Unexpected behavior when creating cache for local modules (symlink to + copies of module)

[agrzegorczyk-leonsoftware]

Terraform Version

Terraform v0.12.10

Terraform Configuration Files

/home/devops
├── infrastructure
│   └── main.tf
└── modules
    └── test
        └── empty.tf

# infrastructure/main.tf:
module "a" {
  source = "/home/devops/modules/test"
}

module "b" {
  source = "/home/devops/modules/test"
}

module "c" {
  source = "/home/devops/modules/test"
}

https://leon-downloads.s3-eu-west-1.amazonaws.com/devops/github/012modules.tar.gz

Debug Output

terraform init

2019/10/08 17:59:39 [INFO] Terraform version: 0.12.10  
2019/10/08 17:59:39 [INFO] Go runtime version: go1.12.9
2019/10/08 17:59:39 [INFO] CLI args: []string{"/usr/local/bin/terraform", "init"}
2019/10/08 17:59:39 [DEBUG] Attempting to open CLI config file: /home/geck/.terraformrc
2019/10/08 17:59:39 [DEBUG] File doesn't exist, but doesn't need to. Ignoring.
2019/10/08 17:59:39 [INFO] CLI command args: []string{"init"}
Initializing modules...
2019/10/08 17:59:39 [TRACE] ModuleInstaller: installing child modules for . into .terraform/modules
2019/10/08 17:59:39 [DEBUG] Module installer: begin a
2019/10/08 17:59:39 [TRACE] ModuleInstaller: a is not yet installed
2019/10/08 17:59:39 [TRACE] ModuleInstaller: cleaning directory .terraform/modules/a prior to install of a
2019/10/08 17:59:39 [TRACE] ModuleInstaller: a address "/home/devops/modules/test" will be handled by go-getter
2019/10/08 17:59:39 [DEBUG] will download "/home/devops/modules/test" to .terraform/modules/a
Downloading /home/devops/modules/test for a...
2019/10/08 17:59:39 [TRACE] go-getter detectors rewrote "/home/devops/modules/test" to "file:///home/devops/modules/test"
2019/10/08 17:59:39 [TRACE] fetching "file:///home/devops/modules/test" to ".terraform/modules/a"
2019/10/08 17:59:39 [TRACE] ModuleInstaller: a "/home/devops/modules/test" was downloaded to .terraform/modules/a
2019/10/08 17:59:39 [DEBUG] Module installer: a installed at .terraform/modules/a
- a in .terraform/modules/a
2019/10/08 17:59:39 [DEBUG] Module installer: begin b
2019/10/08 17:59:39 [TRACE] ModuleInstaller: b is not yet installed
2019/10/08 17:59:39 [TRACE] ModuleInstaller: cleaning directory .terraform/modules/b prior to install of b
2019/10/08 17:59:39 [TRACE] ModuleInstaller: b address "/home/devops/modules/test" will be handled by go-getter
Downloading /home/devops/modules/test for b...
2019/10/08 17:59:39 [DEBUG] will download "/home/devops/modules/test" to .terraform/modules/b
2019/10/08 17:59:39 [TRACE] go-getter detectors rewrote "/home/devops/modules/test" to "file:///home/devops/modules/test"
2019/10/08 17:59:39 [TRACE] copying previous install .terraform/modules/a to .terraform/modules/b
2019/10/08 17:59:39 [TRACE] ModuleInstaller: b "/home/devops/modules/test" was downloaded to .terraform/modules/b
2019/10/08 17:59:39 [DEBUG] Module installer: b installed at .terraform/modules/b
- b in .terraform/modules/b
2019/10/08 17:59:39 [DEBUG] Module installer: begin c
2019/10/08 17:59:39 [TRACE] ModuleInstaller: c is not yet installed
2019/10/08 17:59:39 [TRACE] ModuleInstaller: cleaning directory .terraform/modules/c prior to install of c
2019/10/08 17:59:39 [TRACE] ModuleInstaller: c address "/home/devops/modules/test" will be handled by go-getter
Downloading /home/devops/modules/test for c...
2019/10/08 17:59:39 [DEBUG] will download "/home/devops/modules/test" to .terraform/modules/c
2019/10/08 17:59:39 [TRACE] go-getter detectors rewrote "/home/devops/modules/test" to "file:///home/devops/modules/test"
2019/10/08 17:59:39 [TRACE] copying previous install .terraform/modules/a to .terraform/modules/c
2019/10/08 17:59:39 [TRACE] ModuleInstaller: c "/home/devops/modules/test" was downloaded to .terraform/modules/c
- c in .terraform/modules/c

