provider/aws: Set cloudwatch_metric_alarm treamt_missing_data to missing

This follows what the AWS API does. We had to add a state migration for
this to make sure that the user doesn't see any unexpected activity on
their Terraform plans

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAWSCloudWatchMetricAlarmMigrateState'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/05 14:51:32 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAWSCloudWatchMetricAlarmMigrateState -timeout 120m
=== RUN   TestAWSCloudWatchMetricAlarmMigrateState
2017/04/05 14:52:13 [INFO] Found AWS CloudWatch Metric Alarm State v0; migrating to v1
2017/04/05 14:52:13 [DEBUG] Attributes before migration: map[string]string{}
2017/04/05 14:52:13 [DEBUG] Attributes after migration: map[string]string{"treat_missing_data":"missing"}
--- PASS: TestAWSCloudWatchMetricAlarmMigrateState (0.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	0.018s
```

builtin/providers/aws/resource_aws_cloudwatch_metric_alarm.go

@@ -13,10 +13,13 @@ import (
 
 func resourceAwsCloudWatchMetricAlarm() *schema.Resource {
 	return &schema.Resource{
-		Create: resourceAwsCloudWatchMetricAlarmCreate,
-		Read:   resourceAwsCloudWatchMetricAlarmRead,
-		Update: resourceAwsCloudWatchMetricAlarmUpdate,
-		Delete: resourceAwsCloudWatchMetricAlarmDelete,
+		Create:        resourceAwsCloudWatchMetricAlarmCreate,
+		Read:          resourceAwsCloudWatchMetricAlarmRead,
+		Update:        resourceAwsCloudWatchMetricAlarmUpdate,
+		Delete:        resourceAwsCloudWatchMetricAlarmDelete,
+		SchemaVersion: 1,
+		MigrateState:  resourceAwsCloudWatchMetricAlarmMigrateState,
+
 		Importer: &schema.ResourceImporter{
 			State: schema.ImportStatePassthrough,
 		},
@@ -99,7 +102,7 @@ func resourceAwsCloudWatchMetricAlarm() *schema.Resource {
 			"treat_missing_data": {
 				Type:         schema.TypeString,
 				Optional:     true,
-				Computed:     true,
+				Default:      "missing",
 				ValidateFunc: validation.StringInSlice([]string{"breaching", "notBreaching", "ignore", "missing"}, true),
 			},
 		},
@@ -222,6 +225,7 @@ func getAwsCloudWatchPutMetricAlarmInput(d *schema.ResourceData) cloudwatch.PutM
 		Namespace:          aws.String(d.Get("namespace").(string)),
 		Period:             aws.Int64(int64(d.Get("period").(int))),
 		Threshold:          aws.Float64(d.Get("threshold").(float64)),
+		TreatMissingData:   aws.String(d.Get("treat_missing_data").(string)),
 	}
 
 	if v := d.Get("actions_enabled"); v != nil {
@@ -244,10 +248,6 @@ func getAwsCloudWatchPutMetricAlarmInput(d *schema.ResourceData) cloudwatch.PutM
 		params.ExtendedStatistic = aws.String(v.(string))
 	}
 
-	if v, ok := d.GetOk("treat_missing_data"); ok {
-		params.TreatMissingData = aws.String(v.(string))
-	}
-
 	var alarmActions []*string
 	if v := d.Get("alarm_actions"); v != nil {
 		for _, v := range v.(*schema.Set).List() {

builtin/providers/aws/resource_aws_cloudwatch_metric_alarm_migrate.go

@@ -0,0 +1,33 @@
+package aws
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/hashicorp/terraform/terraform"
+)
+
+func resourceAwsCloudWatchMetricAlarmMigrateState(
+	v int, is *terraform.InstanceState, meta interface{}) (*terraform.InstanceState, error) {
+	switch v {
+	case 0:
+		log.Println("[INFO] Found AWS CloudWatch Metric Alarm State v0; migrating to v1")
+		return migrateCloudWatchMetricAlarmStateV0toV1(is)
+	default:
+		return is, fmt.Errorf("Unexpected schema version: %d", v)
+	}
+}
+
+func migrateCloudWatchMetricAlarmStateV0toV1(is *terraform.InstanceState) (*terraform.InstanceState, error) {
+	if is.Empty() {
+		log.Println("[DEBUG] Empty InstanceState; nothing to migrate.")
+		return is, nil
+	}
+
+	log.Printf("[DEBUG] Attributes before migration: %#v", is.Attributes)
+
+	is.Attributes["treat_missing_data"] = "missing"
+
+	log.Printf("[DEBUG] Attributes after migration: %#v", is.Attributes)
+	return is, nil
+}

builtin/providers/aws/resource_aws_cloudwatch_metric_alarm_migrate_test.go

@@ -0,0 +1,41 @@
+package aws
+
+import (
+	"testing"
+
+	"github.com/hashicorp/terraform/terraform"
+)
+
+func TestAWSCloudWatchMetricAlarmMigrateState(t *testing.T) {
+	cases := map[string]struct {
+		StateVersion int
+		ID           string
+		Attributes   map[string]string
+		Expected     string
+		Meta         interface{}
+	}{
+		"v0_1": {
+			StateVersion: 0,
+			ID:           "some_id",
+			Attributes:   map[string]string{},
+			Expected:     "missing",
+		},
+	}
+
+	for tn, tc := range cases {
+		is := &terraform.InstanceState{
+			ID:         tc.ID,
+			Attributes: tc.Attributes,
+		}
+		is, err := resourceAwsCloudWatchMetricAlarmMigrateState(
+			tc.StateVersion, is, tc.Meta)
+
+		if err != nil {
+			t.Fatalf("bad: %s, err: %#v", tn, err)
+		}
+
+		if is.Attributes["treat_missing_data"] != tc.Expected {
+			t.Fatalf("bad Cloudwatch Metric Alarm Migrate: %s\n\n expected: %s", is.Attributes["treat_missing_data"], tc.Expected)
+		}
+	}
+}

website/source/docs/providers/aws/r/cloudwatch_metric_alarm.html.markdown

@@ -84,7 +84,7 @@ The following arguments are supported:
 * `ok_actions` - (Optional) The list of actions to execute when this alarm transitions into an OK state from any other state. Each action is specified as an Amazon Resource Number (ARN).
 * `unit` - (Optional) The unit for the alarm's associated metric.
 * `extended_statistic` - (Optional) The percentile statistic for the metric associated with the alarm. Specify a value between p0.0 and p100.
-* `treat_missing_data` - (Optional) Sets how this alarm is to handle missing data points. The following values are supported: `missing`, `ignore`, `breaching` and `notBreaching`.
+* `treat_missing_data` - (Optional) Sets how this alarm is to handle missing data points. The following values are supported: `missing`, `ignore`, `breaching` and `notBreaching`. Defaults to `missing`.
 
 ## Attributes Reference