channeldb: add option to re-use existing db transaction for graph traversals

This commit modifies the LightningNode.ForEachChannel method to give
the caller the option of re-using an existing database transaction
instead of always creating a new db transaction with each invocation.
Internally boltdb will run into an error/dead-lock if a nested
transaction is attempted.

Such an action might be attempted if one were to use the traversal
functions in  a path finding algorithm. Therefore in order to avoid
that after, we now allow the re-use of transactions to facilitate
nested calls to ForEachChannel.

channeldb/graph.go

@@ -632,11 +632,14 @@ func (c *ChannelGraph) FetchLightningNode(pub *btcec.PublicKey) (*LightningNode,
 // 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.
-func (l *LightningNode) ForEachChannel(cb func(*ChannelEdge) error) error {
+// propagated back up to the caller. If the caller wishes to re-use an existing
+// boltdb transaction, then it 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 traversal.
+func (l *LightningNode) ForEachChannel(tx *bolt.Tx, cb func(*ChannelEdge) error) error {
 	nodePub := l.PubKey.SerializeCompressed()
 
-	return l.db.View(func(tx *bolt.Tx) error {
+	traversal := func(tx *bolt.Tx) error {
 		nodes := tx.Bucket(nodeBucket)
 		if nodes == nil {
 			return ErrGraphNotFound
@@ -682,7 +685,17 @@ func (l *LightningNode) ForEachChannel(cb func(*ChannelEdge) error) error {
 		}
 
 		return nil
-	})
+	}
+
+	// If no transaction was provided, then we'll create a new transaction
+	// to execute the transaction within.
+	if tx == nil {
+		return l.db.View(traversal)
+	}
+
+	// Otherwise, we re-use the existing transaction to execute the graph
+	// traversal.
+	return traversal(tx)
 }
 
 // ChannelEdge represents a *directed* edge within the channel graph. For each

channeldb/graph_test.go

@@ -489,7 +489,7 @@ 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(func(c *ChannelEdge) error {
+	err = firstNode.ForEachChannel(nil, func(c *ChannelEdge) error {
 		// Each each should indicate that it's outgoing (pointed
 		// towards the second node).
 		if !c.Node.PubKey.IsEqual(secondNode.PubKey) {