quote dot node names

providers can contain quotes, so we need to ensure proper quoting in the
dot format.

dag/marshal.go

@@ -108,9 +108,14 @@ func newMarshalVertex(v Vertex) *marshalVertex {
 		dn = nil
 	}
 
+	// the name will be quoted again later, so we need to ensure it's properly
+	// escaped without quotes.
+	name := strconv.Quote(VertexName(v))
+	name = name[1 : len(name)-1]
+
 	return &marshalVertex{
 		ID:              marshalVertexID(v),
-		Name:            VertexName(v),
+		Name:            name,
 		Attrs:           make(map[string]string),
 		graphNodeDotter: dn,
 	}

dag/marshal_test.go

@@ -32,6 +32,21 @@ func TestGraphDot_basic(t *testing.T) {
 	}
 }
 
+func TestGraphDot_quoted(t *testing.T) {
+	var g Graph
+	quoted := `name["with-quotes"]`
+	other := `other`
+	g.Add(quoted)
+	g.Add(other)
+	g.Connect(BasicEdge(quoted, other))
+
+	actual := strings.TrimSpace(string(g.Dot(nil)))
+	expected := strings.TrimSpace(testGraphDotQuotedStr)
+	if actual != expected {
+		t.Fatalf("\ngot:   %q\nwanted %q\n", actual, expected)
+	}
+}
+
 func TestGraphDot_attrs(t *testing.T) {
 	var g Graph
 	g.Add(&testGraphNodeDotter{
@@ -53,6 +68,14 @@ type testGraphNodeDotter struct{ Result *DotNode }
 func (n *testGraphNodeDotter) Name() string                      { return n.Result.Name }
 func (n *testGraphNodeDotter) DotNode(string, *DotOpts) *DotNode { return n.Result }
 
+const testGraphDotQuotedStr = `digraph {
+	compound = "true"
+	newrank = "true"
+	subgraph "root" {
+		"[root] name[\"with-quotes\"]" -> "[root] other"
+	}
+}`
+
 const testGraphDotBasicStr = `digraph {
 	compound = "true"
 	newrank = "true"