From 72037880688ff9c6f4188a5746c3ab5ce7215628 Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Tue, 12 Apr 2022 12:26:25 +0200
Subject: lib/generators: withRecursion: don't break attr-sets with special
 attrs
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Closes #168327

The issue reported there can be demonstrated with the following
expression:

    → nix-instantiate --eval -E "with import ./. {}; pkgs.lib.options.showDefs [ { file = \"foo\"; value = pkgs.rust.packages.stable.buildRustPackages; } ]"
    error: attempt to call something which is not a function but a string

           at /home/ma27/Projects/nixpkgs/lib/trivial.nix:442:35:

              441|   isFunction = f: builtins.isFunction f ||
              442|     (f ? __functor && isFunction (f.__functor f));
                 |                                   ^
              443|

Basically, if a `__functor` is in an attribute-set at depth-limit,
`__functor` will be set to `"<unevaluated>"`. This however breaks
`lib.isFunction` which checks for a `__functor` by invoking `__functor`
with `f` itself.

The same issue - "magic" attributes being shadowed by `withRecursion` -
also applies to others such as
`__pretty`/`__functionArgs`/`__toString`.

Since these attributes have a low-risk of causing a stack overflow
(because these are flat attr-sets or even functions), ignoring them in
`withRecursion` seems like a valid solution.
---
 lib/tests/misc.nix | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

(limited to 'lib/tests')

diff --git a/lib/tests/misc.nix b/lib/tests/misc.nix
index fcccf89cc888c..ccfa9cc5ec14f 100644
--- a/lib/tests/misc.nix
+++ b/lib/tests/misc.nix
@@ -613,6 +613,21 @@ runTests {
       expected = false;
     };
 
+  testWithRecursionDealsWithFunctors =
+    let
+      functor = {
+        __functor = self: { a, b, }: null;
+      };
+      a = {
+        value = "1234";
+        b = functor;
+        c.d = functor;
+      };
+    in {
+      expr = generators.toPretty { } (generators.withRecursion { depthLimit = 1; throwOnDepthLimit = false; } a);
+      expected = "{\n  b = <function, args: {a, b}>;\n  c = {\n    d = \"<unevaluated>\";\n  };\n  value = \"<unevaluated>\";\n}";
+    };
+
   testToPrettyMultiline = {
     expr = mapAttrs (const (generators.toPretty { })) rec {
       list = [ 3 4 [ false ] ];
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