config: validate that resource references are valid

config/config.go

@@ -89,10 +89,13 @@ func (r *Resource) Id() string {
 
 // Validate does some basic semantic checking of the configuration.
 func (c *Config) Validate() error {
+	var errs []error
+
+	vars := c.allVariables()
+
 	// Check for references to user variables that do not actually
 	// exist and record those errors.
-	var errs []error
-	for source, v := range c.allVariables() {
+	for source, v := range vars {
 		uv, ok := v.(*UserVariable)
 		if !ok {
 			continue
@@ -106,6 +109,27 @@ func (c *Config) Validate() error {
 		}
 	}
 
+	// Check that all references to resources are valid
+	resources := make(map[string]struct{})
+	for _, r := range c.Resources {
+		resources[r.Id()] = struct{}{}
+	}
+	for source, v := range vars {
+		rv, ok := v.(*ResourceVariable)
+		if !ok {
+			continue
+		}
+
+		id := fmt.Sprintf("%s.%s", rv.Type, rv.Name)
+		if _, ok := resources[id]; !ok {
+			errs = append(errs, fmt.Errorf(
+				"%s: unknown resource '%s' referenced in variable %s",
+				source,
+				id,
+				rv.FullKey()))
+		}
+	}
+
 	if len(errs) > 0 {
 		return &MultiError{Errors: errs}
 	}

config/config_test.go

@@ -15,6 +15,13 @@ func TestConfigValidate(t *testing.T) {
 	}
 }
 
+func TestConfigValidate_unknownResourceVar(t *testing.T) {
+	c := testConfig(t, "validate-unknown-resource-var")
+	if err := c.Validate(); err == nil {
+		t.Fatal("should not be valid")
+	}
+}
+
 func TestConfigValidate_unknownVar(t *testing.T) {
 	c := testConfig(t, "validate-unknownvar")
 	if err := c.Validate(); err == nil {

config/test-fixtures/validate-unknown-resource-var/main.tf

@@ -0,0 +1,6 @@
+resource "aws_instance" "web" {
+}
+
+resource "aws_instance" "db" {
+  ami = "${aws_instance.loadbalancer.foo}"
+}