multi: add reset closure to kvdb.View

This commit adds a reset() closure to the kvdb.View function which will
be called before each retry (including the first) of the view
transaction. The reset() closure can be used to reset external state
(eg slices or maps) where the view closure puts intermediate results.

channeldb/migration_01_to_11/db.go

@@ -203,6 +203,8 @@ func (d *DB) FetchClosedChannels(pendingOnly bool) ([]*ChannelCloseSummary, erro
 			chanSummaries = append(chanSummaries, chanSummary)
 			return nil
 		})
+	}, func() {
+		chanSummaries = nil
 	}); err != nil {
 		return nil, err
 	}

channeldb/migration_01_to_11/graph.go

@@ -190,6 +190,8 @@ func (c *ChannelGraph) SourceNode() (*LightningNode, error) {
 		source = node
 
 		return nil
+	}, func() {
+		source = nil
 	})
 	if err != nil {
 		return nil, err

channeldb/migration_01_to_11/invoices.go

@@ -282,6 +282,8 @@ func (d *DB) FetchAllInvoices(pendingOnly bool) ([]Invoice, error) {
 
 			return nil
 		})
+	}, func() {
+		invoices = nil
 	})
 	if err != nil {
 		return nil, err

channeldb/migration_01_to_11/migration_09_legacy_serialization.go

@@ -126,6 +126,8 @@ func (db *DB) fetchAllPayments() ([]*outgoingPayment, error) {
 			payments = append(payments, payment)
 			return nil
 		})
+	}, func() {
+		payments = nil
 	})
 	if err != nil {
 		return nil, err
@@ -144,6 +146,8 @@ func (db *DB) fetchPaymentStatus(paymentHash [32]byte) (PaymentStatus, error) {
 		var err error
 		paymentStatus, err = fetchPaymentStatusTx(tx, paymentHash)
 		return err
+	}, func() {
+		paymentStatus = StatusUnknown
 	})
 	if err != nil {
 		return StatusUnknown, err
@@ -424,6 +428,8 @@ func (db *DB) fetchPaymentsMigration9() ([]*Payment, error) {
 				return nil
 			})
 		})
+	}, func() {
+		payments = nil
 	})
 	if err != nil {
 		return nil, err

channeldb/migration_01_to_11/migrations_test.go

@@ -418,6 +418,8 @@ func TestMigrateOptionalChannelCloseSummaryFields(t *testing.T) {
 						"serialization")
 				}
 				return nil
+			}, func() {
+				dbSummary = nil
 			})
 			if err != nil {
 				t.Fatalf("unable to view DB: %v", err)
@@ -521,6 +523,8 @@ func TestMigrateGossipMessageStoreKeys(t *testing.T) {
 			}
 
 			return nil
+		}, func() {
+			rawMsg = nil
 		})
 		if err != nil {
 			t.Fatal(err)

channeldb/migration_01_to_11/payments.go

@@ -303,6 +303,8 @@ func (db *DB) FetchPayments() ([]*Payment, error) {
 				return nil
 			})
 		})
+	}, func() {
+		payments = nil
 	})
 	if err != nil {
 		return nil, err