Jetty 12: review/remove ByteBufferPool

[sbordet]

Jetty version(s)
12

Description
The Jetty 12 code has now standardized to use RetainableByteBufferPool, so ByteBufferPool should be removed.

This would simplify a lot the codebase, and also clean up the Jetty component tree, because now the ByteBufferPools appear in the component tree, but not the RetainableByteBufferPools.

This is a prerequisite for #9148, because it would then allow to inject leak-tracking Pool instances.

[gregw]

I'm just adding my voice of caution here, that we did try to do this before, but found a few cases that were just too hard to use the stricter Retainable contract for. I can't exactly recall what they were, so this time if we find them again and decide it can't be done, then we should document in the code what those cases are.

Note that I think we can change the API of RetainableByteBuffferPool to make it easier to leak track independently of removing the non retained pool. We can track leaks for just retainable usages.

[gregw]

I'm just adding my voice of caution here, that we did try to do this before, but found a few cases that were just too hard to use the stricter Retainable contract for. I can't exactly recall what they were, so this time if we find them again and decide it can't be done, then we should document in the code what those cases are.

Note that I think we can change the API of RetainableByteBuffferPool to make it easier to leak track independently of removing the non retained pool. We can track leaks for just retainable usages.

[sbordet]

Fixed by #9195.