feat(ec2): Implement UserData methods in MultipartUserData (aws#14347)

Implements: aws#14346

----

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

packages/@aws-cdk/aws-ec2/README.md

@@ -1115,6 +1115,23 @@ new ec2.LaunchTemplate(stack, '', {
 For more information see
 [Specifying Multiple User Data Blocks Using a MIME Multi Part Archive](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/bootstrap_container_instance.html#multi-part_user_data)
 
+#### Using add*Command on MultipartUserData
+
+To use the `add*Command` methods, that are inherited from the `UserData` interface, on `MultipartUserData` you must add a part
+to the `MultipartUserData` and designate it as the reciever for these methods. This is accomplished by using the `addUserDataPart()`
+method on `MultipartUserData` with the `makeDefault` argument set to `true`:
+
+```ts
+const multipartUserData = new ec2.MultipartUserData();
+const commandsUserData = ec2.UserData.forLinux();
+multipartUserData.addUserDataPart(commandsUserData, MultipartBody.SHELL_SCRIPT, true);
+
+// Adding commands to the multipartUserData adds them to commandsUserData, and vice-versa.
+multipartUserData.addCommands('touch /root/multi.txt');
+commandsUserData.addCommands('touch /root/userdata.txt');
+```
+
+When used on an EC2 instance, the above `multipartUserData` will create both `multi.txt` and `userdata.txt` in `/root`.
 
 ## Importing existing subnet
 

packages/@aws-cdk/aws-ec2/lib/user-data.ts

@@ -436,13 +436,15 @@ export interface MultipartUserDataOptions {
  *
  */
 export class MultipartUserData extends UserData {
-  private static readonly USE_PART_ERROR = 'MultipartUserData does not support this operation. Please add part using addPart.';
+  private static readonly USE_PART_ERROR = 'MultipartUserData only supports this operation if it has a default UserData. Call addUserDataPart with makeDefault=true.';
   private static readonly BOUNDRY_PATTERN = '[^a-zA-Z0-9()+,-./:=?]';
 
   private parts: MultipartBody[] = [];
 
   private opts: MultipartUserDataOptions;
 
+  private defaultUserData?: UserData;
+
   constructor(opts?: MultipartUserDataOptions) {
     super();
 
@@ -472,16 +474,28 @@ export class MultipartUserData extends UserData {
   }
 
   /**
-   * Adds a multipart part based on a UserData object
+   * Adds a multipart part based on a UserData object.
+   *
+   * If `makeDefault` is true, then the UserData added by this method
+   * will also be the target of calls to the `add*Command` methods on
+   * this MultipartUserData object.
    *
-   * This is the same as calling:
+   * If `makeDefault` is false, then this is the same as calling:
    *
    * ```ts
    * multiPart.addPart(MultipartBody.fromUserData(userData, contentType));
    * ```
+   *
+   * An undefined `makeDefault` defaults to either:
+   * - `true` if no default UserData has been set yet; or
+   * - `false` if there is no default UserData set.
    */
-  public addUserDataPart(userData: UserData, contentType?: string) {
+  public addUserDataPart(userData: UserData, contentType?: string, makeDefault?: boolean) {
     this.addPart(MultipartBody.fromUserData(userData, contentType));
+    makeDefault = makeDefault ?? (this.defaultUserData === undefined ? true : false);
+    if (makeDefault) {
+      this.defaultUserData = userData;
+    }
   }
 
   public render(): string {
@@ -510,23 +524,43 @@ export class MultipartUserData extends UserData {
     return resultArchive.join('\n');
   }
 
-  public addS3DownloadCommand(_params: S3DownloadOptions): string {
-    throw new Error(MultipartUserData.USE_PART_ERROR);
+  public addS3DownloadCommand(params: S3DownloadOptions): string {
+    if (this.defaultUserData) {
+      return this.defaultUserData.addS3DownloadCommand(params);
+    } else {
+      throw new Error(MultipartUserData.USE_PART_ERROR);
+    }
   }
 
-  public addExecuteFileCommand(_params: ExecuteFileOptions): void {
-    throw new Error(MultipartUserData.USE_PART_ERROR);
+  public addExecuteFileCommand(params: ExecuteFileOptions): void {
+    if (this.defaultUserData) {
+      this.defaultUserData.addExecuteFileCommand(params);
+    } else {
+      throw new Error(MultipartUserData.USE_PART_ERROR);
+    }
   }
 
-  public addSignalOnExitCommand(_resource: Resource): void {
-    throw new Error(MultipartUserData.USE_PART_ERROR);
+  public addSignalOnExitCommand(resource: Resource): void {
+    if (this.defaultUserData) {
+      this.defaultUserData.addSignalOnExitCommand(resource);
+    } else {
+      throw new Error(MultipartUserData.USE_PART_ERROR);
+    }
   }
 
-  public addCommands(..._commands: string[]): void {
-    throw new Error(MultipartUserData.USE_PART_ERROR);
+  public addCommands(...commands: string[]): void {
+    if (this.defaultUserData) {
+      this.defaultUserData.addCommands(...commands);
+    } else {
+      throw new Error(MultipartUserData.USE_PART_ERROR);
+    }
   }
 
-  public addOnExitCommands(..._commands: string[]): void {
-    throw new Error(MultipartUserData.USE_PART_ERROR);
+  public addOnExitCommands(...commands: string[]): void {
+    if (this.defaultUserData) {
+      this.defaultUserData.addOnExitCommands(...commands);
+    } else {
+      throw new Error(MultipartUserData.USE_PART_ERROR);
+    }
   }
 }