provider/aws: Fix reattachment of VPC to VPN gateway.

When VPC is detached from VPN gateway, its VpcAttachment stays in place
just with state changed to "detached". Since terraform was not checking
attachment state, it used to think VPC gateway was still attached.

builtin/providers/aws/resource_aws_vpn_gateway.go

@@ -83,7 +83,7 @@ func resourceAwsVpnGatewayRead(d *schema.ResourceData, meta interface{}) error {
 		return nil
 	}
 
-	if len(vpnGateway.VpcAttachments) == 0 {
+	if len(vpnGateway.VpcAttachments) == 0 || *vpnGateway.VpcAttachments[0].State == "detached" {
 		// Gateway exists but not attached to the VPC
 		d.Set("vpc_id", "")
 	} else {

builtin/providers/aws/resource_aws_vpn_gateway_test.go

@@ -57,6 +57,60 @@ func TestAccAWSVpnGateway_basic(t *testing.T) {
 	})
 }
 
+func TestAccAWSVpnGateway_reattach(t *testing.T) {
+	var v ec2.VpnGateway
+
+	genTestStateFunc := func(expectedState string) func(*terraform.State) error {
+		return func(*terraform.State) error {
+			if len(v.VpcAttachments) == 0 {
+				if expectedState != "detached" {
+					return fmt.Errorf("VPN gateway has no VPC attachments")
+				}
+			} else if len(v.VpcAttachments) == 1 {
+				if *v.VpcAttachments[0].State != expectedState {
+					return fmt.Errorf("Expected VPC gateway VPC attachment to be in '%s' state, but was not: %s", expectedState, v)
+				}
+			} else {
+				return fmt.Errorf("VPN gateway has unexpected number of VPC attachments(more than 1): %s", v)
+			}
+			return nil
+		}
+	}
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:      func() { testAccPreCheck(t) },
+		IDRefreshName: "aws_vpn_gateway.foo",
+		Providers:     testAccProviders,
+		CheckDestroy:  testAccCheckVpnGatewayDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccVpnGatewayConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckVpnGatewayExists(
+						"aws_vpn_gateway.foo", &v),
+					genTestStateFunc("attached"),
+				),
+			},
+			resource.TestStep{
+				Config: testAccVpnGatewayConfigDetach,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckVpnGatewayExists(
+						"aws_vpn_gateway.foo", &v),
+					genTestStateFunc("detached"),
+				),
+			},
+			resource.TestStep{
+				Config: testAccVpnGatewayConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckVpnGatewayExists(
+						"aws_vpn_gateway.foo", &v),
+					genTestStateFunc("attached"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSVpnGateway_delete(t *testing.T) {
 	var vpnGateway ec2.VpnGateway
 
@@ -216,6 +270,16 @@ resource "aws_vpn_gateway" "foo" {
 }
 `
 
+const testAccVpnGatewayConfigDetach = `
+resource "aws_vpc" "foo" {
+	cidr_block = "10.1.0.0/16"
+}
+
+resource "aws_vpn_gateway" "foo" {
+	vpc_id = ""
+}
+`
+
 const testAccCheckVpnGatewayConfigTags = `
 resource "aws_vpc" "foo" {
 	cidr_block = "10.1.0.0/16"