bridgev2: add bridge_remote_leave config option

bridgev2/bridgeconfig/config.go

@@ -76,6 +76,7 @@ type BridgeConfig struct {
 	UnknownErrorAutoReconnect     time.Duration      `yaml:"unknown_error_auto_reconnect"`
 	UnknownErrorMaxAutoReconnects int                `yaml:"unknown_error_max_auto_reconnects"`
 	BridgeMatrixLeave             bool               `yaml:"bridge_matrix_leave"`
+	BridgeRemoteLeave             bool               `yaml:"bridge_remote_leave"`
 	BridgeNotices                 bool               `yaml:"bridge_notices"`
 	TagOnlyOnCreate               bool               `yaml:"tag_only_on_create"`
 	OnlyBridgeTags                []event.RoomTag    `yaml:"only_bridge_tags"`

bridgev2/bridgeconfig/upgrade.go

@@ -35,6 +35,7 @@ func doUpgrade(helper up.Helper) {
 	helper.Copy(up.Str|up.Int|up.Null, "bridge", "unknown_error_auto_reconnect")
 	helper.Copy(up.Int, "bridge", "unknown_error_max_auto_reconnects")
 	helper.Copy(up.Bool, "bridge", "bridge_matrix_leave")
+	helper.Copy(up.Bool, "bridge", "bridge_remote_leave")
 	helper.Copy(up.Bool, "bridge", "bridge_notices")
 	helper.Copy(up.Bool, "bridge", "tag_only_on_create")
 	helper.Copy(up.List, "bridge", "only_bridge_tags")

bridgev2/matrix/mxmain/example-config.yaml

@@ -35,6 +35,12 @@ bridge:
 
     # Should leaving Matrix rooms be bridged as leaving groups on the remote network?
     bridge_matrix_leave: false
+    # Should leaving a group on the remote network kick the user out of the
+    # corresponding Matrix room? When `true`, a remote leave is propagated to
+    # Matrix the same as any other membership change. When `false`, the leave
+    # is dropped so the user keeps access to history and stays in the Matrix
+    # room.
+    bridge_remote_leave: true
     # Should `m.notice` messages be bridged?
     bridge_notices: false
     # Should room tags only be synced when creating the portal? Tags mean things like favorite/pin and archive/low priority.

bridgev2/portal.go

@@ -4618,6 +4618,16 @@ func (portal *Portal) syncParticipants(
 		if member.Membership == "" {
 			member.Membership = event.MembershipJoin
 		}
+		// Symmetric with bridge_matrix_leave: when bridge_remote_leave is
+		// disabled, dropping the leave from the remote network keeps the
+		// user in the Matrix room with their existing history.
+		if !portal.Bridge.Config.BridgeRemoteLeave && member.Membership == event.MembershipLeave {
+			log.Debug().
+				Stringer("user_id", extraUserID).
+				Msg("Dropping remote leave: bridge_remote_leave is disabled")
+			delete(currentMembers, extraUserID)
+			return false
+		}
 		if member.PowerLevel != nil {
 			powerChanged = currentPower.EnsureUserLevelAs(portal.Bridge.Bot.GetMXID(), extraUserID, *member.PowerLevel) || powerChanged
 		}