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Add flake evaluation cache

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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.
This commit is contained in:
Eelco Dolstra 2019-06-07 22:25:48 +02:00
parent 671f16aee0
commit 6644b6099b
No known key found for this signature in database
GPG key ID: 8170B4726D7198DE
9 changed files with 293 additions and 49 deletions
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9
src/libexpr/flake/flake.cc
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@ -601,4 +601,13 @@ const Registries EvalState::getFlakeRegistries()
return registries;
}
Fingerprint ResolvedFlake::getFingerprint() const
{
// FIXME: as an optimization, if the flake contains a lockfile and
// we haven't changed it, then it's sufficient to use
// flake.sourceInfo.storePath for the fingerprint.
return hashString(htSHA256,
fmt("%s;%s", flake.sourceInfo.storePath, lockFile));
}
}