FastMCP server with SSE transport fails to shut down on a signal

使用提供的 FastMCP SSE 服务器和客户端复现关闭行为，先确认请求已处理完毕，再发送信号。然后跟踪 FastMCP SSE 服务器的生命周期及其 Uvicorn 关闭路径；当进程进入应用程序关闭阶段并在不需要第二个信号的情况下结束时，即表示完成。

A very simple server fails to shut down on a signal if it processed at least one request:

❯ python test-server.py --port=8085
INFO:     Started server process [216035]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8085 (Press CTRL+C to quit)
INFO:     127.0.0.1:55188 - "GET /sse HTTP/1.1" 200 OK
INFO:     127.0.0.1:55192 - "POST /messages/?session_id=e690de9733914d09aebf2b0e8c78191c HTTP/1.1" 202 Accepted
INFO:     127.0.0.1:55192 - "POST /messages/?session_id=e690de9733914d09aebf2b0e8c78191c HTTP/1.1" 202 Accepted
INFO:     127.0.0.1:55192 - "POST /messages/?session_id=e690de9733914d09aebf2b0e8c78191c HTTP/1.1" 202 Accepted
Processing request of type CallToolRequest
^CINFO:     Shutting down
INFO:     Waiting for background tasks to complete. (CTRL+C to force quit)

The expected behaviour would be:

❯ python test-server.py --port=8085
INFO:     Started server process [216006]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8085 (Press CTRL+C to quit)
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.
INFO:     Application shutdown complete.
INFO:     Finished server process [216006]

Here is the code for both the server and the client.

What do I miss?

the server

import click
import sys
from pydantic import Field
from mcp.server.fastmcp import FastMCP

@click.command()
@click.option("--port", default=8085, help="Port to listen", type=int)
def main(port: int):
    mcp = FastMCP(
        "mcp-echo-tool",
        debug=True,
        log_level="INFO",
        port=port,
    )

    @mcp.tool(name="echo")
    async def echo_tool(string: str = Field(description="A string to echo back")) -> str:
        """Echoes back the input string"""
        return string

    mcp.run(transport='sse')
    return 0

if __name__ == "__main__":
    sys.exit(main())

and the client

import asyncio
import click
import sys
from mcp.client.session import ClientSession
from mcp.client.sse import sse_client

async def client(url, string):
    async with sse_client(url) as (read, write):
        async with ClientSession(read, write) as session:
            await session.initialize()
            result = await session.call_tool("echo", {"string": string})
            print(result.content)

@click.command()
@click.option('--url', default='http://localhost:8085', help='MCP Server URL', type=str)
@click.argument('string', type=str)
def main(url: str, string: str):
    asyncio.run(client(url, string))

if __name__ == "__main__":
    sys.exit(main())