providers/template: add tests, address review comments

Do directory expansion on filenames.

Add basic acceptance tests. Code coverage is 72.5%.
Uncovered code is uninteresting and/or impossible error cases.

Note that this required adding a knob to
helper/resource.TestStep to allow transient
resources.

builtin/providers/template/resource.go

@@ -10,6 +10,7 @@ import (
 	"github.com/hashicorp/terraform/config/lang"
 	"github.com/hashicorp/terraform/config/lang/ast"
 	"github.com/hashicorp/terraform/helper/schema"
+	"github.com/mitchellh/go-homedir"
 )
 
 func resource() *schema.Resource {
@@ -35,7 +36,6 @@ func resource() *schema.Resource {
 			"rendered": &schema.Schema{
 				Type:        schema.TypeString,
 				Computed:    true,
-				Default:     nil,
 				Description: "rendered template",
 			},
 		},
@@ -55,11 +55,18 @@ func Exists(d *schema.ResourceData, meta interface{}) (bool, error) {
 	return false, nil
 }
 
+var readfile func(string) ([]byte, error) = ioutil.ReadFile // testing hook
+
 func eval(d *schema.ResourceData) error {
 	filename := d.Get("filename").(string)
 	vars := d.Get("vars").(map[string]interface{})
 
-	buf, err := ioutil.ReadFile(filename)
+	path, err := homedir.Expand(filename)
+	if err != nil {
+		return err
+	}
+
+	buf, err := readfile(path)
 	if err != nil {
 		return err
 	}

builtin/providers/template/resource_test.go

@@ -0,0 +1,58 @@
+package template
+
+import (
+	"fmt"
+	"testing"
+
+	r "github.com/hashicorp/terraform/helper/resource"
+	"github.com/hashicorp/terraform/terraform"
+)
+
+var testProviders = map[string]terraform.ResourceProvider{
+	"template": Provider(),
+}
+
+func TestTemplateRendering(t *testing.T) {
+	var cases = []struct {
+		vars     string
+		template string
+		want     string
+	}{
+		{`{}`, `ABC`, `ABC`},
+		{`{a="foo"}`, `${a}`, `foo`},
+		{`{a="hello"}`, `${replace(a, "ello", "i")}`, `hi`},
+		{`{}`, `${1+2+3}`, `6`},
+	}
+
+	for _, tt := range cases {
+		r.Test(t, r.TestCase{
+			PreCheck: func() {
+				readfile = func(string) ([]byte, error) {
+					return []byte(tt.template), nil
+				}
+			},
+			Providers: testProviders,
+			Steps: []r.TestStep{
+				r.TestStep{
+					Config: `
+resource "template_file" "t0" {
+	filename = "mock"
+	vars = ` + tt.vars + `
+}
+output "rendered" {
+    value = "${template_file.t0.rendered}"
+}
+`,
+					Check: func(s *terraform.State) error {
+						got := s.RootModule().Outputs["rendered"]
+						if tt.want != got {
+							return fmt.Errorf("template:\n%s\nvars:\n%s\ngot:\n%s\nwant:\n%s\n", tt.template, tt.vars, got, tt.want)
+						}
+						return nil
+					},
+					TransientResource: true,
+				},
+			},
+		})
+	}
+}

helper/resource/testing.go

@@ -75,6 +75,12 @@ type TestStep struct {
 
 	// Destroy will create a destroy plan if set to true.
 	Destroy bool
+
+	// TransientResource indicates that resources created as part
+	// of this test step are temporary and might be recreated anew
+	// with every planning step. This should only be set for
+	// pseudo-resources, like the null resource or templates.
+	TransientResource bool
 }
 
 // Test performs an acceptance test on a resource.
@@ -260,7 +266,7 @@ func testStep(
 	if p, err := ctx.Plan(); err != nil {
 		return state, fmt.Errorf("Error on second follow-up plan: %s", err)
 	} else {
-		if p.Diff != nil && !p.Diff.Empty() {
+		if p.Diff != nil && !p.Diff.Empty() && !step.TransientResource {
 			return state, fmt.Errorf(
 				"After applying this step and refreshing, the plan was not empty:\n\n%s", p)
 		}