builtins.getFlake fails with "cannot write modified lock file"

[ncfavier]

Describe the bug

builtins.getFlake fails with "cannot write modified lock file" when using it on a repository without a lock file, e.g. home-manager, even when passing --no-write-lock-file.

Steps To Reproduce

$ echo 'builtins.getFlake "github:nix-community/home-manager"' > f.nix
$ nix eval --no-write-lock-file -f f.nix
error: cannot write modified lock file of flake 'github:nix-community/home-manager' (use '--no-write-lock-file' to ignore)
(use '--show-trace' to show detailed location information)

Expected behavior

No such error, i.e. don't attempt to write a lock file in builtins.getFlake.

nix-env --version output

nix (Nix) 2.9.0pre20220505_f4102de

Additional context

nix/src/libexpr/flake/flake.cc

Lines 722 to 729 in fdc97fc

	callFlake(state,
	lockFlake(state, flakeRef,
	LockFlags {
	.updateLockFile = false,
	.useRegistries = !evalSettings.pureEval && fetchSettings.useRegistries,
	.allowMutable = !evalSettings.pureEval,
	}),
	v);

We should probably add .writeLockFile = false here? I'll PR that in a while if there's no response here.

[edolstra]

We do have to make sure that pure evaluation continues to fail, e.g.

nix eval --json --expr 'builtins.getFlake "github:nix-community/home-manager/32a7da69dc53c9eb5ad0675eb7fdc58f7fe35272"'  

has to fail if there are unlocked inputs. I think this will fail before it even gets to writing the lock file though (e.g. error: cannot update flake input 'nixpkgs' in pure mode).

[ncfavier]

Yes, the suggested fix doesn't change that