config: allow applying variables to resources

config/config.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 
 	"github.com/hashicorp/terraform/depgraph"
+	"github.com/mitchellh/reflectwalk"
 )
 
 // ResourceGraphRoot is the name of the resource graph root that should be
@@ -81,6 +82,25 @@ func (r *Resource) Id() string {
 	return fmt.Sprintf("%s.%s", r.Type, r.Name)
 }
 
+// ReplaceVariables replaces the variables in the configuration
+// with the given values.
+//
+// This replacement is not in place. Instead, this function will
+// return a new resource with the variables replaced.
+func (r *Resource) ReplaceVariables(vs map[string]string) *Resource {
+	result := *r
+
+	w := &variableReplaceWalker{
+		Values: vs,
+	}
+
+	if err := reflectwalk.Walk(result.Config, w); err != nil {
+		panic(err)
+	}
+
+	return &result
+}
+
 // ResourceGraph returns a dependency graph of the resources from this
 // Terraform configuration.
 func (c *Config) ResourceGraph() *depgraph.Graph {

config/config_test.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"path/filepath"
+	"reflect"
 	"strings"
 	"testing"
 )
@@ -63,6 +64,49 @@ func TestNewUserVariable(t *testing.T) {
 	}
 }
 
+func TestResourceReplaceVariables(t *testing.T) {
+	r := &Resource{
+		Name: "foo",
+		Type: "bar",
+		Config: map[string]interface{}{
+			"foo": "${var.bar}",
+		},
+	}
+
+	values := map[string]string{
+		"var.bar": "value",
+	}
+
+	r2 := r.ReplaceVariables(values)
+
+	expected := &Resource{
+		Name: "foo",
+		Type: "bar",
+		Config: map[string]interface{}{
+			"foo": "value",
+		},
+	}
+	if !reflect.DeepEqual(r2, expected) {
+		t.Fatalf("bad: %#v", r2)
+	}
+
+	/*
+	TODO(mitchellh): Eventually, preserve original config...
+
+	expectedOriginal := &Resource{
+		Name: "foo",
+		Type: "bar",
+		Config: map[string]interface{}{
+			"foo": "${var.bar}",
+		},
+	}
+
+	if !reflect.DeepEqual(r, expectedOriginal) {
+		t.Fatalf("bad: %#v", r)
+	}
+	*/
+}
+
 const resourceGraphValue = `
 root: root
   root -> aws_security_group.firewall