channeldb: add method initializable to decide initiating payment

This commit adds a new method, `initializable`, to help decide whether
initiating a payment is allowed.

channeldb/payment_control.go

@@ -151,30 +151,8 @@ func (p *PaymentControl) InitPayment(paymentHash lntypes.Hash,
 		// payment. We'll check the status to decide whether we allow
 		// retrying the payment or return a specific error.
 		case err == nil:
-			switch paymentStatus {
-			// We allow retrying failed payments.
-			case StatusFailed:
-
-			// We already have payment creation info for this
-			// payment so we won't allow creating a duplicate one.
-			case StatusInitiated:
-				updateErr = ErrPaymentExists
-				return nil
-
-			// We already have an InFlight payment on the network.
-			// We will disallow any new payments.
-			case StatusInFlight:
-				updateErr = ErrPaymentInFlight
-				return nil
-
-			// We've already succeeded a payment to this payment
-			// hash, forbid the switch from sending another.
-			case StatusSucceeded:
-				updateErr = ErrAlreadyPaid
-				return nil
-
-			default:
-				updateErr = ErrUnknownPaymentStatus
+			if err := paymentStatus.initializable(); err != nil {
+				updateErr = err
 				return nil
 			}
 

channeldb/payment_status.go

@@ -1,5 +1,7 @@
 package channeldb
 
+import "fmt"
+
 // PaymentStatus represent current status of payment.
 type PaymentStatus byte
 
@@ -43,3 +45,33 @@ func (ps PaymentStatus) String() string {
 		return "Unknown"
 	}
 }
+
+// initializable returns an error to specify whether initiating the payment
+// with its current status is allowed. A payment can only be initialized if it
+// hasn't been created yet or already failed.
+func (ps PaymentStatus) initializable() error {
+	switch ps {
+	// The payment has been created already. We will disallow creating it
+	// again in case other goroutines have already been creating HTLCs for
+	// it.
+	case StatusInitiated:
+		return ErrPaymentExists
+
+	// We already have an InFlight payment on the network. We will disallow
+	// any new payments.
+	case StatusInFlight:
+		return ErrPaymentInFlight
+
+	// The payment has been attempted and is succeeded so we won't allow
+	// creating it again.
+	case StatusSucceeded:
+		return ErrAlreadyPaid
+
+	// We allow retrying failed payments.
+	case StatusFailed:
+		return nil
+
+	default:
+		return fmt.Errorf("%w: %v", ErrUnknownPaymentStatus, ps)
+	}
+}