From 636648880971c1ed48da6518c87486d995f445d7 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Fri, 3 Feb 2017 11:26:58 +0100
Subject: [PATCH] dag: Update can be called with a nil graph

---
 dag/walk.go      |  5 ++++-
 dag/walk_test.go | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/dag/walk.go b/dag/walk.go
index 826b7816e..72dad61de 100644
--- a/dag/walk.go
+++ b/dag/walk.go
@@ -136,7 +136,10 @@ func (w *Walker) Wait() error {
 // Multiple Updates can be called in parallel. Update can be called at any
 // time during a walk.
 func (w *Walker) Update(g *Graph) {
-	v, e := g.vertices, g.edges
+	var v, e *Set
+	if g != nil {
+		v, e = g.vertices, g.edges
+	}
 
 	// Grab the change lock so no more updates happen but also so that
 	// no new vertices are executed during this time since we may be
diff --git a/dag/walk_test.go b/dag/walk_test.go
index 04e6ebe3e..5843ca64b 100644
--- a/dag/walk_test.go
+++ b/dag/walk_test.go
@@ -33,6 +33,26 @@ func TestWalker_basic(t *testing.T) {
 	}
 }
 
+func TestWalker_updateNilGraph(t *testing.T) {
+	var g Graph
+	g.Add(1)
+	g.Add(2)
+	g.Connect(BasicEdge(1, 2))
+
+	// Run it a bunch of times since it is timing dependent
+	for i := 0; i < 50; i++ {
+		var order []interface{}
+		w := &Walker{Callback: walkCbRecord(&order)}
+		w.Update(&g)
+		w.Update(nil)
+
+		// Wait
+		if err := w.Wait(); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+}
+
 func TestWalker_error(t *testing.T) {
 	var g Graph
 	g.Add(1)