Make flake input fetching lazy

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.

1 files changed, 11 insertions, 0 deletions

diff --git a/tests/flakes.sh b/tests/flakes.sh
index 998abfd09..29845e5ed 100644
--- a/tests/flakes.sh
+++ b/tests/flakes.sh
@@ -191,6 +191,8 @@ nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth
 # Check whether it saved the lockfile
 [[ ! (-z $(git -C $flake3Dir diff master)) ]]
 
+git -C $flake3Dir commit -m 'Add lockfile'
+
 # Unsupported epochs should be an error.
 sed -i $flake3Dir/flake.nix -e s/201906/201909/
 nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth 2>&1 | grep 'unsupported epoch'
@@ -241,3 +243,12 @@ git -C $flake3Dir commit -m 'Add nonFlakeInputs'
 
 # Check whether `nix build` works with a lockfile which is missing a nonFlakeInputs
 nix build -o $TEST_ROOT/result --flake-registry $registry $flake3Dir:sth
+
+# Check whether flake input fetching is lazy: flake3:sth does not
+# depend on flake2, so this shouldn't fail.
+rm -rf $TEST_HOME/.cache
+clearStore
+mv $flake2Dir $flake2Dir.tmp
+nix build -o $TEST_ROOT/result --flake-registry $registry flake3:sth
+(! nix build -o $TEST_ROOT/result --flake-registry $registry flake3:xyzzy)
+mv $flake2Dir.tmp $flake2Dir