Add failing tests to reproduce POOL-413

[rajucomp]

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In a GenericObjectPool it is possible to configure a maximum number of idle objects to be kept by the pool while they are not in use.

In unfortunate circumstances, if several threads return an object to the pool at the same time, the check on the maximum number of idle objects may be dismissed. This results in the pool keeping more idle objects than configured.

Looking into the source code of the returnObject method of the GenericObjectPool, it seems that there is no synchronization between the moment the check is made for the maxIdle configuration and the moment the object is destroyed:

final int maxIdleSave = getMaxIdle();
if (isClosed() || maxIdleSave > -1 && maxIdleSave <= idleObjects.size()) {
    try {
        destroy(p, DestroyMode.NORMAL);
    } catch (final Exception e) {
        swallowException(e);
    }
    try {
        ensureIdle(1, false);
    } catch (final Exception e) {
        swallowException(e);
    }
}

The PR contains the test to demonstrate the failure. A fix is ready for the same. Once the maintainer confirm the bug, a seperate PR for the fix will be raised.

[rajucomp]

@garydgregory Could you run the pipelines please ? I have added some comments on the JIRA about the fix. Would appreciate your feedback. Thanks@

[garydgregory]

@rajucomp Please rebase on git master.

[rajucomp]

@garydgregory Done. Please run the pipelines. Thanks!

[rajucomp]

This is weird. The test fails when run in isolation but passes when the whole test suite is run. Let me see how to reproduce the failure reliably.

[rajucomp]

@garydgregory Could you trigger the pipelines again ?