Fix parsing of ECS task ID from task ARN (#30)

AWS has undergone a change in the way they format ECS task ARNs.
Previously, task IDs looked like:

 arn:aws:ecs:region:aws_account_id:task/task-id

Now, task IDs look like:

 arn:aws:ecs:region:aws_account_id:task/cluster-name/task-id

To account for both versions, always pull the last element of the
ARN split by /.

Author: hectcastro

CHANGELOG.md

@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Add ASSIGN_PUBLIC_IP configuration option [#28](https://github.com/azavea/django-ecsmanage/pull/28)
 
+### Fixed
+- Fix parsing of ECS task ID from task ARN [#30](https://github.com/azavea/django-ecsmanage/pull/30)
+
 ## [2.0.0] - 2020-10-13
 ### Added
 - Add support for Django 3.x and Python 3.8; no actual code changes were required [#14](https://github.com/azavea/django-ecsmanage/pull/14)

ecsmanage/management/commands/ecsmanage.py

@@ -200,7 +200,11 @@ def run_task(self, config, task_def_arn, security_group_id, subnet_id, cmd):
 
         task = self.parse_response(self.ecs_client.run_task(**kwargs), "tasks", 0)
 
-        # Parse the ask ARN, since ECS doesn't return the task ID.
-        # Task ARNS look like: arn:aws:ecs:<region>:<aws_account_id>:task/<id>
-        task_id = task["taskArn"].split("/")[1]
+        # Task ARNs have at least two formats:
+        #
+        #  - Old: arn:aws:ecs:region:aws_account_id:task/task-id
+        #  - New: arn:aws:ecs:region:aws_account_id:task/cluster-name/task-id
+        #
+        # See: https://docs.aws.amazon.com/AmazonECS/latest/userguide/ecs-account-settings.html#ecs-resource-ids  # NOQA
+        task_id = task["taskArn"].split("/")[-1]
         return task_id