Already applied dependency outputs not used when using --filter flag with Git refs

[SamuelBoerlin]

Describe the bug

When using the filter flag like terragrunt --experiment=filter-flag plan --filter="[HEAD~1...HEAD]" --all outputs of already applied dependency units aren't detected and instead the mock outputs are used.
Applying just the changed unit directly like --filter="dependent" without using Git refs works fine, i.e., the already applied dependency unit's outputs are detected and used.

Steps To Reproduce

// modules/dependency/main.tf
output "test_output" {
  value = "actual_output"
}

// modules/dependent/main.tf 
variable "test_input" {
  type = string
}

resource "null_resource" "echo" {
  for_each = tomap({ (var.test_input) = var.test_input })
  provisioner "local-exec" {
    command = "echo '${each.key}'"
  }
}

// units/dependency/terragrunt.hcl
include "root" {
  path = find_in_parent_folders("root.hcl")
}

terraform {
  source = "${find_in_parent_folders("modules")}/dependency"
}

// units/dependent/terragrunt.hcl
include "root" {
  path = find_in_parent_folders("root.hcl")
}

terraform {
  source = "${find_in_parent_folders("modules")}/dependent"
}

dependency "dependency" {
  config_path = "../dependency"
  
  mock_outputs = {
    test_output = "mock_output"
  }
}

inputs = {
  test_input = dependency.dependency.outputs.test_output
}

// stacks/root.hcl
remote_state {
  backend = "local"

  config = {
    path = "${get_parent_terragrunt_dir()}/.state/${path_relative_to_include()}/terraform.tfstate"
  }

  generate = {
    path = "backend.tf"
    if_exists = "overwrite_terragrunt"
  }
}

// stacks/test/terragrunt.stack.hcl
unit "dependency" {
  source = "${find_in_parent_folders("units")}/dependency"
  path = "dependency"
}

$ git init
$ git add .
$ git commit -m 'initial commit'

$ cd stacks
$ terragrunt apply --all
13:30:00.581 INFO   Generating unit dependency from ./test/terragrunt.stack.hcl
13:30:00.585 INFO   Unit queue will be processed for apply in this order:
- Unit test/.terragrunt-stack/dependency

Are you sure you want to run 'terragrunt apply' in each unit of the run queue displayed above? (y/n) y
13:30:01.759 INFO   [test/.terragrunt-stack/dependency] Downloading Terraform configurations from ../modules/dependency into ./test/.terragrunt-stack/dependency/.terragrunt-cache/9VwxB0LsvyL1V3lDIwI9ntEaav8/DwmkUDhOVSsWXewKJNfHGLZLrb4
13:30:01.810 INFO   [test/.terragrunt-stack/dependency] tofu: Initializing the backend...
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: Initializing provider plugins...
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: OpenTofu has been successfully initialized!
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: 
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: You may now begin working with OpenTofu. Try running "tofu plan" to see
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: any changes that are required for your infrastructure. All OpenTofu commands
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: should now work.
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: If you ever set or change modules or backend configuration for OpenTofu,
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: rerun this command to reinitialize your working directory. If you forget, other
13:30:01.813 INFO   [test/.terragrunt-stack/dependency] tofu: commands will detect it and remind you to do so if necessary.
13:30:01.872 STDOUT [test/.terragrunt-stack/dependency] tofu: Changes to Outputs:
13:30:01.872 STDOUT [test/.terragrunt-stack/dependency] tofu:   + test_output = "actual_output"
13:30:01.872 STDOUT [test/.terragrunt-stack/dependency] tofu: You can apply this plan to save these new output values to the OpenTofu
13:30:01.872 STDOUT [test/.terragrunt-stack/dependency] tofu: state, without changing any real infrastructure.
13:30:01.874 STDOUT [test/.terragrunt-stack/dependency] tofu: 
13:30:01.874 STDOUT [test/.terragrunt-stack/dependency] tofu: Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
13:30:01.874 STDOUT [test/.terragrunt-stack/dependency] tofu: 
13:30:01.874 STDOUT [test/.terragrunt-stack/dependency] tofu: Outputs:
13:30:01.874 STDOUT [test/.terragrunt-stack/dependency] tofu: test_output = "actual_output"

Now add the dependent unit:

// stacks/test/terragrunt.stack.hcl
unit "dependency" {
  source = "${find_in_parent_folders("units")}/dependency"
  path = "dependency"
}

unit "dependent" {
  source = "${find_in_parent_folders("units")}/dependent"
  path = "dependent"
}

git add .
git commit -m 'add dependent unit'

