Replace deprecated asyncio.get_child_watcher()

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

Fixes #583

Author: cryptomilk

pynvim/msgpack_rpc/event_loop/asyncio.py

@@ -188,10 +188,42 @@ async def connect_stdout():
 
     @override
     def _connect_child(self, argv: List[str]) -> None:
+        def can_use_pidfd():
+            # Unix system without pidfd_open?
+            if not hasattr(os, 'pidfd_open'):
+                return False
+
+            # Check that we are not blocked by security policy like SECCOMP
+            try:
+                pid = os.getpid()
+                fd = os.pidfd_open(pid, 0)
+                os.close(fd)
+            except OSError:
+                return False
+
+            return True
+
+        def get_child_watcher():
+            if can_use_pidfd():
+                try:
+                    from asyncio.unix_events import PidfdChildWatcher
+                    return PidfdChildWatcher()
+                except ImportError:
+                    pass
+
+            try:
+                from asyncio.unix_events import ThreadedChildWatcher
+                return ThreadedChildWatcher()
+            except ImportError:
+                pass
+
+            # FIXME Python 3.7, return None if we drop support
+            return asyncio.get_child_watcher()
+
         if os.name != 'nt':
-            # see #238, #241
-            self._child_watcher = asyncio.get_child_watcher()
-            self._child_watcher.attach_loop(self._loop)
+            watcher = get_child_watcher()
+            watcher.attach_loop(self._loop)
+            self._child_watcher = watcher
 
         async def create_subprocess():
             transport: asyncio.SubprocessTransport  # type: ignore