When computing a lock file, we now respect the lock files of flake
inputs. This is important for usability / reproducibility. For
example, the 'nixops' flake depends on the 'nixops-aws' and
'nixops-hetzner' repositories. So when the 'nixops' flake is used in
another flake, we want the versions of 'nixops-aws' and
'nixops-hetzner' locked by the the 'nixops' flake because those
presumably have been tested.
This can lead to a proliferation of versions of flakes like 'nixpkgs'
(since every flake's lock file could depend on a different version of
'nixpkgs'). This is not a major issue when using Nixpkgs overlays or
NixOS modules, since then the top-level flake composes those
overlays/modules into *its* version of Nixpkgs and all other versions
are ignored. Lock file computation has been made a bit more lazy so it
won't try to fetch all those versions of 'nixpkgs'.
However, in case it's necessary to minimize flake versions, there now
are two input attributes that allow this. First, you can copy an input
from another flake, as follows:
inputs.nixpkgs.follows = "dwarffs/nixpkgs";
This states that the calling flake's 'nixpkgs' input shall be the same
as the 'nixpkgs' input of the 'dwarffs' input.
Second, you can override inputs of inputs:
inputs.nixpkgs.url = github:edolstra/nixpkgs/<hash>;
inputs.nixops.inputs.nixpkgs.url = github:edolstra/nixpkgs/<hash>;
or equivalently, using 'follows':
inputs.nixpkgs.url = github:edolstra/nixpkgs/<hash>;
inputs.nixops.inputs.nixpkgs.follows = "nixpkgs";
This states that the 'nixpkgs' input of the 'nixops' input shall be
the same as the calling flake's 'nixpkgs' input.
Finally, at '-v' Nix now prints the changes to the lock file, e.g.
$ nix flake update ~/Misc/eelco-configurations/hagbard
inputs of flake 'git+file:///home/eelco/Misc/eelco-configurations?subdir=hagbard' changed:
updated 'nixpkgs': 'github:edolstra/nixpkgs/7845bf5f4b3013df1cf036e9c9c3a55a30331db9' -> 'github:edolstra/nixpkgs/03f3def66a104a221aac8b751eeb7075374848fd'
removed 'nixops'
removed 'nixops/nixops-aws'
removed 'nixops/nixops-hetzner'
removed 'nixops/nixpkgs'
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.
Instead of a list, inputs are now an attrset like
inputs = {
nixpkgs.uri = github:NixOS/nixpkgs;
};
If 'uri' is omitted, than the flake is a lookup in the flake registry, e.g.
inputs = {
nixpkgs = {};
};
but in that case, you can also just omit the input altogether and
specify it as an argument to the 'outputs' function, as in
outputs = { self, nixpkgs }: ...
This also gets rid of 'nonFlakeInputs', which are now just a special
kind of input that have a 'flake = false' attribute, e.g.
inputs = {
someRepo = {
uri = github:example/repo;
flake = false;
};
};
As long as the flake input is locked, it is now only fetched when it
is evaluated (e.g. "nixpkgs" is fetched when
"inputs.nixpkgs.<something>" is evaluated).
This required adding an "id" attribute to the members of "inputs" in
lockfiles, e.g.
"inputs": {
"nixpkgs/release-19.03": {
"id": "nixpkgs",
"inputs": {},
"narHash": "sha256-eYtxncIMFVmOHaHBtTdPGcs/AnJqKqA6tHCm0UmPYQU=",
"nonFlakeInputs": {},
"uri": "github:edolstra/nixpkgs/e9d5882bb861dc48f8d46960e7c820efdbe8f9c1"
}
}
because the flake ID needs to be known beforehand to construct the
"inputs" attrset.
Fixes#2913.
This is primarily useful for version string generation, where we need
a monotonically increasing number. The revcount is the preferred thing
to use, but isn't available for GitHub flakes (since it requires
fetching the entire history). The last commit timestamp OTOH can be
extracted from GitHub tarballs.
This ensures that flakes don't get garbage-collected, which is
important to get nix-channel-like behaviour.
For example, running
$ nix build hydra:
will create a GC root
~/.cache/nix/flake-closures/hydra -> /nix/store/xarfiqcwa4w8r4qpz1a769xxs8c3phgn-flake-closure
where the contents/references of the linked file in the store are the
flake source trees used by the 'hydra' flake:
/nix/store/n6d5f5lkpfjbmkyby0nlg8y1wbkmbc7i-source
/nix/store/vbkg4zy1qd29fnhflsv9k2j9jnbqd5m2-source
/nix/store/z46xni7d47s5wk694359mq9ay353ar94-source
Note that this in itself is not enough to allow offline use; the
fetcher for the flakeref (e.g. fetchGit or downloadCached) must not
fail if it cannot fetch the latest version of the file, so long as it
knows a cached version.
Issue #2868.
This PR was not intended to be merged until those tests were actually
passing. So disable them for now to unbreak the flakes branch.
https://hydra.nixos.org/eval/1519271
I.e. flake3 depends on flake2 which depends on flake1. Currently this
fails with
error: indirect flake reference 'flake1' is not allowed
because we're not propagating lockfiles downwards properly.
