Port KeepAliveConcat and UnfoldFlow

[alex-bogomaz]

Hi,

please review this port of KeepAliveConcat and UnfoldFlow streams stages.
This is for issue #3234.

[marcpiechura]

Looks good overall 👍

[marcpiechura]

Could you add an test that verifies that the stage keeps going if the source completes but items are still in the buffer ? Previously I’ve called SetKeepGoing In PreStart and I’m not 100% sure if you need to do it too or if it is handled implicitly since you’ve overridden OnUpstreamFinish

[alex-bogomaz]

@marcpiechura - I added 1 tests that checks that KeepAliveConcat still emits buffered elements after upstream completes. Elements go through KeepAliveConcat with sufficiently large time-out to keep all them in buffer, than upstream completes and test check if elements are still emitted after that.

[marcpiechura]

@alex-bogomaz thanks for verifying it !