terraform: return value for resource interpolation on refresh

Instead of returning UnknownVariableValue every time, attempt to return
the real value. If we don't find it, return unknown value. This fixes
removing outputs from state on refresh.

terraform/context_test.go

@@ -1845,6 +1845,54 @@ func TestContext2Refresh_noState(t *testing.T) {
 	}
 }
 
+func TestContext2Refresh_output(t *testing.T) {
+	p := testProvider("aws")
+	m := testModule(t, "refresh-output")
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+		State: &State{
+			Modules: []*ModuleState{
+				&ModuleState{
+					Path: rootModulePath,
+					Resources: map[string]*ResourceState{
+						"aws_instance.web": &ResourceState{
+							Type: "aws_instance",
+							Primary: &InstanceState{
+								ID: "foo",
+								Attributes: map[string]string{
+									"foo": "bar",
+								},
+							},
+						},
+					},
+
+					Outputs: map[string]string{
+						"foo": "foo",
+					},
+				},
+			},
+		},
+	})
+
+	p.RefreshFn = func(info *InstanceInfo, s *InstanceState) (*InstanceState, error) {
+		return s, nil
+	}
+
+	s, err := ctx.Refresh()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(s.String())
+	expected := strings.TrimSpace(testContextRefreshOutputStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s\n\n%s", actual, expected)
+	}
+}
+
 func TestContext2Refresh_outputPartial(t *testing.T) {
 	p := testProvider("aws")
 	m := testModule(t, "refresh-output-partial")
@@ -5995,6 +6043,16 @@ module.child:
     ID = new
 `
 
+const testContextRefreshOutputStr = `
+aws_instance.web:
+  ID = foo
+  foo = bar
+
+Outputs:
+
+foo = bar
+`
+
 const testContextRefreshOutputPartialStr = `
 <no state>
 `

terraform/interpolate.go

@@ -193,7 +193,7 @@ func (i *Interpolater) valueResourceVar(
 	result map[string]ast.Variable) error {
 	// If we're computing all dynamic fields, then module vars count
 	// and we mark it as computed.
-	if i.Operation == walkValidate || i.Operation == walkRefresh {
+	if i.Operation == walkValidate {
 		result[n] = ast.Variable{
 			Value: config.UnknownVariableValue,
 			Type:  ast.TypeString,
@@ -353,6 +353,14 @@ func (i *Interpolater) computeResourceVariable(
 	}
 
 MISSING:
+	// If the operation is refresh, it isn't an error for a value to
+	// be unknown. Instead, we return that the value is computed so
+	// that the graph can continue to refresh other nodes. It doesn't
+	// matter because the config isn't interpolated anyways.
+	if i.Operation == walkRefresh {
+		return config.UnknownVariableValue, nil
+	}
+
 	return "", fmt.Errorf(
 		"Resource '%s' does not have attribute '%s' "+
 			"for variable '%s'",

terraform/test-fixtures/refresh-output/main.tf

@@ -0,0 +1,5 @@
+resource "aws_instance" "web" {}
+
+output "foo" {
+    value = "${aws_instance.web.foo}"
+}