Use CustomizeDiff to infer the provider default values

[rileykarson]

Affected Resource(s)

• google_*

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The provider default project (project field) is substituted in for resources whose project field is empty at apply time across the provider using the getProject method, which checks state and then the provider config for the project. The config object holding provider configuration was only available at apply time, historically, but that's not true with CustomizeDiff around nowadays. We could, presumably, implement a standardised CustomizeDiff for all project-level resources, and determine the final value of the project field (and zone + region) at plan time. Given a config like the following:

provider "google" {
  project = "my-project"
}

resource "google_compute_address" "my-address" {}

Our provider would go from the following terraform plan on creation:

...
      + project            = (known after apply)
...

To the following:

...
      + project            = "my-project"
...

Additionally, we could return a plan-time error when the value is not set.

Note that the current behaviour where the project value is "sticky" was originally due to not having enough information at plan time. We could preserve the existing behaviour by only rewriting the new state to the provider project when both old and new state are empty, and eliminate the behaviour by rewriting whenever new is empty. That's clearly a breaking change, however, I feel that adding the plan-time display+error likely counts as one as well.

In short, I think we could use CustomizeDiff to improve our handling of default projects (but it would be a breaking change to do either of these):

• Writing the provider project into the new state on create when the resource-level one is unset, displaying it in plan, and adding a check to return an error if it is empty
• Writing the provider project into the new state on all plans when the resource-level one is unset, overriding old values and removing the current sticky behaviour

Also, as a note: we'd need to check for unknown values in CustomizeDiff! CD does not protect against them by default, in contrast to other schema functions, and unknown values appear to be empty without special handling.

[wyardley]

Thanks Riley - that would be super helpful!

[wyardley]

Maybe related: #5761?

[rileykarson]

Note: the actual change (or changing just generated resources) is likely size/s or size/m, but applying to all resources makes this size/xl

[c2thorn]

closed with GoogleCloudPlatform/magic-modules#8569

This will release with v5.0.0. Check #15582 for more details on the major release.

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