Guard against different return values for remove_pusher.

changelog.d/7981.misc

@@ -0,0 +1 @@
+Convert various parts of the codebase to async/await.

synapse/push/httppusher.py

@@ -13,6 +13,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import inspect
 import logging
 
 from prometheus_client import Counter
@@ -296,7 +297,11 @@ async def _process_one(self, push_action):
                     )
                 else:
                     logger.info("Pushkey %s was rejected: removing", pk)
-                    await self.hs.remove_pusher(self.app_id, pk, self.user_id)
+                    # remove_pusher might return an awaitable (for the main
+                    # process) or None (for a worker process).
+                    result = self.hs.remove_pusher(self.app_id, pk, self.user_id)
+                    if inspect.isawaitable(result):
+                        await result
         return True
 
     async def _build_notification_dict(self, event, tweaks, badge):