state/remote: Officially Support local backend

This is a rework of pull request #6213 submitted by @joshuaspence,
adjusted to work with the remote state data source. We also add
a deprecation warning for people using the unsupported API, and retain
the ability to refer to "_local" as well as "local" for users in a mixed
version environment.

data_source_state.go

@@ -16,6 +16,16 @@ func dataSourceRemoteState() *schema.Resource {
 			"backend": {
 				Type:     schema.TypeString,
 				Required: true,
+				ValidateFunc: func(v interface{}, k string) (ws []string, errors []error) {
+					if vStr, ok := v.(string); ok && vStr == "_local" {
+						ws = append(ws, "Use of the %q backend is now officially "+
+							"supported as %q. Please update your configuration to ensure "+
+							"compatibility with future versions of Terraform.",
+							"_local", "local")
+					}
+
+					return
+				},
 			},
 
 			"config": {
@@ -38,6 +48,12 @@ func dataSourceRemoteStateRead(d *schema.ResourceData, meta interface{}) error {
 		config[k] = v.(string)
 	}
 
+	// Don't break people using the old _local syntax - but note warning above
+	if backend == "_local" {
+		log.Println(`[INFO] Switching old (unsupported) backend "_local" to "local"`)
+		backend = "local"
+	}
+
 	// Create the client to access our remote state
 	log.Printf("[DEBUG] Initializing remote state client: %s", backend)
 	client, err := remote.NewClient(backend, config)

data_source_state_test.go

@@ -32,7 +32,7 @@ func TestState_complexOutputs(t *testing.T) {
 			{
 				Config: testAccState_complexOutputs,
 				Check: resource.ComposeTestCheckFunc(
-					testAccCheckStateValue("terraform_remote_state.foo", "backend", "_local"),
+					testAccCheckStateValue("terraform_remote_state.foo", "backend", "local"),
 					testAccCheckStateValue("terraform_remote_state.foo", "config.path", "./test-fixtures/complex_outputs.tfstate"),
 					testAccCheckStateValue("terraform_remote_state.foo", "computed_set.#", "2"),
 					testAccCheckStateValue("terraform_remote_state.foo", `map.%`, "2"),
@@ -65,7 +65,7 @@ func testAccCheckStateValue(id, name, value string) resource.TestCheckFunc {
 
 const testAccState_basic = `
 data "terraform_remote_state" "foo" {
-	backend = "_local"
+	backend = "local"
 
 	config {
 		path = "./test-fixtures/basic.tfstate"
@@ -74,7 +74,7 @@ data "terraform_remote_state" "foo" {
 
 const testAccState_complexOutputs = `
 resource "terraform_remote_state" "foo" {
-	backend = "_local"
+	backend = "local"
 
 	config {
 		path = "./test-fixtures/complex_outputs.tfstate"