[3.7] bpo-39191: Don't spawn a task before failing (GH-17796) (#17821)

(cherry picked from commit 3a5de511596f17575de082dcb8d43d63b2bd2da9)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

2 files changed, 10 insertions, 3 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index fdd80bcfdd..f723356344 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -518,14 +518,17 @@ class BaseEventLoop(events.AbstractEventLoop):
                     'asyncgen': agen
                 })
 
-    def run_forever(self):
-        """Run until stop() is called."""
-        self._check_closed()
+    def _check_runnung(self):
         if self.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')
+
+    def run_forever(self):
+        """Run until stop() is called."""
+        self._check_closed()
+        self._check_runnung()
         self._set_coroutine_origin_tracking(self._debug)
         self._thread_id = threading.get_ident()
 
@@ -557,6 +560,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         Return the Future's result, or raise its exception.
         """
         self._check_closed()
+        self._check_runnung()
 
         new_task = not futures.isfuture(future)
         future = tasks.ensure_future(future, loop=self)
diff --git a/Misc/NEWS.d/next/Library/2020-01-02-17-28-03.bpo-39191.ur_scy.rst b/Misc/NEWS.d/next/Library/2020-01-02-17-28-03.bpo-39191.ur_scy.rst
new file mode 100644
index 0000000000..138c93c2e4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-01-02-17-28-03.bpo-39191.ur_scy.rst
@@ -0,0 +1,3 @@
+Perform a check for running loop before starting a new task in
+``loop.run_until_complete()`` to fail fast; it prevents the side effect of
+new task spawning before exception raising.