs"-field in its definition?  Why is it necessary to keep track of
the references?

Thank you!