Allow adding random delay to push (#15516)

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

2 files changed, 26 insertions, 2 deletions

diff --git a/synapse/config/push.py b/synapse/config/push.py
index 3b5378e6ea..8177ff52e2 100644
--- a/synapse/config/push.py
+++ b/synapse/config/push.py
@@ -42,11 +42,17 @@ class PushConfig(Config):
 
         # Now check for the one in the 'email' section and honour it,
         # with a warning.
-        push_config = config.get("email") or {}
-        redact_content = push_config.get("redact_content")
+        email_push_config = config.get("email") or {}
+        redact_content = email_push_config.get("redact_content")
         if redact_content is not None:
             print(
                 "The 'email.redact_content' option is deprecated: "
                 "please set push.include_content instead"
             )
             self.push_include_content = not redact_content
+
+        # Whether to apply a random delay to outbound push.
+        self.push_jitter_delay_ms = None
+        push_jitter_delay = push_config.get("jitter_delay", None)
+        if push_jitter_delay:
+            self.push_jitter_delay_ms = self.parse_duration(push_jitter_delay)
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