tapfreighter: set type for burn keys correctly

In order for burn keys to be stored with the correct type, we need to
detect them before storing the transfer outputs to the database.
We can't set the script key type before, because we're carrying around
the script key in a virtual packet, where that information isn't
available.

Author: guggero

tapfreighter/chain_porter.go

@@ -1296,6 +1296,12 @@ func (p *ChainPorter) stateStep(currentPkg sendPackage) (*sendPackage, error) {
 				"%w", err)
 		}
 
+		// Burn keys are the only keys that we don't explicitly store
+		// in the DB before this point. But we'll want them to have the
+		// correct type when creating the transfer, so we'll set that
+		// now.
+		detectBurnKeys(currentPkg.VirtualPackets)
+
 		// We now need to find out if this is a transfer to ourselves
 		// (e.g. a change output) or an outbound transfer. A key being
 		// local means the lnd node connected to this daemon knows how
@@ -1567,6 +1573,26 @@ func (p *ChainPorter) publishSubscriberEvent(event fn.Event) {
 	}
 }
 
+// detectBurnKeys checks if any of the outputs in the virtual packets are burn
+// keys and sets the appropriate type on the output script key.
+func detectBurnKeys(activeTransfers []*tappsbt.VPacket) {
+	for _, vPkt := range activeTransfers {
+		for _, vOut := range vPkt.Outputs {
+			if vOut.Asset == nil {
+				continue
+			}
+
+			witness := vOut.Asset.PrevWitnesses
+			if len(witness) > 0 && asset.IsBurnKey(
+				vOut.ScriptKey.PubKey, witness[0],
+			) {
+				vOut.Asset.ScriptKey.Type = asset.ScriptKeyBurn
+				vOut.ScriptKey.Type = asset.ScriptKeyBurn
+			}
+		}
+	}
+}
+
 // A compile-time assertion to make sure ChainPorter satisfies the
 // fn.EventPublisher interface.
 var _ fn.EventPublisher[fn.Event, bool] = (*ChainPorter)(nil)