chore(ecs): allow selecting Bottlerocket via machineImage

Previously, Bottlerocket had to be explicitly (and only) selected
via setting `machineImageType`, which would pick an appropriate
`machineImage`.

Setting `machineImage` to `new BottleRocketImage()` would not be
sufficient, since the feature also requires configuring additional
UserData commands which are only added if `machineImageType` was set.

This method of configuration does not allow customization of the AMI,
such as introduced in #16021.

Instead, we reverse the logic: `machineImageType` may still be necessary
to autoconfigure UserData if we can't know what the machineImage is
(for example in case of a preconfigured AutoScalingGroup), but otherwise
is derived from what `machineImage` is being used.

packages/@aws-cdk/aws-ecs/README.md

@@ -141,15 +141,13 @@ purpose-built by AWS for running containers. You can launch Amazon ECS container
 
 The following example will create a capacity with self-managed Amazon EC2 capacity of 2 `c5.large` Linux instances running with `Bottlerocket` AMI.
 
-Note that you must specify either a `machineImage` or `machineImageType`, at least one, not both.
-
 The following example adds Bottlerocket capacity to the cluster:
 
 ```ts
 cluster.addCapacity('bottlerocket-asg', {
   minCapacity: 2,
   instanceType: new ec2.InstanceType('c5.large'),
-  machineImageType: ecs.MachineImageType.BOTTLEROCKET,
+  machineImage: new ecs.BottleRocketImage(),
 });
 ```
 
@@ -214,7 +212,7 @@ some supporting containers which are used to support the main container,
 doings things like upload logs or metrics to monitoring services.
 
 To run a task or service with Amazon EC2 launch type, use the `Ec2TaskDefinition`. For AWS Fargate tasks/services, use the
-`FargateTaskDefinition`. For AWS ECS Anywhere use the `ExternalTaskDefinition`. These classes 
+`FargateTaskDefinition`. For AWS ECS Anywhere use the `ExternalTaskDefinition`. These classes
 provide simplified APIs that only contain properties relevant for each specific launch type.
 
 For a `FargateTaskDefinition`, specify the task size (`memoryLimitMiB` and `cpu`):
@@ -736,7 +734,7 @@ const service = new ecs.Ec2Service(stack, 'Service', {
 });
 ```
 
-With `bridge` or `host` network modes, only `SRV` DNS record types are supported. 
+With `bridge` or `host` network modes, only `SRV` DNS record types are supported.
 By default, `SRV` DNS record types will target the default container and default
 port. However, you may target a different container and port on the same ECS task:
 
@@ -893,7 +891,7 @@ const taskDefinition = new ecs.Ec2TaskDefinition(stack, 'Ec2TaskDef', {
 
 To enable using the inference accelerators in the containers, add `inferenceAcceleratorResources`
 field and set it to a list of device names used for the inference accelerators. Each value in the
-list should match a `DeviceName` for an `InferenceAccelerator` specified in the task definition. 
+list should match a `DeviceName` for an `InferenceAccelerator` specified in the task definition.
 
 ```ts
 const inferenceAcceleratorResources = ['device1'];
@@ -927,11 +925,11 @@ const service = new ecs.Ec2Service(stack, 'Service', {
 You can enable sending logs of your execute session commands to a CloudWatch log group or S3 bucket by configuring
 the `executeCommandConfiguration` property for your cluster. The default configuration will send the
 logs to the CloudWatch Logs using the `awslogs` log driver that is configured in your task definition. Please note,
-when using your own `logConfiguration` the log group or S3 Bucket specified must already be created. 
+when using your own `logConfiguration` the log group or S3 Bucket specified must already be created.
 
