In particular, when building a flake lock file, inputs like 'nixpkgs'
are now downloaded only once. Previously, it would fetch
https://api.github.com/repos/<owner>/<repo>/tarball/<ref> and then
later https://api.github.com/repos/<owner>/<repo>/tarball/<rev>, even
though they produce the same result.
Git and GitHub now also share a cache that maps revs to a store path
and other info.
A command like
$ nix run nixpkgs#hello
will now build the attribute 'packages.${system}.hello' rather than
'packages.hello'. Note that this does mean that the flake needs to
export an attribute for every system type it supports, and you can't
build on unsupported systems. So 'packages' typically looks like this:
packages = nixpkgs.lib.genAttrs ["x86_64-linux" "i686-linux"] (system: {
hello = ...;
});
The 'checks', 'defaultPackage', 'devShell', 'apps' and 'defaultApp'
outputs similarly are now attrsets that map system types to
derivations/apps. 'nix flake check' checks that the derivations for
all platforms evaluate correctly, but only builds the derivations in
'checks.${system}'.
Fixes#2861. (That issue also talks about access to ~/.config/nixpkgs
and --arg, but I think it's reasonable to say that flakes shouldn't
support those.)
The alternative to attribute selection is to pass the system type as
an argument to the flake's 'outputs' function, e.g. 'outputs = { self,
nixpkgs, system }: ...'. However, that approach would be at odds with
hermetic evaluation and make it impossible to enumerate the packages
provided by a flake.
Previously, SANDBOX_SHELL was set to empty when unavailable. This
caused issues when actually generating the sandbox. Instead, just set
SANDBOX_SHELL when --with-sandbox-shell= is non-empty. Alternative
implementation to https://github.com/NixOS/nix/pull/3038.
Pure mode should not try to source the user’s bashrc file. These may
have many impurities that the user does not expect to get into their
shell.
Fixes#3090
Fixes
$ nix build
fatal: bad revision 'HEAD'
error: program 'git' failed with exit code 128
on a new flake. It is now detected as a dirty tree with revCount = 0.
Only variables that were marked as exported are exported in the dev
shell. Also, we no longer try to parse the function section of the env
file, fixing
$ nix dev-shell
error: shell environment '/nix/store/h7ama3kahb8lypf4nvjx34z06g9ncw4h-nixops-1.7pre20190926.4c7acbb-env' has unexpected line '/^[a-z]?"""/ {'
Jonas Chevalier