channeldb: modify ForEachNode/ForEachChannel to accept a db txn

This commit modifies the ForEachNode on the ChannelGraph and
ForEachChannel on the LightningNode struct to accept a database
transaction as its first argument. With this change, it’ll now be
possible to implement graph traversals that typically required a nested
loop with all the vertex loaded into memory using the callback API
instead:
c.ForEachNode(nil, func(tx, node) {
    node.ForEachChannel(tx, func(…) {
    })
})

channeldb/graph.go

@@ -217,11 +217,16 @@ func (c *ChannelGraph) ForEachChannel(cb func(*ChannelEdgeInfo, *ChannelEdgePoli
 // executing the passed callback with each node encountered. If the callback
 // returns an error, then the transaction is aborted and the iteration stops
 // early.
-func (c *ChannelGraph) ForEachNode(cb func(*LightningNode) error) error {
+//
-	// TODO(roasbeef): need to also pass in a transaction? or reverse order
+// If the caller wishes to re-use an existing boltdb transaction, then it
-	// to get all in memory THEN execute callback?
+// should be passed as the first argument.  Otherwise the first argument should
-
+// be nil and a fresh transaction will be created to execute the graph
-	return c.db.View(func(tx *bolt.Tx) error {
+// traversal
+//
+// TODO(roasbeef): add iterator interface to allow for memory efficient graph
+// traversal when graph gets mega
+func (c *ChannelGraph) ForEachNode(tx *bolt.Tx, cb func(*bolt.Tx, *LightningNode) error) error {
+	traversal := func(tx *bolt.Tx) error {
 		// First grab the nodes bucket which stores the mapping from
 		// pubKey to node information.
 		nodes := tx.Bucket(nodeBucket)
@@ -246,9 +251,19 @@ func (c *ChannelGraph) ForEachNode(cb func(*LightningNode) error) error {
 
 			// Execute the callback, the transaction will abort if
 			// this returns an error.
-			return cb(node)
+			return cb(tx, node)
 		})
-	})
+	}
+
+	// If no transaction was provided, then we'll create a new transaction
+	// to execute the transaction within.
+	if tx == nil {
+		return c.db.View(traversal)
+	}
+
+	// Otherwise, we re-use the existing transaction to execute the graph
+	// traversal.
+	return traversal(tx)
 }
 
 // SourceNode returns the source node of the graph. The source node is treated
@@ -962,13 +977,15 @@ func (c *ChannelGraph) HasLightningNode(pub *btcec.PublicKey) (time.Time, bool,
 // ForEachChannel iterates through all the outgoing channel edges from this
 // node, executing the passed callback with each edge as its sole argument. If
 // the callback returns an error, then the iteration is halted with the error
-// propagated back up to the caller. If the caller wishes to re-use an existing
+// propagated back up to the caller.
-// boltdb transaction, then it should be passed as the first argument.
+//
-// Otherwise the first argument should be nil and a fresh transaction will be
+// If the caller wishes to re-use an existing boltdb transaction, then it
-// created to execute the graph traversal.
+// should be passed as the first argument.  Otherwise the first argument should
-func (l *LightningNode) ForEachChannel(tx *bolt.Tx, cb func(*ChannelEdgeInfo, *ChannelEdgePolicy) error) error {
+// be nil and a fresh transaction will be created to execute the graph
-	// TODO(roasbeef): remove the option to pass in a transaction after
+// traversal.
-	// all?
+func (l *LightningNode) ForEachChannel(tx *bolt.Tx,
+	cb func(*bolt.Tx, *ChannelEdgeInfo, *ChannelEdgePolicy) error) error {
+
 	nodePub := l.PubKey.SerializeCompressed()
 
 	traversal := func(tx *bolt.Tx) error {
@@ -1021,7 +1038,7 @@ func (l *LightningNode) ForEachChannel(tx *bolt.Tx, cb func(*ChannelEdgeInfo, *C
 			}
 
 			// Finally, we execute the callback.
-			if err := cb(edgeInfo, edgePolicy); err != nil {
+			if err := cb(tx, edgeInfo, edgePolicy); err != nil {
 				return err
 			}
 		}

channeldb/graph_test.go

@@ -13,6 +13,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/boltdb/bolt"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/roasbeef/btcd/btcec"
@@ -533,7 +534,7 @@ func TestGraphTraversal(t *testing.T) {
 
 	// Iterate over each node as returned by the graph, if all nodes are
 	// reached, then the map created above should be empty.
-	err = graph.ForEachNode(func(node *LightningNode) error {
+	err = graph.ForEachNode(nil, func(_ *bolt.Tx, node *LightningNode) error {
 		delete(nodeIndex, node.Alias)
 		return nil
 	})
@@ -630,7 +631,9 @@ func TestGraphTraversal(t *testing.T) {
 	// Finally, we want to test the ability to iterate over all the
 	// outgoing channels for a particular node.
 	numNodeChans := 0
-	err = firstNode.ForEachChannel(nil, func(_ *ChannelEdgeInfo, c *ChannelEdgePolicy) error {
+	err = firstNode.ForEachChannel(nil, func(_ *bolt.Tx, _ *ChannelEdgeInfo,
+		c *ChannelEdgePolicy) error {
+
 		// Each each should indicate that it's outgoing (pointed
 		// towards the second node).
 		if !c.Node.PubKey.IsEqual(secondNode.PubKey) {