Use PidfdChildWatcher by default when available (#87951)

This is a backport from cpython 3.12

https://docs.python.org/3/library/asyncio-policy.html
> PidfdChildWatcher is a “Goldilocks” child watcher implementation. It doesn’t require signals or threads, doesn’t interfere with any processes launched outside the event loop, and scales linearly with the number of subprocesses launched by the event loop. The main disadvantage is that pidfds are specific to Linux, and only work on recent (5.3+) kernels.

https://github.com/python/cpython/pull/98024
There are some additional fixes in cpython 3.12
in https://github.com/python/cpython/pull/94184
when there is no event loop running in the main thread
but this is not a problem we have

homeassistant/runner.py

@@ -2,8 +2,10 @@
 from __future__ import annotations
 
 import asyncio
+from asyncio import events
 import dataclasses
 import logging
+import os
 import threading
 import traceback
 from typing import Any
@@ -49,6 +51,22 @@ class RuntimeConfig:
     open_ui: bool = False
 
 
+def can_use_pidfd() -> bool:
+    """Check if pidfd_open is available.
+
+    Back ported from cpython 3.12
+    """
+    if not hasattr(os, "pidfd_open"):
+        return False
+    try:
+        pid = os.getpid()
+        os.close(os.pidfd_open(pid, 0))  # pylint: disable=no-member
+    except OSError:
+        # blocked by security policy like SECCOMP
+        return False
+    return True
+
+
 class HassEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
     """Event loop policy for Home Assistant."""
 
@@ -56,6 +74,23 @@ class HassEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
         """Init the event loop policy."""
         super().__init__()
         self.debug = debug
+        self._watcher: asyncio.AbstractChildWatcher | None = None
+
+    def _init_watcher(self) -> None:
+        """Initialize the watcher for child processes.
+
+        Back ported from cpython 3.12
+        """
+        with events._lock:  # type: ignore[attr-defined] # pylint: disable=protected-access
+            if self._watcher is None:  # pragma: no branch
+                if can_use_pidfd():
+                    self._watcher = asyncio.PidfdChildWatcher()
+                else:
+                    self._watcher = asyncio.ThreadedChildWatcher()
+                if threading.current_thread() is threading.main_thread():
+                    self._watcher.attach_loop(
+                        self._local._loop  # type: ignore[attr-defined] # pylint: disable=protected-access
+                    )
 
     @property
     def loop_name(self) -> str: