Replace deprecated asyncio.get_child_watcher()

Use PidfdChildWatcher if os.pidfd_open is available and works, and
otherwise use ThreadedChildWatcher which should work in any case.

Fixes neovim#583

pynvim/msgpack_rpc/event_loop/asyncio.py

@@ -9,6 +9,11 @@
 from collections import deque
 from signal import Signals
 from typing import Any, Callable, Deque, List, Optional, cast
+from asyncio.unix_events import ThreadedChildWatcher
+try:
+    from asyncio.unix_events import PidfdChildWatcher
+except ImportError:
+    PidfdChildWatcher = None
 
 if sys.version_info >= (3, 12):
     from typing import Final, override
@@ -188,18 +193,33 @@ async def connect_stdout():
 
     @override
     def _connect_child(self, argv: List[str]) -> None:
-        if os.name != 'nt':
-            # see #238, #241
-            self._child_watcher = asyncio.get_child_watcher()
-            self._child_watcher.attach_loop(self._loop)
-
         async def create_subprocess():
             transport: asyncio.SubprocessTransport  # type: ignore
             transport, protocol = await self._loop.subprocess_exec(
                 self._protocol_factory, *argv)
             pid = transport.get_pid()
             debug("child subprocess_exec successful, PID = %s", pid)
 
+            if os.name != 'nt':
+                # see #238, #241
+                pidfd_works = False
+                if PidfdChildWatcher is not None and hasattr(os, "pidfd_open"):
+                    try:
+                        fd = os.pidfd_open(pid)
+                    except Exception:
+                        pass
+                    else:
+                        os.close(fd)
+                        pidfd_works = True
+
+                if pidfd_works:
+                    watcher = PidfdChildWatcher()
+                else:
+                    watcher = ThreadedChildWatcher()
+
+                watcher.attach_loop(self._loop)
+                self._child_watcher = watcher
+
             self._transport = cast(asyncio.WriteTransport,
                                    transport.get_pipe_transport(0))  # stdin
             self._protocol = protocol