Initializing the backend...
2019/10/08 17:59:39 [DEBUG] Module installer: c installed at .terraform/modules/c
2019/10/08 17:59:39 [TRACE] modsdir: writing modules manifest to .terraform/modules/modules.json
2019/10/08 17:59:39 [TRACE] Meta.Backend: no config given or present on disk, so returning nil config
2019/10/08 17:59:39 [TRACE] Meta.Backend: backend has not previously been initialized in this working directory
2019/10/08 17:59:39 [DEBUG] New state was assigned lineage "149e705b-17e1-d737-3932-325c45c81f32"
2019/10/08 17:59:39 [TRACE] Meta.Backend: using default local state only (no backend configuration, and no existing initialized backend)
2019/10/08 17:59:39 [TRACE] Meta.Backend: instantiated backend of type <nil>
2019/10/08 17:59:39 [DEBUG] checking for provider in "."
2019/10/08 17:59:39 [DEBUG] checking for provider in "/usr/local/bin"
2019/10/08 17:59:39 [DEBUG] checking for provisioner in "."
2019/10/08 17:59:39 [DEBUG] checking for provisioner in "/usr/local/bin"
2019/10/08 17:59:39 [INFO] Failed to read plugin lock file .terraform/plugins/linux_amd64/lock.json: open .terraform/plugins/linux_amd64/lock.json: no such file or directory
2019/10/08 17:59:39 [TRACE] Meta.Backend: backend <nil> does not support operations, so wrapping it in a local backend
2019/10/08 17:59:39 [TRACE] backend/local: state manager for workspace "default" will:
 - read initial snapshot from terraform.tfstate
 - write new snapshots to terraform.tfstate
 - create any backup at terraform.tfstate.backup
2019/10/08 17:59:39 [TRACE] statemgr.Filesystem: reading initial snapshot from terraform.tfstate
2019/10/08 17:59:39 [TRACE] statemgr.Filesystem: snapshot file has nil snapshot, but that's okay
2019/10/08 17:59:39 [TRACE] statemgr.Filesystem: read nil snapshot
2019/10/08 17:59:39 [DEBUG] checking for provider in "."
2019/10/08 17:59:39 [DEBUG] checking for provider in "/usr/local/bin"

Crash Output

N/A

Expected Behavior

All of referenced modules symlinked (preferably) or copied to local .terraform/modules cache.

Actual Behavior

Terraform during initialization creates one symlink to source of a module and additional 2 copies of it. Exact numer on copies depends of quantity of the module instances
Happens when module source is absolute path to local directory (outside the main terraform working dir).

/home/devops
├── infrastructure
│   ├── .terraform
│   │   └── modules
│   │       ├── a -> /home/devops/modules/test # symlink
│   │       ├── b # copy
│   │       │   └── empty.tf
│   │       ├── c # copy
│   │       │   └── empty.tf
│   │       └── modules.json
│   └── main.tf
└── modules
    └── test
        └── main.tf

Steps to Reproduce

terraform init

Additional Context

References

• Terraform downloads module from git source for every usage #11435 (comment) (possible root cause)

[landintrees]

This has caught us out a few times by exhibiting quite confusing behaviour when a change is made in a module source that is then not pulled into calling modules greater than 1.

For instance, after adding a new variable "cosmosdb_resource_group" to a module definition, the first caller works fine (because it is symlinked), but the 2nd and 3rd error (because they are copies):

Error: Unsupported argument
on main.tf line 116, in module "module_2":
116: cosmosdb_resource_group = local.cosmosdb_resource_group

[hayorov]

Due to closed #11435 I'd like to slightly offtopic and share a small pre-terraform routine utility that optimize init (modules download) for git modules https://github.com/hayorov/terraform-init-booster