Summary

When I run several async requests against Mongo and then cancel after receiving the first result, I get the error:

ERROR reactor.core.publisher.Operators - Operator called default onErrorDropped
reactor.core.Exceptions$ErrorCallbackNotImplemented: com.mongodb.MongoException: next() called after the cursor was closed.
Caused by: com.mongodb.MongoException: next() called after the cursor was closed.

I am using Rx Java .firstElement() in order to cancel unwanted requests.

 
This is using...
driver version: 4.8.0
Mongo version: 6
Topology: replica set

How to Reproduce

I cannot provide exact steps to reproduce the database but the query below matches a couple of entries in the database which are indexed. This code triggers the exception every time. 

Flowable.range(1, 200)
    .flatMapMaybe(
        iter ->
            Flowable.fromIterable(Vector.of(mainCollection, mainCollection))
                .map(
                    dataBaseName ->
                        mongoClient.getDatabase(dataBaseName).getCollection(collectionName))
                .flatMap(collection -> Flowable.fromPublisher(collection.find()))
                .firstElement())
    .collect(Vector.collector())
    .blockingGet();

Additional Background

It appears to be the double nesting of Flowables that helps trigger this bug. With a very large flat list of queries it doesn't trigger.