Expose usePluginCache configuration option to the configuration files.

[mplzik]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

As a platform engineer, I use atlantis to create plans across huge numbers of modules (sometimes hundreds) in parallel. Terraform is configured in a way that it doesn't need to fetch providers from the internet -- it fetches it from a filesystem mirror instead. Since Atlantis configures TF_PLUGIN_CACHE_DIR environment variable, the providers will get symlinked to workspaces. Unfortunately, terraform init running while other modules are already running terraform plan fails because it opens the provider binaries in the cache for writing as well, making it almost impossible for most of the plans to succeed.

Describe the solution you'd like

An option to disable the shared plugin cache would allow to avoid the race condition between terraform init and terraform plan.

Describe the drawbacks of your solution

Without a shared cache, providers need to be fetched from the mirror for every workspace, consuming more disk space and I/O bandwidth. In case of misconfiguration, there is a risk of extraneous requests to network terraform mirrors.

Describe alternatives you've considered

1. Fix the issue in Terraform codebase. Since this is documented in https://developer.hashicorp.com/terraform/cli/config/config-file as inherently not being concurrency safe with no tracking issue, I believe this is a low-priority problem or even a feature.
2. Create a terraform wrapper that would explicitly unset TF_PLUGIN_CACHE_DIR before exec-ing into terraform. This is a fairly awkward and fragile solution.