Cleanup memory pool code add some tests

[pakrym]

This PR adds more descriptive error messages to existing cases and adds two more checks in debug mode:

1. Double dispose of pool
2. Disposing pool with active blocks.

It also adds one check in release mode:

1. Renting from a disposed pool. Current behaviour there is highly unpredictable and hard to correctly handle.

And removes one check in release mode:

1. Disposing (returning) block to a disposed pool.

This PR doesn't address the unpinning while in use issue raised by @benaadams but I don't want to mix up cleanup/diagnostics and functional fixes.

[benaadams]

Kill this exception in Release mode? Its still there...

[halter73]

Would it be better to have an _isDisposed field in MemoryPoolBlock itself that was set in Dispose and unset in Lease? Then there'd be a different exception for using the block after it's returned to the pool and using the block before it's returned to the pool but after the pool is disposed.

[pakrym]

Thought about it too, will do.

[halter73]

Is this ever different than public bool IsActive => !_isDisposed;

[pakrym]

Nope.

[halter73]

I was going to suggest how to calculate the blocksAllocated by a single call to AllocateSlab in a single expression, but I realized the for loop condition is messed up.

I'm pretty sure offset + _blockSize < blockAllocationLength should be either offset + _blockSize <= _slabLength or offset - firstOffset + _blockSize <= blockAllocationLength instead.

[pakrym]

We create additional segment after the loop. It takes the last _blockSize of memory.

[halter73]

Ahhhh. I think it's technically right then but still a little hard to follow. I think it would be a lot easier to grok (if only because less bitwise operations) if written as follows:

// ...

var maxPenultimateOffset = _slabLength - (_blockSize * 2);
var offset = firstOffset;
for (;
    offset <= maxPenultimateOffset;
    offset += _blockSize)
{
    var block = new MemoryPoolBlock(
        this,
        slab,
        offset,
        _blockSize);
    
    // ...

[pakrym]

🆙📅

[pakrym]

🆙📅