fix: nix copy ssh-ng:// not respecting --substitute-on-destination

[SharzyL]

Motivation

When using nix copy with a ssh-ng destination, the flag substitute-on-destination is ignored, instead the builder-use-substitute setting is used.

Refer to #9591 for related source code and reproduction steps.

Context

Closes: #9591, #8744

For legacy ssh destination, --substitute-on-destination is used and builder-use-substitute setting is ignored. It seems reasonable to keep consistency with this strategy since builder-use-substitute looks unrelated to nix copy.

Priorities

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[Ericson2314]

Yup it does seem to be a mistake going back to 3a63fc6. Whereas in the same commit: https://github.com/NixOS/nix/blob/3a63fc6cd515fb009b17d864fede23a356832a5e/src/libstore/legacy-ssh-store.cc

Can you add a test for this, ensuring both protocols do the right thing?

[thufschmitt]

Can you add a test for this, ensuring both protocols do the right thing?

I also wanted to ask for that, but then I realised that I couldn't think of any way of testing it that's not badly contrived. Do you have an idea of how we could do it?

[Ericson2314]

What happens when one tries to copy over a path that they don't have, but the substituter does?

[thufschmitt]

What happens when one tries to copy over a path that they don't have, but the substituter does?

I think it will first try to realise it locally, and fail if it can't

[Ericson2314]

Ah true. Perhaps we are entering contrived now, but: we could corrupt it locally and see which version got copied over. (Copying the corrupted version would fail.)

[Ericson2314]

I'll just give it a shot myself. Since this sounds much harder than the actual fix.

[Ericson2314]

Ok #9604 adds the rest, skipping ssh-ng. @thufschmitt if you approve that then I can make this one no longer skip in the ssh-ng case, and then we are good!

[thufschmitt]

Ok #9604 adds the rest

Yay, thanks

Let's merge it then, thanks a lot for the fix @SharzyL !