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/db.go

@@ -201,12 +201,16 @@ func (db *DB) Update(f func(tx walletdb.ReadWriteTx) error) error {
 
 // View is a wrapper around walletdb.View which calls into the extended
 // backend when available. This call is needed to be able to cast DB to
-// ExtendedBackend.
-func (db *DB) View(f func(tx walletdb.ReadTx) error) error {
+// ExtendedBackend. The passed reset function is called before the start of the
+// transaction and can be used to reset intermediate state. As callers may
+// expect retries of the f closure (depending on the database backend used), the
+// reset function will be called before each retry respectively.
+func (db *DB) View(f func(tx walletdb.ReadTx) error, reset func()) error {
 	if v, ok := db.Backend.(kvdb.ExtendedBackend); ok {
-		return v.View(f)
+		return v.View(f, reset)
 	}
 
+	reset()
 	return walletdb.View(db, f)
 }
 
@@ -389,6 +393,8 @@ func (d *DB) FetchOpenChannels(nodeID *btcec.PublicKey) ([]*OpenChannel, error)
 		var err error
 		channels, err = d.fetchOpenChannels(tx, nodeID)
 		return err
+	}, func() {
+		channels = nil
 	})
 
 	return channels, err
@@ -574,7 +580,7 @@ func (d *DB) FetchChannel(chanPoint wire.OutPoint) (*OpenChannel, error) {
 		})
 	}
 
-	err := kvdb.View(d, chanScan)
+	err := kvdb.View(d, chanScan, func() {})
 	if err != nil {
 		return nil, err
 	}
@@ -741,6 +747,8 @@ func fetchChannels(d *DB, filters ...fetchChannelsFilter) ([]*OpenChannel, error
 			})
 
 		})
+	}, func() {
+		channels = nil
 	})
 	if err != nil {
 		return nil, err
@@ -781,6 +789,8 @@ func (d *DB) FetchClosedChannels(pendingOnly bool) ([]*ChannelCloseSummary, erro
 			chanSummaries = append(chanSummaries, chanSummary)
 			return nil
 		})
+	}, func() {
+		chanSummaries = nil
 	}); err != nil {
 		return nil, err
 	}
@@ -817,6 +827,8 @@ func (d *DB) FetchClosedChannel(chanID *wire.OutPoint) (*ChannelCloseSummary, er
 		chanSummary, err = deserializeCloseChannelSummary(summaryReader)
 
 		return err
+	}, func() {
+		chanSummary = nil
 	}); err != nil {
 		return nil, err
 	}
@@ -865,6 +877,8 @@ func (d *DB) FetchClosedChannelForID(cid lnwire.ChannelID) (
 			return nil
 		}
 		return ErrClosedChannelNotFound
+	}, func() {
+		chanSummary = nil
 	}); err != nil {
 		return nil, err
 	}
@@ -1052,6 +1066,8 @@ func (d *DB) AddrsForNode(nodePub *btcec.PublicKey) ([]net.Addr, error) {
 		}
 
 		return nil
+	}, func() {
+		linkNode = nil
 	})
 	if dbErr != nil {
 		return nil, dbErr
@@ -1261,6 +1277,8 @@ func (db *DB) FetchHistoricalChannel(outPoint *wire.OutPoint) (*OpenChannel, err
 
 		channel, err = fetchOpenChannel(chanBucket, outPoint)
 		return err
+	}, func() {
+		channel = nil
 	})
 	if err != nil {
 		return nil, err