Improve error message when missing remote building feature forbids build

[nh2]

I tried today to build a big package (chromium) remotely on a powerful machine with

nix-store --realise --builders 'ssh://[email protected] x86_64-linux' /nix/store/i0kwyxpihg1gcp9jg4qwp7qcrpagj818-chromium-67.0.3396.87.drv /nix/store/bmigs53iryqpqjsy5w4qjfndlh6hxbms-chromium-67.0.3396.87.drv

but it would build only locally. Trying to force the remote build with -j0:

nix-store --realise --builders 'ssh://[email protected] x86_64-linux' -j0 /nix/store/i0kwyxpihg1gcp9jg4qwp7qcrpagj818-chromium-67.0.3396.87.drv /nix/store/bmigs53iryqpqjsy5w4qjfndlh6hxbms-chromium-67.0.3396.87.drv

got me:

unable to start any build; either increase '--max-jobs' or enable remote builds

But I have remote build enabled -- --builders is given right there!

On IRC I learned that it's because chromium has requiredSystemFeatures = [ "big-parallel" ];, and I need to declare that my remote machine is big-parallel.

Solution:

nix-store --realise --builders 'ssh://[email protected] x86_64-linux - - - big-parallel' -j0 /nix/store/i0kwyxpihg1gcp9jg4qwp7qcrpagj818-chromium-67.0.3396.87.drv /nix/store/bmigs53iryqpqjsy5w4qjfndlh6hxbms-chromium-67.0.3396.87.drv

---

But ideally we would improve the error message, perhaps

• not suggesting or enable remote builds when it's enabled
• printing a helpful warning/info message when when a builder with fitting architecture isn't selected just because it doesn't have a special flag like big-parallel

[shonfeder-da]

The second option seems much more user friendly to me.

An error along these lines would have been the most helpful for me:

considering building on remote machine 'ssh://vm'
hook reply is 'decline'
this build requires system features [foo,...]
the remote machine is only declared as supporting [...]
if the remote machine supports it, add foo to the requiredSystemsFeatures of the `builders` configuration

[domenkozar]

Similarly for local builds: #2238

[colemickens]

I hit another variant of this, sort of. It's easy to make a syntax error and not notice or be warned, unless your Nix evaluator contains a @lheckemann who might then point it out. For those who don't, maybe this will help:

For example, this will work for building my system configuration:

"ssh://azurenixosuser@${azmachine} x86_64-linux - - - big-parallel,kvm"

But this will not:

"ssh://azurenixosuser@${azmachine} x86_64-linux - - - big-parallel kvm"

(Why my system needs kvm to build is a different question for another day...)

[domenkozar]

Fixed!

[colemickens]

I'm stumped again. Why does it seem like it's not even trying the remote builder I'm offering up?

> nix build .#packages.aarch64-linux.drm-howto --builders ssh-ng://[email protected] --verbose
warning: Git tree '/home/cole/code/nixcfg' is dirty
info: --- Remote build --- build-remote
Failed to find a machine for remote build!

derivation: ziz2sfr9h5w1sss2w2bzhjf4vl2g314g-source.drv
required (system, features): (aarch64-linux, )
1 available machines:
(systems, maxjobs, supportedFeatures, mandatoryFeatures)
(x86_64-linux, 1, , )
error: --- Error ------------------------------------------------------------------------------------------------------------------------ nix
error: --- Error --- nix-daemon
a 'aarch64-linux' with features {} is required to build '/nix/store/ziz2sfr9h5w1sss2w2bzhjf4vl2g314g-source.drv', but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test}

[lheckemann]

You need to specify that it's aarch64 in the builders string: --builders ssh-ng://[email protected] aarch64-linux

[573]

You need to specify that it's aarch64 in the builders string: --builders ssh-ng://[email protected] aarch64-linux

Originally posted by @lheckemann in #2238 (comment)

This was what I needed in my ~/.config/nix/nix.conf (or any of the other possible alternative config locations), had builders = ssh-ng://builder and extra-platforms = aarch64-linux there, now with builders = ssh-ng://builder aarch64-linux the remote builder is picked (https://nixos.org/manual/nix/stable/advanced-topics/distributed-builds.html).