Revert "Revert "Merge pull request #6621 from Kha/nested-follows""

This reverts commit a8b3d777fbdaf0b732f129e5be62cd2a1227674b.

This undoes the revert of PR#6621, which allows nested `follows`, i.e.

    {
      inputs = {
        foo.url = "github:bar/foo";
        foo.inputs.bar.inputs.nixpkgs = "nixpkgs";
      };
    }

does the expected thing now. This is useful to avoid the 1000 instances
of nixpkgs problem without having each flake in the dependency tree to
expose all of its transitive dependencies for modification.

This was in fact part of Nix before and the C++ changes applied w/o
conflicts. However, it got reverted then because people didn't want to
merge lazy-trees against it which was supposed to be merged soon back in
October 2022.

Fixes: https://git.lix.systems/lix-project/lix/issues/201

Change-Id: I5ddef914135b695717b2ef88862d57ced5e7aa3c

1 files changed, 60 insertions, 0 deletions

diff --git a/tests/functional/flakes/follow-paths.sh b/tests/functional/flakes/follow-paths.sh
index 183893bde..cd3f75693 100644
--- a/tests/functional/flakes/follow-paths.sh
+++ b/tests/functional/flakes/follow-paths.sh
@@ -230,3 +230,63 @@ git -C "$flakeFollowsOverloadA" add flake.nix flakeB/flake.nix \
 nix flake metadata "$flakeFollowsOverloadA"
 nix flake update --flake "$flakeFollowsOverloadA"
 nix flake lock "$flakeFollowsOverloadA"
+
+# Test nested flake overrides: A overrides B/C/D
+
+cat <<EOF > $flakeFollowsD/flake.nix
+{ outputs = _: {}; }
+EOF
+cat <<EOF > $flakeFollowsC/flake.nix
+{
+  inputs.D.url = "path:nosuchflake";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsB/flake.nix
+{
+  inputs.C.url = "path:$flakeFollowsC";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsA/flake.nix
+{
+  inputs.B.url = "path:$flakeFollowsB";
+  inputs.D.url = "path:$flakeFollowsD";
+  inputs.B.inputs.C.inputs.D.follows = "D";
+  outputs = _: {};
+}
+EOF
+
+nix flake lock $flakeFollowsA
+
+[[ $(jq -c .nodes.C.inputs.D $flakeFollowsA/flake.lock) = '["D"]' ]]
+
+# Test overlapping flake follows: B has D follow C/D, while A has B/C follow C
+
+cat <<EOF > $flakeFollowsC/flake.nix
+{
+  inputs.D.url = "path:$flakeFollowsD";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsB/flake.nix
+{
+  inputs.C.url = "path:nosuchflake";
+  inputs.D.url = "path:nosuchflake";
+  inputs.D.follows = "C/D";
+  outputs = _: {};
+}
+EOF
+cat <<EOF > $flakeFollowsA/flake.nix
+{
+  inputs.B.url = "path:$flakeFollowsB";
+  inputs.C.url = "path:$flakeFollowsC";
+  inputs.B.inputs.C.follows = "C";
+  outputs = _: {};
+}
+EOF
+
+# bug was not triggered without recreating the lockfile
+nix flake update --flake $flakeFollowsA
+
+[[ $(jq -c .nodes.B.inputs.D $flakeFollowsA/flake.lock) = '["B","C","D"]' ]]