test_libevreactor: Restore preparer after cleanup

test_watchers_are_finished calls libev__cleanup(), which stops the shared
libev loop preparer. If a timer test runs later, the stopped preparer means
timers are never scheduled and the test can hang.

Restore _global_loop._shutdown and restart _global_loop._preparer after the
cleanup path runs. Put the restoration in a finally block so the shared loop is
left usable even if the post-cleanup watcher assertions fail.

Author: sylwiaszunejko

tests/unit/io/test_libevreactor.py

@@ -69,24 +69,33 @@ def test_watchers_are_finished(self):
         @test_category connection
         """
         from cassandra.io.libevreactor import _global_loop
-        with patch.object(_global_loop, "_thread"),\
-             patch.object(_global_loop, "notify"):
-
-            self.make_connection()
-
-            # We have to make a copy because the connections shouldn't
-            # be alive when we verify them
-            live_connections = set(_global_loop._live_conns)
-
-            # This simulates the process ending without cluster.shutdown()
-            # being called, then with atexit _cleanup for libevreactor would
-            # be called
-            libev__cleanup(_global_loop)
-            for conn in live_connections:
-                assert conn._write_watcher.stop.mock_calls
-                assert conn._read_watcher.stop.mock_calls
-
-        _global_loop._shutdown = False
+        reactor_needs_restore = False
+        try:
+            with patch.object(_global_loop, "_thread"),\
+                 patch.object(_global_loop, "notify"):
+
+                self.make_connection()
+
+                # We have to make a copy because the connections shouldn't
+                # be alive when we verify them
+                live_connections = set(_global_loop._live_conns)
+
+                # This simulates the process ending without cluster.shutdown()
+                # being called, then with atexit _cleanup for libevreactor would
+                # be called
+                reactor_needs_restore = True
+                libev__cleanup(_global_loop)
+                for conn in live_connections:
+                    assert conn._write_watcher.stop.mock_calls
+                    assert conn._read_watcher.stop.mock_calls
+
+        finally:
+            if reactor_needs_restore:
+                _global_loop._shutdown = False
+                # _cleanup stopped the prepare watcher; restart it so the shared
+                # singleton loop is left in a working state for subsequent tests
+                # (otherwise timers would never be scheduled and tests would hang).
+                _global_loop._preparer.start()
 
 
 class LibevTimerPatcher(unittest.TestCase):