Python test_external_transforms fails on Spark runner

test_external_transforms should theoretically work, but it instead throws a cryptic error:

<_Rendezvous of RPC that terminated with:
status = StatusCode.UNKNOWN
details = ""
debug_error_string = "{"created":"@1557171218.145369401","description":"Error received from peer ipv4:127.0.0.1:48159","file":"src/core/lib/surface/call.cc","file_line":1041,"grpc_message":"","grpc_status":2}"
>

I could just be overlooking some obvious config issue.

EDIT: the previous issue has been fixed. now it seems to fail because the read source is unbounded

https://github.com/apache/beam/blob/23714a335e8a6e0d91106a366e470a3a4820ae27/runners/core-construction-java/src/main/java/org/apache/beam/runners/core/construction/ReadTranslation.java#L92