totalResourcesCount is not properly computed for nonexistent page

[tory-kk]

Hello,
We use NumberSizePagingBehavior with JpaEntityRepositoryBase repository (version 3.4.20210509072026).
And recently, we have discovered the issue that totalResourcesCount is not properly computed when the requested page is greater than actually available.

I will try to describe the issue via an example:
Assume that we have 3 records in the database.

1st query /resource?page[number]=1&page[size]=2:
resourceList.getMeta(PagedMetaInformation.class).getTotalResourceCount() gives 3 (correct, value was fetched from the database).

2nd query /resource?page[number]=2&page[size]=2:
resourceList.getMeta(PagedMetaInformation.class).getTotalResourceCount() gives 3 (correct, value was not fetched from the database: link in source code )

3rd query for nonexistent page /resource?page[number]=5&page[size]=2:
resourceList.getMeta(PagedMetaInformation.class).getTotalResourceCount() gives 8 (wrong).
totalResourcesCount also was not fetched from the database and no errors occurred.

For the same situation, we are getting an error response when using QuerySpec#apply method instead of JpaEntityRepositoryBase queries:

{status=400, title=BAD_REQUEST, detail=page offset out of range, cannot move beyond data set}

In this situation, I would expect consistency with InMemoryEvaluator validation behaviour or at least properly fetched total count.

[dimas]

Our theory is that you tried to optimise things a bit and not run count query where it can be avoided - that is when data is being fetched for the last page - you may get fewer items than the page size and that instantly gives you total count of items.
For example, if pageSize=100 and we are fetching page=5 and JPA returns us 33 items, we can easily derive total item count - we got 4 "full" pages before and 33 items on the last page meaning total = pageSize * (page-1) + itemsFetched = 100 * 4 + 33 = 433

This logic, however, breaks if someone requests page beyond the last one. IMHO, the optimisation should not apply if the JPA result is empty.