From d500d1a66d321c821f5234fe4f47fdfef578cca8 Mon Sep 17 00:00:00 2001
From: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
Date: Mon, 15 Aug 2022 22:32:47 +0530
Subject: [PATCH] GH-95899: fix asyncio.Runner to call set_event_loop only once
 (GH-95900) (cherry picked from commit
 914f6367a0d015986dafa7a9d542e24192753b6b)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
---
 Lib/asyncio/runners.py                             |  8 +++++---
 Lib/test/test_asyncio/test_runners.py              | 14 ++++++++++++++
 .../2022-08-11-18-52-17.gh-issue-95899._Bi4uG.rst  |  1 +
 3 files changed, 20 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2022-08-11-18-52-17.gh-issue-95899._Bi4uG.rst

diff --git a/Lib/asyncio/runners.py b/Lib/asyncio/runners.py
index a19a7f99af064d..b3e0c44b7fb59d 100644
--- a/Lib/asyncio/runners.py
+++ b/Lib/asyncio/runners.py
@@ -115,8 +115,6 @@ def run(self, coro, *, context=None):
 
         self._interrupt_count = 0
         try:
-            if self._set_event_loop:
-                events.set_event_loop(self._loop)
             return self._loop.run_until_complete(task)
         except exceptions.CancelledError:
             if self._interrupt_count > 0:
@@ -137,7 +135,11 @@ def _lazy_init(self):
             return
         if self._loop_factory is None:
             self._loop = events.new_event_loop()
-            self._set_event_loop = True
+            if not self._set_event_loop:
+                # Call set_event_loop only once to avoid calling
+                # attach_loop multiple times on child watchers
+                events.set_event_loop(self._loop)
+                self._set_event_loop = True
         else:
             self._loop = self._loop_factory()
         if self._debug is not None:
diff --git a/Lib/test/test_asyncio/test_runners.py b/Lib/test/test_asyncio/test_runners.py
index 5e1db2357e75f1..4e3acb97c85d37 100644
--- a/Lib/test/test_asyncio/test_runners.py
+++ b/Lib/test/test_asyncio/test_runners.py
@@ -456,6 +456,20 @@ async def coro():
             ):
                 runner.run(coro())
 
+    def test_set_event_loop_called_once(self):
+        # See https://github.com/python/cpython/issues/95736
+        async def coro():
+            pass
+
+        policy = asyncio.get_event_loop_policy()
+        policy.set_event_loop = mock.Mock()
+        runner = asyncio.Runner()
+        runner.run(coro())
+        runner.run(coro())
+
+        self.assertEqual(1, policy.set_event_loop.call_count)
+        runner.close()
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2022-08-11-18-52-17.gh-issue-95899._Bi4uG.rst b/Misc/NEWS.d/next/Library/2022-08-11-18-52-17.gh-issue-95899._Bi4uG.rst
new file mode 100644
index 00000000000000..d2386cf3ae2296
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-08-11-18-52-17.gh-issue-95899._Bi4uG.rst
@@ -0,0 +1 @@
+Fix :class:`asyncio.Runner` to call :func:`asyncio.set_event_loop` only once to avoid calling :meth:`~asyncio.AbstractChildWatcher.attach_loop` multiple times on child watchers. Patch by Kumar Aditya.