NI gRPC Device Server reports DAQmx event registration errors as events, not as errors

[Brad Keryan]

The NI gRPC Device Server uses server streaming to return a response for each event generated by DAQmx. However, when a DAQmx event registration function (like DAQmxRegisterDoneEvent) returns an error, the server reports the error by sending a streaming response where response.status is the error code.

When the rest of the NI gRPC Device Server APIs were updated to report errors by returning a gRPC status, the DAQmx event registration functions were not updated.

Implications:

• It is difficult for clients to synchronously detect whether event registration failed.
  • In the success case, no responses are generated until DAQmx notifies the server of an event, so waiting indefinitely for a response only works in the error case.
  • Waiting indefinitely for initial metadata does not guarantee that the error will be reported at the same time because the server sends initial metadata before reporting the error.
• Rich error descriptions are not reported for event registration errors, only an error code.
• For done events, a fatal response.status is ambiguous: it can mean that event registration failed or that the acquisition/generation encountered an error.
• The server does not close the stream when a event registration error occurs, but it will never send any responses other than the one containing the error code.

Addressing this is required to fix nidaqmx-python bug AB#2395958: "GrpcStubInterpreter does not report event registration errors to caller". nidaqmx-python reports event registration errors by raising an exception from the register_done_event method, not by invoking the event callback. We will have the same problem when supporting gRPC in other versions of the DAQmx API.

AB#2396353