Hi there 👋🏼

Thank you for working on this. I ran a few tests using this branch, and unfortunately the problem persists. Going to try and share as much information as possible, progressively, as time allows me to investigate further.

Current knowledge: we see infinite recursion in trace_message_refs/3, prior to the code modified in this PR. Still digging, as time allows, to figure out why.

Edit: It looks like the smallest reproducible test case might be any message that references google.protobuf.Struct. This message type is indirectly self-referential.

Edit 2: It's possible that the solution is related to adding a conditional based on State.has_symbol?/2 in the reduce call within trace_message_fields/4:

    |> Enum.reduce(state, fn %{mod: mod, symbol: symbol}, state ->
      symbol = Util.trim_symbol(symbol)

+     if State.has_symbol?(state, symbol) do
+       state
+     else
        response =
          if symbol in nested_types do
            build_response(parent_symbol, module)
          else
            build_response(symbol, mod)
          end

        state
        |> Extensions.add_extensions(symbol, mod)
        |> State.add_symbols(%{symbol => response})
        |> State.add_files(%{(symbol <> ".proto") => response})
        |> trace_message_refs(symbol, mod)
+     end
    end)

Unfortunately, while this addition avoids infinite recursion in my test cases, we get a different error (that may be completely unrelated, because it happens in this branch and on the latest release).

Edit 3: I see now that #59 is a better version of this suggestion.

@aj-foster yeah, I had some work in that direction in #59 already, which I'm embarrassed I didn't think to combine these before your comment. They are both in this PR now. It does make sense that we would need both solutions as the recursion occurs in two places.

Thanks for helping keep an eye on this, it is really appreciated.