VPC shouldn't select 0 subnets in "dummy" mode

[davidh83110]

When I was using ec2.Vpc.fromLookup with my RDS, it couldn't fetch the correct VPC and keep showing me vpc-12345 which is obviously not my VPC ID. However, after I removed the new rds.DatabaseCluster() creation code which only left Vpc.fromLookup, it backed to normal.

Reproduction Steps

My code:

   // VPC
    const vpc = ec2.Vpc.fromLookup(this, 'vpc', {
      vpcName: vpcName
    })

    console.log(vpc.vpcId)

    // RDS - Aurora MySQL
    new rds.DatabaseCluster(this, 'rds', {
      clusterIdentifier: dbName,
      engine: rds.DatabaseClusterEngine.AURORA_MYSQL,
      engineVersion: dbEngineVersion,
      storageEncrypted: true,
      parameterGroup: rdsParameterGroup,
      masterUser: {
          username: 'root',
          password: cdk.SecretValue.ssmSecure('Password', '1')
      },
      instanceProps: {
        instanceType: ec2.InstanceType.of(ec2.InstanceClass.T3, ec2.InstanceSize.MEDIUM),
        vpc: vpcXrex,
        vpcSubnets: {
          subnetGroupName: 'db'
        },
        securityGroup: rdsSG
      }
    })

And I ran cdk diff, I got this.

➜  rds-aurora git:(develop) ✗ aws-vault exec staging -- cdk diff
vpc-12345
Cluster requires at least 2 subnets, got 0
Subprocess exited with error 1

After I removed new rds.DatabaseCluster, it backed to normal.

➜  rds-aurora git:(develop) ✗ aws-vault exec staging -- cdk diff
vpc-12345
vpc-03eaf74111
Stack RdsAuroraStack
There were no differences

Then I was trying to run again

➜  rds-aurora git:(develop) ✗ aws-vault exec staging -- cdk diff
vpc-03eaf74111
Stack RdsAuroraStack
There were no differences

At lastly, I added the rds backed. It becomes normal.

➜  rds-aurora git:(develop) ✗ aws-vault exec staging -- cdk diff
vpc-03eaf74111
Stack RdsAuroraStack

Resources
[+] AWS::RDS::DBSubnetGroup rds/Subnets rdsSubnet111
[+] AWS::RDS::DBCluster rds rds111
[+] AWS::RDS::DBInstance rds/Instance1 rdsInstance111
[+] AWS::RDS::DBInstance rds/Instance2 rdsInstance111

Environment

• CLI Version : 1.22.0
• Framework Version: typescript 3.7.5
• OS : macOS-Catalina 10.15.2
• Language : typescript

Other

Is there anyone who also encountered the same issue on ec2.Vpc.fromLookup?
The workaround to me is to remove rds.DatabaseCluster first, then run cdk diff and add it back and run again.

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This is 🐛 Bug Report