lnwallet: send invalid commitment secret if restored chan in ChanSyncMsg

In this commit, we modify the `ChanSyncMsg` to send an invalid
commitment secret in `ChanSyncMsg`. We do this in order to force the
remote party to force close off-chain, if we're restoring a channel from
scratch and we never had any state updates within the channel. We need
to do this, as otherwise the remote party will think we can resume as
they're able to verify their own commit secret for state zero.

lnwallet/channel.go

@@ -3531,7 +3531,14 @@ func (lc *LightningChannel) ProcessChanSyncMsg(
 //      it.
 //   3. We didn't get the last RevokeAndAck message they sent, so they'll
 //      re-send it.
-func ChanSyncMsg(c *channeldb.OpenChannel) (*lnwire.ChannelReestablish, error) {
+//
+// The isRestoredChan bool indicates if we need to craft a chan sync message
+// for a channel that's been restored. If this is a restored channel, then
+// we'll modify our typical chan sync  message to ensure they force close even
+// if we're on the very first state.
+func ChanSyncMsg(c *channeldb.OpenChannel,
+	isRestoredChan bool) (*lnwire.ChannelReestablish, error) {
+
 	c.Lock()
 	defer c.Unlock()
 
@@ -3572,6 +3579,13 @@ func ChanSyncMsg(c *channeldb.OpenChannel) (*lnwire.ChannelReestablish, error) {
 		return nil, err
 	}
 
+	// If we've restored this channel, then we'll purposefully give them an
+	// invalid LocalUnrevokedCommitPoint so they'll force close the channel
+	// allowing us to sweep our funds.
+	if isRestoredChan {
+		currentCommitSecret[0] ^= 1
+	}
+
 	return &lnwire.ChannelReestablish{
 		ChanID: lnwire.NewChanIDFromOutPoint(
 			&c.FundingOutpoint,
@@ -3632,7 +3646,7 @@ func (lc *LightningChannel) computeView(view *htlcView, remoteChain bool,
 	// will remove their corresponding added HTLCs.  The resulting filtered
 	// view will only have Add entries left, making it easy to compare the
 	// channel constraints to the final commitment state. If any fee
-	// updates are found in the logs, the comitment fee rate should be
+	// updates are found in the logs, the commitment fee rate should be
 	// changed, so we'll also set the feePerKw to this new value.
 	filteredHTLCView := lc.evaluateHTLCView(view, &ourBalance,
 		&theirBalance, nextHeight, remoteChain, updateState)
@@ -5086,12 +5100,14 @@ func NewUnilateralCloseSummary(chanState *channeldb.OpenChannel, signer input.Si
 	// Next, we'll obtain HTLC resolutions for all the outgoing HTLC's we
 	// had on their commitment transaction.
 	htlcResolutions, err := extractHtlcResolutions(
-		SatPerKWeight(remoteCommit.FeePerKw), false, signer, remoteCommit.Htlcs,
-		keyRing, &chanState.LocalChanCfg, &chanState.RemoteChanCfg,
-		*commitSpend.SpenderTxHash, pCache,
+		SatPerKWeight(remoteCommit.FeePerKw), false, signer,
+		remoteCommit.Htlcs, keyRing, &chanState.LocalChanCfg,
+		&chanState.RemoteChanCfg, *commitSpend.SpenderTxHash,
+		pCache,
 	)
 	if err != nil {
-		return nil, fmt.Errorf("unable to create htlc resolutions: %v", err)
+		return nil, fmt.Errorf("unable to create htlc "+
+			"resolutions: %v", err)
 	}
 
 	commitTxBroadcast := commitSpend.SpendingTx
@@ -5101,7 +5117,8 @@ func NewUnilateralCloseSummary(chanState *channeldb.OpenChannel, signer input.Si
 	// transaction.
 	selfP2WKH, err := input.CommitScriptUnencumbered(keyRing.NoDelayKey)
 	if err != nil {
-		return nil, fmt.Errorf("unable to create self commit script: %v", err)
+		return nil, fmt.Errorf("unable to create self commit "+
+			"script: %v", err)
 	}
 
 	var (
@@ -5159,7 +5176,10 @@ func NewUnilateralCloseSummary(chanState *channeldb.OpenChannel, signer input.Si
 	}
 
 	// Attempt to add a channel sync message to the close summary.
-	chanSync, err := ChanSyncMsg(chanState)
+	chanSync, err := ChanSyncMsg(
+		chanState,
+		chanState.HasChanStatus(channeldb.ChanStatusRestored),
+	)
 	if err != nil {
 		walletLog.Errorf("ChannelPoint(%v): unable to create channel sync "+
 			"message: %v", chanState.FundingOutpoint, err)