Request awaitSingleOrNull for reactive

[JasonMing]

kotlin-coroutines awaitSingle() can assert the Flux contains one and only one element and returning first element.
But seems there is no operators can assert the Flux contains zero or one element, i.e., awaitSingleOrNull().

[elizarov]

Also related to #2079

[dkhalanskyjb]

Hi @JasonMing! In fact, a method with that same name has been present for a while as an extension on arbitrary Publisher, but we decided to deprecate that method, as it has a confusing name: "xOrY" in Kotlin typically means that if the operation finishes with an error, then "Y" will be returned instead, but for awaitSingleOrNull, this would require that null is returned in cases of both zero elements and more than one element, which can be surprising.

Maybe this method could be added with another name, like awaitAtMostOne, but we would like first to know the use cases for this methods. Could you please share yours?

[JasonMing]

Hi @dkhalanskyjb! This issue seems has been obsoleted after kotlinx.coroutines 1.4.0 released at 27 Oct 2020.
If only focus on this issue, the reason I'd like to request awaitSingleOrNull() feature is, awaiting on a Mono which I don't care about its value but only status, single may provide better readability and semantic than first.
This await operation is common in bridging reactive api in coroutine context.

[dkhalanskyjb]

This issue being obsoleted will become obsoleted after kotlinx.coroutines 1.5.0, as we plan to remove awaitSingleOrNull :) #2628
That said, your use case should not be affected, as awaitSingleOrNull on Mono will still be present. So, this issue can be closed then?

[JasonMing]

ok👌