gh-130145: fix loop.run_forever when loop is already running

Lib/asyncio/base_events.py

@@ -671,8 +671,8 @@ def _run_forever_cleanup(self):
 
     def run_forever(self):
         """Run until stop() is called."""
+        self._run_forever_setup()
         try:
-            self._run_forever_setup()
             while True:
                 self._run_once()
                 if self._stopping:

Lib/test/test_asyncio/test_events.py

@@ -3004,6 +3004,22 @@ async def main():
                 self.loop.run_until_complete(main()),
                 'hello')
 
+    def test_get_running_loop_already_running(self):
+        async def main():
+            running_loop = asyncio.get_running_loop()
+            loop = asyncio.new_event_loop()
+            try:
+                loop.run_forever()
+            except RuntimeError:
+                pass
+            else:
+                self.fail("RuntimeError not raised")
+
+            self.assertIs(asyncio.get_running_loop(), running_loop)
+
+        self.loop.run_until_complete(main())
+
+
     def test_get_event_loop_returns_running_loop(self):
         class TestError(Exception):
             pass

Misc/NEWS.d/next/Library/2025-02-15-07-50-37.gh-issue-130145.I0CkV0.rst

@@ -0,0 +1 @@
+Fix :meth:`!asyncio.AbstractEventloop.run_forever` when another loop is already running.