[BUG]: github_repository_environment CustomizeDiff panics on nil reviewers after upgrade from v6.11.x to v6.12.x

[mwielgosz-esky]

Expected Behavior

Upgrading from terraform-provider-github v6.11.x to v6.12.x should work seamlessly for existing github_repository_environment resources, including those that have no reviewers block configured.

Actual Behavior

terraform plan crashes with a nil pointer dereference panic in the new CustomizeDiff function when processing existing github_repository_environment resources that have no reviewers:

Stack trace from the terraform-provider-github_v6.12.1 plugin:

panic: interface conversion: interface {} is nil, not map[string]interface {}

goroutine 203 [running]:
github.com/integrations/terraform-provider-github/v6/github.resourceGithubRepositoryEnvironmentDiff({0x1260dc8?, 0x20d058af4c90?}, 0x20d058930820?, {0xf42c40?, 0x20d0587b6e80?})
	github.com/integrations/terraform-provider-github/v6/github/resource_github_repository_environment.go:133 +0x1c5
github.com/integrations/terraform-provider-github/v6/github.resourceGithubRepositoryEnvironment.All.func3(...)
	github.com/hashicorp/terraform-plugin-sdk/v2@v2.38.2/helper/customdiff/compose.go:52 +0xd9
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).PlanResourceChange(...)
	github.com/hashicorp/terraform-plugin-sdk/v2@v2.38.2/helper/schema/grpc_provider.go:1133 +0xeff

Terraform Version

Terraform v1.15.4
on linux_amd64
+ provider registry.terraform.io/integrations/github v6.12.1

GitHub Installation Type

• GitHub Enterprise Cloud with Personal Accounts (github.com)

Affected Resource(s)

• github_repository_environment

Terraform Configuration Files

resource "github_repository_environment" "example" {
  repository  = "my-repo"
  environment = "staging"
  # no reviewers block — this is the trigger
}

Any github_repository_environment resource without a reviewers block that was created under v6.11.x triggers the panic when upgrading to v6.12.x.

Steps to Reproduce

1. Create a github_repository_environment resource without a reviewers block using provider v6.11.x
2. Upgrade the provider to v6.12.0 or v6.12.1
3. Run terraform plan
4. The provider panics during PlanResourceChange

Debug Output

Root cause analysis:

The CustomizeDiff function introduced in v6.12.0 (PR #3162) does not handle the case where reviewers is empty or contains a nil element after state migration.

In resource_github_repository_environment.go line 133:

o := v.([]any)[0]          // returns nil when reviewers list has a nil element
o.(map[string]any)["teams"] // panics: nil is not map[string]any

The StateUpgrader (SchemaVersion 0→1) only adds repository_id — it does not transform the reviewers attribute. When the SDK coerces the old state into the new schema (where reviewers changed from MaxItems: 6 to MaxItems: 1), it can produce a list like [nil], which the CustomizeDiff doesn't guard against.

Note that expandReviewers in the same file does have a proper nil check (if m != nil), but CustomizeDiff lacks it.

Suggested fix — add a nil guard before the type assertion:

if v != nil {
    items := v.([]any)
    if len(items) > 0 && items[0] != nil {
        o := items[0].(map[string]any)
        // ... rest of the logic
    }
}

---

This issue was filed with AI assistance (analysis and writeup). The bug, stack trace, and root cause were verified by a human engineer.

[github-actions]

👋 Hi! Thank you for this contribution! Just to let you know, our GitHub SDK team does a round of issue and PR reviews twice a week, every Monday and Friday! We have a process in place for prioritizing and responding to your input. Because you are a part of this community please feel free to comment, add to, or pick up any issues/PRs that are labeled with Status: Up for grabs. You & others like you are the reason all of this works! So thank you & happy coding! 🚀

[deiga]

Sorry that you are bumping into this! Could you add the DEBUG level plan output as well?