ECS: implement IEventRuleTarget

[eladb]

The ECS library needs to be able to be used as an event rule target.

@thomasdeml wrote on Gitter:

I have an ECS task definition and want to schedule my task every 5 minutes. I create a scheduled event

var scheduled_event = new EventRule(this, sdk + '_scheduled_event', { 
  scheduleExpression: "rate(5 minutes)"
});

and then I try to attach the task definition to it. I need an IEventRuleTarget object however. I looked around in the docs, but I have no idea where I could get it from.

[eladb]

@thomasdeml this is needed a missing feature in our ECS library, in the meantime, it shouldn't be too hard to define a class that implements IEventRuleTarget for ECS:

Here's a sketch (updated and tested by @thomasdeml):

import cdk = require('@aws-cdk/cdk');
import ecs = require('@aws-cdk/aws-ecs');
import iam = require('@aws-cdk/aws-iam');
import events = require('@aws-cdk/aws-events');

// https://github.com/awslabs/aws-cdk/issues/1370
export class TaskEventRuleTarget implements events.IEventRuleTarget {
  private readonly clusterArn: string;

  constructor(private readonly cluster: ecs.ICluster, private readonly task: ecs.TaskDefinition, private readonly count = 1) {
    // https://github.com/awslabs/aws-cdk/issues/1371
    this.clusterArn = cdk.ArnUtils.fromComponents({
      service: 'ecs',
      resource: 'cluster',
      resourceName: this.cluster.clusterName
    });
  }

  public asEventRuleTarget(_ruleArn: string, _ruleUniqueId: string): events.EventRuleTargetProps {
    let role = this.task.findChild('EventRuleRole') as iam.Role;
    if (!role) {
      role = new iam.Role(this.task, 'EventRuleRole', {
        assumedBy: new iam.ServicePrincipal('cloudwatch.amazonaws.com'),
      });

      // https://github.com/awslabs/aws-cdk/issues/1372
      role.addToPolicy(new iam.PolicyStatement()
        .addAction('ecs:RunTask')
        .addResource(`${this.task.taskDefinitionArn}:*`)
        .addCondition('ArnEquals', {
          'ecs:cluster': this.clusterArn
        }));
    }

    return {
      id: this.task.id,
      arn: this.clusterArn,
      ecsParameters: {
        taskCount: this.count,
        taskDefinitionArn: this.task.taskDefinitionArn
      },
      roleArn: role.roleArn
    };
  }
}