Merge pull request lightningnetwork#6217 from liviu-ln/psbt-finalize-…

…check

walletrpc: return an error when finalizing an already complete PSBT

docs/release-notes/release-notes-0.15.0.md

@@ -106,6 +106,8 @@
 * [Add ForAll implementation for etcd to speed up
   graph cache at startup](https://github.com/lightningnetwork/lnd/pull/6136)
 
+* [Improve validation of a PSBT packet when handling a request to finalize it.](https://github.com/lightningnetwork/lnd/pull/6217)
+
 ## Documentation
 
 * Improved instructions on [how to build lnd for mobile](https://github.com/lightningnetwork/lnd/pull/6085).

lnrpc/walletrpc/walletkit_server.go

@@ -1254,15 +1254,20 @@ func (w *WalletKit) FinalizePsbt(_ context.Context,
 		account = req.Account
 	}
 
-	// Parse the funded PSBT. No additional checks are required at this
-	// level as the wallet will perform all of them.
+	// Parse the funded PSBT.
 	packet, err := psbt.NewFromRawBytes(
 		bytes.NewReader(req.FundedPsbt), false,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("error parsing PSBT: %v", err)
 	}
 
+	// The only check done at this level is to validate that the PSBT is
+	// not complete. The wallet performs all other checks.
+	if packet.IsComplete() {
+		return nil, fmt.Errorf("PSBT is already fully signed")
+	}
+
 	// Let the wallet do the heavy lifting. This will sign all inputs that
 	// we have the UTXO for. If some inputs can't be signed and don't have
 	// witness data attached, this will fail.