Merge pull request #1765 from matrix-org/markjh/timeout_get_missing_events

cherrypick #1744: limit total timeout for get_missing_events to 10s

1 files changed, 21 insertions, 0 deletions

diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index f4c60e67e3..800f04189f 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -425,6 +425,7 @@ class FederationServer(FederationBase):
                 " limit: %d, min_depth: %d",
                 earliest_events, latest_events, limit, min_depth
             )
+
             missing_events = yield self.handler.on_get_missing_events(
                 origin, room_id, earliest_events, latest_events, limit, min_depth
             )
@@ -567,6 +568,25 @@ class FederationServer(FederationBase):
                                 len(prevs - seen), pdu.room_id, list(prevs - seen)[:5]
                             )
 
+                            # XXX: we set timeout to 10s to help workaround
+                            # https://github.com/matrix-org/synapse/issues/1733.
+                            # The reason is to avoid holding the linearizer lock
+                            # whilst processing inbound /send transactions, causing
+                            # FDs to stack up and block other inbound transactions
+                            # which empirically can currently take up to 30 minutes.
+                            #
+                            # N.B. this explicitly disables retry attempts.
+                            #
+                            # N.B. this also increases our chances of falling back to
+                            # fetching fresh state for the room if the missing event
+                            # can't be found, which slightly reduces our security.
+                            # it may also increase our DAG extremity count for the room,
+                            # causing additional state resolution?  See #1760.
+                            # However, fetching state doesn't hold the linearizer lock
+                            # apparently.
+                            #
+                            # see https://github.com/matrix-org/synapse/pull/1744
+
                             missing_events = yield self.get_missing_events(
                                 origin,
                                 pdu.room_id,
@@ -574,6 +594,7 @@ class FederationServer(FederationBase):
                                 latest_events=[pdu],
                                 limit=10,
                                 min_depth=min_depth,
+                                timeout=10000,
                             )
 
                             # We want to sort these by depth so we process them and