asyncio: Make sure sys.set_coroutine_wrapper is called *only* when loop is running.

Previous approach of installing coroutine wrapper in loop.set_debug() and
uninstalling it in loop.close() was very fragile.  Most of asyncio tests
do not call loop.close() at all.  Since coroutine wrapper is a global
setting, we have to make sure that it's only set when the loop is
running, and is automatically unset when it stops running.

Issue #24017.

Lib/asyncio/base_events.py

@@ -198,6 +198,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         self.slow_callback_duration = 0.1
         self._current_handle = None
         self._task_factory = None
+        self._coroutine_wrapper_set = False
 
     def __repr__(self):
         return ('<%s running=%s closed=%s debug=%s>'
@@ -291,6 +292,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         self._check_closed()
         if self.is_running():
             raise RuntimeError('Event loop is running.')
+        self._set_coroutine_wrapper(self._debug)
         self._thread_id = threading.get_ident()
         try:
             while True:
@@ -300,6 +302,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                     break
         finally:
             self._thread_id = None
+            self._set_coroutine_wrapper(False)
 
     def run_until_complete(self, future):
         """Run until the Future is done.
@@ -360,7 +363,6 @@ class BaseEventLoop(events.AbstractEventLoop):
             return
         if self._debug:
             logger.debug("Close %r", self)
-        try:
         self._closed = True
         self._ready.clear()
         self._scheduled.clear()
@@ -368,10 +370,6 @@ class BaseEventLoop(events.AbstractEventLoop):
         if executor is not None:
             self._default_executor = None
             executor.shutdown(wait=False)
-        finally:
-            # It is important to unregister "sys.coroutine_wrapper"
-            # if it was registered.
-            self.set_debug(False)
 
     def is_closed(self):
         """Returns True if the event loop was closed."""
@@ -1199,19 +1197,20 @@ class BaseEventLoop(events.AbstractEventLoop):
                 handle._run()
         handle = None  # Needed to break cycles when an exception occurs.
 
-    def get_debug(self):
-        return self._debug
-
-    def set_debug(self, enabled):
-        self._debug = enabled
-        wrapper = coroutines.debug_wrapper
-
+    def _set_coroutine_wrapper(self, enabled):
         try:
             set_wrapper = sys.set_coroutine_wrapper
+            get_wrapper = sys.get_coroutine_wrapper
         except AttributeError:
-            pass
-        else:
-            current_wrapper = sys.get_coroutine_wrapper()
+            return
+
+        enabled = bool(enabled)
+        if self._coroutine_wrapper_set is enabled:
+            return
+
+        wrapper = coroutines.debug_wrapper
+        current_wrapper = get_wrapper()
+
         if enabled:
             if current_wrapper not in (None, wrapper):
                 warnings.warn(
@@ -1220,6 +1219,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                     current_wrapper, RuntimeWarning)
             else:
                 set_wrapper(wrapper)
+                self._coroutine_wrapper_set = True
         else:
             if current_wrapper not in (None, wrapper):
                 warnings.warn(
@@ -1228,3 +1228,13 @@ class BaseEventLoop(events.AbstractEventLoop):
                     current_wrapper, RuntimeWarning)
             else:
                 set_wrapper(None)
+                self._coroutine_wrapper_set = False
+
+    def get_debug(self):
+        return self._debug
+
+    def set_debug(self, enabled):
+        self._debug = enabled
+
+        if self.is_running():
+            self._set_coroutine_wrapper(enabled)