channeldb: isolate duplicate payments

Duplicate payments is legacy that we keep alive for accounting purposes.
This commit isolates the deserialization logic for duplicate payments in
its own file, so that regular payment logic and db structure can evolve
without needing to handle/migrate the legacy data.

channeldb/payments.go

@@ -54,13 +54,6 @@ var (
 	//
 	paymentsRootBucket = []byte("payments-root-bucket")
 
-	// paymentDublicateBucket is the name of a optional sub-bucket within
-	// the payment hash bucket, that is used to hold duplicate payments to
-	// a payment hash. This is needed to support information from earlier
-	// versions of lnd, where it was possible to pay to a payment hash more
-	// than once.
-	paymentDuplicateBucket = []byte("payment-duplicate-bucket")
-
 	// paymentSequenceKey is a key used in the payment's sub-bucket to
 	// store the sequence number of the payment.
 	paymentSequenceKey = []byte("payment-sequence-key")
@@ -308,28 +301,14 @@ func (db *DB) FetchPayments() ([]*MPPayment, error) {
 			// payment has was possible. These will be found in a
 			// sub-bucket indexed by their sequence number if
 			// available.
-			dup := bucket.Bucket(paymentDuplicateBucket)
-			if dup == nil {
-				return nil
+			duplicatePayments, err := fetchDuplicatePayments(bucket)
+			if err != nil {
+				return err
 			}
 
-			return dup.ForEach(func(k, v []byte) error {
-				subBucket := dup.Bucket(k)
-				if subBucket == nil {
-					// We one bucket for each duplicate to
-					// be found.
-					return fmt.Errorf("non bucket element" +
-						"in duplicate bucket")
-				}
+			payments = append(payments, duplicatePayments...)
 
-				p, err := fetchPayment(subBucket)
-				if err != nil {
-					return err
-				}
-
-				payments = append(payments, p)
-				return nil
-			})
+			return nil
 		})
 	})
 	if err != nil {