rpcwallet+itest: fix incomplete key info problem

Fixes an issue with SignOutputRaw in remote signing mode where we
weren't able to sign on the remote signer if we only provided the public
key or only the family/index (and not both).

Fixes part of an issue detected in lightninglabs/loop#457.

lntest/itest/lnd_remote_signer_test.go

@@ -99,6 +99,12 @@ func testRemoteSigner(net *lntest.NetworkHarness, t *harnessTest) {
 			runPsbtChanFunding(net, tt, carol, wo)
 			runSignPsbt(tt, net, wo)
 		},
+	}, {
+		name:      "sign output raw",
+		sendCoins: true,
+		fn: func(tt *harnessTest, wo, carol *lntest.HarnessNode) {
+			runSignOutputRaw(tt, net, wo)
+		},
 	}}
 
 	for _, st := range subTests {

lnwallet/rpcwallet/rpcwallet.go

@@ -17,6 +17,7 @@ import (
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
+	"github.com/btcsuite/btcwallet/waddrmgr"
 	basewallet "github.com/btcsuite/btcwallet/wallet"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -585,6 +586,79 @@ func (r *RPCKeyRing) remoteSign(tx *wire.MsgTx, signDesc *input.SignDescriptor,
 	in := &packet.Inputs[signDesc.InputIndex]
 	txIn := tx.TxIn[signDesc.InputIndex]
 
+	// Things are a bit tricky with the sign descriptor. There basically are
+	// four ways to describe a key:
+	//   1. By public key only. To match this case both family and index
+	//      must be set to 0.
+	//   2. By family and index only. To match this case the public key
+	//      must be nil and either the family or index must be non-zero.
+	//   3. All values are set and locator is non-empty. To match this case
+	//      the public key must be set and either the family or index must
+	//      be non-zero.
+	//   4. All values are set and locator is empty. This is a special case
+	//      for the very first channel ever created (with the multi-sig key
+	//      family which is 0 and the index which is 0 as well). This looks
+	//      identical to case 1 and will also be handled like that case.
+	// We only really handle case 1 and 2 here, since 3 is no problem and 4
+	// is identical to 1.
+	switch {
+	// Case 1: Public key only. We need to find out the derivation path for
+	// this public key by asking the wallet. This is only possible for our
+	// internal, custom 1017 scope since we know all keys derived there are
+	// internally stored as p2wkh addresses.
+	case signDesc.KeyDesc.PubKey != nil && signDesc.KeyDesc.IsEmpty():
+		pubKeyBytes := signDesc.KeyDesc.PubKey.SerializeCompressed()
+		addr, err := btcutil.NewAddressWitnessPubKeyHash(
+			btcutil.Hash160(pubKeyBytes), r.netParams,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("error deriving address from "+
+				"public key %x: %v", pubKeyBytes, err)
+		}
+
+		managedAddr, err := r.AddressInfo(addr)
+		if err != nil {
+			return nil, fmt.Errorf("error fetching address info "+
+				"for public key %x: %v", pubKeyBytes, err)
+		}
+
+		pubKeyAddr, ok := managedAddr.(waddrmgr.ManagedPubKeyAddress)
+		if !ok {
+			return nil, fmt.Errorf("address derived for public "+
+				"key %x is not a p2wkh address", pubKeyBytes)
+		}
+
+		scope, path, _ := pubKeyAddr.DerivationInfo()
+		if scope.Purpose != keychain.BIP0043Purpose {
+			return nil, fmt.Errorf("address derived for public "+
+				"key %x is not in custom key scope %d'",
+				pubKeyBytes, keychain.BIP0043Purpose)
+		}
+
+		// We now have all the information we need to complete our key
+		// locator information.
+		signDesc.KeyDesc.KeyLocator = keychain.KeyLocator{
+			Family: keychain.KeyFamily(path.InternalAccount),
+			Index:  path.Index,
+		}
+
+	// Case 2: Family and index only. This case is easy, we can just go
+	// ahead and derive the public key from the family and index and then
+	// supply that information in the BIP32 derivation field.
+	case signDesc.KeyDesc.PubKey == nil && !signDesc.KeyDesc.IsEmpty():
+		fullDesc, err := r.watchOnlyKeyRing.DeriveKey(
+			signDesc.KeyDesc.KeyLocator,
+		)
+		if err != nil {
+			return nil, fmt.Errorf("error deriving key with "+
+				"family %d and index %d from the watch-only "+
+				"wallet: %v",
+				signDesc.KeyDesc.KeyLocator.Family,
+				signDesc.KeyDesc.KeyLocator.Index, err)
+		}
+		signDesc.KeyDesc.PubKey = fullDesc.PubKey
+	}
+
 	// Make sure we actually know about the input. We either have been
 	// watching the UTXO on-chain or we have been given all the required
 	// info in the sign descriptor.