Cancel Supervisor startup task on early shutdown signal (#6175)

When receiving a shutdown signal during startup, the Supervisor should
cancel its startup task to ensure a graceful shutdown. This prevents
Supervisor accidentally accessing the Event loop after it has been
closed by the stop procedure:

RuntimeError: Event loop stopped before Future completed.

supervisor/__main__.py

@@ -66,10 +66,23 @@ if __name__ == "__main__":
     _LOGGER.info("Setting up Supervisor")
     loop.run_until_complete(coresys.core.setup())
 
-    bootstrap.register_signal_handlers(loop, coresys)
+    # Create startup task that can be cancelled gracefully
+    startup_task = loop.create_task(coresys.core.start())
+
+    def shutdown_handler() -> None:
+        """Handle shutdown signals gracefully during startup."""
+        if not startup_task.done():
+            _LOGGER.warning("Supervisor startup interrupted by shutdown signal")
+            startup_task.cancel()
+
+        coresys.create_task(coresys.core.stop())
+
+    bootstrap.register_signal_handlers(loop, shutdown_handler)
 
     try:
-        loop.run_until_complete(coresys.core.start())
+        loop.run_until_complete(startup_task)
+    except asyncio.CancelledError:
+        _LOGGER.warning("Supervisor startup cancelled")
     except Exception as err:  # pylint: disable=broad-except
         # Supervisor itself is running at this point, just something didn't
         # start as expected. Log with traceback to get more insights for

supervisor/bootstrap.py

@@ -2,6 +2,7 @@
 
 # ruff: noqa: T100
 import asyncio
+from collections.abc import Callable
 from importlib import import_module
 import logging
 import os
@@ -289,26 +290,22 @@ def check_environment() -> None:
         _LOGGER.critical("Can't find Docker socket!")
 
 
-def register_signal_handlers(loop: asyncio.AbstractEventLoop, coresys: CoreSys) -> None:
+def register_signal_handlers(
+    loop: asyncio.AbstractEventLoop, shutdown_handler: Callable[[], None]
+) -> None:
     """Register SIGTERM, SIGHUP and SIGKILL to stop the Supervisor."""
     try:
-        loop.add_signal_handler(
-            signal.SIGTERM, lambda: loop.create_task(coresys.core.stop())
-        )
+        loop.add_signal_handler(signal.SIGTERM, shutdown_handler)
     except (ValueError, RuntimeError):
         _LOGGER.warning("Could not bind to SIGTERM")
 
     try:
-        loop.add_signal_handler(
-            signal.SIGHUP, lambda: loop.create_task(coresys.core.stop())
-        )
+        loop.add_signal_handler(signal.SIGHUP, shutdown_handler)
     except (ValueError, RuntimeError):
         _LOGGER.warning("Could not bind to SIGHUP")
 
     try:
-        loop.add_signal_handler(
-            signal.SIGINT, lambda: loop.create_task(coresys.core.stop())
-        )
+        loop.add_signal_handler(signal.SIGINT, shutdown_handler)
     except (ValueError, RuntimeError):
         _LOGGER.warning("Could not bind to SIGINT")