flattenTree returns {} for a simple nested attrset (perhaps user error?)

[gautammohan]

Describe the bug

I am trying to use flattenTree to collapse a nested attrset for my flake checks, and it returns an empty set instead of concatenating the keys.

To Reproduce

Steps to reproduce the behavior:
A minimal case in nix repl:

nix-repl> :lf github:numtide/flake-utils/v1.0.0
nix-repl> nixpkgs = import <nixpkgs> {}
nix-repl> pkgs = nixpkgs.pkgs
nix-repl> lib.flattenTree {zip = nixpkgs.pkgs.runCommand "foo" {} "bar";}     
{
  zip = «derivation /nix/store/j43kxmmazclshcr9xzdhic5r3s090ind-foo.drv»;
}

nix-repl> lib.flattenTree {zip.zap = nixpkgs.pkgs.runCommand "foo" {} "bar";}
{ }

Expected behavior

I expect the second command to output an attrset with {zip/zap: <...foo.drv>}, instead of an empty set.

System information

I am using the determinate nix installer (version 0.32.2) with a pinned nixpkgs of 24.05 for my flake. I am unsure about the version of <nixpkgs> in the repl, but this behavior is the same if I run my flake checks directly.

Additional context

I apologize in advance if this is a simple error or misuse of the function; I am still quite new to nix. However, perhaps this is still a useful issue since it feels like "unexpected" behavior, unless I am completely misinterpreting the purpose of flattenTree.

[dermetfan]

flattenTree only recurses into an attribute set if it has recurseForDerivations = true, as documented:

(by respecting recurseIntoAttrs)

It can be a bit surprising that it defaults to false, and unfortunately there is no way change the default.

Here's a function that recursively calls recurseIntoAttrs for all non-derivation attribute sets:

recurseIntoDeepAttrs = attrs:
  lib.recurseIntoAttrs (lib.mapAttrs (_: v:
    if builtins.typeOf v == "set" && !lib.isDerivation v
    then recurseIntoDeepAttrs v
    else v
  ) attrs);

With that, flattenTree behaves as you expected:

nix-repl> utils.lib.flattenTree (recurseIntoDeepAttrs {
            zip.zap = pkgs.runCommand "foo" {} "bar";
          })
{
  "zip/zap" = «derivation /nix/store/ds9cnsp8yrja1gm9wqiyhqhqwg64zqsq-foo.drv»;
}

[TheNeikos]

I just fell into this.

Maybe on could check if the function input is a set, and actually has the recurse property set. And if it doesn't, emit a warning/error? Cause I don't see how calling the function could be useful in that situation.

[gautammohan]

@dermetfan Thank you so much for the explanation! I suspected my issue was not fully understanding something about nix.

Perhaps this can be added as a cautionary use-case in the documentation for flattenTree. If any maintainers think it's a good idea, I would be happy to work on a PR for it.