provider/aws: Support updating of aws_db_instance db_subnet_group_name (#10818)

Fixes: #10811

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBInstance_subnetGroup'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/12/18 12:52:21 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSDBInstance_subnetGroup -timeout 120m
=== RUN   TestAccAWSDBInstance_subnetGroup
--- PASS: TestAccAWSDBInstance_subnetGroup (992.05s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	992.073s
```

builtin/providers/aws/resource_aws_db_instance.go

@@ -216,7 +216,6 @@ func resourceAwsDbInstance() *schema.Resource {
 			"db_subnet_group_name": {
 				Type:     schema.TypeString,
 				Optional: true,
-				ForceNew: true,
 				Computed: true,
 			},
 
@@ -936,6 +935,11 @@ func resourceAwsDbInstanceUpdate(d *schema.ResourceData, meta interface{}) error
 		req.DBPortNumber = aws.Int64(int64(d.Get("port").(int)))
 		requestUpdate = true
 	}
+	if d.HasChange("db_subnet_group_name") {
+		d.SetPartial("db_subnet_group_name")
+		req.DBSubnetGroupName = aws.String(d.Get("db_subnet_group_name").(string))
+		requestUpdate = true
+	}
 
 	log.Printf("[DEBUG] Send DB Instance Modification request: %t", requestUpdate)
 	if requestUpdate {
@@ -949,7 +953,7 @@ func resourceAwsDbInstanceUpdate(d *schema.ResourceData, meta interface{}) error
 
 		stateConf := &resource.StateChangeConf{
 			Pending: []string{"creating", "backing-up", "modifying", "resetting-master-credentials",
-				"maintenance", "renaming", "rebooting", "upgrading", "configuring-enhanced-monitoring"},
+				"maintenance", "renaming", "rebooting", "upgrading", "configuring-enhanced-monitoring", "moving-to-vpc"},
 			Target:     []string{"available"},
 			Refresh:    resourceAwsDbInstanceStateRefreshFunc(d, meta),
 			Timeout:    80 * time.Minute,

builtin/providers/aws/resource_aws_db_instance_test.go

@@ -78,6 +78,35 @@ func TestAccAWSDBInstance_kmsKey(t *testing.T) {
 	})
 }
 
+func TestAccAWSDBInstance_subnetGroup(t *testing.T) {
+	var v rds.DBInstance
+	rName := acctest.RandString(10)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSDBInstanceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSDBInstanceConfigWithSubnetGroup(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBInstanceExists("aws_db_instance.bar", &v),
+					resource.TestCheckResourceAttr(
+						"aws_db_instance.bar", "db_subnet_group_name", "foo"),
+				),
+			},
+			{
+				Config: testAccAWSDBInstanceConfigWithSubnetGroupUpdated(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBInstanceExists("aws_db_instance.bar", &v),
+					resource.TestCheckResourceAttr(
+						"aws_db_instance.bar", "db_subnet_group_name", "bar"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSDBInstance_optionGroup(t *testing.T) {
 	var v rds.DBInstance
 
@@ -766,3 +795,131 @@ resource "aws_db_instance" "bar" {
   apply_immediately = true
 }`, rName)
 }
+
+func testAccAWSDBInstanceConfigWithSubnetGroup(rName string) string {
+	return fmt.Sprintf(`
+resource "aws_vpc" "foo" {
+	cidr_block = "10.1.0.0/16"
+}
+
+resource "aws_subnet" "foo" {
+	cidr_block = "10.1.1.0/24"
+	availability_zone = "us-west-2a"
+	vpc_id = "${aws_vpc.foo.id}"
+	tags {
+		Name = "tf-dbsubnet-test-1"
+	}
+}
+
+resource "aws_subnet" "bar" {
+	cidr_block = "10.1.2.0/24"
+	availability_zone = "us-west-2b"
+	vpc_id = "${aws_vpc.foo.id}"
+	tags {
+		Name = "tf-dbsubnet-test-2"
+	}
+}
+
+resource "aws_db_subnet_group" "foo" {
+	name = "foo"
+	subnet_ids = ["${aws_subnet.foo.id}", "${aws_subnet.bar.id}"]
+	tags {
+		Name = "tf-dbsubnet-group-test"
+	}
+}
+
+resource "aws_db_instance" "bar" {
+  identifier           = "mydb-rds-%s"
+  engine               = "mysql"
+  engine_version       = "5.6.23"
+  instance_class       = "db.t2.micro"
+  name                 = "mydb"
+  username             = "foo"
+  password             = "barbarbar"
+  parameter_group_name = "default.mysql5.6"
+  db_subnet_group_name = "${aws_db_subnet_group.foo.name}"
+  port = 3305
+  allocated_storage = 10
+
+  apply_immediately = true
+}`, rName)
+}
+
+func testAccAWSDBInstanceConfigWithSubnetGroupUpdated(rName string) string {
+	return fmt.Sprintf(`
+resource "aws_vpc" "foo" {
+	cidr_block = "10.1.0.0/16"
+}
+
+resource "aws_vpc" "bar" {
+	cidr_block = "10.10.0.0/16"
+}
+
+resource "aws_subnet" "foo" {
+	cidr_block = "10.1.1.0/24"
+	availability_zone = "us-west-2a"
+	vpc_id = "${aws_vpc.foo.id}"
+	tags {
+		Name = "tf-dbsubnet-test-1"
+	}
+}
+
+resource "aws_subnet" "bar" {
+	cidr_block = "10.1.2.0/24"
+	availability_zone = "us-west-2b"
+	vpc_id = "${aws_vpc.foo.id}"
+	tags {
+		Name = "tf-dbsubnet-test-2"
+	}
+}
+
+resource "aws_subnet" "test" {
+	cidr_block = "10.10.3.0/24"
+	availability_zone = "us-west-2c"
+	vpc_id = "${aws_vpc.bar.id}"
+	tags {
+		Name = "tf-dbsubnet-test-3"
+	}
+}
+
+resource "aws_subnet" "another_test" {
+	cidr_block = "10.10.4.0/24"
+	availability_zone = "us-west-2a"
+	vpc_id = "${aws_vpc.bar.id}"
+	tags {
+		Name = "tf-dbsubnet-test-4"
+	}
+}
+
+resource "aws_db_subnet_group" "foo" {
+	name = "foo"
+	subnet_ids = ["${aws_subnet.foo.id}", "${aws_subnet.bar.id}"]
+	tags {
+		Name = "tf-dbsubnet-group-test"
+	}
+}
+
+resource "aws_db_subnet_group" "bar" {
+	name = "bar"
+	subnet_ids = ["${aws_subnet.test.id}", "${aws_subnet.another_test.id}"]
+	tags {
+		Name = "tf-dbsubnet-group-test-updated"
+	}
+}
+
+resource "aws_db_instance" "bar" {
+  identifier           = "mydb-rds-%s"
+  engine               = "mysql"
+  engine_version       = "5.6.23"
+  instance_class       = "db.t2.micro"
+  name                 = "mydb"
+  username             = "foo"
+  password             = "barbarbar"
+  parameter_group_name = "default.mysql5.6"
+  db_subnet_group_name = "${aws_db_subnet_group.bar.name}"
+  port = 3305
+  allocated_storage = 10
+
+  apply_immediately = true
+}`, rName)
+}