Fix grpc middleware interceptor not PostCall-ing when a streaming RPC with non-streaming server finishes successfully.

[alexandrupitis1]

Current Client-side Stream Interceptor implementation is bugged - it assumes that if the RPC is streaming, then the Server must be streaming responses too. This is incorrect, as a Streaming RPC can be either a Bidi stream ( which are currently handled properly ), or a Client-side only stream. In this case, if the RPC finished successfully, then PostCall never fires as Interceptor assumes it should get an io.EOF ( which obviously does not make sense when Server is not sending a response stream ).

This most obviously manifests when using it with Prometheus reporter - client side metrics for such RPCs that depend on PostCall ( grpc_client_handled_total, grpc_client_handling_seconds ) are not exported.

Changes

• Client StreamInterceptor now handles cases when Server is streaming or non-streaming separately.

Verification

• I created a POC with a Client-streaming, Server non-streaming RPC and modified the example in this repo to run it. Verified that the example does not export certain prometheus metrics ( grpc_client_handled_total, grpc_client_handling_seconds ) for said RPC.
• Applied the fix, verified again that the metrics are now exported

.

[johanbrandhorst]

Nice! Could you add a test for this?

[alexandrupitis1]

Hi there Johan,

Sure - I'll take a stab next week, pretty loaded right now with other things.