fix(lambda): cannot create lambda in public subnets (#9468)

----
Closes #8935

*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-lambda/lib/function.ts

@@ -299,6 +299,15 @@ export interface FunctionProps extends FunctionOptions {
    * @default - will not mount any filesystem
    */
   readonly filesystem?: FileSystem;
+
+  /**
+   * Lambda Functions in a public subnet can NOT access the internet.
+   * Use this property to acknowledge this limitation and still place the function in a public subnet.
+   * @see https://stackoverflow.com/questions/52992085/why-cant-an-aws-lambda-function-inside-a-public-subnet-in-a-vpc-connect-to-the/52994841#52994841
+   *
+   * @default false
+   */
+  readonly allowPublicSubnet?: boolean;
 }
 
 /**
@@ -819,15 +828,13 @@ export class Function extends FunctionBase {
       }
     }
 
-    // Pick subnets, make sure they're not Public. Routing through an IGW
-    // won't work because the ENIs don't get a Public IP.
-    // Why are we not simply forcing vpcSubnets? Because you might still be choosing
-    // Isolated networks or selecting among 2 sets of Private subnets by name.
+    const allowPublicSubnet = props.allowPublicSubnet ?? false;
     const { subnetIds } = props.vpc.selectSubnets(props.vpcSubnets);
     const publicSubnetIds = new Set(props.vpc.publicSubnets.map(s => s.subnetId));
     for (const subnetId of subnetIds) {
-      if (publicSubnetIds.has(subnetId)) {
-        throw new Error('Not possible to place Lambda Functions in a Public subnet');
+      if (publicSubnetIds.has(subnetId) && !allowPublicSubnet) {
+        throw new Error('Lambda Functions in a public subnet can NOT access the internet. ' +
+          'If you are aware of this limitation and would still like to place the function int a public subnet, set `allowPublicSubnet` to true');
       }
     }
 

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

@@ -212,22 +212,133 @@ export = {
     test.done();
   },
 
-  'picking public subnets is not allowed'(test: Test) {
+  'can pick public subnet for Lambda'(test: Test) {
     // GIVEN
     const stack = new cdk.Stack();
     const vpc = new ec2.Vpc(stack, 'VPC');
 
+    // WHEN
+    new lambda.Function(stack, 'PublicLambda', {
+      allowPublicSubnet: true,
+      code: new lambda.InlineCode('foo'),
+      handler: 'index.handler',
+      runtime: lambda.Runtime.NODEJS_10_X,
+      vpc,
+      vpcSubnets: { subnetType: ec2.SubnetType.PUBLIC },
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::Lambda::Function', {
+      VpcConfig: {
+        SecurityGroupIds: [
+          {'Fn::GetAtt': [ 'PublicLambdaSecurityGroup61D896FD', 'GroupId' ]},
+        ],
+        SubnetIds: [
+          {Ref: 'VPCPublicSubnet1SubnetB4246D30'},
+          {Ref: 'VPCPublicSubnet2Subnet74179F39'},
+        ],
+      },
+    }));
+    test.done();
+  },
+
+  'can pick private subnet for Lambda'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'VPC');
+
+    // WHEN
+    new lambda.Function(stack, 'PrivateLambda', {
+      code: new lambda.InlineCode('foo'),
+      handler: 'index.handler',
+      runtime: lambda.Runtime.NODEJS_10_X,
+      vpc,
+      vpcSubnets: { subnetType: ec2.SubnetType.PRIVATE },
+    });
+
+    // THEN
+
+    expect(stack).to(haveResource('AWS::Lambda::Function', {
+      VpcConfig: {
+        SecurityGroupIds: [
+          {'Fn::GetAtt': [ 'PrivateLambdaSecurityGroupF53C8342', 'GroupId' ]},
+        ],
+        SubnetIds: [
+          {Ref: 'VPCPrivateSubnet1Subnet8BCA10E0'},
+          {Ref: 'VPCPrivateSubnet2SubnetCFCDAA7A'},
+        ],
+      },
+    }));
+    test.done();
+  },
+
+  'can pick isolated subnet for Lambda'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'VPC', {
+      subnetConfiguration: [
+        {
+          name: 'Isolated',
+          subnetType: ec2.SubnetType.ISOLATED,
+        },
+      ],
+    });
+
+    // WHEN
+    new lambda.Function(stack, 'IsolatedLambda', {
+      code: new lambda.InlineCode('foo'),
+      handler: 'index.handler',
+      runtime: lambda.Runtime.NODEJS_10_X,
+      vpc,
+      vpcSubnets: { subnetType: ec2.SubnetType.ISOLATED },
+    });
+
+    // THEN
+
+    expect(stack).to(haveResource('AWS::Lambda::Function', {
+      VpcConfig: {
+        SecurityGroupIds: [
+          {'Fn::GetAtt': [ 'IsolatedLambdaSecurityGroupCE25B6A9', 'GroupId' ]},
+        ],
+        SubnetIds: [
+          {Ref: 'VPCIsolatedSubnet1SubnetEBD00FC6'},
+          {Ref: 'VPCIsolatedSubnet2Subnet4B1C8CAA'},
+        ],
+      },
+    }));
+    test.done();
+  },
+
+  'picking public subnet type is not allowed if not overriding allowPublicSubnet'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'VPC', {
+      subnetConfiguration: [
+        {
+          name: 'Public',
+          subnetType: ec2.SubnetType.PUBLIC,
+        },
+        {
+          name: 'Private',
+          subnetType: ec2.SubnetType.PRIVATE,
+        },
+        {
+          name: 'Isolated',
+          subnetType: ec2.SubnetType.ISOLATED,
+        },
+      ],
+    });
+
     // WHEN
     test.throws(() => {
-      new lambda.Function(stack, 'Lambda', {
+      new lambda.Function(stack, 'PublicLambda', {
         code: new lambda.InlineCode('foo'),
         handler: 'index.handler',
         runtime: lambda.Runtime.NODEJS_10_X,
         vpc,
         vpcSubnets: { subnetType: ec2.SubnetType.PUBLIC },
       });
-    });
-
+    }, /Lambda Functions in a public subnet/);
     test.done();
   },
 };