Allow adding random delay to push (#15516)

This is to discourage timing based profiling on the push gateways.

1 files changed, 18 insertions, 0 deletions

diff --git a/synapse/push/httppusher.py b/synapse/push/httppusher.py
index a01445e374..e628b484a9 100644
--- a/synapse/push/httppusher.py
+++ b/synapse/push/httppusher.py
@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import random
 import urllib.parse
 from typing import TYPE_CHECKING, Dict, List, Optional, Union
 
@@ -114,6 +115,8 @@ class HttpPusher(Pusher):
         )
         self._pusherpool = hs.get_pusherpool()
 
+        self.push_jitter_delay_ms = hs.config.push.push_jitter_delay_ms
+
         self.data = pusher_config.data
         if self.data is None:
             raise PusherConfigException("'data' key can not be null for HTTP pusher")
@@ -327,6 +330,21 @@ class HttpPusher(Pusher):
         event = await self.store.get_event(push_action.event_id, allow_none=True)
         if event is None:
             return True  # It's been redacted
+
+        # Check if we should delay sending out the notification by a random
+        # amount.
+        #
+        # Note: we base the delay off of when the event was sent, rather than
+        # now, to handle the case where we need to send out many notifications
+        # at once. If we just slept the random amount each loop then the last
+        # push notification in the set could be delayed by many times the max
+        # delay.
+        if self.push_jitter_delay_ms:
+            delay_ms = random.randint(1, self.push_jitter_delay_ms)
+            diff_ms = event.origin_server_ts + delay_ms - self.clock.time_msec()
+            if diff_ms > 0:
+                await self.clock.sleep(diff_ms / 1000)
+
         rejected = await self.dispatch_push_event(event, tweaks, badge)
         if rejected is False:
             return False