Segfault when displaying a self-referencing attrset as JSON

[hgl]

Describe the bug

When displaying a self-referencing attrset as JSON, it would be great if nix could show an error message along with the stack.

Steps To Reproduce

test.nix

let
  a = {
    b = a;
  };
in
a

$ nix eval --json -f test.nix .
fish: Job 1, 'nix eval --json -f test.nix .' terminated by signal SIGSEGV (Address boundary error)

Expected behavior

An error message showing the stack

Metadata

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-01-22-nix-team-meeting-minutes-210/59239/1

[llakala]

Oh hey, I just found out about this in #11576!

However, I do get an error message with the same setup:

❯ nix eval --json -f test.nix .
error: stack overflow (possible infinite recursion)

And, inlining the expression rather than storing it in a file, same result:

❯ nix eval --json --expr "let a = { b = a; }; in a"
error: stack overflow (possible infinite recursion)

I did some binary search on old Nix versions (see my comment in the linked issue). But TLDR, even though the behavior changed between 2.11 and 2.12, in both cases, I get a different output from what you got. Maybe it's a flaky output based on the amount of cache you have or something.