Tesla Response Stream is not compatible with Tesla Multipart upload

[elijahkim]

I've been trying to stream an upload from GCP Cloud Storage to OpenAi using Tesla and I'm running into issues that I think has to do with Tesla. The code looks similar to this:

def get_stream(bucket, object) do
    {:ok, token} = Goth.Token.for_scope("https://www.googleapis.com/auth/cloud-platform")

    Tesla.client(
      [{Tesla.Middleware.Headers, [{"authorization", "Bearer #{token.token}"}]}],
      {Tesla.Adapter.Mint, [body_as: :stream]}
    )
    |> GoogleApi.Storage.V1.Api.Objects.storage_objects_get(
      bucket,
      object,
      [alt: "media"]
    )
end

def upload(stream) do
    body =
      Multipart.new()
      |> Multipart.add_file_content(stream, "file.mp4")
      |> Multipart.add_field("model", "whisper-1")
      |> Multipart.add_field("response_format", "json")

    post("v1/audio/transcriptions", body)
end

{:ok, resp} = get_stream("foo", "bar")

upload(resp.body)

The response from GCP returns a stream that is a function/2 and if I try to use that directly in upload/1 I get an error due to https://github.com/elixir-tesla/tesla/blob/master/lib/tesla/multipart.ex#L189. I also tried turning the function into a stream doing Stream.map(env.body, &Function.identity/1) but that seems to just hang before doing the upload. The last thing I tried was adding is_function into the guard that checks if the part is a valid type but that hangs as well.

I tried spelunking a bit deeper into Tesla but can't seem to figure out exactly how it works or else I would've tried opening a PR. Any help will be appreciated.

Thanks!