Merge 3.5 (issue #28613)

3 files changed, 47 insertions, 2 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index 68e676b..85cc1e4 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -393,7 +393,10 @@ class BaseEventLoop(events.AbstractEventLoop):
         """Run until stop() is called."""
         self._check_closed()
         if self.is_running():
-            raise RuntimeError('Event loop is running.')
+            raise RuntimeError('This event loop is already running')
+        if events._get_running_loop() is not None:
+            raise RuntimeError(
+                'Cannot run the event loop while another loop is running')
         self._set_coroutine_wrapper(self._debug)
         self._thread_id = threading.get_ident()
         if self._asyncgens is not None:
@@ -401,6 +404,7 @@ class BaseEventLoop(events.AbstractEventLoop):
             sys.set_asyncgen_hooks(firstiter=self._asyncgen_firstiter_hook,
                                    finalizer=self._asyncgen_finalizer_hook)
         try:
+            events._set_running_loop(self)
             while True:
                 self._run_once()
                 if self._stopping:
@@ -408,6 +412,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         finally:
             self._stopping = False
             self._thread_id = None
+            events._set_running_loop(None)
             self._set_coroutine_wrapper(False)
             if self._asyncgens is not None:
                 sys.set_asyncgen_hooks(*old_agen_hooks)
diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index b89b4b2..8575e2c 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -607,6 +607,30 @@ _event_loop_policy = None
 _lock = threading.Lock()
 
 
+# A TLS for the running event loop, used by _get_running_loop.
+class _RunningLoop(threading.local):
+    _loop = None
+_running_loop = _RunningLoop()
+
+
+def _get_running_loop():
+    """Return the running event loop or None.
+
+    This is a low-level function intended to be used by event loops.
+    This function is thread-specific.
+    """
+    return _running_loop._loop
+
+
+def _set_running_loop(loop):
+    """Set the running event loop.
+
+    This is a low-level function intended to be used by event loops.
+    This function is thread-specific.
+    """
+    _running_loop._loop = loop
+
+
 def _init_event_loop_policy():
     global _event_loop_policy
     with _lock:
@@ -632,7 +656,17 @@ def set_event_loop_policy(policy):
 
 
 def get_event_loop():
-    """Equivalent to calling get_event_loop_policy().get_event_loop()."""
+    """Return an asyncio event loop.
+
+    When called from a coroutine or a callback (e.g. scheduled with call_soon
+    or similar API), this function will always return the running event loop.
+
+    If there is no running event loop set, the function will return
+    the result of `get_event_loop_policy().get_event_loop()` call.
+    """
+    current_loop = _get_running_loop()
+    if current_loop is not None:
+        return current_loop
     return get_event_loop_policy().get_event_loop()
 
 
diff --git a/Lib/asyncio/test_utils.py b/Lib/asyncio/test_utils.py
index fdd3ba0..99e3839 100644
--- a/Lib/asyncio/test_utils.py
+++ b/Lib/asyncio/test_utils.py
@@ -449,7 +449,13 @@ class TestCase(unittest.TestCase):
         self.set_event_loop(loop)
         return loop
 
+    def setUp(self):
+        self._get_running_loop = events._get_running_loop
+        events._get_running_loop = lambda: None
+
     def tearDown(self):
+        events._get_running_loop = self._get_running_loop
+
         events.set_event_loop(None)
 
         # Detect CPython bug #23353: ensure that yield/yield-from is not used