Return `ErrRoomNoExists` if insufficient state is available for a `buildEvent` to succeed when joining a room (#2210)

This may help cases like #2206, since it should prompt us to try a federated join again instead.

4 files changed, 18 insertions, 3 deletions

diff --git a/roomserver/internal/perform/perform_join.go b/roomserver/internal/perform/perform_join.go
index 9d2a66d4..a40f66d2 100644
--- a/roomserver/internal/perform/perform_join.go
+++ b/roomserver/internal/perform/perform_join.go
@@ -29,6 +29,7 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/internal/input"
 	"github.com/matrix-org/dendrite/roomserver/internal/query"
 	"github.com/matrix-org/dendrite/roomserver/storage"
+	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/dendrite/setup/config"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/sirupsen/logrus"
@@ -367,7 +368,15 @@ func buildEvent(
 		StateToFetch: eventsNeeded.Tuples(),
 	}, &queryRes)
 	if err != nil {
-		return nil, nil, fmt.Errorf("QueryLatestEventsAndState: %w", err)
+		switch err.(type) {
+		case types.MissingStateError:
+			// We know something about the room but the state seems to be
+			// insufficient to actually build a new event, so in effect we
+			// had might as well treat the room as if it doesn't exist.
+			return nil, nil, eventutil.ErrRoomNoExists
+		default:
+			return nil, nil, fmt.Errorf("QueryLatestEventsAndState: %w", err)
+		}
 	}
 
 	ev, err := eventutil.BuildEvent(ctx, builder, cfg, time.Now(), &eventsNeeded, &queryRes)
diff --git a/roomserver/storage/postgres/state_snapshot_table.go b/roomserver/storage/postgres/state_snapshot_table.go
index ce9f2463..8ed88603 100644
--- a/roomserver/storage/postgres/state_snapshot_table.go
+++ b/roomserver/storage/postgres/state_snapshot_table.go
@@ -134,7 +134,7 @@ func (s *stateSnapshotStatements) BulkSelectStateBlockNIDs(
 		return nil, err
 	}
 	if i != len(stateNIDs) {
-		return nil, fmt.Errorf("storage: state NIDs missing from the database (%d != %d)", i, len(stateNIDs))
+		return nil, types.MissingStateError(fmt.Sprintf("storage: state NIDs missing from the database (%d != %d)", i, len(stateNIDs)))
 	}
 	return results, nil
 }
diff --git a/roomserver/storage/sqlite3/state_snapshot_table.go b/roomserver/storage/sqlite3/state_snapshot_table.go
index 3c4bde3f..01df31e9 100644
--- a/roomserver/storage/sqlite3/state_snapshot_table.go
+++ b/roomserver/storage/sqlite3/state_snapshot_table.go
@@ -137,7 +137,7 @@ func (s *stateSnapshotStatements) BulkSelectStateBlockNIDs(
 		}
 	}
 	if i != len(stateNIDs) {
-		return nil, fmt.Errorf("storage: state NIDs missing from the database (%d != %d)", i, len(stateNIDs))
+		return nil, types.MissingStateError(fmt.Sprintf("storage: state NIDs missing from the database (%d != %d)", i, len(stateNIDs)))
 	}
 	return results, nil
 }
diff --git a/roomserver/types/types.go b/roomserver/types/types.go
index 5d52ccfc..65fbee04 100644
--- a/roomserver/types/types.go
+++ b/roomserver/types/types.go
@@ -213,6 +213,12 @@ type MissingEventError string
 
 func (e MissingEventError) Error() string { return string(e) }
 
+// A MissingStateError is an error that happened because the roomserver was
+// missing requested state snapshots from its databases.
+type MissingStateError string
+
+func (e MissingStateError) Error() string { return string(e) }
+
 // A RejectedError is returned when an event is stored as rejected. The error
 // contains the reason why.
 type RejectedError string