Don't go into federation catch up mode so easily (#9561)

Federation catch up mode is very inefficient if the number of events
that the remote server has missed is small, since handling gaps can be
very expensive, c.f. #9492.

Instead of going into catch up mode whenever we see an error, we instead
do so only if we've backed off from trying the remote for more than an
hour (the assumption being that in such a case it is more than a
transient failure).

2 files changed, 182 insertions, 153 deletions

diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index deb519f3ef..cc0d765e5f 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -17,6 +17,7 @@ import datetime
 import logging
 from typing import TYPE_CHECKING, Dict, Hashable, Iterable, List, Optional, Tuple, cast
 
+import attr
 from prometheus_client import Counter
 
 from synapse.api.errors import (
@@ -93,6 +94,10 @@ class PerDestinationQueue:
         self._destination = destination
         self.transmission_loop_running = False
 
+        # Flag to signal to any running transmission loop that there is new data
+        # queued up to be sent.
+        self._new_data_to_send = False
+
         # True whilst we are sending events that the remote homeserver missed
         # because it was unreachable. We start in this state so we can perform
         # catch-up at startup.
@@ -108,7 +113,7 @@ class PerDestinationQueue:
         # destination (we are the only updater so this is safe)
         self._last_successful_stream_ordering = None  # type: Optional[int]
 
-        # a list of pending PDUs
+        # a queue of pending PDUs
         self._pending_pdus = []  # type: List[EventBase]
 
         # XXX this is never actually used: see
@@ -208,6 +213,10 @@ class PerDestinationQueue:
         transaction in the background.
         """
 
+        # Mark that we (may) have new things to send, so that any running
+        # transmission loop will recheck whether there is stuff to send.
+        self._new_data_to_send = True
+
         if self.transmission_loop_running:
             # XXX: this can get stuck on by a never-ending
             # request at which point pending_pdus just keeps growing.
@@ -250,125 +259,41 @@ class PerDestinationQueue:
 
             pending_pdus = []
             while True:
-                # We have to keep 2 free slots for presence and rr_edus
-                limit = MAX_EDUS_PER_TRANSACTION - 2
-
-                device_update_edus, dev_list_id = await self._get_device_update_edus(
-                    limit
-                )
-
-                limit -= len(device_update_edus)
-
-                (
-                    to_device_edus,
-                    device_stream_id,
-                ) = await self._get_to_device_message_edus(limit)
-
-                pending_edus = device_update_edus + to_device_edus
-
-                # BEGIN CRITICAL SECTION
-                #
-                # In order to avoid a race condition, we need to make sure that
-                # the following code (from popping the queues up to the point
-                # where we decide if we actually have any pending messages) is
-                # atomic - otherwise new PDUs or EDUs might arrive in the
-                # meantime, but not get sent because we hold the
-                # transmission_loop_running flag.
-
-                pending_pdus = self._pending_pdus
+                self._new_data_to_send = False
 
-                # We can only include at most 50 PDUs per transactions
-                pending_pdus, self._pending_pdus = pending_pdus[:50], pending_pdus[50:]
+                async with _TransactionQueueManager(self) as (
+                    pending_pdus,
+                    pending_edus,
+                ):
+                    if not pending_pdus and not pending_edus:
+                        logger.debug("TX [%s] Nothing to send", self._destination)
+
+                        # If we've gotten told about new things to send during
+                        # checking for things to send, we try looking again.
+                        # Otherwise new PDUs or EDUs might arrive in the meantime,
+                        # but not get sent because we hold the
+                        # `transmission_loop_running` flag.
+                        if self._new_data_to_send:
+                            continue
+                        else:
+                            return
 
