How to test fixed output derivations in the bordeaux build farm?

[Christopher Baines]

Hey!

I'd like to do more testing of fixed output derivations, both in general
and for patches/branches via QA.

In particular, it would be useful to test specific operations in the
derivation, e.g. downloading just from upstream. Being able to control
this is also necessary to prevent the bordeaux build farm downloading a
previous result from itself via the content addressed mirror or download
nar fallbacks in fixed output derivations.

I've had a look at the GUIX_DOWNLOAD_METHODS environment variable, which
is looked at by the url-fetch procedure, but this seems to work by
generating different derivations, rather than changing the behaviour of
the derivation at build time.

It seems sensible to me to add download-methods to the impureEnvVars so
that this can be passed in when the derivation is built. As far as I can
see though, the only way to set these impureEnvVars is in the
environment when you start the daemon, right? That's very inflexible,
but given most of these variables look like they could/should come from
the client environment (LC_ALL, LC_MESSAGES, LANG adn COLUMNS) maybe
that part of the problem can be addressed in the protocol/daemon.

Have I missed anything?

Thanks,

Chris

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