Allow relative paths in --store option

[meditans]

In nix commands which accept --store options, we can now specify a
relative path, which will be canonicalized. The relative path must start
with ./ like in the nix language.

Fixes #3734

[meditans]

It works, but it's marked as WIP as some tests should be added for this.

[thufschmitt]

Might make sense to also allow . as a path

[meditans]

The cost of that would be divergence from the nix expression language though, right?

[matthewbauer]

this already happens with cli args. for instance:

nix build -f . hello

[Ericson2314]

but does -f <url> work? Because we are overloading URL and relative path syntax, and URL syntax already is infamous for having parser mistakes/ambiguities/vulnerabilities, I rather move slowly. We can always add . later.

Of course, if @edolstra wants / there's concensus to go ahead and do ., we'll happily add it, but I rather advocate for the smallest change that gets the job done.

[matthewbauer]

url's do work (although they have to be in tar format):

nix build -f https://github.com/NixOS/nixpkgs/archive/master.tar.gz hello

[matthewbauer]

rules for it are:

nix/src/libexpr/common-eval-args.cc

Lines 74 to 85 in 3f01fa1

	Path lookupFileArg(EvalState & state, string s)
	{
	if (isUri(s)) {
	return state.store->toRealPath(
	fetchers::downloadTarball(
	state.store, resolveUri(s), "source", false).first.storePath);
	} else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
	Path p = s.substr(1, s.size() - 2);
	return state.findFile(p);
	} else
	return absPath(s);
	}

[Ericson2314]

OK fair, but it as wrong of me to compare -f <url> in the first place, because this sort of "each store parses the thing" means having else cases is no good either---everything should be if (...) { ... } else return nullptr.

Now we could still do ., yes. My above does not work argument against that, I will totally grant.

[Ericson2314]

@edolstra's weak preference is if it doesn't contain :// and does contain / then it is deemed a path and is turned into a LocalStore.

[Ericson2314]

In meeting we got @edolstra's preference, and I just updated the PR accordingly.

[matthewbauer]

Can you just use !isUri from file-transfer?