Add a 'static_tags' option to disable automatic merging of tags

[Makeshift]

what

• Adds a new variable static_tags, set to false by default, which stops the module from merging in new tags and instead uses the tags variable in the input as-is, including handing it off to other modules as necessary

why

• Helps avoid default_tags always shows an update hashicorp/terraform-provider-aws#18311
• Being able to precompute labels which are then passed into cloudposse modules can be useful in certain circumstances

I have a fork of terraform-null-label that removes tags that interfere with provider default_tags, but this isn't obeyed by downstream cloudposse modules. This PR should make it possible to pass context in and force null_label to not make any tags, or only make a subset of the tags :)

references

• default_tags always shows an update hashicorp/terraform-provider-aws#18311

[Makeshift]

/test all

[Nuru]

@Makeshift Thank you for this contribution.

I think I understand the problem, though maybe you want to explain it in further detail, because it does not look to me like this PR solves the problem.

As I understand it, the issue is that the current AWS provider does not like it when a resource's tags have the same keys as the provider's default_tags. However, in some cases, modules add tags that are critical to proper functioning of the module, such as tagging resources as managed by an autoscaler or belonging to a Kubernetes cluster. So we cannot allow an option that blocks modules from adding tags.

To me this suggest 2 reasonable solutions:

1. Tell people not to use the default_tags option to the provider and instead implement default_tags in null-label and have people use null-label's tags output. This has the added advantage of properly allowing tags to be overridden, which the AWS provider seem not to support. Of course the disadvantage is that this does not cover any resources created by modules that do not use null-label. And in fact, we do not even need a new input for this. Simply setting the current tags input with the set of default tags in a base null-label module in the root module would do it.
2. Add some input like suppress_tags which provides a list of tag keys to filter out of the tags output, so those tags are not set in resources. The trade-off here is that while this ensures all resources get the default tags via the provider, the default tags cannot be overridden.

@Makeshift What do you think of those solutions?

[Makeshift]

You're correct in your assessment - Thank you for pointing out the flaw in disabling tags, it didn't occur to me that they could be integral to the working of certain resources/services. Do you have an example of a module that does this so I can make sure that any changes I make doesn't break it?

1. Tell people not to use the default_tags option to the provider and instead implement default_tags in null-label and have people use null-label's tags output. This has the added advantage of properly allowing tags to be overridden, which the AWS provider seem not to support. Of course the disadvantage is that this does not cover any resources created by modules that do not use null-label. And in fact, we do not even need a new input for this. Simply setting the current tags input with the set of default tags in a base null-label module in the root module would do it.

That disadvantage is rather a large one, unfortunately, I believe that default_tags is simply too convenient of an option to ignore, even if it's a little broken currently. Being able to generate tags for an entire stack without manually setting them is invaluable for costcentering.

2. Add some input like suppress_tags which provides a list of tag keys to filter out of the tags output, so those tags are not set in resources. The trade-off here is that while this ensures all resources get the default tags via the provider, the default tags cannot be overridden.

This seems like a reasonable compromise. You're suggesting something along the lines of allowing the user to set suppress_tags = ["environment", "stage", "name"], then tags becomes:

tags = { for k, v in merge(local.generated_tags, local.input.tags): k => v if ! contains([for i in local.suppress_tags: lower(i)], lower(k)) }

?
@Nuru

Edit: I've implemented and committed what I think you were proposing, let me know :)

[Makeshift]

/test all

[Nuru]

@Makeshift I hope you do not mind, I am going to steal this idea and some of this code and re-implement it in my current PR, rather than ask you to make additional changes.