Root Block Device Potential Breaking Change

[grubernaut]

A potentially breaking change was released in Terraform v0.8.6 from (#11619).

Since #1445 (v0.7.7) the following issue (#3941) has been open, where modifying attributes inside of a root_block_device configuration would no longer update the schema values in the statefile.

This has the potential to form a breaking change for any infrastructure that was created with Terraform v0.7.7 or older, with a direct upgrade to Terraform v0.8.6 or newer.

Potential break:

# terraform <v0.7.7
root_block_device {
  volume_size = 10
  delete_on_termination = false
}

Terraform will create the root block device, with delete_on_terminate correctly set.

# Terraform v0.7.7
root_block_device {
  volume_size = 10
}

Terraform will update the statefile to show delete_on_terminate=true, but will silently fail to actually update the root_block_device in AWS.

# Terraform v0.8.6
root_block_device {
  volume_size = 10
}

Upon running an apply for the latest config with the latest version of Terraform, a diff will not show for delete_on_terminate, but the AWS API will actually be updated, whereas previously the root_block_device would not be updated.

This can, of course, cause fairly unexpected changes.

tl;dr: The bugfix from (#11619) allows root_block_device parameters to actually be updated in the AWS Console, potentially causing an unknown diff between a users statefile and reality, for any infrastructures managed with Terraform <v0.7.7, and updating to >v0.8.6