bug#30143: UX: print warning if substitute server is not authorized

[Chris Marusich <cmmarusich@gmail.com>]

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Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de> writes:

> Suppose I add example.com as a substitute server by passing
> “--substitute-urls=https://example.com” to the daemon or the Guix
> command line.  I haven’t authorized the signing key, so Guix won’t
> accept any of the substitutes from example.com.
>
> Currently, Guix does not make it obvious to the user that a requested
> substitute server is ignored because its key is not authorized.  We
> should print a clear warning in this case.
>
> (guix scripts authenticate) already includes “validate-signature”, which
> aborts with an error if the key is not authorized, but we don’t seem to
> use it.

What if example.com serves substitutes that are signed by another
server, such as hydra.gnu.org?  No matter where a substitute comes from,
if it was signed with an authorized key and its signature checks out,
then it's OK to use, right?

-- 
Chris

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