Overview of the Issue

In order to set image_signatures_db, the UEFI_COMPATIBLE flag must be passed as a guest OS feature:

source "googlecompute" "gcp_re_debian12_x86_64" {
   arch = "x86_64"
  
  image_guest_os_features = ["UEFI_COMPATIBLE"]
  image_signatures_db     = local.image_signatures_db
}

However, when using VMs that inherit from others, specify the image_guest_os_features would overwrite the ones that get used. The gcloud SDK actually doesn't check this guest OS features (https://github.com/google-cloud-sdk-unofficial/google-cloud-sdk/blob/master/lib/surface/compute/images/create.py#L306-L319) and allows the image signatures to be passed in regardless (note how the initial shielded VM state is set outside of what guest os features are provided).

Reproduction Steps

Don't set image_guest_os_features = ["UEFI_COMPATIBLE"] when setting image_signatures_db

Plugin and Packer version

Packer 1.2.5 verison

Operating system and Environment details

Linux x86 and arm64