gh-128340: add thread safe handle for loop.call_soon_threadsafe (#128369)

Adds `_ThreadSafeHandle` to be used for callbacks scheduled with `loop.call_soon_threadsafe`.

Lib/asyncio/events.py

@@ -113,6 +113,34 @@ class Handle:
             self._loop.call_exception_handler(context)
         self = None  # Needed to break cycles when an exception occurs.
 
+# _ThreadSafeHandle is used for callbacks scheduled with call_soon_threadsafe
+# and is thread safe unlike Handle which is not thread safe.
+class _ThreadSafeHandle(Handle):
+
+    __slots__ = ('_lock',)
+
+    def __init__(self, callback, args, loop, context=None):
+        super().__init__(callback, args, loop, context)
+        self._lock = threading.RLock()
+
+    def cancel(self):
+        with self._lock:
+            return super().cancel()
+
+    def cancelled(self):
+        with self._lock:
+            return super().cancelled()
+
+    def _run(self):
+        # The event loop checks for cancellation without holding the lock
+        # It is possible that the handle is cancelled after the check
+        # but before the callback is called so check it again after acquiring
+        # the lock and return without calling the callback if it is cancelled.
+        with self._lock:
+            if self._cancelled:
+                return
+            return super()._run()
+
 
 class TimerHandle(Handle):
     """Object returned by timed callback registration methods."""