12.0.x: Rework consumeAvailable() mechanism

[lorban]

Started with a fix for a comment detailing the consumeAvailable() behavior that is partially incorrect, ended up reworking the consumeAvailable() mechanism.

See #13934 (comment)

See #14026 for the 12.1.x version of this PR.

[gregw]

I'm a bit more concerned about this now, as I didn't realise that HttpStreamOverHttp1#consumeAvailable made the connection non-persistent.

If this is the case, it is a little bit redundant as we have the method org.eclipse.jetty.server.ResponseUtils#ensureConsumeAvailableOrNotPersistent, which does a much better job of being non persistent.

I think we need to look a little closer before moving on, and maybe document a b it more

[lorban]

I'm a bit more concerned about this now, as I didn't realise that HttpStreamOverHttp1#consumeAvailable made the connection non-persistent.

If this is the case, it is a little bit redundant as we have the method org.eclipse.jetty.server.ResponseUtils#ensureConsumeAvailableOrNotPersistent, which does a much better job of being non persistent.

I think we need to look a little closer before moving on, and maybe document a b it more

ResponseUtils#ensureConsumeAvailableOrNotPersistent works at the Request level, so eventually that work is delegated to HttpStream#consumeAvailable.

Out of curiosity, I tried disabling the ensureNotPersistent call from ensureConsumeAvailableOrNotPersistent and that didn't break a single test, so I'm tempted to believe ResponseUtils#ensureConsumeAvailableOrNotPersistent could be replaced with Request#consumeAvailable, and ResponseUtils removed.

[lorban]

I pushed the removal of ResponseUtils and added the missing javadoc.

How does that look?

[lorban]

@gregw I've reworked the whole consumeAvailable() mechanism to get rid of ResponseUtils, and fixed ServletCoreRequest.consumeAvailable() to make sure it never blocks even when the servlet request is wrapped.

[gregw]

Thinking on this one for a day...

[gregw]

This is good. The HttpGenerator does all the headers required.