$ terragrunt --experiment=filter-flag plan --filter="[HEAD~1...HEAD]" --all
13:31:28.352 INFO   Generating unit dependent from /tmp/terragrunt-worktree-HEAD-856930344/stacks/test/terragrunt.stack.hcl
13:31:28.352 INFO   Generating unit dependency from ./test/terragrunt.stack.hcl
13:31:28.352 INFO   Generating unit dependency from /tmp/terragrunt-worktree-HEAD-856930344/stacks/test/terragrunt.stack.hcl
13:31:28.352 INFO   Generating unit dependency from /tmp/terragrunt-worktree-HEAD_1-3379350842/stacks/test/terragrunt.stack.hcl
13:31:28.352 INFO   Generating unit dependent from ./test/terragrunt.stack.hcl
13:31:28.405 INFO   Unit queue will be processed for plan in this order:
- Unit stacks/test/.terragrunt-stack/dependent

13:31:28.464 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] Downloading Terraform configurations from /tmp/terragrunt-worktree-HEAD-856930344/modules/dependency into /tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency/.terragrunt-cache/BdNKsXEZb5ujffbRTD3hSq6ehT0/PTqHdvF9VzAsgxrLwGqFHnRwFx8
13:31:28.516 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: Initializing the backend...
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: Initializing provider plugins...
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: OpenTofu has been successfully initialized!
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: 
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: You may now begin working with OpenTofu. Try running "tofu plan" to see
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: any changes that are required for your infrastructure. All OpenTofu commands
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: should now work.
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: If you ever set or change modules or backend configuration for OpenTofu,
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: rerun this command to reinitialize your working directory. If you forget, other
13:31:28.519 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency] tofu: commands will detect it and remind you to do so if necessary.
13:31:28.580 WARN   [stacks/test/.terragrunt-stack/dependent] Config /tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependency/terragrunt.hcl is a dependency of /tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent/terragrunt.hcl that has no outputs, but mock outputs provided and returning those in dependency output.
13:31:28.581 INFO   [stacks/test/.terragrunt-stack/dependent] Downloading Terraform configurations from /tmp/terragrunt-worktree-HEAD-856930344/modules/dependent into /tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent/.terragrunt-cache/iphAX9Mw-kasr0DubbrO3kX69wg/11C1Ewc9XzjthK26Br374jeQsew
13:31:28.633 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: Initializing the backend...
13:31:28.635 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: Initializing provider plugins...
13:31:28.635 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: - Reusing previous version of hashicorp/null from the dependency lock file
13:31:28.762 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: - Detected previously-installed hashicorp/null v3.2.4 in the shared cache directory
13:31:28.763 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: - Using hashicorp/null v3.2.4 from the shared cache directory
13:31:28.763 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: OpenTofu has been successfully initialized!
13:31:28.763 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: 
13:31:28.763 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: You may now begin working with OpenTofu. Try running "tofu plan" to see
13:31:28.763 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: any changes that are required for your infrastructure. All OpenTofu commands
13:31:28.763 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: should now work.
13:31:28.764 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: If you ever set or change modules or backend configuration for OpenTofu,
13:31:28.764 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: rerun this command to reinitialize your working directory. If you forget, other
13:31:28.764 INFO   [/tmp/terragrunt-worktree-HEAD-856930344/stacks/test/.terragrunt-stack/dependent] tofu: commands will detect it and remind you to do so if necessary.
13:31:29.102 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: OpenTofu used the selected providers to generate the following execution
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: plan. Resource actions are indicated with the following symbols:
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu:   + create
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: OpenTofu will perform the following actions:
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu:   # null_resource.echo["mock_output"] will be created
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu:   + resource "null_resource" "echo" {
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu:       + id = (known after apply)
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu:     }
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: Plan: 1 to add, 0 to change, 0 to destroy.
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: 
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: ─────────────────────────────────────────────────────────────────────────────
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: Note: You didn't use the -out option to save this plan, so OpenTofu can't
13:31:29.103 STDOUT [stacks/test/.terragrunt-stack/dependent] tofu: guarantee to take exactly these actions if you run "tofu apply" now.

