After pushing a new image of my container I use Terraform apply to update the task definition. This seems to work fine but in the ECS service list of tasks I can see the task as inactive and I have an event:
service blahblah was unable to place a task because no container instance met all of its requirements. The closest matching container-instance [guid here] is already using a port required by your task.
The thing is, the site is still active and working.
This is more of an ECS issue than a Terraform issue because Terraform is updating your task definition and updating the service to use the new task definition but ECS is unable to schedule new tasks on to the container instances because you're (presumably) defining a specific port that the container must run on and directly mapping it to the host or using host networking instead of bridge (or the new aws-vpc CNI plugin).
ECS has a couple of parameters to control the behaviour of an update to the service: minimum healthy percent and maximum healthy percent. By default these are set to 100% and 200% respectively meaning that ECS will attempt to deploy a new task matching the new task definition and wait for it to be considered healthy (such as passing ELB health checks) before terminating the old tasks.
In your case you have as many tasks as you have container instances in your cluster and so when it attempts to schedule a new task on to the cluster it is unable to place it because the port is already bound to by the old task. You could also find yourself in this position if you had placement constraints on your task/service.
Because the minimum healthy percent is set to 100% it is unable to schedule the removal of any of the old tasks that would then free up a placement option for a new task.
You could have more container instances in the cluster than you have instances of the task running which would allow ECS to deploy new tasks before removing old tasks from the other instances or you could change the minimum healthy percent (deployment_minimum_healthy_percent in Terraform's ECS service resource) to a number less than 100 that allows deployments to happen.
For example, if you normally deploy 3 instances of the task in the service then setting the minimum healthy percent to 50% would allow ECS to remove one task from the service before scheduling a new task matching the new task definition. It would then proceed with a rolling upgrade, making sure the new task is healthy before replacing the old task.
Setting the minimum healthy percent to 0% would mean that ECS can stop all of the tasks running before starting new tasks but this would obviously lead to a potential (but not guaranteed) service interruption.
Alternatively you could remove the placement constraint by switching away from host networking if that is viable for your service.