fixing an edge case for 'START' container dependency

[singholt]

Summary

ECS supports specifying container ordering dependencies in task definitions. documentation

This PR fixes an edge case with the START container ordering dependency. Consider the following customer use case:

• An ECS task definition specifies 2 containers - A and B.
• ctr A is essential and it depends on ctr B to "START".
• ctr B is non-essential. It can be a sidecar that shouldn't impact the task's uptime, for example, a logging sidecar like firelens or a miscellaneous cleanup job runner.

The edge case we're trying to solve here becomes apparent when ctr B starts and stops even before the ctr A has reached >= PULLED state. By definition of the START dependency condition, ctr A should proceed to run as long as ctr B has started (it shouldn't matter when/how ctr B stops. Other dependency checks like COMPLETE, SUCCESS, HEALTHY let customers control the stop behaviors).

Implementation details

Updated the start condition checks with the following: return true (i.e. dependency is resolved) when the dependency container's known status is stopped.

Testing

New tests cover the changes: yes

1. Added new unit test cases when dependency container known status is stopped.
2. Added a new integration test for testing the START condition. We seem to have integration tests for all other conditions except start. I ran this integration test on an AL2 instance about a 100 times to ensure its not flaky.
3. Manually tested that this change fixes the corner case, while not breaking existing use-cases. This case is reproducible by using 2 different container images; ctr A depends on START of B; ctr image for A is considerably larger than that of B; B exits even before A's image was finished pulling. The task stays in pending forever. If the images are cached then it runs fine, exhibiting two different inconsistent task behaviors.
4. Also verified that a container's known status is STOPPED only when it has transitioned from RUNNING -> STOPPED. This implies that since it reached RUNNING, the START dependency condition has been fulfilled.

Description for the changelog

bugfix: fixing an edge case for 'START' container dependency

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.