Succeed creating aws_volume_attachment if identical attachment exists (#11060)

If an `aws_volume_attachment` is identical to one that already exists in
the API, don't attempt to re-create it (which fails), simply act as
though the creation command had already been run and continue.

This allows Terraform to cleanly recover from a situation where a volume
attachment action hangs indefinitely, possibly due to a bad instance
state, requiring manual intervention such as an instance reboot. In such
a situation, Terraform believes the attachment has failed, when in fact
it succeeded after the timeout had expired. On the subsequent retry run,
attempting to re-create the attachment will fail outright, due to the
AttachVolume API call being non-idempotent. This patch implements the
idempotency client-side by matching the (name, vID, iID) tuple.

Note that volume attachments are not assigned an ID by the API.

builtin/providers/aws/resource_aws_volume_attachment.go

@@ -59,6 +59,25 @@ func resourceAwsVolumeAttachmentCreate(d *schema.ResourceData, meta interface{})
 	iID := d.Get("instance_id").(string)
 	vID := d.Get("volume_id").(string)
 
+	// Find out if the volume is already attached to the instance, in which case
+	// we have nothing to do
+	request := &ec2.DescribeVolumesInput{
+		VolumeIds: []*string{aws.String(vID)},
+		Filters: []*ec2.Filter{
+			&ec2.Filter{
+				Name:   aws.String("attachment.instance-id"),
+				Values: []*string{aws.String(iID)},
+			},
+			&ec2.Filter{
+				Name:   aws.String("attachment.device"),
+				Values: []*string{aws.String(name)},
+			},
+		},
+	}
+
+	vols, err := conn.DescribeVolumes(request)
+	if (err != nil) || (len(vols.Volumes) == 0) {
+		// not attached
 		opts := &ec2.AttachVolumeInput{
 			Device:     aws.String(name),
 			InstanceId: aws.String(iID),
@@ -74,6 +93,7 @@ func resourceAwsVolumeAttachmentCreate(d *schema.ResourceData, meta interface{})
 			}
 			return err
 		}
+	}
 
 	stateConf := &resource.StateChangeConf{
 		Pending:    []string{"attaching"},