Revert "Don't use threads in this test"

This reverts commit 78cd4161f0.

tests/test_basic.py

@@ -1180,9 +1180,8 @@ def test_test_app_proper_environ():
     assert rv.data == b'Foo SubDomain'
 
 
-@pytest.mark.parametrize('config_key',
-                         ['TESTING', 'PROPAGATE_EXCEPTIONS', 'DEBUG', None])
-def test_exception_propagation(config_key):
+def test_exception_propagation():
+    def apprunner(configkey):
         app = flask.Flask(__name__)
         app.config['LOGGER_HANDLER_POLICY'] = 'never'
 
@@ -1190,22 +1189,26 @@ def test_exception_propagation(config_key):
         def index():
             1 // 0
         c = app.test_client()
-
         if config_key is not None:
             app.config[config_key] = True
-        with pytest.raises(Exception):
+            try:
                 c.get('/')
-
+            except Exception:
+                pass
+            else:
+                assert False, 'expected exception'
         else:
             assert c.get('/').status_code == 500
 
-    # If the debug flag is set to true and an exception happens the context
-    # is not torn down.  This causes other tests that run after this fail
+    # we have to run this test in an isolated thread because if the
+    # debug flag is set to true and an exception happens the context is
+    # not torn down.  This causes other tests that run after this fail
     # when they expect no exception on the stack.
-    while flask._request_ctx_stack.top is not None:
-        flask._request_ctx_stack.pop()
-    while flask._app_ctx_stack.top is not None:
-        flask._app_ctx_stack.pop()
+    for config_key in 'TESTING', 'PROPAGATE_EXCEPTIONS', 'DEBUG', None:
+        t = Thread(target=apprunner, args=(config_key,))
+        t.start()
+        t.join()
+
 
 def test_max_content_length():
     app = flask.Flask(__name__)