fix(rds): fix unresolved endpoint socket address (#2846)

Convert port to a string token before using it in `socketAddress` otherwise the number token is
converted to a string and cannot be resolved.

Fixes #2711

packages/@aws-cdk/aws-rds/lib/endpoint.ts

@@ -25,7 +25,7 @@ export class Endpoint {
     this.hostname = address;
     this.port = port;
 
-    const portDesc = Token.isUnresolved(port) ? '{IndirectPort}' : port;
+    const portDesc = Token.isUnresolved(port) ? Token.asString(port) : port;
     this.socketAddress = `${address}:${portDesc}`;
   }
 }

packages/@aws-cdk/aws-rds/test/test.instance.ts

@@ -375,6 +375,37 @@ export = {
       statistic: 'Average'
     });
 
+    test.done();
+  },
+
+  'can resolve endpoint port and socket address'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'VPC');
+
+    // WHEN
+    const instance = new rds.DatabaseInstance(stack, 'Instance', {
+      engine: rds.DatabaseInstanceEngine.Mysql,
+      instanceClass: new ec2.InstanceTypePair(ec2.InstanceClass.Burstable2, ec2.InstanceSize.Small),
+      masterUsername: 'admin',
+      vpc
+    });
+
+    test.deepEqual(stack.resolve(instance.instanceEndpoint.port), {
+      'Fn::GetAtt': ['InstanceC1063A87', 'Endpoint.Port']
+    });
+
+    test.deepEqual(stack.resolve(instance.instanceEndpoint.socketAddress), {
+      'Fn::Join': [
+        '',
+        [
+          { 'Fn::GetAtt': ['InstanceC1063A87', 'Endpoint.Address'] },
+          ':',
+          { 'Fn::GetAtt': ['InstanceC1063A87', 'Endpoint.Port'] },
+        ]
+      ]
+    });
+
     test.done();
   }
 };