Possible Race Condition in ModelObjectPool

[mksergiy]

A possible race condition exists in the ModelObjectPool implementation when multiple threads concurrently call the Return method. The issue arises due to the non-atomic _availableObjects.Count check and the subsequent _availableObjects.Add().
In my specific use case, this issue has resulted in RabbitMQ channel leaks.

I fixed it by replacing the _availableObjects.Count logic with an atomic counter (_currentCount) managed using Interlocked. Here's the sample:

private int _currentCount = 0;

public void Return(IModel model)
{
    if (Interlocked.Increment(ref _currentCount) > _maxEntries)
    {
        Interlocked.Decrement(ref _currentCount);
        model.SafeDrop();
    }
    else
    {
        _availableObjects.Add(model);
    }
}

public IModel Get()
{
    if (_availableObjects.TryTake(out var model))
    {
        Interlocked.Decrement(ref _currentCount);
        return model;
    }

    return _policy.Create();
}