Catch-up after Federation Outage (split, 4): catch-up loop

changelog.d/8272.bugfix

@@ -0,0 +1 @@
+Fix messages over federation being lost until an event is sent into the same room.

synapse/federation/sender/per_destination_queue.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 import datetime
 import logging
-from typing import TYPE_CHECKING, Dict, Hashable, Iterable, List, Tuple
+from typing import TYPE_CHECKING, Dict, Hashable, Iterable, List, Optional, Tuple, cast
 
 from prometheus_client import Counter
 
@@ -92,6 +92,23 @@ def __init__(
         self._destination = destination
         self.transmission_loop_running = False
 
+        # True whilst we are sending events that the remote homeserver missed
+        # because it was unreachable.
+        # False whilst we are known to not be catching up.
+        # None when it has not been determined if we are catching-up yet (we
+        # do so lazily).
+        # New events will only be sent once this is finished, at which point
+        # _catching_up is flipped to False.
+        self._catching_up = None  # type: Optional[bool]
+
+        # The stream_ordering of the most recent PDU that was discarded due to
+        # being in catch-up mode, or None if not applicable.
+        self._catchup_last_skipped = None  # type: Optional[int]
+
+        # Cache of the last successfully-transmitted stream ordering for this
+        # destination (we are the only updater so this is safe)
+        self._last_successful_stream_ordering = None  # type: Optional[int]
+
         # a list of pending PDUs
         self._pending_pdus = []  # type: List[EventBase]
 
@@ -138,7 +155,11 @@ def send_pdu(self, pdu: EventBase) -> None:
         Args:
             pdu: pdu to send
         """
-        self._pending_pdus.append(pdu)
+        if not self._catching_up:
+            # only enqueue the PDU if we are not catching up (False) or do not
+            # yet know if we are to catch up (None)
+            self._pending_pdus.append(pdu)
+
         self.attempt_new_transaction()
 
     def send_presence(self, states: Iterable[UserPresenceState]) -> None:
@@ -218,6 +239,13 @@ async def _transaction_transmission_loop(self) -> None:
             # hence why we throw the result away.
             await get_retry_limiter(self._destination, self._clock, self._store)
 
+            if self._catching_up is None or self._catching_up is True:
+                # we potentially need to catch-up first
+                await self._catch_up_transmission_loop()
+                if self._catching_up:
+                    # not caught up yet
+                    return
+
             pending_pdus = []
             while True:
                 # We have to keep 2 free slots for presence and rr_edus
@@ -351,8 +379,9 @@ async def _transaction_transmission_loop(self) -> None:
             if e.retry_interval > 60 * 60 * 1000:
                 # we won't retry for another hour!
                 # (this suggests a significant outage)
-                # We drop pending PDUs and EDUs because otherwise they will
+                # We drop pending EDUs because otherwise they will
                 # rack up indefinitely.
+                # (Dropping PDUs is already performed by `_start_catching_up`.)
                 # Note that:
                 # - the EDUs that are being dropped here are those that we can
                 #   afford to drop (specifically, only typing notifications,
@@ -364,11 +393,12 @@ async def _transaction_transmission_loop(self) -> None:
 
                 # dropping read receipts is a bit sad but should be solved
                 # through another mechanism, because this is all volatile!
-                self._pending_pdus = []
                 self._pending_edus = []
                 self._pending_edus_keyed = {}
                 self._pending_presence = {}
                 self._pending_rrs = {}
+
+            self._start_catching_up()
         except FederationDeniedError as e:
             logger.info(e)
         except HttpResponseException as e:
@@ -378,6 +408,8 @@ async def _transaction_transmission_loop(self) -> None:
                 e.code,
                 e,
             )
+
+            self._start_catching_up()
         except RequestSendFailed as e:
             logger.warning(
                 "TX [%s] Failed to send transaction: %s", self._destination, e
@@ -387,16 +419,92 @@ async def _transaction_transmission_loop(self) -> None:
                 logger.info(
                     "Failed to send event %s to %s", p.event_id, self._destination
                 )
+
+            self._start_catching_up()
         except Exception:
             logger.exception("TX [%s] Failed to send transaction", self._destination)
             for p in pending_pdus:
                 logger.info(
                     "Failed to send event %s to %s", p.event_id, self._destination
                 )
+
+            self._start_catching_up()
         finally:
             # We want to be *very* sure we clear this after we stop processing
             self.transmission_loop_running = False
 
