JAVA-2934: Handle empty non-final pages in ReactiveResultSetSubscription

[dssysolyatin]

Hi !

I tried to use this driver with scylladb database, because it supports CQL protocol. But code started to hang for some queries:

val source = Source.fromPublisher(session.executeReactive(statement))
Await.result(source.runWith(Sink.ignore), scala.concurrent.duration.Duration.Inf))
println("finished")

The reason is that scylladb sometime returns a page with no data but with pagingState != null and it leads to code hanging

For instance, if pages queue contains three pages:

• first page with data and with pagingState != null
• second page without data and pagingState != null
• third page without data and pagingState = null

Akka stream calls request(1) if it needs data and request calls drain that sends one row to akka stream subscriber. It works well for pages with data. But if there is page without data and pagingState != null the following situation appears:

Akka stream calls requests(1) that calls drain that calls tryNext. tryNext method detects that there is no data anymore in page 1 and it gets second page from pages queue. But because of second page has no data, tryNext() will return null and no data will be sent to akka stream subscriber. Because no data will be sent to akka subscriber, akka stream will not call request(1) anymore and drain will not be called from anywhere because all pages are already loaded.

I don't know why scylladb returns pages without data and pagingState != null. I will ask scylladb team, but it also would be good to fix this case in driver as well

[adutra]

Nice and elegant :-)

Could you please:

• Add a unit test for this specific case in ReactiveResultSetSubscriptionTest
• Change the commit and PR name to JAVA-2934: Handle empty non-final pages in ReactiveResultSetSubscription
• Add an entry to the changelog:

4.12.0 (in progress)
- [bug] JAVA-2934: Handle empty non-final pages in ReactiveResultSetSubscription

Thanks!

[dssysolyatin]

@adutra I fixed PR. Could you re-review my PR ?

[adutra]

Looking good, please do the changes I suggested and this should be good to go. Thank you for your time and involvement!

[adutra]

Thank you so much @dssysolyatin !

[dssysolyatin]

Thank you for reviewing of PR @adutra

[adutra]

@dssysolyatin sorry I forgot the legal part :-) I need you to sign our CLA please:

https://cla.datastax.com/

Please ping me when you've signed it. Many thanks!

[dssysolyatin]

@adutra No problem. Done ;)