Limit size of presence EDUs (#17371)

Otherwise they are unbounded.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

3 files changed, 140 insertions, 11 deletions

diff --git a/changelog.d/17371.misc b/changelog.d/17371.misc
new file mode 100644
index 0000000000..0fbf19f4fb
--- /dev/null
+++ b/changelog.d/17371.misc
@@ -0,0 +1 @@
+Limit size of presence EDUs to 50 entries.
diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index d9f2f017ed..9f1c2fe22a 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -21,6 +21,7 @@
 #
 import datetime
 import logging
+from collections import OrderedDict
 from types import TracebackType
 from typing import TYPE_CHECKING, Dict, Hashable, Iterable, List, Optional, Tuple, Type
 
@@ -68,6 +69,10 @@ sent_edus_by_type = Counter(
 # If the retry interval is larger than this then we enter "catchup" mode
 CATCHUP_RETRY_INTERVAL = 60 * 60 * 1000
 
+# Limit how many presence states we add to each presence EDU, to ensure that
+# they are bounded in size.
+MAX_PRESENCE_STATES_PER_EDU = 50
+
 
 class PerDestinationQueue:
     """
@@ -144,7 +149,7 @@ class PerDestinationQueue:
 
         # Map of user_id -> UserPresenceState of pending presence to be sent to this
         # destination
-        self._pending_presence: Dict[str, UserPresenceState] = {}
+        self._pending_presence: OrderedDict[str, UserPresenceState] = OrderedDict()
 
         # List of room_id -> receipt_type -> user_id -> receipt_dict,
         #
@@ -399,7 +404,7 @@ class PerDestinationQueue:
                 # through another mechanism, because this is all volatile!
                 self._pending_edus = []
                 self._pending_edus_keyed = {}
-                self._pending_presence = {}
+                self._pending_presence.clear()
                 self._pending_receipt_edus = []
 
                 self._start_catching_up()
@@ -721,22 +726,26 @@ class _TransactionQueueManager:
 
         # Add presence EDU.
         if self.queue._pending_presence:
+            # Only send max 50 presence entries in the EDU, to bound the amount
+            # of data we're sending.
+            presence_to_add: List[JsonDict] = []
+            while (
+                self.queue._pending_presence
+                and len(presence_to_add) < MAX_PRESENCE_STATES_PER_EDU
+            ):
+                _, presence = self.queue._pending_presence.popitem(last=False)
+                presence_to_add.append(
+                    format_user_presence_state(presence, self.queue._clock.time_msec())
+                )
+
             pending_edus.append(
                 Edu(
                     origin=self.queue._server_name,
                     destination=self.queue._destination,
                     edu_type=EduTypes.PRESENCE,
-                    content={
-                        "push": [
-                            format_user_presence_state(
-                                presence, self.queue._clock.time_msec()
-                            )
-                            for presence in self.queue._pending_presence.values()
-                        ]
-                    },
+                    content={"push": presence_to_add},
                 )
             )
-            self.queue._pending_presence = {}
 
         # Add read receipt EDUs.
         pending_edus.extend(self.queue._get_receipt_edus(force_flush=False, limit=5))
diff --git a/tests/federation/test_federation_sender.py b/tests/federation/test_federation_sender.py
index 9073afc70e..6a8887fe74 100644
--- a/tests/federation/test_federation_sender.py
+++ b/tests/federation/test_federation_sender.py
@@ -27,6 +27,8 @@ from twisted.internet import defer
 from twisted.test.proto_helpers import MemoryReactor
 
 from synapse.api.constants import EduTypes, RoomEncryptionAlgorithms
+from synapse.api.presence import UserPresenceState
+from synapse.federation.sender.per_destination_queue import MAX_PRESENCE_STATES_PER_EDU
 from synapse.federation.units import Transaction
 from synapse.handlers.device import DeviceHandler
 from synapse.rest import admin
@@ -266,6 +268,123 @@ class FederationSenderReceiptsTestCases(HomeserverTestCase):
         )
 
 
+class FederationSenderPresenceTestCases(HomeserverTestCase):
+    """
+    Test federation sending for presence updates.
+    """
+
+    def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
+        self.federation_transport_client = Mock(spec=["send_transaction"])
+        self.federation_transport_client.send_transaction = AsyncMock()
+        hs = self.setup_test_homeserver(
+            federation_transport_client=self.federation_transport_client,
+        )
+
+        return hs
+
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+        config["federation_sender_instances"] = None
+        return config
+
+    def test_presence_simple(self) -> None:
+        "Test that sending a single presence update works"
+
+        mock_send_transaction: AsyncMock = (
+            self.federation_transport_client.send_transaction
+        )
+        mock_send_transaction.return_value = {}
+
+        sender = self.hs.get_federation_sender()
+        self.get_success(
+            sender.send_presence_to_destinations(
+                [UserPresenceState.default("@user:test")],
+                ["server"],
+            )
+        )
+
+        self.pump()
+
+        # expect a call to send_transaction
+        mock_send_transaction.assert_awaited_once()
+
+        json_cb = mock_send_transaction.call_args[0][1]
+        data = json_cb()
+        self.assertEqual(
+            data["edus"],
+            [
+                {
+                    "edu_type": EduTypes.PRESENCE,
+                    "content": {
+                        "push": [
+                            {
+                                "presence": "offline",
+                                "user_id": "@user:test",
+                            }
+                        ]
+                    },
+                }
+            ],
+        )
+
+    def test_presence_batched(self) -> None:
+        """Test that sending lots of presence updates to a destination are
+        batched, rather than having them all sent in one EDU."""
+
+        mock_send_transaction: AsyncMock = (
+            self.federation_transport_client.send_transaction
+        )
+        mock_send_transaction.return_value = {}
+
+        sender = self.hs.get_federation_sender()
+
+        # We now send lots of presence updates to force the federation sender to
+        # batch the mup.
+        number_presence_updates_to_send = MAX_PRESENCE_STATES_PER_EDU * 2
+        self.get_success(
+            sender.send_presence_to_destinations(
+                [
+                    UserPresenceState.default(f"@user{i}:test")
+                    for i in range(number_presence_updates_to_send)
+                ],
+                ["server"],
+            )
+        )
+
+        self.pump()
+
+        # We should have seen at least one transcation be sent by now.
+        mock_send_transaction.assert_called()
+
+        # We don't want to specify exactly how the presence EDUs get sent out,
+        # could be one per transaction or multiple per transaction. We just want
+        # to assert that a) each presence EDU has bounded number of updates, and
+        # b) that all updates get sent out.
+        presence_edus = []
+        for transaction_call in mock_send_transaction.call_args_list:
+            json_cb = transaction_call[0][1]
+            data = json_cb()
+
+            for edu in data["edus"]:
+                self.assertEqual(edu.get("edu_type"), EduTypes.PRESENCE)
+                presence_edus.append(edu)
+
+        # A set of all user presence we see, this should end up matching the
+        # number we sent out above.
+        seen_users: Set[str] = set()
+
+        for edu in presence_edus:
+            presence_states = edu["content"]["push"]
+
+            # This is where we actually check that the number of presence
+            # updates is bounded.
+            self.assertLessEqual(len(presence_states), MAX_PRESENCE_STATES_PER_EDU)
+
+            seen_users.update(p["user_id"] for p in presence_states)
+
+        self.assertEqual(len(seen_users), number_presence_updates_to_send)
+
+
 class FederationSenderDevicesTestCases(HomeserverTestCase):
     """
     Test federation sending to update devices.