Streams with prefecth_first_result do not convert grpc errors

[daniel-sanche]

one task of grpc_helpers is to wrap grpc channels in error_remapped_callable, converting grpc errors into api_core.exception types. But this error remapping is skipped when processing a streaming rpc with prefetch_first enabled.

When prefetch_first is enabled, the StreamingResponseIterator calls next as part of its init. But if there is an error as part of the next call, the grpc error is raised immediately, without being wrapped

Because the error is not wrapped, and it is raised outside the usual call loop, this will cause retryable errors to not be retried properly

---

To solve this, we should probably add a case to this try/catch:

       except grpc.RpcError as exc:
            self._stored_first_result = exc

And then raise it as part of __next__ if present here

related: b/368027810

[daniel-sanche]

Actually, looking closer, the exception should be handled up higher in the stack by this catch. It's not clear why we're seeing the errors we see in b/368027810