tapfreighter: set type for burn/tombstone keys correctly

In order for burn/tombstone 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 and tombstone 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.
+		detectUnSpendableKeys(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 +1573,42 @@ func (p *ChainPorter) publishSubscriberEvent(event fn.Event) {
 	}
 }
 
+// detectUnSpendableKeys checks if any of the outputs in the virtual packets are
+// burn or tombstone keys and sets the appropriate type on the output script
+// key.
+func detectUnSpendableKeys(activeTransfers []*tappsbt.VPacket) {
+	setScriptKeyType := func(vOut *tappsbt.VOutput,
+		scriptKeyType asset.ScriptKeyType) {
+
+		if vOut.Asset.ScriptKey.TweakedScriptKey != nil {
+			vOut.Asset.ScriptKey.Type = scriptKeyType
+		}
+		if vOut.ScriptKey.TweakedScriptKey != nil {
+			vOut.ScriptKey.Type = scriptKeyType
+		}
+	}
+
+	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],
+			) {
+				setScriptKeyType(vOut, asset.ScriptKeyBurn)
+			}
+
+			unSpendable, _ := vOut.ScriptKey.IsUnSpendable()
+			if unSpendable {
+				setScriptKeyType(vOut, asset.ScriptKeyTombstone)
+			}
+		}
+	}
+}
+
 // A compile-time assertion to make sure ChainPorter satisfies the
 // fn.EventPublisher interface.
 var _ fn.EventPublisher[fn.Event, bool] = (*ChainPorter)(nil)