start_tls: AssertionError if writing is paused

First found in the CPython port of sslproto (see https://github.com/python/cpython/issues/109051 but disregard the title, it's not platform-specific), and it can be reproduced in uvloop.

protocol.resume_writing() failed
protocol: <uvloop.loop.SSLProtocol object at 0x1033910c0>
transport: <TCPTransport closed=False reading=True 0x1098351c0>
Traceback (most recent call last):
  File "uvloop/handles/basetransport.pyx", line 96, in uvloop.loop.UVBaseTransport._maybe_resume_protocol
    run_in_context(
  File "uvloop/loop.pyx", line 101, in uvloop.loop.run_in_context
    return context.run(method)
  File "uvloop/sslproto.pyx", line 922, in uvloop.loop.SSLProtocol.resume_writing
    assert self._ssl_writing_paused
AssertionError

Simplest way to reproduce is to increase payload size in test_start_tls_server_1. Send will block and transport will request to pause writing. After start_tls(), the transport will attempt to resume the now-switched protocol, which was not aware it was ever supposed to be paused.

I commented further in the original bug with some notes. The CPython implementation is a bit different (it has a redundant buffering mechanism with a deque which I believe is unnecessary) but the fundamental issue is the same.

Question: what is the motivation for the extra flow control layer in SSLProtocol? It seems it was introduced in uvloop first. I suggested it can be removed and let the original protocol and original transport deal with flow control (which would also fix the bug). But it wouldn't be a good idea to remove it if it's solving a known problem.

Bắt đầu với tests/test_tcp.py, đặc biệt là test_start_tls_server_1, và tái hiện trường hợp payload lớn được mô tả trong issue. Sau đó kiểm tra uvloop/sslproto.pyx quanh SSLProtocol.resume_writing và start_tls để hiểu quá trình chuyển đổi kiểm soát luồng. Phần hoàn thành cần bao gồm một bản sửa đã được xác nhận cho assertion và một bài kiểm thử hồi quy bao quát trường hợp thao tác ghi bị tạm dừng trong khi khởi động TLS.