I’m trying to create a CloudFormation template that has, among other things, an EC2 Instance with an attached EBS volume. The user data in the EC2 instance should mount that volume and write some data into it:

Resources:
  #More resources in the real template
  EBSVolume:
    Type: AWS::EC2::Volume
    Properties:
      AvailabilityZone: !GetAtt Subnet.AvailabilityZone
      VolumeType: gp3
      Size: 10
  EBSVolumeMount:
    Type: AWS::EC2::VolumeAttachment
    Properties:
      Device: /dev/sdf
      InstanceId: !Ref Instance
      VolumeId: !Ref EBSVolume
  Instance:
    Type: AWS::EC2::Instance
    CreationPolicy:
      ResourceSignal:
        Timeout: PT5M
    Properties:
      InstanceType: t2.micro
      ImageId: !Ref LatestAmiId      
      SubnetId: !Ref Subnet
      SecurityGroupIds: 
        - !Ref InstanceSecurityGroup
      UserData:
        Fn::Base64: !Sub |
          #!/bin/bash -xe
          yum -y update
          yum -y upgrade
          yum install -y aws-cfn-bootstrap

          #Create the file system for the EBS volume and mount it
          mkfs -t xfs /dev/xvdf
          mkdir /ebs
          mount /dev/xvdf /ebs
          blkid -s UUID -o value /dev/xvdf | xargs -I {} sudo sh -c "echo 'UUID={}  /ebs  xfs  defaults,nofail  0  2' >> /etc/fstab"

          (more bootstrapping that involves writing data into /ebs)

          #Signal CloudFormation
          /opt/aws/bin/cfn-signal -e $? --stack ${AWS::StackName} --resource Instance --region ${AWS::Region}

The problem that I’m encountering with the above setup is that the AWS::EC2::VolumeAttachment resource depends on the AWS::EC2::Instance resource but its user data expects the volume to be created. If it isn’t, the script fails and the CFN signal is never sent, therefore failing the stack.

If I don’t use cfn-signal it works because the volume is provisioned and available before the user data is run as the CREATE_COMPLETE signal is sent before running the user data. Is there a way for me to use cfn-signal in this scenario?