hydra-queue-runner: fix building on machines specified with the ssh-ng protocol

[Ma27]

Most of my machines in /etc/nix/machines are specified as
ssh-ng://foobar where foobar is defined in e.g. ~/.ssh/config. The
problem here is that while it's possible to use the remote Nix
installation via SSH, hydra-queue-runner refuses to do so since ssh(1)
can't resolve hostnames that begin with ssh-ng://.

To work around this problem, this prefix will be drop if it exists on a
machine declaration that was chosen for a build step.

cc @grahamc @edolstra

[grahamc]

It seems like we would be better served by using Nix's own store abstractions instead of running our own ssh incantation. I don't suppose there is a way we could test this behavior?

[Ma27]

I don't suppose there is a way we could test this behavior?

Well, I have an entry like this in my /etc/nix/machines:

ssh-ng://builder-ng x86_64-linux - 10 4 big-parallel,kvm,nixos-test

With this patch on a Hydra, hydra-queue-runner happily picks up this server to build derivations :)

It seems like we would be better served by using Nix's own store abstractions instead of running our own ssh incantation

Not sure if I agree here. First of all, this nix-store --serve via ssh approach is also used by e.g. nix.sshServe (IIRC). Also, to me this currently sounds like yet another thing to catch up with (rather rapid) changes in nixUnstable.

[knedlsepp]

Fixes: #688

[edolstra]

This PR just makes hydra-queue-runner treat ssh-ng:// as ssh://, which IMHO is not all that useful and probably isn't what users expect.

Currently hydra-queue-runner uses the nix-store --serve protocol, so it can only use ssh://. I have a branch somewhere that makes it use the Store abstraction. However, we can't use it because of some build log issues (fixable) and lack of build step cancellation (not fixable without some major changes to the Store API - currently build steps are cancelled by sending a signal to the corresponding SSH process).

[Ma27]

Hmm yeah, so I know the difference between ssh-ng and ssh, however this was supposed to be a cheap fix for existing /etc/nix/machines where I use ssh-ng by default for good reasons.

But if you've got already a draft for using Nix's store abstraction in Hydra (and hence allow both protocols), that's way better, so 👍 for closing it then :)

[edolstra]

To be clear, the code that I wrote isn't usable because it doesn't support cancellation.

[ajs124]

The current code supports cancellation? I'm not sure that's ever worked for me.

[grahamc]

It does support cancellation, and it often does work :)

[dasJ]

I thought the exact log message was "maybe cancelling build" and it finishes the current step and aborts the rest?

[Ericson2314]

To be clear, the code that I wrote isn't usable because it doesn't support cancellation.

Where is that branch again? I've been thinking if some of legacy ssh store and remote store methods were split up to separate the parts hydra needs to do separately, it could work. That split would be of no use to Nix itself, but I think the separation could be worth it for hydra's sake alone.

[edolstra]

Looks like I never committed anything, but I recovered this from my stash: https://pastebin.com/suP2ZX2D