Fixing Terraform Azure Container App Errors: Sub-Path Issue

Terraform Azure Container Apps: Solving the Sub-Path Problem

Hey everyone! Let's dive into a specific issue that can pop up when you're using Terraform to manage Azure Container Apps. We're talking about a bug related to the volume_mnt schema and how it handles sub_path, especially in version 8.2.0 of the module. If you've run into errors about missing attributes when deploying container apps, this might be exactly what you need to read. We'll cover the problem, the fix, a workaround, and how to apply it using OpenTofu, including the specific configuration files. This guide is designed to be easy to understand, even if you're new to the whole Terraform and Azure world.

The Core Problem: volume_mnt and sub_path

The root of the issue lies in how the volume_mnt object is defined within the modules/container-app/variables-aca.tf file of the module. In version 8.2.0, the volume_mnt was set up without a sub_path attribute. This is a problem because the modules/container-app/r-container-app.tf file unconditionally tries to reference volume_mounts.value.sub_path. So, when you configure a secret-backed volume and omit sub_path, which is perfectly valid if you don't need to mount a specific file, Terraform throws an error.

This is the error you might see:

Error: Unsupported attribute

on modules/container-app/r-container-app.tf line 135, in resource "azurerm_container_app" "main":
   135:             sub_path = volume_mounts.value.sub_path
     ├────────────────
     │ volume_mounts.value is object with 2 attributes

This object does not have an attribute named "sub_path".

Basically, Terraform is looking for something that isn't there, and it crashes. This error happens when you're trying to deploy a container app that uses volume mounts, especially those backed by secrets where you don't specify a sub_path. You'll hit this when you're using the AzureRM provider, and your configuration files include a volume_mnt configuration that's not quite aligned with what the module expects.

How to Fix It: Step-by-Step

Fortunately, there's a straightforward fix. The key is to update the schema in modules/container-app/variables-aca.tf to allow for the sub_path attribute and then guard access to it in modules/container-app/r-container-app.tf.

Here's how to do it. First, update the schema in variables-aca.tf to include sub_path:

volume_mnt = optional(list(object({
  name     = string
  path     = string
  sub_path = optional(string, null)
})), [])

This change allows you to specify sub_path if you need it, but it's still optional. Existing configurations won't break because sub_path defaults to null if not specified. Next, guard the access to sub_path in r-container-app.tf:

sub_path = lookup(volume_mounts.value, "sub_path", null)

This change uses lookup, which safely checks for the existence of sub_path before trying to access it. If sub_path isn't present, it defaults to null, preventing the error. With these fixes, the module becomes much more flexible. It supports users who need file mounts using sub_path while staying compatible with existing configurations that don't require it.

Workaround Until the Official Fix

Until a new release is published, we have a workaround. This involves patching the cached module during terragrunt init. This approach uses a terraform block with an after_hook that runs a bash script to modify the necessary files. Here's how the workaround looks:

terraform {
  after_hook "patch_sub_path" {
    commands = ["init"]
    execute = [
      "bash",
      "-c",
      "cat <<'PY' | python3\nfrom pathlib import Path\nrca = Path('modules/container-app/r-container-app.tf')\nif rca.exists():\n    text = rca.read_text()\n    text = text.replace('sub_path = volume_mounts.value.sub_path', 'sub_path = lookup(volume_mounts.value, \"sub_path\", null)')\n    rca.write_text(text)\nvars_file = Path('modules/container-app/variables-aca.tf')\nif vars_file.exists():\n    text = vars_file.read_text()\n    marker = '      path = string'\n    insertion = '      path = string\\n      sub_path = optional(string, null)'\n    if 'sub_path = optional' not in text:\n        text = text.replace(marker, insertion)\n        vars_file.write_text(text)\nPY"
    ]
  }
}

This code snippet runs a Python script during terraform init. The script checks if r-container-app.tf and variables-aca.tf exist. If they do, it replaces the problematic line with the safer lookup function and adds the sub_path attribute. The PY block contains a Python script. It uses the pathlib module to handle file paths safely. This workaround ensures that your module is patched before Terraform attempts to use it, avoiding the error.

Configuration File Example

To help visualize how this fits into your configuration, here’s a sample of how you might structure your containers and volumes blocks:

containers = [
    {
      name   = "prometheus"
      image  = "docker.io/prom/prometheus:v2.49.1"
      cpu    = 0.25
      memory = "0.5Gi"
      envs   = []
      volume_mnt = [
        {
          name     = "config-volume"
          path     = "/etc/prometheus"
          sub_path = "prometheus.yml"
        }
      ]
    }
  ]

  volumes = [
    {
      name         = "config-volume"
      storage_type = "Secret"
    }
  ]

In this example, we're deploying a Prometheus container. The prometheus.yml configuration file is mounted into the container at /etc/prometheus. The volume config-volume uses a secret. If sub_path weren’t correctly configured, this would trigger the error.

Quick Recap and Best Practices

• Understand the Error: The core issue is a missing sub_path attribute in the Terraform module, causing an "Unsupported attribute" error.
• Implement the Fix: Update the schema in variables-aca.tf and guard the sub_path access in r-container-app.tf. This keeps everything backward-compatible while adding functionality.
• Use the Workaround: If a new version of the module isn’t available, use the terragrunt init hook to patch the module during the initialization phase.
• Review Configuration: Make sure your containers and volumes blocks are correctly structured and that you're using sub_path where necessary.

That's the gist of it! This issue highlights the importance of understanding the schemas that Terraform uses and how to adapt when there are changes. Hope this helps you to avoid these types of problems in the future and make your deployments smoother.