-                pending_edus.extend(self._get_rr_edus(force_flush=False))
-                pending_presence = self._pending_presence
-                self._pending_presence = {}
-                if pending_presence:
-                    pending_edus.append(
-                        Edu(
-                            origin=self._server_name,
-                            destination=self._destination,
-                            edu_type="m.presence",
-                            content={
-                                "push": [
-                                    format_user_presence_state(
-                                        presence, self._clock.time_msec()
-                                    )
-                                    for presence in pending_presence.values()
-                                ]
-                            },
+                    if pending_pdus:
+                        logger.debug(
+                            "TX [%s] len(pending_pdus_by_dest[dest]) = %d",
+                            self._destination,
+                            len(pending_pdus),
                         )
-                    )
 
-                pending_edus.extend(
-                    self._pop_pending_edus(MAX_EDUS_PER_TRANSACTION - len(pending_edus))
-                )
-                while (
-                    len(pending_edus) < MAX_EDUS_PER_TRANSACTION
-                    and self._pending_edus_keyed
-                ):
-                    _, val = self._pending_edus_keyed.popitem()
-                    pending_edus.append(val)
-
-                if pending_pdus:
-                    logger.debug(
-                        "TX [%s] len(pending_pdus_by_dest[dest]) = %d",
-                        self._destination,
-                        len(pending_pdus),
+                    await self._transaction_manager.send_new_transaction(
+                        self._destination, pending_pdus, pending_edus
                     )
 
-                if not pending_pdus and not pending_edus:
-                    logger.debug("TX [%s] Nothing to send", self._destination)
-                    self._last_device_stream_id = device_stream_id
-                    return
-
-                # if we've decided to send a transaction anyway, and we have room, we
-                # may as well send any pending RRs
-                if len(pending_edus) < MAX_EDUS_PER_TRANSACTION:
-                    pending_edus.extend(self._get_rr_edus(force_flush=True))
-
-                # END CRITICAL SECTION
-
-                success = await self._transaction_manager.send_new_transaction(
-                    self._destination, pending_pdus, pending_edus
-                )
-                if success:
                     sent_transactions_counter.inc()
                     sent_edus_counter.inc(len(pending_edus))
                     for edu in pending_edus:
                         sent_edus_by_type.labels(edu.edu_type).inc()
-                    # Remove the acknowledged device messages from the database
-                    # Only bother if we actually sent some device messages
-                    if to_device_edus:
-                        await self._store.delete_device_msgs_for_remote(
-                            self._destination, device_stream_id
-                        )
 
-                    # also mark the device updates as sent
-                    if device_update_edus:
-                        logger.info(
-                            "Marking as sent %r %r", self._destination, dev_list_id
-                        )
-                        await self._store.mark_as_sent_devices_by_remote(
-                            self._destination, dev_list_id
-                        )
-
-                    self._last_device_stream_id = device_stream_id
-                    self._last_device_list_stream_id = dev_list_id
-
-                    if pending_pdus:
-                        # we sent some PDUs and it was successful, so update our
-                        # last_successful_stream_ordering in the destinations table.
-                        final_pdu = pending_pdus[-1]
-                        last_successful_stream_ordering = (
-                            final_pdu.internal_metadata.stream_ordering
-                        )
-                        assert last_successful_stream_ordering
-                        await self._store.set_destination_last_successful_stream_ordering(
-                            self._destination, last_successful_stream_ordering
-                        )
-                else:
-                    break
         except NotRetryingDestination as e:
             logger.debug(
                 "TX [%s] not ready for retry yet (next retry at %s) - "
@@ -401,7 +326,7 @@ class PerDestinationQueue:
                 self._pending_presence = {}
                 self._pending_rrs = {}
 
-            self._start_catching_up()
+                self._start_catching_up()
         except FederationDeniedError as e:
             logger.info(e)
         except HttpResponseException as e:
@@ -412,7 +337,6 @@ class PerDestinationQueue:
                 e,
             )
 
-            self._start_catching_up()
         except RequestSendFailed as e:
             logger.warning(
                 "TX [%s] Failed to send transaction: %s", self._destination, e
@@ -422,16 +346,12 @@ class PerDestinationQueue:
                 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
@@ -499,13 +419,10 @@ class PerDestinationQueue:
                 rooms = [p.room_id for p in catchup_pdus]
                 logger.info("Catching up rooms to %s: %r", self._destination, rooms)
 
