data-source/aws_api_gateway_rest_api: Prevent error with VPC Endpoint…

… configured APIs

Reference: #12350
Reference: #8223

Previously:

```
    TestAccDataSourceAwsApiGatewayRestApi_EndpointConfiguration_VpcEndpointIds: testing.go:669: Step 0 error: errors during apply:

        Error: error setting endpoint_configuration: endpoint_configuration.0.vpc_endpoint_ids: '': source data must be an array or slice, got struct

          on /var/folders/w8/05f3x02n27x72g0mc2jy6_180000gp/T/tf-test660234204/main.tf line 62:
          (source code not available)
```

Output from acceptance testing:

```
--- PASS: TestAccDataSourceAwsApiGatewayRestApi_basic (18.71s)
--- PASS: TestAccDataSourceAwsApiGatewayRestApi_EndpointConfiguration_VpcEndpointIds (151.75s)

--- PASS: TestAccAWSAPIGatewayRestApi_tags (66.14s)
--- PASS: TestAccAWSAPIGatewayRestApi_EndpointConfiguration (89.86s)
--- PASS: TestAccAWSAPIGatewayRestApi_disappears (118.54s)
--- PASS: TestAccAWSAPIGatewayRestApi_api_key_source (187.17s)
--- PASS: TestAccAWSAPIGatewayRestApi_openapi (239.32s)
--- PASS: TestAccAWSAPIGatewayRestApi_EndpointConfiguration_VPCEndpoint (370.75s)
--- PASS: TestAccAWSAPIGatewayRestApi_policy (459.60s)
--- PASS: TestAccAWSAPIGatewayRestApi_basic (505.99s)
--- PASS: TestAccAWSAPIGatewayRestApi_EndpointConfiguration_Private (690.91s)
```

aws/data_source_aws_api_gateway_rest_api.go

@@ -59,7 +59,7 @@ func dataSourceAwsApiGatewayRestApi() *schema.Resource {
 							Elem:     &schema.Schema{Type: schema.TypeString},
 						},
 						"vpc_endpoint_ids": {
-							Type:     schema.TypeList,
+							Type:     schema.TypeSet,
 							Computed: true,
 							Elem:     &schema.Schema{Type: schema.TypeString},
 						},

aws/data_source_aws_api_gateway_rest_api_test.go

@@ -1,14 +1,13 @@
 package aws
 
 import (
-	"fmt"
 	"testing"
 
 	"github.com/hashicorp/terraform-plugin-sdk/helper/acctest"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/resource"
 )
 
-func TestAccDataSourceAwsApiGatewayRestApi(t *testing.T) {
+func TestAccDataSourceAwsApiGatewayRestApi_basic(t *testing.T) {
 	rName := acctest.RandString(8)
 	dataSourceName := "data.aws_api_gateway_rest_api.test"
 	resourceName := "aws_api_gateway_rest_api.test"
@@ -17,7 +16,10 @@ func TestAccDataSourceAwsApiGatewayRestApi(t *testing.T) {
 		Providers: testAccProviders,
 		Steps: []resource.TestStep{
 			{
-				Config: testAccDataSourceAwsApiGatewayRestApiConfig(rName),
+				Config: composeConfig(
+					testAccAWSAPIGatewayRestAPIConfig_Name(rName),
+					testAccDataSourceAwsApiGatewayRestApiConfigName(),
+				),
 				Check: resource.ComposeTestCheckFunc(
 					resource.TestCheckResourceAttrPair(dataSourceName, "name", resourceName, "name"),
 					resource.TestCheckResourceAttrPair(dataSourceName, "arn", resourceName, "arn"),
@@ -36,14 +38,41 @@ func TestAccDataSourceAwsApiGatewayRestApi(t *testing.T) {
 	})
 }
 
-func testAccDataSourceAwsApiGatewayRestApiConfig(r string) string {
-	return fmt.Sprintf(`
-resource "aws_api_gateway_rest_api" "test" {
-  name = "%[1]s"
+func TestAccDataSourceAwsApiGatewayRestApi_EndpointConfiguration_VpcEndpointIds(t *testing.T) {
+	rName := acctest.RandString(8)
+	dataSourceName := "data.aws_api_gateway_rest_api.test"
+	resourceName := "aws_api_gateway_rest_api.test"
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:  func() { testAccPreCheck(t) },
+		Providers: testAccProviders,
+		Steps: []resource.TestStep{
+			{
+				Config: composeConfig(
+					testAccAWSAPIGatewayRestAPIConfig_VPCEndpointConfiguration(rName),
+					testAccDataSourceAwsApiGatewayRestApiConfigName(),
+				),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttrPair(dataSourceName, "name", resourceName, "name"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "arn", resourceName, "arn"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "root_resource_id", resourceName, "root_resource_id"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "tags", resourceName, "tags"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "description", resourceName, "description"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "policy", resourceName, "policy"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "api_key_source", resourceName, "api_key_source"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "minimum_compression_size", resourceName, "minimum_compression_size"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "binary_media_types", resourceName, "binary_media_types"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "endpoint_configuration", resourceName, "endpoint_configuration"),
+					resource.TestCheckResourceAttrPair(dataSourceName, "execution_arn", resourceName, "execution_arn"),
+				),
+			},
+		},
+	})
 }
 
+func testAccDataSourceAwsApiGatewayRestApiConfigName() string {
+	return `
 data "aws_api_gateway_rest_api" "test" {
-  name = "${aws_api_gateway_rest_api.test.name}"
+  name = aws_api_gateway_rest_api.test.name
 }
-`, r)
+`
 }

aws/resource_aws_api_gateway_rest_api.go

@@ -483,7 +483,7 @@ func flattenApiGatewayEndpointConfiguration(endpointConfiguration *apigateway.En
 	}
 
 	if len(endpointConfiguration.VpcEndpointIds) > 0 {
-		m["vpc_endpoint_ids"] = flattenStringSet(endpointConfiguration.VpcEndpointIds)
+		m["vpc_endpoint_ids"] = aws.StringValueSlice(endpointConfiguration.VpcEndpointIds)
 	}
 
 	return []interface{}{m}