bpo-34769: Thread safety for _asyncgen_finalizer_hook().

Lib/asyncio/base_events.py

@@ -477,10 +477,7 @@ def _check_closed(self):
     def _asyncgen_finalizer_hook(self, agen):
         self._asyncgens.discard(agen)
         if not self.is_closed():
-            self.create_task(agen.aclose())
-            # Wake up the loop if the finalizer was called from
-            # a different thread.
-            self._write_to_self()
+            self.call_soon_threadsafe(self.create_task, agen.aclose())
 
     def _asyncgen_firstiter_hook(self, agen):
         if self._asyncgens_shutdown_called:

Lib/test/test_asyncio/test_base_events.py

@@ -926,6 +926,76 @@ def test_run_forever_pre_stopped(self):
         self.loop.run_forever()
         self.loop._selector.select.assert_called_once_with(0)
 
+    async def leave_unfinalized_asyncgen(self):
+        # The following should create an async generator, iterate
+        # it partially, and leave it to be garbage collected.
+        # This depends on implementation details of the garbage
+        # collector, and may stop working in future versions of
+        # cpython or in other implementations. In that case,
+        # the tests below on async generator finalization may
+        # break as well.
+        status = {'started': False,
+                  'stopped': False,
+                  'finalized': False}
+
+        async def agen():
+            status['started'] = True
+            try:
+                for item in ['ZERO', 'ONE', 'TWO', 'THREE', 'FOUR']:
+                    yield item
+            finally:
+                status['finalized'] = True
+
+        ag = agen()
+        ai = ag.__aiter__()
+
+        async def iter_one():
+            try:
+                item = await ai.__anext__()
+            except StopAsyncIteration:
+                return
+            if item == 'THREE':
+                status['stopped'] = True
+                return
+            asyncio.create_task(iter_one())
+
+        asyncio.create_task(iter_one())
+        return status
+
+    def test_asyncgen_finalization_by_gc(self):
+        # Async generators should be finalized when garbage collected.
+        self.loop._process_events = mock.Mock()
+        self.loop._write_to_self = mock.Mock()
+        with support.disable_gc():
+            status = self.loop.run_until_complete(self.leave_unfinalized_asyncgen())
+            while not status['stopped']:
+                test_utils.run_briefly(self.loop)
+            self.assertTrue(status['started'])
+            self.assertTrue(status['stopped'])
+            self.assertFalse(status['finalized'])
+            support.gc_collect()
+            test_utils.run_briefly(self.loop)
+            self.assertTrue(status['finalized'])
+
+    def test_asyncgen_finalization_by_gc_in_other_thread(self):
+        # Python issue 34769: If garbage collector runs in another
+        # thread, async generators will not finalize in debug
+        # mode.
+        self.loop._process_events = mock.Mock()
+        self.loop._write_to_self = mock.Mock()
+        self.loop.set_debug(True)
+        with support.disable_gc():
+            status = self.loop.run_until_complete(self.leave_unfinalized_asyncgen())
+            while not status['stopped']:
+                test_utils.run_briefly(self.loop)
+            self.assertTrue(status['started'])
+            self.assertTrue(status['stopped'])
+            self.assertFalse(status['finalized'])
+            self.loop.run_until_complete(
+                self.loop.run_in_executor(None, support.gc_collect))
+            test_utils.run_briefly(self.loop)
+            self.assertTrue(status['finalized'])
+
 
 class MyProto(asyncio.Protocol):
     done = None

Misc/NEWS.d/next/Library/2018-10-09-11-01-16.bpo-34769.cSkkZt.rst

@@ -0,0 +1,2 @@
+Fix for async generators not finalizing when event loop in debug mode and
+garbage collector runs in another thread.