Fix removeEdge test failures

The removeEdge test could fail intermittently with the wrong order.

The precondition of a 1->2->3 order wasn't met, because there was no
edge from 1->3, so 3->1->2 was also a valid ordering.

The other failure was a bookkeeping error, were the recorded order may
not match the visited order. What happened in this case was the gateCh
was closed by V2, allowing V3 to run which could beat V2 to recording
its visit. Now the visit is recorded as part of the vertex walk, and the
gate is released as the final operation.

The order is deterministic now, so remove the brute-force test loop.

dag/walk_test.go

@@ -215,63 +215,63 @@ func TestWalker_newEdge(t *testing.T) {
 }
 
 func TestWalker_removeEdge(t *testing.T) {
-	// Run it a bunch of times since it is timing dependent
-	for i := 0; i < 50; i++ {
-		var g AcyclicGraph
-		g.Add(1)
-		g.Add(2)
-		g.Add(3)
-		g.Connect(BasicEdge(1, 2))
-		g.Connect(BasicEdge(3, 2))
-
-		// Record function
-		var order []interface{}
-		recordF := walkCbRecord(&order)
-
-		// The way this works is that our original graph forces
-		// the order of 1 => 3 => 2. During the execution of 1, we
-		// remove the edge forcing 3 before 2. Then, during the execution
-		// of 3, we wait on a channel that is only closed by 2, implicitly
-		// forcing 2 before 3 via the callback (and not the graph). If
-		// 2 cannot execute before 3 (edge removal is non-functional), then
-		// this test will timeout.
-		var w *Walker
-		gateCh := make(chan struct{})
-		cb := func(v Vertex) error {
-			if v == 1 {
-				g.RemoveEdge(BasicEdge(3, 2))
-				w.Update(&g)
-			}
+	var g AcyclicGraph
+	g.Add(1)
+	g.Add(2)
+	g.Add(3)
+	g.Connect(BasicEdge(1, 2))
+	g.Connect(BasicEdge(1, 3))
+	g.Connect(BasicEdge(3, 2))
 
-			if v == 2 {
-				close(gateCh)
-			}
+	// Record function
+	var order []interface{}
+	recordF := walkCbRecord(&order)
 
-			if v == 3 {
-				select {
-				case <-gateCh:
-				case <-time.After(50 * time.Millisecond):
-					return fmt.Errorf("timeout 3 waiting for 2")
-				}
-			}
+	// The way this works is that our original graph forces
+	// the order of 1 => 3 => 2. During the execution of 1, we
+	// remove the edge forcing 3 before 2. Then, during the execution
+	// of 3, we wait on a channel that is only closed by 2, implicitly
+	// forcing 2 before 3 via the callback (and not the graph). If
+	// 2 cannot execute before 3 (edge removal is non-functional), then
+	// this test will timeout.
+	var w *Walker
+	gateCh := make(chan struct{})
+	cb := func(v Vertex) error {
+		switch v {
+		case 1:
+			g.RemoveEdge(BasicEdge(3, 2))
+			w.Update(&g)
 
-			return recordF(v)
+		case 2:
+			// this visit isn't completed until we've recorded it
+			// Once the visit is official, we can then close the gate to
+			// let 3 continue.
+			defer close(gateCh)
+
+		case 3:
+			select {
+			case <-gateCh:
+			case <-time.After(50 * time.Millisecond):
+				return fmt.Errorf("timeout 3 waiting for 2")
+			}
 		}
 
-		// Add the initial vertices
-		w = &Walker{Callback: cb}
-		w.Update(&g)
+		return recordF(v)
+	}
 
-		// Wait
-		if err := w.Wait(); err != nil {
-			t.Fatalf("err: %s", err)
-		}
+	// Add the initial vertices
+	w = &Walker{Callback: cb}
+	w.Update(&g)
 
-		// Check
-		expected := []interface{}{1, 2, 3}
-		if !reflect.DeepEqual(order, expected) {
-			t.Fatalf("bad: %#v", order)
-		}
+	// Wait
+	if err := w.Wait(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	// Check
+	expected := []interface{}{1, 2, 3}
+	if !reflect.DeepEqual(order, expected) {
+		t.Fatalf("bad: %#v", order)
 	}
 }