Terraform Crash - "Unexpected EOF" when using azurerm_virtual_machine_scale_set

Problem

When using the azurerm_virtual_machine_scale_set resource, Terraform may crash with an unexpected EOF error.

!!!!!!!!!!!!!!!!!!!!!!!!!!! TERRAFORM CRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!

* module.mymodulename.azurerm_virtual_machine_scale_set.main: 1 error occurred:
* module.mymodulename.azurerm_virtual_machine_scale_set.main: azurerm_virtual_machine_scale_set.main: unexpected EOF

panic: runtime error: invalid memory address or nil pointer dereference

Cause

This crash can occur when using the deprecated azurerm_virtual_machine_scale_set resource with newer Azure features that it does not support. Because the Azure API does not support these operations with the older resource, Terraform panics.

The azurerm_virtual_machine_scale_set resource is feature-frozen and has been superseded by two new resources: azurerm_linux_virtual_machine_scale_set and azurerm_windows_virtual_machine_scale_set.

As noted in the provider documentation:

The azurerm_virtual_machine_scale_set resource has been superseded by the azurerm_linux_virtual_machine_scale_set and azurerm_windows_virtual_machine_scale_set resources. The existing azurerm_virtual_machine_scale_set resource will continue to be available throughout the 2.x releases however is in a feature-frozen state to maintain compatibility - new functionality will instead be added to the azurerm_linux_virtual_machine_scale_set and azurerm_windows_virtual_machine_scale_set resources.

Additional explanation from the provider engineering team is available in the related GitHub issue.

Solution

To resolve this issue, you must replace the deprecated azurerm_virtual_machine_scale_set resource in your configuration with either azurerm_linux_virtual_machine_scale_set or azurerm_windows_virtual_machine_scale_set, depending on your use case.

You can migrate your existing infrastructure to the new resource by using the terraform state mv command or by importing the existing scale set into the new resource definition.

Additional Information

• From AzureRM provider v2.86 onwards, the azurerm_linux_virtual_machine_scale_set and azurerm_windows_virtual_machine_scale_set resources use only the Uniform Orchestration Mode.
• For the Flexible Orchestration Mode, use the azurerm_orchestrated_virtual_machine_scale_set resource instead. This resource also exclusively uses Flexible Orchestration Mode from v2.86 onwards.
• For a detailed comparison of Virtual Machine Scale Set (VMSS) Orchestration Modes, refer to the official Azure documentation.
• If you are using an AzureRM 1.x provider release, you must upgrade to a newer version. The replacement scale set resources were released in v2.0.0. Refer to the AzureRM v2.0 upgrade guide for assistance.