provider/template: Fix panic in cloudinit config

Fixes an uncaught panic during an interface cast in the `template_cloudinit_config` data source.

Fixes: #13572

```
$ make test TEST=./builtin/providers/template TESTARGS="-v -run=TestRender_handlePanic"
==> Checking that code complies with gofmt requirements...
==> Checking AWS provider for unchecked errors...
==> NOTE: at this time we only look for uncheck errors in the AWS package
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/12 10:46:33 Generated command/internal_plugin_list.go
go test -i ./builtin/providers/template || exit 1
echo ./builtin/providers/template | \
                xargs -t -n4 go test -v -run=TestRender_handlePanic -timeout=60s -parallel=4
go test -v -run=TestRender_handlePanic -timeout=60s -parallel=4 ./builtin/providers/template
=== RUN   TestRender_handlePanic
--- PASS: TestRender_handlePanic (0.00s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/template       0.028s
```

builtin/providers/template/datasource_cloudinit_config.go

@@ -19,41 +19,41 @@ func dataSourceCloudinitConfig() *schema.Resource {
 		Read: dataSourceCloudinitConfigRead,
 
 		Schema: map[string]*schema.Schema{
-			"part": &schema.Schema{
+			"part": {
 				Type:     schema.TypeList,
 				Required: true,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
-						"content_type": &schema.Schema{
+						"content_type": {
 							Type:     schema.TypeString,
 							Optional: true,
 						},
-						"content": &schema.Schema{
+						"content": {
 							Type:     schema.TypeString,
 							Required: true,
 						},
-						"filename": &schema.Schema{
+						"filename": {
 							Type:     schema.TypeString,
 							Optional: true,
 						},
-						"merge_type": &schema.Schema{
+						"merge_type": {
 							Type:     schema.TypeString,
 							Optional: true,
 						},
 					},
 				},
 			},
-			"gzip": &schema.Schema{
+			"gzip": {
 				Type:     schema.TypeBool,
 				Optional: true,
 				Default:  true,
 			},
-			"base64_encode": &schema.Schema{
+			"base64_encode": {
 				Type:     schema.TypeBool,
 				Optional: true,
 				Default:  true,
 			},
-			"rendered": &schema.Schema{
+			"rendered": {
 				Type:        schema.TypeString,
 				Computed:    true,
 				Description: "rendered cloudinit configuration",
@@ -84,7 +84,10 @@ func renderCloudinitConfig(d *schema.ResourceData) (string, error) {
 
 	cloudInitParts := make(cloudInitParts, len(partsValue.([]interface{})))
 	for i, v := range partsValue.([]interface{}) {
-		p := v.(map[string]interface{})
+		p, castOk := v.(map[string]interface{})
+		if !castOk {
+			return "", fmt.Errorf("Unable to parse parts in cloudinit resource declaration")
+		}
 
 		part := cloudInitPart{}
 		if p, ok := p["content_type"]; ok {

builtin/providers/template/datasource_cloudinit_config_test.go

@@ -3,6 +3,8 @@ package template
 import (
 	"testing"
 
+	"regexp"
+
 	r "github.com/hashicorp/terraform/helper/resource"
 )
 
@@ -58,7 +60,7 @@ func TestRender(t *testing.T) {
 		r.UnitTest(t, r.TestCase{
 			Providers: testProviders,
 			Steps: []r.TestStep{
-				r.TestStep{
+				{
 					Config: tt.ResourceBlock,
 					Check: r.ComposeTestCheckFunc(
 						r.TestCheckResourceAttr("data.template_cloudinit_config.foo", "rendered", tt.Expected),
@@ -69,12 +71,23 @@ func TestRender(t *testing.T) {
 	}
 }
 
-var testCloudInitConfig_basic = `
-data "template_cloudinit_config" "config" {
-  part {
-    content_type = "text/x-shellscript"
-    content      = "baz"
-  }
-}`
+// From GH-13572, Correctly handle panic on a misconfigured cloudinit part
+func TestRender_handlePanic(t *testing.T) {
+	r.UnitTest(t, r.TestCase{
+		Providers: testProviders,
+		Steps: []r.TestStep{
+			{
+				Config:      testCloudInitConfig_misconfiguredParts,
+				ExpectError: regexp.MustCompile("Unable to parse parts in cloudinit resource declaration"),
+			},
+		},
+	})
+}
 
-var testCloudInitConfig_basic_expected = `Content-Type: multipart/mixed; boundary=\"MIMEBOUNDARY\"\nMIME-Version: 1.0\r\n--MIMEBOUNDARY\r\nContent-Transfer-Encoding: 7bit\r\nContent-Type: text/x-shellscript\r\nMime-Version: 1.0\r\n\r\nbaz\r\n--MIMEBOUNDARY--\r\n`
+var testCloudInitConfig_misconfiguredParts = `
+data "template_cloudinit_config" "foo" {
+  part {
+    content = ""
+  }
+}
+`