Detecting drift on aws_lb_target_group_attachment

aws/resource_aws_lb_target_group_attachment.go

@@ -126,6 +126,7 @@ func resourceAwsLbAttachmentRead(d *schema.ResourceData, meta interface{}) error
 		TargetGroupArn: aws.String(d.Get("target_group_arn").(string)),
 		Targets:        []*elbv2.TargetDescription{target},
 	})
+
 	if err != nil {
 		if isAWSErr(err, elbv2.ErrCodeTargetGroupNotFoundException, "") {
 			log.Printf("[WARN] Target group does not exist, removing target attachment %s", d.Id())
@@ -140,6 +141,29 @@ func resourceAwsLbAttachmentRead(d *schema.ResourceData, meta interface{}) error
 		return fmt.Errorf("Error reading Target Health: %s", err)
 	}
 
+	for _, targetDesc := range resp.TargetHealthDescriptions {
+		if targetDesc == nil || targetDesc.Target == nil {
+			continue
+		}
+
+		if *targetDesc.Target.Id == d.Get("target_id").(string) {
+			// These will catch targets being removed by hand (draining as we plan) or that have been removed for a while
+			// without trying to re-create ones that are just not in use. For example, a target can be `unused` if the
+			// target group isnt assigned to anything, a scenario where we don't want to continuously recreate the resource.
+			if targetDesc.TargetHealth == nil {
+				continue
+			}
+
+			reason := aws.StringValue(targetDesc.TargetHealth.Reason)
+
+			if reason == elbv2.TargetHealthReasonEnumTargetNotRegistered || reason == elbv2.TargetHealthReasonEnumTargetDeregistrationInProgress {
+				log.Printf("[WARN] Target Attachment does not exist, recreating attachment")
+				d.SetId("")
+				return nil
+			}
+		}
+	}
+
 	if len(resp.TargetHealthDescriptions) != 1 {
 		log.Printf("[WARN] Target does not exist, removing target attachment %s", d.Id())
 		d.SetId("")

aws/resource_aws_lb_target_group_attachment_test.go

@@ -32,6 +32,26 @@ func TestAccAWSLBTargetGroupAttachment_basic(t *testing.T) {
 	})
 }
 
+func TestAccAWSLBTargetGroupAttachment_missingAttachmentDrift(t *testing.T) {
+	targetGroupName := fmt.Sprintf("test-target-group-%s", acctest.RandStringFromCharSet(10, acctest.CharSetAlphaNum))
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:      func() { testAccPreCheck(t) },
+		IDRefreshName: "aws_lb_target_group.test",
+		Providers:     testAccProviders,
+		CheckDestroy:  testAccCheckAWSLBTargetGroupAttachmentDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSLBTargetGroupAttachmentConfig_basic(targetGroupName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSLBTargetGroupAttachmentExists("aws_lb_target_group_attachment.test"),
+					deregisterTarget("aws_lb_target_group_attachment.test"),
+				),
+				ExpectNonEmptyPlan: true,
+			},
+		},
+	})
+}
+
 func TestAccAWSLBTargetGroupAttachmentBackwardsCompatibility(t *testing.T) {
 	targetGroupName := fmt.Sprintf("test-target-group-%s", acctest.RandStringFromCharSet(10, acctest.CharSetAlphaNum))
 
@@ -108,6 +128,40 @@ func TestAccAWSALBTargetGroupAttachment_lambda(t *testing.T) {
 	})
 }
 
+func deregisterTarget(n string) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		rs, ok := s.RootModule().Resources[n]
+		if !ok {
+			return fmt.Errorf("Attachment not found: %s", n)
+		}
+
+		conn := testAccProvider.Meta().(*AWSClient).elbv2conn
+		targetGroupArn := rs.Primary.Attributes["target_group_arn"]
+
+		target := &elbv2.TargetDescription{
+			Id: aws.String(rs.Primary.Attributes["target_id"]),
+		}
+
+		_, hasPort := rs.Primary.Attributes["port"]
+		if hasPort {
+			port, _ := strconv.Atoi(rs.Primary.Attributes["port"])
+			target.Port = aws.Int64(int64(port))
+		}
+
+		params := &elbv2.DeregisterTargetsInput{
+			TargetGroupArn: aws.String(targetGroupArn),
+			Targets:        []*elbv2.TargetDescription{target},
+		}
+
+		_, err := conn.DeregisterTargets(params)
+		if err != nil && !isAWSErr(err, elbv2.ErrCodeTargetGroupNotFoundException, "") {
+			return fmt.Errorf("Error deregistering Targets: %s", err)
+		}
+
+		return err
+	}
+}
+
 func testAccCheckAWSLBTargetGroupAttachmentExists(n string) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		rs, ok := s.RootModule().Resources[n]