Docker容器内FastAPI lifespan的shutdown事件未执行的问题

以下FastAPI代码在Docker容器和本地终端直接运行时，收到Ctrl+C后的表现不一致：

from contextlib import asynccontextmanager
from fastapi import FastAPI
from uvicorn import run

@asynccontextmanager
async def lifespan(app: FastAPI):
    # startup
    logger.info("[IN SYSTEM STARTUP]")     
    logger.info("[STARTUP DONE]")
    
    yield
    # shutdown
    logger.info("[IN SHUTDOWN]")
    logger.info("[IN SHUTDOWN] Done")

app = FastAPI(lifespan=lifespan)

容器内运行日志（Ctrl+C未执行shutdown逻辑） #

event_deliver | INFO:     Waiting for application startup.
event_deliver | 2023-09-05 02:50:22,092 - logger INFO - [IN SYSTEM STARTUP]
event_deliver | 2023-09-05 02:50:22,092 - logger INFO - [STARTUP DONE]
event_deliver | INFO:     Application startup complete.
event_deliver | INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
^CGracefully stopping... (press Ctrl+C again to force)
Stopping event_deliver   ... done
Stopping redis           ... done

本地终端运行日志（Ctrl+C正常执行shutdown逻辑） #

2023-09-05 02:57:48,307 - logger INFO - [IN SYSTEM STARTUP]
2023-09-05 02:57:48,307 - logger INFO - [STARTUP DONE]
INFO:     Application startup complete.
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
^CINFO:     Shutting down
INFO:     Waiting for application shutdown.
2023-09-05 02:57:52,523 - logger INFO - [IN SHUTDOWN]
2023-09-05 02:57:52,523 - logger INFO - [IN SHUTDOWN] Done
INFO:     Application shutdown complete.
INFO:     Finished server process [70464]

请问造成这种差异的原因是什么？

• PID 1进程的信号处理特性：Docker容器默认将启动命令作为PID 1进程，而Linux系统中PID 1进程不会自动转发SIGINT（Ctrl+C发送的信号）给子进程。如果容器内的启动逻辑没有正确处理信号，Uvicorn进程收不到终止信号，就无法触发shutdown代码。而本地终端中Uvicorn是非PID 1进程，能正常捕获SIGINT。
• Docker Compose的停止机制：从日志看使用了Docker Compose，按下Ctrl+C时，Compose会先向容器发送SIGTERM信号，若进程未在指定时间内退出，就会发送SIGKILL强制终止。如果Uvicorn未配置处理SIGTERM，或者容器内的PID 1进程没有把SIGTERM转发给Uvicorn，进程会被直接杀死，来不及执行shutdown逻辑。
• 启动方式的信号传递差异：本地直接用uvicorn命令运行时，Uvicorn是前台进程，能直接接收终端发送的信号。但容器中如果用shell脚本启动、或者启动命令未用exec形式执行，会导致Uvicorn成为shell的子进程，而作为PID 1的shell不会转发信号，最终Uvicorn收不到终止信号，无法执行shutdown逻辑。

内容的提问来源于stack exchange，提问作者coder_boy