Feature/cleanup and recursion

[mjheilmann]

This is an attempt to resolve the circular recursion case, in this PR:

1. Rework the state to store the descriptor once, so symbols reference a filename instead of holding a duplicate descriptor
2. Some state simplification, making some operations stricter
3. Add a function that shrinks the state by combining cycles

When files participate in a cycle, causing the reflection tree to instead be a graph, we combine those participating file descriptors into a single descriptor.

• minimal change from each-module-as-a-file
• clients should now resolve schemas that contain cycles

[aj-foster]

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.