Extensibility framework

HTTP/2 is extendable: it's one of its great strengths. It would be really nice if H2 was relatively easily extensible. We should aim to start supporting pluggable HTTP/2 extensions.

What do we need to do this? Well, the answer is: what can extensions do?

• They can add new frames. This means we need to be able to add:
  • new frame types that can be successfully parsed;
  • new transitions in the state machine, both connection and stream, that occur when a new frame is received;
  • specific frame callbacks, both connection and stream, that allow manipulating the connection and stream objects;
  • allow new frames to follow the state machine transitions and callbacks of a pre-existing frame;
• They can add new state machine transitions in the absence of new frames, or adjust the existing state machine transitions;
• They can add new settings. We need to be able to inject useful representations of settings, and allow callbacks for those settings if they're changed;
• They can add new error codes.

This will also require work in hyperframe.

首先梳理 h2 中现有的 HTTP/2 状态机、帧解析、回调、设置以及错误代码处理。然后审查 hyperframe 中所需的相关工作，并确定扩展如何在不破坏现有行为的情况下添加帧、状态转换、设置、回调和错误代码。完成的标准是：定义好可扩展性设计，并在两个项目中完成配套实现。