-            success = await self._transaction_manager.send_new_transaction(
+            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(
@@ -584,3 +501,135 @@ class PerDestinationQueue:
         """
         self._catching_up = True
         self._pending_pdus = []
+
+
+@attr.s(slots=True)
+class _TransactionQueueManager:
+    """A helper async context manager for pulling stuff off the queues and
+    tracking what was last successfully sent, etc.
+    """
+
+    queue = attr.ib(type=PerDestinationQueue)
+
+    _device_stream_id = attr.ib(type=Optional[int], default=None)
+    _device_list_id = attr.ib(type=Optional[int], default=None)
+    _last_stream_ordering = attr.ib(type=Optional[int], default=None)
+    _pdus = attr.ib(type=List[EventBase], factory=list)
+
+    async def __aenter__(self) -> Tuple[List[EventBase], List[Edu]]:
+        # First we calculate the EDUs we want to send, if any.
+
+        # We start by fetching device related EDUs, i.e device updates and to
+        # device messages. We have to keep 2 free slots for presence and rr_edus.
+        limit = MAX_EDUS_PER_TRANSACTION - 2
+
+        device_update_edus, dev_list_id = await self.queue._get_device_update_edus(
+            limit
+        )
+
+        if device_update_edus:
+            self._device_list_id = dev_list_id
+        else:
+            self.queue._last_device_list_stream_id = dev_list_id
+
+        limit -= len(device_update_edus)
+
+        (
+            to_device_edus,
+            device_stream_id,
+        ) = await self.queue._get_to_device_message_edus(limit)
+
+        if to_device_edus:
+            self._device_stream_id = device_stream_id
+        else:
+            self.queue._last_device_stream_id = device_stream_id
+
+        pending_edus = device_update_edus + to_device_edus
+
+        # Now add the read receipt EDU.
+        pending_edus.extend(self.queue._get_rr_edus(force_flush=False))
+
+        # And presence EDU.
+        if self.queue._pending_presence:
+            pending_edus.append(
+                Edu(
+                    origin=self.queue._server_name,
+                    destination=self.queue._destination,
+                    edu_type="m.presence",
+                    content={
+                        "push": [
+                            format_user_presence_state(
+                                presence, self.queue._clock.time_msec()
+                            )
+                            for presence in self.queue._pending_presence.values()
+                        ]
+                    },
+                )
+            )
+            self.queue._pending_presence = {}
+
+        # Finally add any other types of EDUs if there is room.
+        pending_edus.extend(
+            self.queue._pop_pending_edus(MAX_EDUS_PER_TRANSACTION - len(pending_edus))
+        )
+        while (
+            len(pending_edus) < MAX_EDUS_PER_TRANSACTION
+            and self.queue._pending_edus_keyed
+        ):
+            _, val = self.queue._pending_edus_keyed.popitem()
+            pending_edus.append(val)
+
+        # Now we look for any PDUs to send, by getting up to 50 PDUs from the
+        # queue
+        self._pdus = self.queue._pending_pdus[:50]
+
+        if not self._pdus and not pending_edus:
+            return [], []
+
+        # if we've decided to send a transaction anyway, and we have room, we
+        # may as well send any pending RRs
+        if len(pending_edus) < MAX_EDUS_PER_TRANSACTION:
+            pending_edus.extend(self.queue._get_rr_edus(force_flush=True))
+
+        if self._pdus:
+            self._last_stream_ordering = self._pdus[
+                -1
+            ].internal_metadata.stream_ordering
+            assert self._last_stream_ordering
+
+        return self._pdus, pending_edus
+
+    async def __aexit__(self, exc_type, exc, tb):
+        if exc_type is not None:
+            # Failed to send transaction, so we bail out.
+            return
+
+        # Successfully sent transactions, so we remove pending PDUs from the queue
+        if self._pdus:
+            self.queue._pending_pdus = self.queue._pending_pdus[len(self._pdus) :]
+
+        # Succeeded to send the transaction so we record where we have sent up
+        # to in the various streams
+
+        if self._device_stream_id:
+            await self.queue._store.delete_device_msgs_for_remote(
+                self.queue._destination, self._device_stream_id
+            )
+            self.queue._last_device_stream_id = self._device_stream_id
+
+        # also mark the device updates as sent
+        if self._device_list_id:
+            logger.info(
+                "Marking as sent %r %r", self.queue._destination, self._device_list_id
+            )
+            await self.queue._store.mark_as_sent_devices_by_remote(
+                self.queue._destination, self._device_list_id
+            )
+            self.queue._last_device_list_stream_id = self._device_list_id
+
+        if self._last_stream_ordering:
+            # we sent some PDUs and it was successful, so update our
+            # last_successful_stream_ordering in the destinations table.
+            await self.queue._store.set_destination_last_successful_stream_ordering(
+                self.queue._destination, self._last_stream_ordering
+            )
diff --git a/synapse/federation/sender/transaction_manager.py b/synapse/federation/sender/transaction_manager.py
index 2a9cd063c4..07b740c2f2 100644
--- a/synapse/federation/sender/transaction_manager.py
+++ b/synapse/federation/sender/transaction_manager.py
@@ -69,15 +69,12 @@ class TransactionManager:
         destination: str,
         pdus: List[EventBase],
         edus: List[Edu],
-    ) -> bool:
+    ) -> None:
         """
         Args:
             destination: The destination to send to (e.g. 'example.org')
             pdus: In-order list of PDUs to send
             edus: List of EDUs to send
-
-        Returns:
-            True iff the transaction was successful
         """
 
