This exploits the hermetic nature of flake evaluation to speed up
repeated evaluations of a flake output attribute.
For example (doing 'nix build' on an already present package):
$ time nix build nixpkgs:firefox
real 0m1.497s
user 0m1.160s
sys 0m0.139s
$ time nix build nixpkgs:firefox
real 0m0.052s
user 0m0.038s
sys 0m0.007s
The cache is ~/.cache/nix/eval-cache-v1.sqlite, which has entries like
INSERT INTO Attributes VALUES(
X'92a907d4efe933af2a46959b082cdff176aa5bfeb47a98fabd234809a67ab195',
'packages.firefox',
1,
'/nix/store/pbalzf8x19hckr8cwdv62rd6g0lqgc38-firefox-67.0.drv /nix/store/g6q0gx0v6xvdnizp8lrcw7c4gdkzana0-firefox-67.0 out');
where the hash 92a9... is a fingerprint over the flake store path and
the contents of the lockfile. Because flakes are evaluated in pure
mode, this uniquely identifies the evaluation result.
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.
