Generic support for 1xx responses and early hints in particular

[markt-asf]

This issue arose from discussions around dropping HTTP/2 push support in #538.

Do we reuse sendError(), introduce a new method or something else?

Given the Javadoc for sendError(), I think a new method (name?) would be better.

Given the number of different methods that can set status codes, I think we should also add a requirement to throw IllegalArgumentException if any such method is used with an inappropriate status code (e.g. a 4xx code with sendRedirect)

[gregw]

Note that eclipse jetty has had a proprietary extension to sendError for some time to support 102 Processing and now 103 Early Hints. We also treat -1 specially as a request to abort/reset the response. This is useful for trying to communicate failure when an error has been detected, but the response is already committed.

I think a sendInformationalStatus(int) method would be fine... although we probably would need to block 101, as that has other implications beyond what can be done in a servlet. Perhaps an abort() method to cover the -1 case above?

[markt-asf]

By abort() do you mean close the connection?

[markt-asf]

Support for early hints was added via #672

The discussion around #672 concluded generic 1xx support was not viable.

Leaving this issue open for the abort() method and the use of inappropriate codes with methods like sendRedirect().