93 lines
3.4 KiB
Python
93 lines
3.4 KiB
Python
"""TCP 是字节流：演示“粘包 / 拆包”现象，以及用“长度前缀”协议正确分帧。只用标准库，`python framing.py` 运行。
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TCP 只保证字节按顺序可靠到达，不保留“消息边界”：发送方 send 了 3 次，接收方可能 1 次 recv 全部收到，
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也可能把一条消息分成多次收到。应用层必须自己定义边界：换行分隔（如 SSE、JSON Lines）或长度前缀（如很多 RPC 协议）。
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"""
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from __future__ import annotations
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import json
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import socket
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import struct
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import threading
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HEADER = struct.Struct("!I") # 4 字节无符号整数，网络字节序（大端）
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def encode(msg: dict) -> bytes:
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body = json.dumps(msg, ensure_ascii=False).encode()
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return HEADER.pack(len(body)) + body
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class FrameDecoder:
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"""增量解码：不管数据被怎样切分送进来，都能还原出完整的消息。"""
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def __init__(self, max_frame: int = 1 << 20):
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self.buf = bytearray()
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self.max_frame = max_frame
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def feed(self, data: bytes) -> list[dict]:
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self.buf += data
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out = []
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while len(self.buf) >= HEADER.size:
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(n,) = HEADER.unpack_from(self.buf)
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if n > self.max_frame:
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raise ValueError(f"帧长度 {n} 超过上限，可能是攻击或协议错误")
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if len(self.buf) < HEADER.size + n:
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break # 数据还不完整，等下一次
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body = bytes(self.buf[HEADER.size:HEADER.size + n])
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del self.buf[:HEADER.size + n]
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out.append(json.loads(body))
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return out
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def demo_sticky() -> int:
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"""连续 send 三次小消息，看接收端一次 recv 拿到几段。"""
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srv = socket.create_server(("127.0.0.1", 0))
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port = srv.getsockname()[1]
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received: list[bytes] = []
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def server() -> None:
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conn, _ = srv.accept()
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with conn:
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threading.Event().wait(0.2) # 等发送方把三条都发完
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received.append(conn.recv(4096))
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t = threading.Thread(target=server)
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t.start()
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with socket.create_connection(("127.0.0.1", port)) as c:
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for i in range(3):
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c.sendall(f"msg{i}".encode())
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t.join()
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srv.close()
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return len(received[0])
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def demo_framed() -> list[dict]:
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srv = socket.create_server(("127.0.0.1", 0))
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port = srv.getsockname()[1]
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got: list[dict] = []
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def server() -> None:
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conn, _ = srv.accept()
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dec = FrameDecoder()
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with conn:
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while chunk := conn.recv(3): # 故意每次只读 3 字节，模拟拆包
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got.extend(dec.feed(chunk))
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t = threading.Thread(target=server)
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t.start()
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with socket.create_connection(("127.0.0.1", port)) as c:
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c.sendall(b"".join(encode({"seq": i, "text": f"第{i}条"}) for i in range(3))) # 一次发出，模拟粘包
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t.join()
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srv.close()
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return got
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if __name__ == "__main__":
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n = demo_sticky()
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note = "三条消息粘在一起，边界丢失" if n > 4 else "这次恰好没有粘包，但不能依赖这一点"
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print(f"发送方 send 了 3 次（每次 4 字节），接收方一次 recv 收到 {n} 字节 → {note}")
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msgs = demo_framed()
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assert [m["seq"] for m in msgs] == [0, 1, 2], msgs
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print(f"长度前缀分帧：每次只读 3 字节，仍然还原出 {len(msgs)} 条完整消息：{[m['text'] for m in msgs]}")
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