htlcswitch/link: inspect sync errors, force close channel

This commit makes the link inspect the error encountered during channel
sync, force closing the channel if we detect a remote data loss.

htlcswitch/link.go

@@ -613,10 +613,7 @@ func (l *channelLink) syncChanStates() error {
 		msgsToReSend, openedCircuits, closedCircuits, err =
 			l.channel.ProcessChanSyncMsg(remoteChanSyncMsg)
 		if err != nil {
-			// TODO(roasbeef): check concrete type of error, act
-			// accordingly
-			return fmt.Errorf("unable to handle upstream reestablish "+
-				"message: %v", err)
+			return err
 		}
 
 		// Repopulate any identifiers for circuits that may have been
@@ -810,13 +807,71 @@ func (l *channelLink) htlcManager() {
 	if l.cfg.SyncStates {
 		err := l.syncChanStates()
 		if err != nil {
-			l.errorf("unable to synchronize channel states: %v", err)
-			if err != ErrLinkShuttingDown {
-				// TODO(halseth): must be revisted when
-				// data-loss protection is in.
-				l.fail(LinkFailureError{code: ErrSyncError},
-					err.Error())
+			switch {
+			case err == ErrLinkShuttingDown:
+				log.Debugf("unable to sync channel states, " +
+					"link is shutting down")
+				return
+
+			// We failed syncing the commit chains, probably
+			// because the remote has lost state. We should force
+			// close the channel.
+			// TODO(halseth): store sent chanSync message to
+			// database, such that it can be resent to peer in case
+			// it tries to sync the channel again.
+			case err == lnwallet.ErrCommitSyncRemoteDataLoss:
+				fallthrough
+
+			// The remote sent us an invalid last commit secret, we
+			// should force close the channel.
+			// TODO(halseth): and permanently ban the peer?
+			case err == lnwallet.ErrInvalidLastCommitSecret:
+				fallthrough
+
+			// The remote sent us a commit point different from
+			// what they sent us before.
+			// TODO(halseth): ban peer?
+			case err == lnwallet.ErrInvalidLocalUnrevokedCommitPoint:
+				l.fail(
+					LinkFailureError{
+						code:       ErrSyncError,
+						ForceClose: true,
+					},
+					"unable to synchronize channel "+
+						"states: %v", err,
+				)
+				return
+
+			// We have lost state and cannot safely force close the
+			// channel. Fail the channel and wait for the remote to
+			// hopefully force close it. The remote has sent us its
+			// latest unrevoked commitment point, that we stored in
+			// the database, that we can use to retrieve the funds
+			// when the remote closes the channel.
+			// TODO(halseth): mark this, such that we prevent
+			// channel from being force closed by the user or
+			// contractcourt etc.
+			case err == lnwallet.ErrCommitSyncLocalDataLoss:
+
+			// We determined the commit chains were not possible to
+			// sync. We cautiously fail the channel, but don't
+			// force close.
+			// TODO(halseth): can we safely force close in any
+			// cases where this error is returned?
+			case err == lnwallet.ErrCannotSyncCommitChains:
+
+			// Other, unspecified error.
+			default:
 			}
+
+			l.fail(
+				LinkFailureError{
+					code:       ErrSyncError,
+					ForceClose: false,
+				},
+				"unable to synchronize channel "+
+					"states: %v", err,
+			)
 			return
 		}
 	}