Terraform: Solving the double slash mystery with Terragrunt

The solution I found to my particular case of “Error: Unreadable module directory” when using Terragrunt.

Terms

A quick note on wording

• Concrete service: A service in TF using a shared module. Example: a specific web site site.example.com
• Shared module or module: Encapsulated TF provisioning logic to be reused as required. Example: the “webserver” module used by the above web site concrete service

Details

Where: Concrete service definition.

I was baffled by the following error message from Terraform when standing up a concrete service (using contrived module names):

Initializing modules...
- sibling1_mod in
- sibling2_mod in
-
Error: Unreadable module directory

Unable to evaluate directory symlink: lstat ../sibling1: no such file or directory

This is triggered when including a module as follows when trying to instantiate the concrete service

terraform {
  source = "../../../../../modules/example1"
}

The fix was simply to add a missing slash (/) to the base of the repository relative path above. So if ../../../../../ takes you to the Git repo root then you need a double slash at the end ../../../../..// followed by the module path. Full example:

terraform {
  source = "../../../../..//modules/example1"
  #                       ^^ Double slash here!
}

I should say Terragrunt also offers a useful hint; the implications of which I did not fully understand initially:

WARNING: no double-slash (//) found in source URL /Users/fclausen/freddiegit/tf12-mod-source-issue/modules/example1. Relative paths in downloaded Terraform code may not work.

Circumstances in which this happen

This will happen under the following conditions

• Using in-repo Terraform modules; either part of the repo or placed there some other way
• Forgetting the double slash as described above (obviously)
• The module you are sourceing includes transitive dependencies at the same level in the directory tree. E.g.

  .
  ├── accounts
  │   └── account1
  │       └── earth
  │           └── dev
  │               └── service1 # <-- source =  "../../../../..//modules/example1"
  ├── common
  └── modules
    ├── example1 # Depends on sibling1 and sibling2 via `source ../sibling1`, `source ../sibling2`
    ├── sibling1
    └── sibling2
  

Example Repo

I have put an example repo to demonstrate this issue here:

https://github.com/ftclausen/tf12-mod-source-issue

Specifically you can check out the demonstrate_incorrect_slash_usage tag to replicate