gh-109538: Avoid RuntimeError when StreamWriter is deleted with closed loop (#111983)

Issue a ResourceWarning instead.

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

3 files changed, 45 insertions, 5 deletions

diff --git a/Lib/asyncio/streams.py b/Lib/asyncio/streams.py
index f82b10c..ffb48b9 100644
--- a/Lib/asyncio/streams.py
+++ b/Lib/asyncio/streams.py
@@ -406,9 +406,11 @@ class StreamWriter:
 
     def __del__(self, warnings=warnings):
         if not self._transport.is_closing():
-            self.close()
-            warnings.warn(f"unclosed {self!r}", ResourceWarning)
-
+            if self._loop.is_closed():
+                warnings.warn("loop is closed", ResourceWarning)
+            else:
+                self.close()
+                warnings.warn(f"unclosed {self!r}", ResourceWarning)
 
 class StreamReader:
 
diff --git a/Lib/test/test_asyncio/test_streams.py b/Lib/test/test_asyncio/test_streams.py
index 3fea7b9..ccb7dbf 100644
--- a/Lib/test/test_asyncio/test_streams.py
+++ b/Lib/test/test_asyncio/test_streams.py
@@ -1082,10 +1082,11 @@ os.close(fd)
             self.assertEqual(data, b'HTTP/1.0 200 OK\r\n')
             data = await rd.read()
             self.assertTrue(data.endswith(b'\r\n\r\nTest message'))
-            with self.assertWarns(ResourceWarning):
+            with self.assertWarns(ResourceWarning) as cm:
                 del wr
                 gc.collect()
-
+                self.assertEqual(len(cm.warnings), 1)
+                self.assertTrue(str(cm.warnings[0].message).startswith("unclosed <StreamWriter"))
 
         messages = []
         self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))
@@ -1095,6 +1096,42 @@ os.close(fd)
 
         self.assertEqual(messages, [])
 
+    def test_loop_is_closed_resource_warnings(self):
+        async def inner(httpd):
+            rd, wr = await asyncio.open_connection(*httpd.address)
+
+            wr.write(b'GET / HTTP/1.0\r\n\r\n')
+            data = await rd.readline()
+            self.assertEqual(data, b'HTTP/1.0 200 OK\r\n')
+            data = await rd.read()
+            self.assertTrue(data.endswith(b'\r\n\r\nTest message'))
+
+            # Make "loop is closed" occur first before "del wr" for this test.
+            self.loop.stop()
+            wr.close()
+            while not self.loop.is_closed():
+                await asyncio.sleep(0.0)
+
+            with self.assertWarns(ResourceWarning) as cm:
+                del wr
+                gc.collect()
+                self.assertEqual(len(cm.warnings), 1)
+                self.assertEqual("loop is closed", str(cm.warnings[0].message))
+
+        messages = []
+        self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))
+
+        with test_utils.run_test_server() as httpd:
+            try:
+                self.loop.run_until_complete(inner(httpd))
+            # This exception is caused by `self.loop.stop()` as expected.
+            except RuntimeError:
+                pass
+            finally:
+                gc.collect()
+
+        self.assertEqual(messages, [])
+
     def test_unhandled_exceptions(self) -> None:
         port = socket_helper.find_unused_port()
 
diff --git a/Misc/NEWS.d/next/Library/2023-11-11-16-42-48.gh-issue-109538.cMG5ux.rst b/Misc/NEWS.d/next/Library/2023-11-11-16-42-48.gh-issue-109538.cMG5ux.rst
new file mode 100644
index 0000000..d1ee4c0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-11-11-16-42-48.gh-issue-109538.cMG5ux.rst
@@ -0,0 +1 @@
+Issue warning message instead of having :class:`RuntimeError` be displayed when event loop has already been closed at :meth:`StreamWriter.__del__`.