bug#57978: [bug#58017] [PATCH 2/2] substitute: Retry downloading when a nar is unavailable.

[Maxime Devos <maximedevos@telenet.be>]

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> +(test-equal "substitute, first URL has narinfo but nar is 404, both URLs authorized"
> +  "Substitutable data."
> +  (with-narinfo*
> +      (string-append %narinfo "Signature: "
> +                     (signature-field %narinfo))
> +      %main-substitute-directory
> +
> +    (with-http-server `((200 ,(string-append %narinfo "Signature: "
> +                                             (signature-field %narinfo)))
> +                        (404 "Sorry, nar is missing!"))
> +      (dynamic-wind
> +        (const #t)
> +        (lambda ()
> +          (parameterize ((substitute-urls
> +                          (list (%local-url)
> +                                (string-append "file://"
> +                                               %main-substitute-directory))))
> +            (request-substitution (string-append (%store-prefix)
> +                                                 "/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-foo")
> +                                  "substitute-retrieved"))
> +          (call-with-input-file "substitute-retrieved" get-string-all))
> +        (lambda ()
> +          (false-if-exception (delete-file "substitute-retrieved")))))))

Shouldn't it only ignore 'file not found' (ENOENT?) exceptions?
If the exception handling is refined a bit, it becomes a bit more 
complicated, and could be simplified to (when [exists] [delete]), as 
there are no atomicity concerns.

This test, and some others, can be improved by also checking the URI. 
While currently 'with-http-server' does not support that, there are (5 
months, with the v1 having seen some reviewing and a v2 available) 
patches for that at <https://issues.guix.gnu.org/53389>.

That patch also _requires_ always mentioning the URI, if the cover 
letter is correct.  It also allows simplifying the use of '%local-url' a 
bit.

Greetings,
Maxime.

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