Migrate Testing Provider Code to terraform-plugin-go

[bflad]

Description

In order to appropriately (unit) test the functionality offered to provider developers with helper/resource.TestCase and friends, it generally requires the need for real provider implementations. Currently, some of the tests utilize the lingering direct dependency on terraform-plugin-sdk. For example:

{
	Config: `resource "null_resource" "test" {}`,
	ProviderFactories: map[string]func() (*schema.Provider, error){
		"null": func() (*schema.Provider, error) { //nolint:unparam // required signature
			return &schema.Provider{
				ResourcesMap: map[string]*schema.Resource{
					"null_resource": {
						CreateContext: func(_ context.Context, d *schema.ResourceData, _ interface{}) diag.Diagnostics {
							d.SetId("test")
							return nil
						},
						DeleteContext: func(_ context.Context, _ *schema.ResourceData, _ interface{}) diag.Diagnostics {
							return nil
						},
						ReadContext: func(_ context.Context, _ *schema.ResourceData, _ interface{}) diag.Diagnostics {
							return nil
						},
						Schema: map[string]*schema.Schema{
							"triggers": {
								Elem:     &schema.Schema{Type: schema.TypeString},
								ForceNew: true,
								Optional: true,
								Type:     schema.TypeMap,
							},
						},
					},
				},
			}, nil
		},
	},
},

A long term goal of this Go module would be to remove that direct dependency, which is only based on supporting the legacy Providers and ProviderFactories fields which work with the terraform-plugin-sdk/helper/schema.Provider type.

Other tests rely on downloading real providers, such as hashicorp/random that happen to include the desired functionality for testing.

Proposal

The summary of this proposal is:

• Introduce a relatively straightforward terraform-plugin-go based "provider SDK" for creating testing providers in this codebase
• Remove any testing provider code that is directly terraform-plugin-sdk based by migrating to the terraform-plugin-go equivalent.
• Real, externally hosted providers, such as hashicorp/random etc., can and should still be used for testing ExternalProviders field functionality, but not in other cases. Migrate those other cases to the terraform-plugin-go equivalent.

Some additional notes/assumptions:

• Provider developers should always want to test in-development provider Go code, rather than being forced to host/publish them elsewhere first.
• The terraform-plugin-go dependency should always exist in this Go module as long as that Go module is the one that contains the provider protocol implementation in Go or at least the lowest-level provider server implementation.
• The goal of creating a terraform-plugin-go "provider SDK" is to enable consistency across testing, since correctly implementing the full lifecycle of a provider server is non-trivial.
• The terraform-plugin-go "provider SDK" can be fairly lightweight, but it should abstract away all the lowest level details of directly handling the protocol RPCs and instead can present more familiar Create, Read, etc. methods.
• The terraform-plugin-go "provider SDK" can/should use the tftypes type system to reduce complexity in that area.

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