Add methods to annotate Graph vertices and edges

The AnnotateVertex and AnnotateEdge Graph methods will allow terraform
to insert arbitray information into the encoded graph stream for later
processing.

dag/graph.go

@@ -337,6 +337,16 @@ func (g *Graph) SetDebugWriter(w io.Writer) {
 	g.debug.Encode(newMarshalGraph("root", g))
 }
 
+func (g *Graph) AnnotateVertex(v Vertex, info string) {
+	va := newVertexAnnotation(v, info)
+	g.debug.Encode(va)
+}
+
+func (g *Graph) AnnotateEdge(e Edge, info string) {
+	ea := newEdgeAnnotation(e, info)
+	g.debug.Encode(ea)
+}
+
 // VertexName returns the name of a vertex.
 func VertexName(raw Vertex) string {
 	switch v := raw.(type) {

dag/marshal.go

@@ -467,3 +467,33 @@ func decodeGraph(r io.Reader) (*marshalGraph, error) {
 	}
 	return g, nil
 }
+
+// *Annotation structs allow encoding arbitrary information about the graph in
+// the logs.
+type vertexAnnotation struct {
+	Type   string
+	Vertex *marshalVertex
+	Info   string
+}
+
+func newVertexAnnotation(v Vertex, info string) *vertexAnnotation {
+	return &vertexAnnotation{
+		Type:   "VertexAnnotation",
+		Vertex: newMarshalVertex(v),
+		Info:   info,
+	}
+}
+
+type edgeAnnotation struct {
+	Type string
+	Edge *marshalEdge
+	Info string
+}
+
+func newEdgeAnnotation(e Edge, info string) *edgeAnnotation {
+	return &edgeAnnotation{
+		Type: "EdgeAnnotation",
+		Edge: newMarshalEdge(e),
+		Info: info,
+	}
+}

dag/marshal_test.go

@@ -129,6 +129,84 @@ func TestGraphJSON_basicRecord(t *testing.T) {
 	}
 }
 
+// Verify that Vertex and Edge annotations appear in the debug output
+func TestGraphJSON_annotations(t *testing.T) {
+	var g Graph
+	var buf bytes.Buffer
+	g.SetDebugWriter(&buf)
+
+	g.Add(1)
+	g.Add(2)
+	g.Add(3)
+	g.Connect(BasicEdge(1, 2))
+
+	g.AnnotateVertex(2, "2")
+	g.AnnotateVertex(3, "3")
+	g.AnnotateEdge(BasicEdge(1, 2), "1|2")
+
+	dec := json.NewDecoder(bytes.NewReader(buf.Bytes()))
+
+	var found2, found3, foundEdge bool
+	for dec.More() {
+		var d streamDecode
+
+		err := dec.Decode(&d)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		switch d.Type {
+		case "VertexAnnotation":
+			va := &vertexAnnotation{}
+			err := json.Unmarshal(d.JSON, va)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			switch va.Info {
+			case "2":
+				if va.Vertex.Name != "2" {
+					t.Fatalf("wrong vertex annotated 2: %#v", va)
+				}
+				found2 = true
+			case "3":
+				if va.Vertex.Name != "3" {
+					t.Fatalf("wrong vertex annotated 3: %#v", va)
+				}
+				found3 = true
+			default:
+				t.Fatalf("unexpected annotation:", va)
+			}
+		case "EdgeAnnotation":
+			ea := &edgeAnnotation{}
+			err := json.Unmarshal(d.JSON, ea)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			switch ea.Info {
+			case "1|2":
+				if ea.Edge.Name != "1|2" {
+					t.Fatalf("incorrect edge annotation: %#v\n", ea)
+				}
+				foundEdge = true
+			default:
+				t.Fatalf("unexpected edge Info: %#v", ea)
+			}
+		}
+	}
+
+	if !found2 {
+		t.Fatal("annotation 2 not found")
+	}
+	if !found3 {
+		t.Fatal("annotation 3 not found")
+	}
+	if !foundEdge {
+		t.Fatal("edge annotation not found")
+	}
+}
+
 const testGraphJSONEmptyStr = `{
   "Type": "Graph",
   "Name": "root",