feat(aws-ec2): Add SubnetFilter for Id and CIDR netmask (aws#15373)

This PR adds a couple of basic SubnetFilters into the CDK directly so they don't have to be needlessly reimplemented.  The `SubnetIdSubnetFilter` filters subnets by ID and the `CidrMaskSubnetFilter` filters subnets based on the CIDR netmask.

closes aws#15228
----

*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

@@ -166,7 +166,13 @@ Which subnets are selected is evaluated as follows:
   * `onePerAz`: per availability zone, a maximum of one subnet will be returned (Useful for resource
     types that do not allow creating two ENIs in the same availability zone).
 * `subnetFilters`: additional filtering on subnets using any number of user-provided filters which
-  extend the SubnetFilter class.
+  extend `SubnetFilter`.  The following methods on the `SubnetFilter` class can be used to create
+  a filter:
+  * `byIds`: chooses subnets from a list of ids
+  * `availabilityZones`: chooses subnets in the provided list of availability zones
+  * `onePerAz`: chooses at most one subnet per availability zone
+  * `containsIpAddresses`: chooses a subnet which contains *any* of the listed ip addresses
+  * `byCidrMask`: chooses subnets that have the provided CIDR netmask
 
 ### Using NAT instances
 

packages/@aws-cdk/aws-ec2/lib/subnet.ts

@@ -8,6 +8,13 @@ import { ISubnet } from './vpc';
  */
 export abstract class SubnetFilter {
 
+  /**
+   * Chooses subnets by id.
+   */
+  public static byIds(subnetIds: string[]): SubnetFilter {
+    return new SubnetIdSubnetFilter(subnetIds);
+  }
+
   /**
    * Chooses subnets which are in one of the given availability zones.
   */
@@ -29,6 +36,13 @@ export abstract class SubnetFilter {
     return new ContainsIpAddressesSubnetFilter(ipv4addrs);
   }
 
+  /**
+   * Chooses subnets which have the provided CIDR netmask.
+   */
+  public static byCidrMask(mask: number): SubnetFilter {
+    return new CidrMaskSubnetFilter(mask);
+  }
+
   /**
    * Executes the subnet filtering logic, returning a filtered set of subnets.
    */
@@ -112,4 +126,46 @@ class ContainsIpAddressesSubnetFilter extends SubnetFilter {
       return cidrBlockObjs.some(cidr => subnetCidrBlock.containsCidr(cidr));
     });
   }
-}
+}
+
+/**
+ * Chooses subnets based on the subnetId
+ */
+class SubnetIdSubnetFilter extends SubnetFilter {
+
+  private readonly subnetIds: string[];
+
+  constructor(subnetIds: string[]) {
+    super();
+    this.subnetIds = subnetIds;
+  }
+
+  /**
+   * Executes the subnet filtering logic.
+   */
+  public selectSubnets(subnets: ISubnet[]): ISubnet[] {
+    return subnets.filter(subnet => this.subnetIds.includes(subnet.subnetId));
+  }
+}
+
+/**
+ * Chooses subnets based on the CIDR Netmask
+ */
+class CidrMaskSubnetFilter extends SubnetFilter {
+  private readonly mask: number
+
+  constructor(mask: number) {
+    super();
+    this.mask = mask;
+  }
+
+  /**
+   * Executes the subnet filtering logic.
+   */
+  public selectSubnets(subnets: ISubnet[]): ISubnet[] {
+    return subnets.filter(subnet => {
+      const subnetCidr = new CidrBlock(subnet.ipv4CidrBlock);
+      return subnetCidr.mask === this.mask;
+    });
+  }
+}

packages/@aws-cdk/aws-ec2/test/vpc.test.ts

@@ -1688,6 +1688,58 @@ nodeunitShim({
       }));
       test.done();
     },
+
+    'can filter by Subnet Ids'(test: Test) {
+      // GIVEN
+      const stack = getTestStack();
+
+      const vpc = Vpc.fromVpcAttributes(stack, 'VPC', {
+        vpcId: 'vpc-1234',
+        vpcCidrBlock: '192.168.0.0/16',
+        availabilityZones: ['dummy1a', 'dummy1b', 'dummy1c'],
+        privateSubnetIds: ['priv-1', 'priv-2', 'priv-3'],
+      });
+
+      // WHEN
+      new InterfaceVpcEndpoint(stack, 'VPC Endpoint', {
+        vpc,
+        service: new InterfaceVpcEndpointService('com.amazonaws.vpce.us-east-1.vpce-svc-uuddlrlrbastrtsvc', 443),
+        subnets: {
+          subnetFilters: [SubnetFilter.byIds(['priv-1', 'priv-2'])],
+        },
+      });
+
+      // THEN
+      cdkExpect(stack).to(haveResource('AWS::EC2::VPCEndpoint', {
+        ServiceName: 'com.amazonaws.vpce.us-east-1.vpce-svc-uuddlrlrbastrtsvc',
+        SubnetIds: ['priv-1', 'priv-2'],
+      }));
+      test.done();
+    },
+
+    'can filter by Cidr Netmask'(test: Test) {
+      // GIVEN
+      const stack = getTestStack();
+      const vpc = new Vpc(stack, 'VpcNetwork', {
+        maxAzs: 1,
+        subnetConfiguration: [
+          { name: 'normalSn1', subnetType: SubnetType.PUBLIC, cidrMask: 20 },
+          { name: 'normalSn2', subnetType: SubnetType.PUBLIC, cidrMask: 20 },
+          { name: 'smallSn', subnetType: SubnetType.PUBLIC, cidrMask: 28 },
+        ],
+      });
+
+      // WHEN
+      const { subnetIds } = vpc.selectSubnets(
+        { subnetFilters: [SubnetFilter.byCidrMask(20)] },
+      );
+
+      // THEN
+      test.deepEqual(subnetIds.length, 2);
+      const expected = vpc.publicSubnets.filter(s => s.ipv4CidrBlock.endsWith('/20'));
+      test.deepEqual(subnetIds, expected.map(s => s.subnetId));
+      test.done();
+    },
   },
 });