fix(ec2): userData in launchTemplate is created automatically when machineImege is provided (#23593)

closes #23592

Reading through the discussion in PR #12385, which introduced the original code, I could not find any reason not to create userData when machineImage is provided. They also agreed with the design [here](#12385 (comment)), but it seems it accidentally became out of scope at the time.

This change should not be considered as a breaking change because we are just adding empty userData to launchTemplates whose userData is not specified explicitly, and it will not have any effect on the existing behavior.

* Users who already sets a userData explicitly:
   * will not see any change to their synthesized template, as this PR only modifies userData when it is not set explicitly.
* Users who is not using userData:
   * will see a userData is added to their template. But the userData is empty and does nothing. It should not have any effect on the previous behavior.

----

### All Submissions:

* [X] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Construct Runtime Dependencies:

* [ ] This PR adds new construct runtime dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-construct-runtime-dependencies)

### New Features

* [X] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [X] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: tmokmss

packages/@aws-cdk/aws-autoscaling/test/integ.asg-lt.js.snapshot/aws-cdk-asg-integ.assets.json

@@ -1,15 +1,15 @@
 {
   "version": "22.0.0",
   "files": {
-    "768b8ff8b1178a04dbfca488da9459f4f402bfad643db0b4791787ef23ec4db5": {
+    "2f0699c7863c01ee4fb7ccca4b192b311d9d6723f012b40f36bdf25a5db22f0c": {
       "source": {
         "path": "aws-cdk-asg-integ.template.json",
         "packaging": "file"
       },
       "destinations": {
         "current_account-current_region": {
           "bucketName": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}",
-          "objectKey": "768b8ff8b1178a04dbfca488da9459f4f402bfad643db0b4791787ef23ec4db5.json",
+          "objectKey": "2f0699c7863c01ee4fb7ccca4b192b311d9d6723f012b40f36bdf25a5db22f0c.json",
           "assumeRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-file-publishing-role-${AWS::AccountId}-${AWS::Region}"
         }
       }

packages/@aws-cdk/aws-autoscaling/test/integ.asg-lt.js.snapshot/aws-cdk-asg-integ.template.json

@@ -27,7 +27,10 @@
         }
        ]
       }
-     ]
+     ],
+     "UserData": {
+      "Fn::Base64": "#!/bin/bash"
+     }
     },
     "TagSpecifications": [
      {
@@ -69,7 +72,10 @@
         }
        ]
       }
-     ]
+     ],
+     "UserData": {
+      "Fn::Base64": "#!/bin/bash"
+     }
     },
     "TagSpecifications": [
      {

packages/@aws-cdk/aws-autoscaling/test/integ.asg-lt.js.snapshot/manifest.json

@@ -17,7 +17,7 @@
         "validateOnSynth": false,
         "assumeRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-deploy-role-${AWS::AccountId}-${AWS::Region}",
         "cloudFormationExecutionRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-cfn-exec-role-${AWS::AccountId}-${AWS::Region}",
-        "stackTemplateAssetObjectUrl": "s3://cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}/768b8ff8b1178a04dbfca488da9459f4f402bfad643db0b4791787ef23ec4db5.json",
+        "stackTemplateAssetObjectUrl": "s3://cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}/2f0699c7863c01ee4fb7ccca4b192b311d9d6723f012b40f36bdf25a5db22f0c.json",
         "requiresBootstrapStackVersion": 6,
         "bootstrapStackVersionSsmParameter": "/cdk-bootstrap/hnb659fds/version",
         "additionalDependencies": [

packages/@aws-cdk/aws-autoscaling/test/integ.asg-lt.js.snapshot/tree.json

@@ -42,7 +42,10 @@
                             }
                           ]
                         }
-                      ]
+                      ],
+                      "userData": {
+                        "Fn::Base64": "#!/bin/bash"
+                      }
                     },
                     "tagSpecifications": [
                       {
@@ -118,7 +121,10 @@
                             }
                           ]
                         }
-                      ]
+                      ],
+                      "userData": {
+                        "Fn::Base64": "#!/bin/bash"
+                      }
                     },
                     "tagSpecifications": [
                       {
@@ -1275,7 +1281,7 @@
         "path": "Tree",
         "constructInfo": {
           "fqn": "constructs.Construct",
-          "version": "10.1.168"
+          "version": "10.1.189"
         }
       }
     },

packages/@aws-cdk/aws-ec2/lib/launch-template.ts

@@ -12,7 +12,9 @@ import {
   TagType,
   Tags,
   Token,
+  FeatureFlags,
 } from '@aws-cdk/core';
+import * as cxapi from '@aws-cdk/cx-api';
 import { Construct } from 'constructs';
 import { Connections, IConnectable } from './connections';
 import { CfnLaunchTemplate } from './ec2.generated';
@@ -591,8 +593,19 @@ export class LaunchTemplate extends Resource implements ILaunchTemplate, iam.IGr
       },
     });
 
