Fix write to check poll_ready before sending data

[0x676e67]

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[gemini-code-assist]

Code Review

This pull request introduces support for QPACK dynamic tables in the HTTP/3 implementation, including state synchronization, encoder/decoder stream handling, and blocked stream management. It also transitions the QUIC stream sending API from a synchronous send_data to an asynchronous poll_send_data model across h3, h3-quinn, and h3-webtransport. Feedback on these changes highlights a critical issue in h3/src/connection.rs where calling self.waker().wake() unconditionally inside poll_qpack_encoder can trigger an infinite busy-loop and 100% CPU usage when a partial QPACK instruction is received; the waker should instead only be signaled when progress is actually made.

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h3/src/connection.rs (599-611)

Calling self.waker().wake() unconditionally on every iteration of the loop in poll_qpack_encoder can cause an infinite busy-loop (100% CPU usage) when a partial QPACK instruction is received.

Specifically, if encoder_recv contains some bytes but not enough to form a complete instruction, decoder.on_encoder_recv will consume 0 bytes and return Ok. Since encoder_recv.buf().remaining() == before is true, the loop will break and return Poll::Pending. However, because self.waker().wake() was already called, the executor will immediately poll the connection task again, repeating the same process and busy-looping until more data arrives to complete the instruction.

We should only call self.waker().wake() if we actually made progress (i.e., after checking that encoder_recv.buf().remaining() != before).

                if let Err(err) = decode_result {
                    return Poll::Ready(Err(self.handle_connection_error(
                        InternalConnectionError::new(
                            Code::QPACK_ENCODER_STREAM_ERROR,
                            format!("invalid QPACK encoder stream instruction: {}", err),
                        ),
                    )));
                }

                if encoder_recv.buf().remaining() == before {
                    break;
                }

                self.waker().wake();