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

From: Lukas Gradl
Subject: Re: inputs vs. native-inputs vs. propagated-inputs
Date: Tue, 21 Jun 2016 08:37:27 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

address@hidden (Ludovic Courtès) writes:

> Lukas Gradl <address@hidden> skribis:
>> address@hidden (Ludovic Courtès) writes:
>>> Leo Famulari <address@hidden> 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/  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?
> The “references” of a store item are its run-time dependencies, a subset
> of the ‘inputs’ etc. fields, which are themselves the compile-time
> dependencies.
> If those run-time dependencies were not inferred automatically by the
> daemon, we’d have to maintain them individually, and this would be
> error-prone and imprecise.
> Run-time dependency information is what allows Guix to know which
> substitutes need to be downloaded when installing from substitutes, and
> it’s what allows the garbage collector to determine which store items
> are “live”, and which ones are not.

OK, Thank you for the explanation, this makes more sense to me now!


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