feat(lambda): Expose $LATEST function version (#2792)

Sometimes, one wants to create an `Alias` to the `$LATEST` version of a
`Function`, but this requires access to an `IVersion` representing that.
This features adds a `latestVersion` property to `IFunction` that offers
simple access to `$LATEST`. Prior to this, each user needing to access
this would have had to roll theiw own implementation of `IVersion`,
which is rather cumbersome.

Fixes #2776

packages/@aws-cdk/aws-lambda/lib/function-base.ts

@@ -4,6 +4,7 @@ import iam = require('@aws-cdk/aws-iam');
 import { IResource, Resource } from '@aws-cdk/cdk';
 import { IEventSource } from './event-source';
 import { EventSourceMapping, EventSourceMappingOptions } from './event-source-mapping';
+import { IVersion } from './lambda-version';
 import { CfnPermission } from './lambda.generated';
 import { Permission } from './permission';
 
@@ -40,6 +41,11 @@ export interface IFunction extends IResource, ec2.IConnectable, iam.IGrantable {
    */
   readonly isBoundToVpc: boolean;
 
+  /**
+   * The `$LATEST` version of this function.
+   */
+  readonly latestVersion: IVersion;
+
   /**
    * Adds an event source that maps to this AWS Lambda function.
    * @param id construct ID
@@ -139,6 +145,11 @@ export abstract class FunctionBase extends Resource implements IFunction  {
    */
   public abstract readonly role?: iam.IRole;
 
+  /**
+   * The $LATEST version of this function.
+   */
+  public readonly latestVersion: IVersion = new LatestVersion(this);
+
   /**
    * Whether the addPermission() call adds any permissions
    *
@@ -277,3 +288,16 @@ export abstract class FunctionBase extends Resource implements IFunction  {
       'Supported: AccountPrincipal, ServicePrincipal');
   }
 }
+
+/**
+ * The $LATEST version of a function, useful when attempting to create aliases.
+ */
+class LatestVersion extends Resource implements IVersion {
+  public readonly lambda: IFunction;
+  public readonly version = '$LATEST';
+
+  constructor(lambda: FunctionBase) {
+    super(lambda, '$LATEST');
+    this.lambda = lambda;
+  }
+}

packages/@aws-cdk/aws-lambda/test/test.alias.ts

@@ -40,6 +40,29 @@ export = {
     test.done();
   },
 
+  'can create an alias to $LATEST'(test: Test): void {
+    const stack = new Stack();
+    const fn = new lambda.Function(stack, 'MyLambda', {
+      code: new lambda.InlineCode('hello()'),
+      handler: 'index.hello',
+      runtime: lambda.Runtime.NodeJS810,
+    });
+
+    new lambda.Alias(stack, 'Alias', {
+      aliasName: 'latest',
+      version: fn.latestVersion,
+    });
+
+    expect(stack).to(haveResource('AWS::Lambda::Alias', {
+      FunctionName: { Ref: "MyLambdaCCE802FB" },
+      FunctionVersion: '$LATEST',
+      Name: 'latest',
+    }));
+    expect(stack).notTo(haveResource('AWS::Lambda::Version'));
+
+    test.done();
+  },
+
   'can use newVersion to create a new Version'(test: Test) {
     const stack = new Stack();
     const fn = new lambda.Function(stack, 'MyLambda', {