Add flake evaluation cache

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.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index bb0543541..0018a0d07 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -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));
+}
+
 }