-    if (props.userData) {
-      this.userData = props.userData;
+    const imageConfig: MachineImageConfig | undefined = props.machineImage?.getImage(this);
+    if (imageConfig) {
+      this.osType = imageConfig.osType;
+      this.imageId = imageConfig.imageId;
+    }
+
+    if (FeatureFlags.of(this).isEnabled(cxapi.EC2_LAUNCH_TEMPLATE_DEFAULT_USER_DATA)) {
+      // priority: prop.userData -> userData from machineImage -> undefined
+      this.userData = props.userData ?? imageConfig?.userData;
+    } else {
+      if (props.userData) {
+        this.userData = props.userData;
+      }
     }
     const userDataToken = Lazy.string({
       produce: () => {
@@ -603,12 +616,6 @@ export class LaunchTemplate extends Resource implements ILaunchTemplate, iam.IGr
       },
     });
 
-    const imageConfig: MachineImageConfig | undefined = props.machineImage?.getImage(this);
-    if (imageConfig) {
-      this.osType = imageConfig.osType;
-      this.imageId = imageConfig.imageId;
-    }
-
     this.instanceType = props.instanceType;
 
     let marketOptions: any = undefined;

packages/@aws-cdk/aws-ec2/test/integ.launch-template.js.snapshot/LambdaTestDefaultTestDeployAssert1AF2B360.assets.json

@@ -1,5 +1,5 @@
 {
-  "version": "21.0.0",
+  "version": "22.0.0",
   "files": {
     "21fbb51d7b23f6a6c262b46a9caee79d744a3ac019fd45422d988b96d44b2a22": {
       "source": {

packages/@aws-cdk/aws-ec2/test/integ.launch-template.js.snapshot/TestStack.assets.json

@@ -1,15 +1,15 @@
 {
-  "version": "21.0.0",
+  "version": "22.0.0",
   "files": {
-    "ea313fee581c8e898c158e9d123dd48345192689bb08a3f7e84716db61247c6c": {
+    "534a1fbecaccb7e2a071086c8085be5c15b2501781767cdeddf754fe3a0ceecb": {
       "source": {
         "path": "aws-cdk-ec2-lt-metadata-1.template.json",
         "packaging": "file"
       },
       "destinations": {
         "current_account-current_region": {
           "bucketName": "cdk-hnb659fds-assets-${AWS::AccountId}-${AWS::Region}",
-          "objectKey": "ea313fee581c8e898c158e9d123dd48345192689bb08a3f7e84716db61247c6c.json",
+          "objectKey": "534a1fbecaccb7e2a071086c8085be5c15b2501781767cdeddf754fe3a0ceecb.json",
           "assumeRoleArn": "arn:${AWS::Partition}:iam::${AWS::AccountId}:role/cdk-hnb659fds-file-publishing-role-${AWS::AccountId}-${AWS::Region}"
         }
       }

packages/@aws-cdk/aws-ec2/test/integ.launch-template.js.snapshot/TestStack.template.json

@@ -44,9 +44,57 @@
      }
     ]
    }
+  },
+  "LTWithMachineImageAAC227A5": {
+   "Type": "AWS::EC2::LaunchTemplate",
+   "Properties": {
+    "LaunchTemplateData": {
+     "ImageId": {
+      "Ref": "SsmParameterValueawsserviceamiamazonlinuxlatestamzn2amihvmx8664gp2C96584B6F00A464EAD1953AFF4B05118Parameter"
+     },
+     "TagSpecifications": [
+      {
+       "ResourceType": "instance",
+       "Tags": [
+        {
+         "Key": "Name",
+         "Value": "aws-cdk-ec2-lt-metadata-1/LTWithMachineImage"
+        }
+       ]
+      },
+      {
+       "ResourceType": "volume",
+       "Tags": [
+        {
+         "Key": "Name",
+         "Value": "aws-cdk-ec2-lt-metadata-1/LTWithMachineImage"
+        }
+       ]
+      }
+     ],
+     "UserData": {
+      "Fn::Base64": "#!/bin/bash"
+     }
+    },
+    "TagSpecifications": [
+     {
+      "ResourceType": "launch-template",
+      "Tags": [
+       {
+        "Key": "Name",
+        "Value": "aws-cdk-ec2-lt-metadata-1/LTWithMachineImage"
+       }
+      ]
+     }
+    ]
+   }
   }
  },
  "Parameters": {
+  "SsmParameterValueawsserviceamiamazonlinuxlatestamzn2amihvmx8664gp2C96584B6F00A464EAD1953AFF4B05118Parameter": {
+   "Type": "AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>",
+   "Default": "/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2"
+  },
   "BootstrapVersion": {
    "Type": "AWS::SSM::Parameter::Value<String>",
    "Default": "/cdk-bootstrap/hnb659fds/version",