#421 - Handle modifying query methods returning kotlin.Unit.

[ada-waffles]

Added check for kotlin.Unit to AbstractR2dbcQuery#getExecutionToWrap. This case is essentially equivalent to a return type of Void, but the singleton Unit instance needs to be returned instead of discarding the result entirely.

It was also necessary to add a check to R2dbcQueryMethod#getEntityInformation, as otherwise a PersistentEntity is created for Unit, which leads to a new instance being created via reflection down the pipeline (which is probably not a thing that should happen).

Added AbstractR2dbcQueryUnitTests to test the functionality surrounding the main change. Also added integration tests surrounding modifying query return types.

A couple notes:

• While I do believe the check added to R2dbcQueryMethod#getEntityInformation is sufficient to prevent a PersistentEntity or new instance of Unit from being created for this particular case, I feel like it might be a good idea to add an additional check for Unit to SimpleTypeHolder, alongside the one for java.lang.*. I'm not sure whether there's a need for a broader kotlin.* check, though.
• I'm not exactly thrilled about the idea of peppering KotlinDetector.isKotlinPresent() && Unit.class.isAssignableFrom(someClass) all over the place. I don't think a check for Unit belongs in ClassUtils.isPrimitiveOrWrapper as, strictly speaking, Unit is neither of those, but perhaps an isKotlinUnit(Class) method could be added to KotlinDetector.

(I decided to submit this PR without either of those improvements since those are both in other repos and the solution works as-is for now. I'd like to know your thoughts on this PR and those/any other potential improvements before I go submitting any additional PRs elsewhere.)

[mp911de]

Thanks for submitting a pull request. We had a look and indeed checking for kotlin.Unit to be a simple type (in SimpleTypeHolder in combination with a isVoid(…) method hosted in commons we can really simplify the amount of changes that is needed to make a store module discard results for Flow<Unit> (that is how Kotlin compiles coroutine methods). It's not required to emit Unit.INSTANCE, instead completion without a value leads to the same behavior (at least what we've observed in the tests).

I'm going to merge test and documentation changes from this pull request.

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.