ASGI app SIGTERM handler silently replaces the hosting server's handler

Read python/restate/server.py:254-270 and inspect how asgi_app registers the SIGTERM handler for request and lifespan scopes. Reproduce the displacement with the provided minimal repro, then check behavior with both loop.add_signal_handler and signal.signal. Done means the host server's shutdown handler still runs after restate.app(...) is called, while active receive channels are notified.

On the first call into restate.app(...) of any scope, asgi_app registers loop.add_signal_handler(signal.SIGTERM, _on_sigterm) (python/restate/server.py:254-267). Because loop.add_signal_handler replaces the existing handler without chaining, SIGTERM no longer reaches the hosting ASGI server's shutdown path after the first request.

Observed behavior

Measured on restate-sdk 1.0.5 (main 63d2fe5), Python 3.13:

• Hypercorn 0.18: serving a plain ASGI app exits in ~0.1 s on SIGTERM. Serving restate.app(...), hitting /health once, and sending SIGTERM leaves the process running past a 15 s grace period until SIGKILL.
• Uvicorn 0.54.0: serving a plain ASGI app exits in ~0.1 s on SIGTERM. Serving restate.app(...) after one request hangs the same way until SIGKILL.
• Lifespan scope: a single lifespan.startup call is enough to displace the host handler, because the signal handler is installed before the scope["type"] == "lifespan" check (python/restate/server.py:269-270).

_on_sigterm only calls notify_shutdown() on active ReceiveChannel instances. Nothing notifies the host server to stop accepting connections or exit.

Why both loop.add_signal_handler and signal.signal matter

ASGI servers register SIGTERM in two ways on Unix:

1. Uvicorn uses signal.signal(signal.SIGTERM, ...).
2. Hypercorn uses loop.add_signal_handler(signal.SIGTERM, ...) (hypercorn/asyncio/run.py). Once loop.add_signal_handler is active, Python's signal.getsignal(signal.SIGTERM) returns asyncio.unix_events._sighandler_noop, and the actual host callback lives in loop._signal_handlers[signal.SIGTERM] (an asyncio.Handle storing the callback and its arguments).

PR #226 adds re-dispatch using signal.getsignal(signal.SIGTERM) before loop.add_signal_handler. Tested on top of 1.0.5, #226 fixes Uvicorn, but Hypercorn still hangs for 15 s because signal.getsignal(signal.SIGTERM) only captures _sighandler_noop rather than the previous loop handler.

Suggested fix

Before calling loop.add_signal_handler(signal.SIGTERM, _on_sigterm):

1. Check getattr(loop, "_signal_handlers", {}).get(signal.SIGTERM) first. If present, save that asyncio.Handle and invoke prev_handle._run() from _on_sigterm after notifying the channels.
2. Otherwise, fall back to signal.getsignal(signal.SIGTERM) and invoke it as prev(signal.SIGTERM, None) when callable and not asyncio.unix_events._sighandler_noop.
3. Optionally move the registration after the scope["type"] == "lifespan" check so a rejected lifespan call does not touch process signals.

Minimal repro (no external server required)

# uv run --no-project --python 3.13 --with restate-sdk==1.0.5 python repro.py
import asyncio
import signal
import restate

calls: list[str] = []


def build_app():
    svc = restate.Service("probe")

    @svc.handler()
    async def noop(ctx: restate.Context) -> str:
        return "ok"

    return restate.app(services=[svc])


async def one_call(app):
    async def receive():
        return {"type": "lifespan.startup"}

    async def send(msg):
        pass

    try:
        await app({"type": "lifespan", "asgi": {"version": "3.0"}}, receive, send)
    except Exception:
        pass


def run_case(style: str) -> bool:
    calls.clear()
    loop = asyncio.new_event_loop()
    asyncio.set_event_loop(loop)
    if style == "loop.add_signal_handler":
        loop.add_signal_handler(signal.SIGTERM, lambda: calls.append("host_ran"))
    elif style == "signal.signal":
        signal.signal(signal.SIGTERM, lambda *_: calls.append("host_ran"))

    app = build_app()
    loop.run_until_complete(one_call(app))
    signal.raise_signal(signal.SIGTERM)
    loop.run_until_complete(asyncio.sleep(0.05))
    print(f"{style:<24} -> {calls or ['displaced (nothing ran)']}")
    return "host_ran" in calls


for mode in ("loop.add_signal_handler", "signal.signal"):
    run_case(mode)

Output on restate-sdk 1.0.5:

loop.add_signal_handler  -> ['displaced (nothing ran)']
signal.signal            -> ['displaced (nothing ran)']