channeldb/migrations: skip migration for empty outgoing payments bucket

Previously the migration would fail if the source node was not set in
the database. Since we know that the source node must have been set
before making any payments, we check whether there actually are any
payments to migrate, and return early if not.

channeldb/migrations.go

@@ -746,9 +746,14 @@ func migrateGossipMessageStoreKeys(tx *bbolt.Tx) error {
 // payments) we delete those statuses, so only Completed payments remain in the
 // new bucket structure.
 func migrateOutgoingPayments(tx *bbolt.Tx) error {
-	oldPayments, err := tx.CreateBucketIfNotExists(paymentBucket)
-	if err != nil {
-		return err
+	log.Infof("Migrating outgoing payments to new bucket structure")
+
+	oldPayments := tx.Bucket(paymentBucket)
+
+	// Return early if there are no payments to migrate.
+	if oldPayments == nil {
+		log.Infof("No outgoing payments found, nothing to migrate.")
+		return nil
 	}
 
 	newPayments, err := tx.CreateBucket(paymentsRootBucket)
@@ -756,20 +761,22 @@ func migrateOutgoingPayments(tx *bbolt.Tx) error {
 		return err
 	}
 
-	// Get the source pubkey.
-	nodes := tx.Bucket(nodeBucket)
-	if nodes == nil {
-		return ErrGraphNotFound
-	}
+	// Helper method to get the source pubkey. We define it such that we
+	// only attempt to fetch it if needed.
+	sourcePub := func() ([33]byte, error) {
+		var pub [33]byte
+		nodes := tx.Bucket(nodeBucket)
+		if nodes == nil {
+			return pub, ErrGraphNotFound
+		}
 
-	selfPub := nodes.Get(sourceKey)
-	if selfPub == nil {
-		return ErrSourceNodeNotSet
+		selfPub := nodes.Get(sourceKey)
+		if selfPub == nil {
+			return pub, ErrSourceNodeNotSet
+		}
+		copy(pub[:], selfPub[:])
+		return pub, nil
 	}
-	var sourcePubKey [33]byte
-	copy(sourcePubKey[:], selfPub[:])
-
-	log.Infof("Migrating outgoing payments to new bucket structure")
 
 	err = oldPayments.ForEach(func(k, v []byte) error {
 		// Ignores if it is sub-bucket.
@@ -800,6 +807,11 @@ func migrateOutgoingPayments(tx *bbolt.Tx) error {
 			return err
 		}
 
+		sourcePubKey, err := sourcePub()
+		if err != nil {
+			return err
+		}
+
 		// Do the same for the PaymentAttemptInfo.
 		totalAmt := payment.Terms.Value + payment.Fee
 		rt := route.Route{