CloudWatch: Ensure we can tag new Alarms that are currently tag-free (#5614)

moto/cloudwatch/models.py

@@ -665,7 +665,10 @@ class CloudWatchBackend(BaseBackend):
         return self.tagger.get_tag_dict_for_resource(arn)
 
     def tag_resource(self, arn, tags):
-        if arn not in self.tagger.tags.keys():
+        # From boto3:
+        # Currently, the only CloudWatch resources that can be tagged are alarms and Contributor Insights rules.
+        all_arns = [alarm.alarm_arn for alarm in self.get_all_alarms()]
+        if arn not in all_arns:
             raise ResourceNotFoundException
 
         self.tagger.tag_resource(arn, tags)

tests/test_cloudwatch/test_cloudwatch_tags.py

@@ -102,6 +102,28 @@ def test_tag_resource():
     )
 
 
+@mock_cloudwatch
+def test_tag_resource_on_resource_without_tags():
+    cw = boto3.client("cloudwatch", region_name="eu-central-1")
+    cw.put_metric_alarm(
+        AlarmName="testalarm",
+        EvaluationPeriods=1,
+        ComparisonOperator="GreaterThanThreshold",
+        Period=60,
+        MetricName="test",
+        Namespace="test",
+    )
+    alarms = cw.describe_alarms()
+    alarm_arn = alarms["MetricAlarms"][0]["AlarmArn"]
+
+    # List 0 tags - none have been added
+    cw.list_tags_for_resource(ResourceARN=alarm_arn)["Tags"].should.equal([])
+
+    # Tag the Alarm for the first time
+    cw.tag_resource(ResourceARN=alarm_arn, Tags=[{"Key": "tk", "Value": "tv"}])
+    assert len(cw.list_tags_for_resource(ResourceARN=alarm_arn)["Tags"]) == 1
+
+
 @mock_cloudwatch
 def test_tag_resource_error_not_exists():
     # given