Module within module silently ignored until `terraform init`

[benhoskings]

Terraform Version

0.12.2

G'day, I've found on terraform 0.12.x that a module within a module won't be detected during plan and will be silently ignored until terraform init is run, rather than failing plan like a top-level module does.

Terraform Configuration Files

$ cat target/target.tf
resource "null_resource" "test" {
  provisioner "local-exec" {
    command = "true"
  }
}

module "mod" {
  source = "../module"
}

$ cat module/module.tf
resource "null_resource" "mod_test" {
  provisioner "local-exec" {
    command = "true"
  }
}

$ cat submod/submod.tf
resource "null_resource" "submod_test" {
  provisioner "local-exec" {
    command = "true"
  }
}

If you initialise the above structure, and then modify module/module.tf like so without re-initing:

$ cat module/module.tf
resource "null_resource" "mod_test" {
  provisioner "local-exec" {
    command = "true"
  }
}

module "submod" {
  source = "../submod"
}

Expected behaviour

Error: Module not installed

  on module.tf line 7:
   7: module "submod" {

Actual behaviour

$ terraform plan

# ...

Terraform will perform the following actions:

  # null_resource.test will be created
  + resource "null_resource" "test" {
      + id = (known after apply)
    }

  # module.mod.null_resource.mod_test will be created
  + resource "null_resource" "mod_test" {
      + id = (known after apply)
    }

Plan: 2 to add, 0 to change, 0 to destroy.

$ terraform init

# ...

$ terraform plan

# ...

Terraform will perform the following actions:

  # null_resource.test will be created
  + resource "null_resource" "test" {
      + id = (known after apply)
    }

  # module.mod.null_resource.mod_test will be created
  + resource "null_resource" "mod_test" {
      + id = (known after apply)
    }

  # module.mod.module.submod.null_resource.submod_test will be created
  + resource "null_resource" "submod_test" {
      + id = (known after apply)
    }

Plan: 3 to add, 0 to change, 0 to destroy.

[jbardin]

HI @benhoskings,

Thanks for filing the issue. This may seem unusual at first, but this is the expected behavior as new modules always need to be initialized via init. Terraform needs to analyze the module, ensure that all versions constraints match, the providers are available, and record the module location for reference.

The init command by itself is meant to be idempotent, and can be run whenever you have a change to the configuration to ensure everything is correctly initialized.

[benhoskings]

@jbardin Absolutely, I understand that the init is required; the problem is that terraform doesn't. :)

I would say terraform should fail with an error in this case, as shown in "expected output" above; instead it silently omits module changes.

[jbardin]

Apologies @benhoskings, I saw your "expected output" and missed that it wasn't what you got ;).
That is unusual, we'll have to take a closer look.

[gchamon]

My first thought when I came across this issue was that modules within modules wasn't supported, because while terraform silently ignores modules within modules, it fails instantly if the module is declared in the project's root folder.

It is actually just that terraform won't search for submodules within modules when planning to ask for terraform init, just for modules declared in .tf files in the root folder.

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.