Remove fallback from get_missing_events.

get_missing_events used to fallback to fetching the missing events
individually requesting from every server in the room, one by one.e

This could be unacceptably slow, possibly causing #1732

synapse/federation/federation_client.py

@@ -27,7 +27,6 @@ from synapse.util.caches.expiringcache import ExpiringCache
 from synapse.util.logutils import log_function
 from synapse.util.logcontext import preserve_fn, preserve_context_over_deferred
 from synapse.events import FrozenEvent
-from synapse.types import get_domain_from_id
 import synapse.metrics
 
 from synapse.util.retryutils import get_retry_limiter, NotRetryingDestination
@@ -741,8 +740,6 @@ class FederationClient(FederationBase):
             signed_events = yield self._check_sigs_and_hash_and_fetch(
                 destination, events, outlier=False
             )
-
-            have_gotten_all_from_destination = True
         except HttpResponseException as e:
             if not e.code == 400:
                 raise
@@ -750,72 +747,6 @@ class FederationClient(FederationBase):
             # We are probably hitting an old server that doesn't support
             # get_missing_events
             signed_events = []
-            have_gotten_all_from_destination = False
-
-        if len(signed_events) >= limit:
-            defer.returnValue(signed_events)
-
-        users = yield self.state.get_current_user_in_room(room_id)
-        servers = set(get_domain_from_id(u) for u in users)
-
-        servers = set(servers)
-        servers.discard(self.server_name)
-
-        failed_to_fetch = set()
-
-        while len(signed_events) < limit:
-            # Are we missing any?
-
-            seen_events = set(earliest_events_ids)
-            seen_events.update(e.event_id for e in signed_events if e)
-
-            missing_events = {}
-            for e in itertools.chain(latest_events, signed_events):
-                if e.depth > min_depth:
-                    missing_events.update({
-                        e_id: e.depth for e_id, _ in e.prev_events
-                        if e_id not in seen_events
-                        and e_id not in failed_to_fetch
-                    })
-
-            if not missing_events:
-                break
-
-            have_seen = yield self.store.have_events(missing_events)
-
-            for k in have_seen:
-                missing_events.pop(k, None)
-
-            if not missing_events:
-                break
-
-            # Okay, we haven't gotten everything yet. Lets get them.
-            ordered_missing = sorted(missing_events.items(), key=lambda x: x[0])
-
-            if have_gotten_all_from_destination:
-                servers.discard(destination)
-
-            def random_server_list():
-                srvs = list(servers)
-                random.shuffle(srvs)
-                return srvs
-
-            deferreds = [
-                preserve_fn(self.get_pdu)(
-                    destinations=random_server_list(),
-                    event_id=e_id,
-                )
-                for e_id, depth in ordered_missing[:limit - len(signed_events)]
-            ]
-
-            res = yield preserve_context_over_deferred(
-                defer.DeferredList(deferreds, consumeErrors=True)
-            )
-            for (result, val), (e_id, _) in zip(res, ordered_missing):
-                if result and val:
-                    signed_events.append(val)
-                else:
-                    failed_to_fetch.add(e_id)
 
         defer.returnValue(signed_events)