add signal service, building block for a full gracefull shutdown impl…

…ementation

async_utils/sig_service.py

@@ -0,0 +1,71 @@
+#   Copyright 2020-present Michael Hall
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+from __future__ import annotations
+
+import select
+import signal
+import socket
+import sys
+from collections.abc import Callable
+from types import FrameType
+from typing import Any
+
+type SignalCallback = Callable[[signal.Signals], Any]
+type StartStopCall = Callable[[], Any]
+type _HANDLER = Callable[[int, FrameType | None], Any] | int | signal.Handlers | None
+
+__all__ = ["SignalService"]
+
+possible = "SIGINT", "SIGTERM", "SIGBREAK", "SIGHUP"
+actual = tuple(e for name, e in signal.Signals.__members__.items() if name in possible)
+
+
+class SignalService:
+    """Meant for graceful signal handling where the main thread is only used for signal handling.
+    This should be paired with event loops being run in threads."""
+
+    def __init__(self, *, startup: list[StartStopCall], signal_cbs: list[SignalCallback], joins: list[StartStopCall]) -> None:
+        self._startup: list[StartStopCall] = startup
+        self._cbs: list[SignalCallback] = signal_cbs
+        self._joins: list[StartStopCall] = joins
+
+    def run(self):
+        ss, cs = socket.socketpair()
+        ss.setblocking(False)
+        cs.setblocking(False)
+        signal.set_wakeup_fd(cs.fileno())
+
+        original_handlers: list[_HANDLER] = []
+
+        for sig in actual:
+            original_handlers.append(signal.getsignal(sig))
+            signal.signal(sig, lambda s, f: None)
+            if sys.platform != "win32":
+                signal.siginterrupt(sig, False)
+
+        for task_start in self._startup:
+            task_start()
+
+        select.select([ss], [], [])
+        data, *_ = ss.recv(4096)
+
+        for cb in self._cbs:
+            cb(signal.Signals(data))
+
+        for join in self._joins:
+            join()
+
+        for sig, original in zip(actual, original_handlers):
+            signal.signal(sig, original)