provider/aws: Allow `port` on `aws_db_instance` to be updated (#7441)

Fixes #2439

Port used to ForceNew, it has now been changed to allow it to be
updated. 2 changes were needed:

1. Setting the port back to state
2. Adding the wait for state function to the Update func

```
make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSDBInstance_portUpdate'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /vendor/)
TF_ACC=1 go test ./builtin/providers/aws -v
-run=TestAccAWSDBInstance_portUpdate -timeout 120m
=== RUN   TestAccAWSDBInstance_portUpdate
--- PASS: TestAccAWSDBInstance_portUpdate (699.84s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    699.861s
```

builtin/providers/aws/resource_aws_db_instance.go

@@ -160,7 +160,6 @@ func resourceAwsDbInstance() *schema.Resource {
 				Type:     schema.TypeInt,
 				Optional: true,
 				Computed: true,
-				ForceNew: true,
 			},
 
 			"publicly_accessible": &schema.Schema{
@@ -654,6 +653,7 @@ func resourceAwsDbInstanceRead(d *schema.ResourceData, meta interface{}) error {
 	d.Set("publicly_accessible", v.PubliclyAccessible)
 	d.Set("multi_az", v.MultiAZ)
 	d.Set("kms_key_id", v.KmsKeyId)
+	d.Set("port", v.DbInstancePort)
 	if v.DBSubnetGroup != nil {
 		d.Set("db_subnet_group_name", v.DBSubnetGroup.DBSubnetGroupName)
 	}
@@ -923,6 +923,12 @@ func resourceAwsDbInstanceUpdate(d *schema.ResourceData, meta interface{}) error
 		requestUpdate = true
 	}
 
+	if d.HasChange("port") {
+		d.SetPartial("port")
+		req.DBPortNumber = aws.Int64(int64(d.Get("port").(int)))
+		requestUpdate = true
+	}
+
 	log.Printf("[DEBUG] Send DB Instance Modification request: %t", requestUpdate)
 	if requestUpdate {
 		log.Printf("[DEBUG] DB Instance Modification request: %s", req)
@@ -930,6 +936,24 @@ func resourceAwsDbInstanceUpdate(d *schema.ResourceData, meta interface{}) error
 		if err != nil {
 			return fmt.Errorf("Error modifying DB Instance %s: %s", d.Id(), err)
 		}
+
+		log.Println("[INFO] Waiting for DB Instance to be available")
+
+		stateConf := &resource.StateChangeConf{
+			Pending: []string{"creating", "backing-up", "modifying", "resetting-master-credentials",
+				"maintenance", "renaming", "rebooting", "upgrading"},
+			Target:     []string{"available"},
+			Refresh:    resourceAwsDbInstanceStateRefreshFunc(d, meta),
+			Timeout:    80 * time.Minute,
+			MinTimeout: 10 * time.Second,
+			Delay:      30 * time.Second, // Wait 30 secs before starting
+		}
+
+		// Wait, catching any errors
+		_, dbStateErr := stateConf.WaitForState()
+		if dbStateErr != nil {
+			return dbStateErr
+		}
 	}
 
 	// separate request to promote a database

builtin/providers/aws/resource_aws_db_instance_test.go

@@ -213,6 +213,37 @@ func TestAccAWSDBInstance_iops_update(t *testing.T) {
 	})
 }
 
+func TestAccAWSDBInstance_portUpdate(t *testing.T) {
+	var v rds.DBInstance
+
+	rName := acctest.RandString(5)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSDBInstanceDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccSnapshotInstanceConfig_mysqlPort(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBInstanceExists("aws_db_instance.bar", &v),
+					resource.TestCheckResourceAttr(
+						"aws_db_instance.bar", "port", "3306"),
+				),
+			},
+
+			resource.TestStep{
+				Config: testAccSnapshotInstanceConfig_updateMysqlPort(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSDBInstanceExists("aws_db_instance.bar", &v),
+					resource.TestCheckResourceAttr(
+						"aws_db_instance.bar", "port", "3305"),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckAWSDBInstanceDestroy(s *terraform.State) error {
 	conn := testAccProvider.Meta().(*AWSClient).rdsconn
 
@@ -701,3 +732,39 @@ resource "aws_db_instance" "bar" {
   iops              = %d
 }`, rName, iops)
 }
+
+func testAccSnapshotInstanceConfig_mysqlPort(rName string) string {
+	return fmt.Sprintf(`
+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"
+  port = 3306
+  allocated_storage = 10
+
+  apply_immediately = true
+}`, rName)
+}
+
+func testAccSnapshotInstanceConfig_updateMysqlPort(rName string) string {
+	return fmt.Sprintf(`
+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"
+  port = 3305
+  allocated_storage = 10
+
+  apply_immediately = true
+}`, rName)
+}