provider/aws: Fix reading dimensions on cloudwatch alarms

They're structs that need to be unrolled and d.Set was silently failing
on them before. This enhances the basic test to cover the change.

builtin/providers/aws/resource_aws_cloudwatch_metric_alarm.go

@@ -129,7 +129,9 @@ func resourceAwsCloudWatchMetricAlarmRead(d *schema.ResourceData, meta interface
 	d.Set("alarm_description", a.AlarmDescription)
 	d.Set("alarm_name", a.AlarmName)
 	d.Set("comparison_operator", a.ComparisonOperator)
-	d.Set("dimensions", a.Dimensions)
+	if err := d.Set("dimensions", flattenDimensions(a.Dimensions)); err != nil {
+		return err
+	}
 	d.Set("evaluation_periods", a.EvaluationPeriods)
 
 	if err := d.Set("insufficient_data_actions", _strArrPtrToList(a.InsufficientDataActions)); err != nil {
@@ -282,3 +284,11 @@ func _strArrPtrToList(strArrPtr []*string) []string {
 	}
 	return result
 }
+
+func flattenDimensions(dims []*cloudwatch.Dimension) map[string]interface{} {
+	flatDims := make(map[string]interface{})
+	for _, d := range dims {
+		flatDims[*d.Name] = *d.Value
+	}
+	return flatDims
+}

builtin/providers/aws/resource_aws_cloudwatch_metric_alarm_test.go

@@ -24,12 +24,32 @@ func TestAccAWSCloudWatchMetricAlarm_basic(t *testing.T) {
 					testAccCheckCloudWatchMetricAlarmExists("aws_cloudwatch_metric_alarm.foobar", &alarm),
 					resource.TestCheckResourceAttr("aws_cloudwatch_metric_alarm.foobar", "metric_name", "CPUUtilization"),
 					resource.TestCheckResourceAttr("aws_cloudwatch_metric_alarm.foobar", "statistic", "Average"),
+					testAccCheckCloudWatchMetricAlarmDimension(
+						"aws_cloudwatch_metric_alarm.foobar", "InstanceId", "i-abc123"),
 				),
 			},
 		},
 	})
 }
 
+func testAccCheckCloudWatchMetricAlarmDimension(n, k, v string) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		rs, ok := s.RootModule().Resources[n]
+		if !ok {
+			return fmt.Errorf("Not found: %s", n)
+		}
+		key := fmt.Sprintf("dimensions.%s", k)
+		val, ok := rs.Primary.Attributes[key]
+		if !ok {
+			return fmt.Errorf("Could not find dimension: %s", k)
+		}
+		if val != v {
+			return fmt.Errorf("Expected dimension %s => %s; got: %s", k, v, val)
+		}
+		return nil
+	}
+}
+
 func testAccCheckCloudWatchMetricAlarmExists(n string, alarm *cloudwatch.MetricAlarm) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		rs, ok := s.RootModule().Resources[n]
@@ -81,15 +101,18 @@ func testAccCheckAWSCloudWatchMetricAlarmDestroy(s *terraform.State) error {
 
 var testAccAWSCloudWatchMetricAlarmConfig = fmt.Sprintf(`
 resource "aws_cloudwatch_metric_alarm" "foobar" {
-    alarm_name = "terraform-test-foobar5"
-    comparison_operator = "GreaterThanOrEqualToThreshold"
-    evaluation_periods = "2"
-    metric_name = "CPUUtilization"
-    namespace = "AWS/EC2"
-    period = "120"
-    statistic = "Average"
-    threshold = "80"
-    alarm_description = "This metric monitor ec2 cpu utilization"
-    insufficient_data_actions = []
+  alarm_name                = "terraform-test-foobar5"
+  comparison_operator       = "GreaterThanOrEqualToThreshold"
+  evaluation_periods        = "2"
+  metric_name               = "CPUUtilization"
+  namespace                 = "AWS/EC2"
+  period                    = "120"
+  statistic                 = "Average"
+  threshold                 = "80"
+  alarm_description         = "This metric monitors ec2 cpu utilization"
+  insufficient_data_actions = []
+  dimensions {
+    InstanceId = "i-abc123"
+  }
 }
 `)