Issue #28368: Refuse monitoring processes if the child watcher has no loop attached.

Patch by Vincent Michel.

Lib/asyncio/unix_events.py

@@ -746,6 +746,7 @@ class BaseChildWatcher(AbstractChildWatcher):
 
     def __init__(self):
         self._loop = None
+        self._callbacks = {}
 
     def close(self):
         self.attach_loop(None)
@@ -759,6 +760,12 @@ class BaseChildWatcher(AbstractChildWatcher):
     def attach_loop(self, loop):
         assert loop is None or isinstance(loop, events.AbstractEventLoop)
 
+        if self._loop is not None and loop is None and self._callbacks:
+            warnings.warn(
+                'A loop is being detached '
+                'from a child watcher with pending handlers',
+                RuntimeWarning)
+
         if self._loop is not None:
             self._loop.remove_signal_handler(signal.SIGCHLD)
 
@@ -807,10 +814,6 @@ class SafeChildWatcher(BaseChildWatcher):
     big number of children (O(n) each time SIGCHLD is raised)
     """
 
-    def __init__(self):
-        super().__init__()
-        self._callbacks = {}
-
     def close(self):
         self._callbacks.clear()
         super().close()
@@ -822,6 +825,11 @@ class SafeChildWatcher(BaseChildWatcher):
         pass
 
     def add_child_handler(self, pid, callback, *args):
+        if self._loop is None:
+            raise RuntimeError(
+                "Cannot add child handler, "
+                "the child watcher does not have a loop attached")
+
         self._callbacks[pid] = (callback, args)
 
         # Prevent a race condition in case the child is already terminated.
@@ -886,7 +894,6 @@ class FastChildWatcher(BaseChildWatcher):
     """
     def __init__(self):
         super().__init__()
-        self._callbacks = {}
         self._lock = threading.Lock()
         self._zombies = {}
         self._forks = 0
@@ -918,6 +925,12 @@ class FastChildWatcher(BaseChildWatcher):
 
     def add_child_handler(self, pid, callback, *args):
         assert self._forks, "Must use the context manager"
+
+        if self._loop is None:
+            raise RuntimeError(
+                "Cannot add child handler, "
+                "the child watcher does not have a loop attached")
+
         with self._lock:
             try:
                 returncode = self._zombies.pop(pid)