terraform: some basic validation

terraform/graph.go

@@ -77,6 +77,10 @@ type GraphNodeResourceProvider struct {
 //     configured at this point.
 //
 func Graph(opts *GraphOpts) (*depgraph.Graph, error) {
+	if opts.Config == nil {
+		return nil, errors.New("Config is required for Graph")
+	}
+
 	g := new(depgraph.Graph)
 
 	// First, build the initial resource graph. This only has the resources

terraform/graph_test.go

@@ -21,6 +21,12 @@ func TestGraph(t *testing.T) {
 	}
 }
 
+func TestGraph_configRequired(t *testing.T) {
+	if _, err := Graph(new(GraphOpts)); err == nil {
+		t.Fatal("should error")
+	}
+}
+
 func TestGraph_cycle(t *testing.T) {
 	config := testConfig(t, "graph-cycle")