fix: race condition between close() and _encode_sync in H264LiveEncoder

[zackzmai]

Problem

H264LiveEncoder.close() sets self._codec = None on the main thread to signal concurrent _encode_sync calls to stop. However, _encode_sync may have already passed the assert self._codec is not None guard and be about to call self._codec.encode(frame) — at which point self._codec becomes None, causing:

'NoneType' object has no attribute 'encode'

This triggers whenever a WebSocket video-stream client disconnects while a frame encode is in-flight, and is visible in logs as repeated transcode encode error entries.

Fix

Capture self._codec into a local variable right after the assertion and use that local for the encode loop. close() can safely set self._codec = None without affecting an in-flight encode.

One-line semantic change, no API or behavioral difference.

[github-actions]

👋 感谢提交 PR @zackzmai！维护者会尽快 review。

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• CI 全绿（test / lint / build）
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[yangbaofu007]

🟡 [Review] backend/miloco/src/miloco/miot/transcoder.py:134-137 — 局部捕获放在断言之后，竞态窗口缩小但没消除，同一个异常仍可复现

背景：_encode_sync 跑在 h264-enc 编码线程，close() 在事件循环主线程上执行 self._codec = None（L179）。两者真并发——即 PR 描述里"客户端断开时某帧编码进行中"的场景。

问题：修复对 self._codec 做了两次独立读取——先断言、再捕获：

assert self._codec is not None   # L134 ← 第 1 次读 self._codec
# Capture codec ref locally ...
codec = self._codec              # L137 ← 第 2 次读 self._codec
...
for packet in codec.encode(frame):   # L151

CPython 可在任意两条字节码之间切线程。若主线程在 L134 和 L137 之间执行 self._codec = None，捕获到的 codec 就是 None，L151 抛出和修复前一模一样的 'NoneType' object has no attribute 'encode'：

编码线程 (_encode_sync)	主线程 (close())
L134 assert self._codec is not None → 通过（codec 已设置）
	L179 self._codec = None
L137 codec = self._codec → codec = None
L151 codec.encode(frame) → 'NoneType' object has no attribute 'encode'

窗口比修复前小（原窗口 L134→L151 含 from_ndarray/reformat/pts 赋值多步；现窗口只剩 L134→L137 一次属性 load），复现概率大幅下降但没归零，所以 "close() can safely set self._codec = None without affecting an in-flight encode" 目前兑现不了。

补充：Python 断言在 -O / PYTHONOPTIMIZE 下被整条剥除。若后端以优化模式运行，L134 断言不存在，唯一读取是 L137 捕获，捕获到的 codec 仍可能是 None——断言救不了这个局部捕获。

改进：只读一次 self._codec，把判空作用在捕获到的局部上。因为 close() 置 None 是正常并发关闭、不是编程错误，用早返回比断言更贴语义：

    # Capture codec ref locally FIRST so a single read is checked and used —
    # close() setting self._codec = None on the main thread can't slip in
    # between the check and the encode.
    codec = self._codec
    if codec is None:
        return []

    frame = av.VideoFrame.from_ndarray(bgr, format="bgr24")
    frame = frame.reformat(format="yuv420p")
    ...
    out: list[tuple[bytes, bool]] = []
    for packet in codec.encode(frame):
        out.append((bytes(packet), bool(packet.is_keyframe)))
    return out

若想保留断言来捕捉"非 close 路径下 codec 不应为 None"的编程错误，也要让断言作用在局部上，确保只读一次：

    codec = self._codec
    assert codec is not None   # 校验即将使用的同一个局部，不再二次读 self._codec

[zackzmai]

@yangbaofu007 采纳，已按建议改为单次读取 + 早返回（commit 40ca318）：

codec = self._codec
if codec is None:
    return []

原 assert + 二次读取捕获的窗口已消除：整个 encode 循环只引用局部 codec，主线程 close() 置 self._codec = None 不再影响在途 encode；PYTHONOPTIMIZE 下断言被剥除的问题也随之规避。close 是正常并发关闭，早返回比断言更贴语义。CI 全绿。