         # Make a transaction-sending opentracing span. This span follows on from
@@ -96,8 +93,6 @@ class TransactionManager:
                 edu.strip_context()
 
         with start_active_span_follows_from("send_transaction", span_contexts):
-            success = True
-
             logger.debug("TX [%s] _attempt_new_transaction", destination)
 
             txn_id = str(self._next_txn_id)
@@ -152,44 +147,29 @@ class TransactionManager:
                 response = await self._transport_layer.send_transaction(
                     transaction, json_data_cb
                 )
-                code = 200
             except HttpResponseException as e:
                 code = e.code
                 response = e.response
 
-                if e.code in (401, 404, 429) or 500 <= e.code:
-                    logger.info(
-                        "TX [%s] {%s} got %d response", destination, txn_id, code
-                    )
-                    raise e
-
-            logger.info("TX [%s] {%s} got %d response", destination, txn_id, code)
-
-            if code == 200:
-                for e_id, r in response.get("pdus", {}).items():
-                    if "error" in r:
-                        logger.warning(
-                            "TX [%s] {%s} Remote returned error for %s: %s",
-                            destination,
-                            txn_id,
-                            e_id,
-                            r,
-                        )
-            else:
-                for p in pdus:
+                set_tag(tags.ERROR, True)
+
+                logger.info("TX [%s] {%s} got %d response", destination, txn_id, code)
+                raise
+
+            logger.info("TX [%s] {%s} got 200 response", destination, txn_id)
+
+            for e_id, r in response.get("pdus", {}).items():
+                if "error" in r:
                     logger.warning(
-                        "TX [%s] {%s} Failed to send event %s",
+                        "TX [%s] {%s} Remote returned error for %s: %s",
                         destination,
                         txn_id,
-                        p.event_id,
+                        e_id,
+                        r,
                     )
-                success = False
 
-            if success and pdus and destination in self._federation_metrics_domains:
+            if pdus and destination in self._federation_metrics_domains:
                 last_pdu = pdus[-1]
                 last_pdu_ts_metric.labels(server_name=destination).set(
                     last_pdu.origin_server_ts / 1000
                 )
-
-            set_tag(tags.ERROR, not success)
-            return success