asyncio: Fix race in FastChildWatcher (by its original author, Anthony Baire).

Lib/asyncio/unix_events.py

@@ -641,21 +641,15 @@ class FastChildWatcher(BaseChildWatcher):
 
     def add_child_handler(self, pid, callback, *args):
         assert self._forks, "Must use the context manager"
-
-        self._callbacks[pid] = callback, args
-
+        with self._lock:
             try:
-            # Ensure that the child is not already terminated.
-            # (raise KeyError if still alive)
                 returncode = self._zombies.pop(pid)
-
-            # Child is dead, therefore we can fire the callback immediately.
-            # First we remove it from the dict.
-            # (raise KeyError if .remove_child_handler() was called in-between)
-            del self._callbacks[pid]
             except KeyError:
-            pass
-        else:
+                # The child is running.
+                self._callbacks[pid] = callback, args
+                return
+
+        # The child is dead already. We can fire the callback.
         callback(pid, returncode, *args)
 
     def remove_child_handler(self, pid):
@@ -681,16 +675,18 @@ class FastChildWatcher(BaseChildWatcher):
 
                 returncode = self._compute_returncode(status)
 
+            with self._lock:
                 try:
                     callback, args = self._callbacks.pop(pid)
                 except KeyError:
                     # unknown child
-                with self._lock:
                     if self._forks:
                         # It may not be registered yet.
                         self._zombies[pid] = returncode
                         continue
+                    callback = None
 
+            if callback is None:
                 logger.warning(
                     "Caught subprocess termination from unknown pid: "
                     "%d -> %d", pid, returncode)