 To encrypt data using your own KMS Customer Key (CMK), you must create a CMK and provide the key in the `kmsKey` field
 of the `executeCommandConfiguration`. To use this key for encrypting CloudWatch log data or S3 bucket, make sure to associate the key
-to these resources on creation. 
+to these resources on creation.
 
 ```ts
 const kmsKey = new kms.Key(stack, 'KmsKey');

packages/@aws-cdk/aws-ecs/lib/cluster.ts

@@ -294,12 +294,13 @@ export class Cluster extends Resource implements ICluster {
    * @deprecated Use {@link Cluster.addAsgCapacityProvider} instead.
    */
   public addCapacity(id: string, options: AddCapacityOptions): autoscaling.AutoScalingGroup {
-    if (options.machineImage && options.machineImageType) {
-      throw new Error('You can only specify either machineImage or machineImageType, not both.');
-    }
+    // Do 2-way defaulting here: if the machineImageType is BOTTLEROCKET, pick the right AMI.
+    // Otherwise, determine the machineImageType from the given AMI.
+    const machineImage = options.machineImage ??
+      (options.machineImageType === MachineImageType.BOTTLEROCKET ? new BottleRocketImage() : new EcsOptimizedAmi());
 
-    const machineImage = options.machineImage ?? options.machineImageType === MachineImageType.BOTTLEROCKET ?
-      new BottleRocketImage() : new EcsOptimizedAmi();
+    const machineImageType = options.machineImageType ??
+      (isBottleRocketImage(machineImage) ? MachineImageType.BOTTLEROCKET : MachineImageType.AMAZON_LINUX_2);
 
     const autoScalingGroup = new autoscaling.AutoScalingGroup(this, id, {
       vpc: this.vpc,
@@ -309,7 +310,7 @@ export class Cluster extends Resource implements ICluster {
     });
 
     this.addAutoScalingGroup(autoScalingGroup, {
-      machineImageType: options.machineImageType,
+      machineImageType: machineImageType,
       ...options,
     });
 
@@ -1016,11 +1017,18 @@ export interface AddAutoScalingGroupCapacityOptions {
    */
   readonly topicEncryptionKey?: kms.IKey;
 
-
   /**
-   * Specify the machine image type.
+   * What type of machine image this is
+   *
+   * Depending on the setting, different UserData will automatically be added
+   * to the `AutoScalingGroup` to configure it properly for use with ECS.
    *
-   * @default MachineImageType.AMAZON_LINUX_2
+   * If you create an `AutoScalingGroup` yourself and are adding it via
+   * `addAutoScalingGroup()`, you must specify this value. If you are adding an
+   * `autoScalingGroup` via `addCapacity`, this value will be determined
+   * from the `machineImage` you pass.
+   *
+   * @default - Automatically determined from `machineImage`, if available, otherwise `MachineImageType.AMAZON_LINUX_2`.
    */
   readonly machineImageType?: MachineImageType;
 }
@@ -1356,3 +1364,8 @@ class MaybeCreateCapacityProviderAssociations implements IAspect {
     }
   }
 }
+
+
+function isBottleRocketImage(image: ec2.IMachineImage) {
+  return image instanceof BottleRocketImage;
+}

packages/@aws-cdk/aws-ecs/test/cluster.test.ts

@@ -1589,7 +1589,7 @@ nodeunitShim({
     test.done();
   },
 
-  'cluster capacity with bottlerocket AMI'(test: Test) {
+  'cluster capacity with bottlerocket AMI, by setting machineImageType'(test: Test) {
     // GIVEN
     const app = new cdk.App();
     const stack = new cdk.Stack(app, 'test');
@@ -1682,20 +1682,34 @@ nodeunitShim({
     test.done();
   },
 
-  'throws when machineImage and machineImageType both specified'(test: Test) {
+  'cluster capacity with bottlerocket AMI, by setting the machineImage'(test: Test) {
     // GIVEN
     const app = new cdk.App();
     const stack = new cdk.Stack(app, 'test');
+
     const cluster = new ecs.Cluster(stack, 'EcsCluster');
+    cluster.addCapacity('bottlerocket-asg', {
+      instanceType: new ec2.InstanceType('c5.large'),
+      machineImage: new ecs.BottleRocketImage(),
+    });
 
     // THEN
-    test.throws(() => {
-      cluster.addCapacity('bottlerocket-asg', {
-        instanceType: new ec2.InstanceType('c5.large'),
-        machineImageType: ecs.MachineImageType.BOTTLEROCKET,
-        machineImage: new ecs.EcsOptimizedAmi(),
-      });
-    }, /You can only specify either machineImage or machineImageType, not both./);
+    expect(stack).to(haveResourceLike('AWS::AutoScaling::LaunchConfiguration', {
+      UserData: {
+        'Fn::Base64': {
+          'Fn::Join': [
+            '',
+            [
+              '\n[settings.ecs]\ncluster = "',
+              {
+                Ref: 'EcsCluster97242B84',
+              },
+              '"',
+            ],
+          ],
+        },
+      },
+    }));
     test.done();
   },