fix(rds): pass the ARN of master instead of its ID in DatabaseInstanc…

…eReadReplica (#5702)

This works around the
"DbSubnetGroupName should not be specified for read replicas that are
created in the same region as the master" RDS validation.

See hashicorp/terraform-provider-aws#528 (comment)
for details.

Fixes #5530

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

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

@@ -965,7 +965,8 @@ export class DatabaseInstanceReadReplica extends DatabaseInstanceNew implements
 
     const instance = new CfnDBInstance(this, 'Resource', {
       ...this.newCfnProps,
-      sourceDbInstanceIdentifier: props.sourceDatabaseInstance.instanceIdentifier,
+      // this must be ARN, not ID, because of https://github.com/terraform-providers/terraform-provider-aws/issues/528#issuecomment-391169012
+      sourceDbInstanceIdentifier: props.sourceDatabaseInstance.instanceArn,
       kmsKeyId: props.kmsKey && props.kmsKey.keyArn,
       storageEncrypted: props.kmsKey ? true : props.storageEncrypted,
     });

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

@@ -301,7 +301,7 @@ export = {
     test.done();
   },
 
-  'create a read replica'(test: Test) {
+  'create a read replica in the same region - with the subnet group name'(test: Test) {
     // GIVEN
     const stack = new cdk.Stack();
     const vpc = new ec2.Vpc(stack, 'VPC');
@@ -323,8 +323,20 @@ export = {
     // THEN
     expect(stack).to(haveResource('AWS::RDS::DBInstance', {
       SourceDBInstanceIdentifier: {
-        Ref: 'InstanceC1063A87'
-      }
+        "Fn::Join": ["", [
+          "arn:",
+          { Ref: "AWS::Partition" },
+          ":rds:",
+          { Ref: "AWS::Region" },
+          ":",
+          { Ref: "AWS::AccountId" },
+          ":db:",
+          { Ref: "InstanceC1063A87" },
+        ]],
+      },
+      DBSubnetGroupName: {
+        Ref: 'ReadReplicaSubnetGroup680C605C',
+      },
     }));
 
     test.done();