Ensure that we do not cache empty sync responses after a timeout (#10158)

Fixes #8518 by telling the ResponseCache not to cache the /sync response if the next_batch param is the same as the since token.

2 files changed, 53 insertions, 5 deletions

diff --git a/tests/rest/client/v2_alpha/test_sync.py b/tests/rest/client/v2_alpha/test_sync.py
index b52f78ba69..012910f136 100644
--- a/tests/rest/client/v2_alpha/test_sync.py
+++ b/tests/rest/client/v2_alpha/test_sync.py
@@ -558,3 +558,53 @@ class UnreadMessagesTestCase(unittest.HomeserverTestCase):
 
         # Store the next batch for the next request.
         self.next_batch = channel.json_body["next_batch"]
+
+
+class SyncCacheTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        synapse.rest.admin.register_servlets,
+        login.register_servlets,
+        sync.register_servlets,
+    ]
+
+    def test_noop_sync_does_not_tightloop(self):
+        """If the sync times out, we shouldn't cache the result
+
+        Essentially a regression test for #8518.
+        """
+        self.user_id = self.register_user("kermit", "monkey")
+        self.tok = self.login("kermit", "monkey")
+
+        # we should immediately get an initial sync response
+        channel = self.make_request("GET", "/sync", access_token=self.tok)
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # now, make an incremental sync request, with a timeout
+        next_batch = channel.json_body["next_batch"]
+        channel = self.make_request(
+            "GET",
+            f"/sync?since={next_batch}&timeout=10000",
+            access_token=self.tok,
+            await_result=False,
+        )
+        # that should block for 10 seconds
+        with self.assertRaises(TimedOutException):
+            channel.await_result(timeout_ms=9900)
+        channel.await_result(timeout_ms=200)
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # we expect the next_batch in the result to be the same as before
+        self.assertEqual(channel.json_body["next_batch"], next_batch)
+
+        # another incremental sync should also block.
+        channel = self.make_request(
+            "GET",
+            f"/sync?since={next_batch}&timeout=10000",
+            access_token=self.tok,
+            await_result=False,
+        )
+        # that should block for 10 seconds
+        with self.assertRaises(TimedOutException):
+            channel.await_result(timeout_ms=9900)
+        channel.await_result(timeout_ms=200)
+        self.assertEqual(channel.code, 200, channel.json_body)
diff --git a/tests/server.py b/tests/server.py
index 9df8cda24f..f32d8dc375 100644
--- a/tests/server.py
+++ b/tests/server.py
@@ -138,21 +138,19 @@ class FakeChannel:
     def transport(self):
         return self
 
-    def await_result(self, timeout: int = 100) -> None:
+    def await_result(self, timeout_ms: int = 1000) -> None:
         """
         Wait until the request is finished.
         """
+        end_time = self._reactor.seconds() + timeout_ms / 1000.0
         self._reactor.run()
-        x = 0
 
         while not self.is_finished():
             # If there's a producer, tell it to resume producing so we get content
             if self._producer:
                 self._producer.resumeProducing()
 
-            x += 1
-
-            if x > timeout:
+            if self._reactor.seconds() > end_time:
                 raise TimedOutException("Timed out waiting for request to finish.")
 
             self._reactor.advance(0.1)