+    async def _catch_up_transmission_loop(self) -> None:
+        if self._last_successful_stream_ordering is None:
+            # first catch-up, so get from database
+            self._last_successful_stream_ordering = await self._store.get_destination_last_successful_stream_ordering(
+                self._destination
+            )
+
+        if self._last_successful_stream_ordering is None:
+            # if it's still None, then this means we don't have the information
+            # in our database (oh, the perils of being a new feature).
+            # So we can't actually do anything here, and in this case, we don't
+            # know what to catch up, sadly.
+            # Trying to catch up right now is futile, so let's stop.
+            self._catching_up = False
+            return
+
+        # get at most 50 catchup room/PDUs
+        while True:
+            event_ids = await self._store.get_catch_up_room_event_ids(
+                self._destination, self._last_successful_stream_ordering,
+            )
+
+            if not event_ids:
+                # No more events to catch up on, but we can't ignore the chance
+                # of a race condition, so we check that no new events have been
+                # skipped due to us being in catch-up mode
+
+                if (
+                    self._catchup_last_skipped is not None
+                    and self._catchup_last_skipped
+                    > self._last_successful_stream_ordering
+                ):
+                    # another event has been skipped because we were in catch-up mode
+                    continue
+
+                # we are done catching up!
+                self._catching_up = False
+                break
+
+            if self._catching_up is None:
+                # we didn't know if we're in catch-up mode yet but now we know
+                # we are — so mark us as in catch-up mode and drop the queue.
+                self._start_catching_up()
+
+            # fetch the relevant events from the event store
+            # - redacted behaviour of REDACT is fine, since we only send metadata
+            #   of redacted events to the destination.
+            # - don't need to worry about rejected events as we do not actively
+            #   forward received events over federation.
+            catchup_pdus = await self._store.get_events_as_list(event_ids)
+            if not catchup_pdus:
+                raise AssertionError(
+                    "No events retrieved when we asked for %r. "
+                    "This should not happen." % event_ids
+                )
+
+            success = await self._transaction_manager.send_new_transaction(
+                self._destination, catchup_pdus, []
+            )
+
+            if not success:
+                return
+
+            sent_transactions_counter.inc()
+            final_pdu = catchup_pdus[-1]
+            self._last_successful_stream_ordering = cast(
+                int, final_pdu.internal_metadata.stream_ordering
+            )
+            await self._store.set_destination_last_successful_stream_ordering(
+                self._destination, self._last_successful_stream_ordering
+            )
+
     def _get_rr_edus(self, force_flush: bool) -> Iterable[Edu]:
         if not self._pending_rrs:
             return
@@ -457,3 +565,12 @@ async def _get_to_device_message_edus(self, limit: int) -> Tuple[List[Edu], int]
         ]
 
         return (edus, stream_id)
+
+    def _start_catching_up(self) -> None:
+        """
+        Marks this destination as being in catch-up mode.
+
+        This throws away the PDU queue.
+        """
+        self._catching_up = True
+        self._pending_pdus = []

synapse/storage/databases/main/transactions.py

@@ -15,7 +15,7 @@
 
 import logging
 from collections import namedtuple
-from typing import Iterable, Optional, Tuple
+from typing import Iterable, List, Optional, Tuple
 
 from canonicaljson import encode_canonical_json
 
@@ -371,3 +371,44 @@ async def set_destination_last_successful_stream_ordering(
             values={"last_successful_stream_ordering": last_successful_stream_ordering},
             desc="set_last_successful_stream_ordering",
         )
+
+    async def get_catch_up_room_event_ids(
+        self, destination: str, last_successful_stream_ordering: int,
+    ) -> List[str]:
+        """
+        Returns at most 50 event IDs and their corresponding stream_orderings
+        that correspond to the oldest events that have not yet been sent to
+        the destination.
+
+        Args:
+            destination: the destination in question
+            last_successful_stream_ordering: the stream_ordering of the
+                most-recently successfully-transmitted event to the destination
+
+        Returns:
+            list of event_ids
+        """
+        return await self.db_pool.runInteraction(
+            "get_catch_up_room_event_ids",
+            self._get_catch_up_room_event_ids_txn,
+            destination,
+            last_successful_stream_ordering,
+        )
+
+    @staticmethod
+    def _get_catch_up_room_event_ids_txn(
+        txn, destination: str, last_successful_stream_ordering: int,
+    ) -> List[str]:
+        q = """
+                SELECT event_id FROM destination_rooms
+                 JOIN events USING (stream_ordering)
+                WHERE destination = ?
+                  AND stream_ordering > ?
+                ORDER BY stream_ordering
+                LIMIT 50
+            """
+        txn.execute(
+            q, (destination, last_successful_stream_ordering),
+        )
+        event_ids = [row[0] for row in txn]
+        return event_ids