❯❯ Run Summary  1 units  704ms
   ────────────────────────────
   Succeeded    1

Note the mock output being used instead of the already applied dependency unit's output actual_output:

# null_resource.echo["mock_output"] will be created

Expected behavior

I'd expect --filter="[HEAD~1...HEAD]" to detect outputs and to work exactly the same as filtering the changed unit directly with --filter="dependent":

terragrunt --experiment=filter-flag plan --filter="dependent" --all
13:45:53.538 INFO   Generating unit dependent from ./test/terragrunt.stack.hcl
13:45:53.538 INFO   Generating unit dependency from ./test/terragrunt.stack.hcl
13:45:53.590 INFO   Unit queue will be processed for plan in this order:
- Unit test/.terragrunt-stack/dependent

13:45:53.698 INFO   [test/.terragrunt-stack/dependency] tofu: Initializing the backend...
13:45:53.700 INFO   [test/.terragrunt-stack/dependency] tofu: Initializing provider plugins...
13:45:53.700 INFO   [test/.terragrunt-stack/dependency] tofu: OpenTofu has been successfully initialized!
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: 
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: You may now begin working with OpenTofu. Try running "tofu plan" to see
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: any changes that are required for your infrastructure. All OpenTofu commands
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: should now work.
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: If you ever set or change modules or backend configuration for OpenTofu,
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: rerun this command to reinitialize your working directory. If you forget, other
13:45:53.701 INFO   [test/.terragrunt-stack/dependency] tofu: commands will detect it and remind you to do so if necessary.
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: OpenTofu used the selected providers to generate the following execution
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: plan. Resource actions are indicated with the following symbols:
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu:   + create
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: OpenTofu will perform the following actions:
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu:   # null_resource.echo["actual_output"] will be created
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu:   + resource "null_resource" "echo" {
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu:       + id = (known after apply)
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu:     }
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: Plan: 1 to add, 0 to change, 0 to destroy.
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: 
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: ─────────────────────────────────────────────────────────────────────────────
13:45:54.100 STDOUT [test/.terragrunt-stack/dependent] tofu: Note: You didn't use the -out option to save this plan, so OpenTofu can't
13:45:54.101 STDOUT [test/.terragrunt-stack/dependent] tofu: guarantee to take exactly these actions if you run "tofu apply" now.

❯❯ Run Summary  1 units  517ms
   ────────────────────────────
   Succeeded    1

Note that the correct output actual_output is used:

# null_resource.echo["actual_output"] will be created

Must haves

• Steps for reproduction provided.

Nice to haves

• Terminal output
• Screenshots

Versions

• Terragrunt version: v0.95.1
• OpenTofu/Terraform version: OpenTofu v1.10.7 on linux_amd64
• Environment details (Ubuntu 20.04, Windows 10, etc.): Ubuntu 24.04.3 LTS

Additional context

This is the issue I mentioned in #4060 (comment) .

[yhakbar]

Hey @SamuelBoerlin ,

Are you using remote state? If you take a look at the docs here under "How it works":
https://terragrunt.gruntwork.io/docs/features/filter/#git-based-expressions

You can see that runs will be performed in different worktrees, depending on the content being managed. I wonder if that's influencing what you're seeing here? If you have local state, that might impact things.

[SamuelBoerlin]

Hey @SamuelBoerlin ,

Are you using remote state? If you take a look at the docs here under "How it works": https://terragrunt.gruntwork.io/docs/features/filter/#git-based-expressions

You can see that runs will be performed in different worktrees, depending on the content being managed. I wonder if that's influencing what you're seeing here? If you have local state, that might impact things.

Right, sorry, forgot to add that I'm using local state. Updated the repro steps.

[yhakbar]

So I think two things we can do are:

1. Add a warning to make sure that users are aware using local state can result in messed up runs with Git-based filters.
2. Update docs with a call out and a workaround for users trying out Git-based filters with local state.

Does that sound like a good plan of action?

[SamuelBoerlin]

Okay so after changing the local state path to an absolute path instead of using get_parent_terragrunt_dir() it does detect the output.

Yes, that sounds reasonable, thanks.

I just looked over the docs again and they do already call out this caveat re running in the worktrees:

Then, for any unit that is discovered within those worktrees, Terragrunt will enqueue that unit for a run in the run queue in the worktree where it was discovered.
...
As a consequence, however, you may find that usage of absolute paths don’t work how you expect, as you will be performing runs in the temporary directories created for the relevant worktrees.

It might be worth adding an extra remark specifically regarding local state.