lnrpc: add new locked balance field for WalletBalance

In this commit, we add a new field to the WalletBalance call that
permits users to account for the set of outputs that may be locked due
to a pending transaction. Without this field any time users locked
outputs for things like PSBT signing, then they disappear from the
WalletBalance call, which may cause a panic.

lnrpc/lightning.proto

@@ -2502,6 +2502,10 @@ message WalletBalanceResponse {
     // The unconfirmed balance of a wallet(with 0 confirmations)
     int64 unconfirmed_balance = 3;
 
+    // The total amount of wallet UTXOs held in outputs that are locked for
+    // other usage.
+    int64 locked_balance = 5;
+
     // A mapping of each wallet account's name to its balance.
     map<string, WalletAccountBalance> account_balance = 4;
 }

lnrpc/lightning.swagger.json

@@ -6540,6 +6540,11 @@
           "format": "int64",
           "title": "The unconfirmed balance of a wallet(with 0 confirmations)"
         },
+        "locked_balance": {
+          "type": "string",
+          "format": "int64",
+          "description": "The total amount of wallet UTXOs held in outputs that are locked for\nother usage."
+        },
         "account_balance": {
           "type": "object",
           "additionalProperties": {

rpcserver.go

@@ -3087,6 +3087,27 @@ func (r *rpcServer) WalletBalance(ctx context.Context,
 		}
 	}
 
+	// Now that we have the base balance accounted for with each account,
+	// we'll look at the set of locked UTXOs to tally that as well. If we
+	// don't display this, then anytime we attempt a funding reservation,
+	// the outputs will chose as being "gone" until they're confirmed on
+	// chain.
+	var lockedBalance btcutil.Amount
+	leases, err := r.server.cc.Wallet.ListLeasedOutputs()
+	if err != nil {
+		return nil, err
+	}
+	for _, leasedOutput := range leases {
+		utxoInfo, err := r.server.cc.Wallet.FetchInputInfo(
+			&leasedOutput.Outpoint,
+		)
+		if err != nil {
+			return nil, err
+		}
+
+		lockedBalance += utxoInfo.Value
+	}
+
 	rpcsLog.Debugf("[walletbalance] Total balance=%v (confirmed=%v, "+
 		"unconfirmed=%v)", totalBalance, confirmedBalance,
 		unconfirmedBalance)
@@ -3095,6 +3116,7 @@ func (r *rpcServer) WalletBalance(ctx context.Context,
 		TotalBalance:       int64(totalBalance),
 		ConfirmedBalance:   int64(confirmedBalance),
 		UnconfirmedBalance: int64(unconfirmedBalance),
+		LockedBalance:      int64(lockedBalance),
 		AccountBalance:     rpcAccountBalances,
 	}, nil
 }