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

[Chris Marusich]

Lukas Gradl <address@hidden> writes:

> 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/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?
>>
>> 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!
>
> Best,
> Lukas

If you are curious, there is a more detailed explanation of "references"
in Eelco Dolstra's PhD thesis, "The Purely Functional Software
Deployment Model", available here:

https://nixos.org/%7Eeelco/pubs/phd-thesis.pdf

You'll find more information about Nix here, much of which applies to
Guix also:

http://nixos.org/docs/papers.html

You'll find more links about Guix here:

https://www.gnu.org/software/guix/contribute/

And this repo contains talks etc. about Guix:

http://git.savannah.gnu.org/cgit/guix/maintenance.git/tree/

-- 
Chris

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