3.x: Flowable.publish() should not drop items if there are no consumers

[akarnokd]

Unlike Observable.publish(), the Flowable.publish() has an internal queue and fetches data from upstream based on consumption. For historical reasons, the 2.x implementation drops items when there are no consumers, which can lead to unnecessary dataloss when switching between consumers on an item boundary.

I propose changing the operator in 3.x to not drop items when there are no consumers. The 2.x behavior can be achieved via an additional unbounded consumer which stays subscribed until the operator completes.

[davidmoten]

I propose changing the operator in 3.x to not drop items when there are no consumers.

Does this only apply after the first subscription has happened?

[akarnokd]

If there are no subscribers, don't consume the queue. The 2.x version has dedicated logic to keep consuming the queue.

[akarnokd]

Closing via #6519.