[workaround]lnwallet: set remote update log blinding point from pending

This commit adds a workaround for the fact that we don't have our extra
htlc data (ie, that provided in tlvs) stored in channeldb.HTLC. It
ensures that we have the blinding point populated on restart for the
following set of circumstances:
* The incoming HTLC is irrevocably committed to in our local commitment,
  ie: we have received a CommitSig and sent a RevokeAndAck for a
  commitment that includes the incoming HTLC.
* The incoming HTLC is still pending on the sender, ie: we have not yet
  sent the remote party a CommitSig covering the incoming HTLC.

If we restart at this point, the htlc will be stored as an incoming htlc
on our local commitment (with no blinding point) and the full log update
will be saved as a LogUpdate in our remote pending updates (because we
have not yet provided the remote party with a signature).

We restore our remoteUpdateLog from the local commit's incoming htlcs,
so before this change these htlcs would be loaded into our in-memory log
without their blinding point set (and operation would incorrectly resume
as usual without it). This commit updates our logic to restore blinding
points (if set) to the htlcs in the remoteUpdateLog.

Author: carlaKC

lnwallet/channel.go

@@ -107,6 +107,11 @@ var (
 	// ErrOutputIndexOutOfRange is returned when an output index is greater
 	// than or equal to the length of a given transaction's outputs.
 	ErrOutputIndexOutOfRange = errors.New("output index is out of range")
+
+	// ErrModDescriptorNotFound indicates that a payment descriptor we
+	// aimed to modify was not found.
+	ErrModDescriptorNotFound = errors.New("payment descriptor for " +
+		"modification not found")
 )
 
 // ErrCommitSyncLocalDataLoss is returned in the case that we receive a valid
@@ -1112,6 +1117,23 @@ func (u *updateLog) restoreUpdate(pd *PaymentDescriptor) {
 	u.updateIndex[pd.LogIndex] = u.PushBack(pd)
 }
 
+// restoreExtraData looks up a log update and updates additional data that
+// would not have been recovered from our on-disk commitments. It will fail if
+// the update index provided is not in the log.
+//
+// Note: this is a workaround for channeldb.HTLC not storing additional data
+// passed in update_add_htlc tlvs.
+func (u *updateLog) restoreExtraAddData(i uint64, b *btcec.PublicKey) error {
+	pd := u.lookupHtlc(i)
+	if pd == nil {
+		return fmt.Errorf("%w: index: %v", ErrModDescriptorNotFound, i)
+	}
+
+	pd.BlindingPoint = b
+
+	return nil
+}
+
 // appendHtlc appends a new HTLC offer to the tip of the update log. The entry
 // is also added to the offer index accordingly.
 func (u *updateLog) appendHtlc(pd *PaymentDescriptor) {
@@ -2039,7 +2061,17 @@ func (lc *LightningChannel) restorePendingRemoteUpdates(
 		// but this Add restoration was a no-op as every single one of
 		// these Adds was already restored since they're all incoming
 		// htlcs on the local commitment.
+		//
+		// Since our on-disk commitment does not store extra data for
+		// htlcs, we restore the update on the local log from our log
+		// update.
 		if payDesc.EntryType == Add {
+			if err = lc.remoteUpdateLog.restoreExtraAddData(
+				payDesc.HtlcIndex, payDesc.BlindingPoint,
+			); err != nil {
+				return err
+			}
+
 			continue
 		}