pubsub: use unary calls to send acks/modacks #3123
Merged
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Previously we send ack/modack requests on the StreamingPull streams.
However, if pull responses are buffered, gRPC cannot promptly tell us
that the stream is broken, so we'll keep sending requests on the dead
stream.
This commit implements a temporary solution. We send requests by unary
RPCs instead and only use streaming for receiving messages.
Load test shows no regression. However, modacks seem to take longer to
take effect. The timeout for nack test had to be increased.
cc @mdietz94