fix: pass timeout to 'PartialRowsData.response_iterator'

[mf2199]

This offers a minimally invasive solution to the client hanging issue. As reported by the author and reproduced using the emulator, the client hangs when there is no response from the CBT server. One of the causes is the specified deadline not propagating to the corresponding gRPC calls. The proposed change links this input with the _grpc._UnaryStreamMultiCallable.__call__.timeout parameter by adding it as a keyword argument to the PartialRowsData constructor.

An alternative way would invoke gapic wrap_method to wrap the bigtable_data_client.transport.read_rows, along with the user-supplied timeout value. But that would bind Table.read_rows to gapic retry engine instead of the 'smart' retries offered by 'raw' gRPC. The latter supposedly avoids repeating the whole read sequence in a case of a transient failure, the lack of which may become an issue with large reads, when chances of error become significant while the time needed to complete the sequence gets increasingly large. Another possible approach is to call gapic's bigtable_data_client.read_rows instead of bigtable_data_client.transport.read_rows. But that would also invoke gapic retries and may cause the same problem as described above.

The timeout parameter must be somewhat greater than the value contained in self.retry, in order to avoid race-like condition and allow registering the first deadline error before invoking the retry. Otherwise there is a risk of getting stuck in an infinite loop that resets the timeout counter just before it being triggered. The increment by 1 second here is customary but should not be much less than that.

*** NOTE: The kokoro tests are bound to fail as the deadline property of the Retry class has not been exposed yet. This is being addressed in a separate PR. ***

The coverage test does not register these changes, hence adding a unit test for now has been left as an option.

Fixes#6 🦕

[mf2199]

@kolea2 Friendly ping.

[kolea2]

why is this + 1? Can you add a comment explaining?

[mf2199]

@kolea2 Sure, copied from the PR description.