substitute: Retry downloading when a nar is unavailable.

Fixes <https://issues.guix.gnu.org/57978>
Reported by Attila Lendvai <attila@lendvai.name>.

Previously, if a narinfo was available but its corresponding nar was
missing (for instance because the narinfo was cached and the server
became unreachable in the meantime), 'guix substitute --substitute'
would try to download the nar from its preferred location and abort when
that fails.  This change forces one retry with each of the URLs.

* guix/scripts/substitute.scm (download-nar): Do not catch
'http-get-error?' exceptions.
(system-error?, network-error?, process-substitution/fallback): New
procedures.
(process-substitution): Call 'process-substitution/fallback' upon
'network-error?'.
* tests/substitute.scm ("substitute, first URL has narinfo but lacks nar, second URL unauthorized")
("substitute, first URL has narinfo but nar is 404, both URLs authorized")
("substitute, first URL has narinfo but nar is 404, one URL authorized")
("substitute, narinfo is available but nar is missing"): New tests.

0 files changed, 0 insertions, 0 deletions