Add util_register resource for storing a value

* Set a register value to persist in state, even after
the value is no longer set in the Terraform resource
* Setting the value to `null` or "" (empty string)
won't alter the resource value (or show a diff)
* Allow the value to be updated (plan shows a diff)

This differs from conditional expressions that check that
a value isn't null or default to a value (which may be a
data reference to the old value):

```tf
locals {
  out = var.foo == null ? data.old_foo : var.foo
}

data "old_foo" ...
```

Instead, it allows a Terraform resource value to retain
a value from a previous declarative state. This can be
useful in cases where querying and referencing the value
again is expensive and the value cannot change externally.

```tf
resource "util_register" "example" {
  set = null         # previously "bar"
}

output "out" {
  value = util_register.example.value  # "bar"
}
```

CHANGES.md

@@ -4,6 +4,10 @@ Notable changes between releases.
 
 ## Latest
 
+## v0.2.0
+
+* Add `util_register` resource for storing values
+
 ## v0.1.0
 
-* Add `util_replace` function
+* Add `util_replace` data source function

README.md

@@ -13,7 +13,7 @@ terraform {
   required_providers {
     ct = {
       source  = "poseidon/util"
-      version = "0.1.0"
+      version = "0.2.0"
     }
   }
 }
@@ -36,6 +36,26 @@ output "example" {
 }
 ```
 
+Store a value in state that persists until changed to a non-empty value.
+
+```tf
+resource "util_register" "example" {
+  set = "a1b2c3"
+}
+```
+
+Later, the register's value may be updated, but setting it to `null` or `""` is ignored.
+
+```tf
+resource "util_register" "example" {
+  set = null
+}
+
+output "sha" {
+  value = util_register.example.value  # "a1b2c3"
+}
+```
+
 Run `terraform init` to ensure plugin version requirements are met.
 
 ```
@@ -55,4 +75,3 @@ To develop the provider plugin locally, build an executable with Go v1.16+.
 ```
 make
 ```
-

docs/index.md

@@ -13,7 +13,7 @@ terraform {
   required_providers {
     ct = {
       source  = "poseidon/util"
-      version = "0.1.0"
+      version = "0.2.0"
     }
   }
 }

docs/resources/register.md

@@ -0,0 +1,29 @@
+# Register
+
+Store a value in state that persists until changed to a non-empty value.
+
+```tf
+resource "util_register" "example" {
+  set = "a1b2c3"
+}
+```
+
+Later, the register's value may be updated, but setting it to `null` or `""` is ignored.
+
+```tf
+resource "util_register" "example" {
+  set = null
+}
+
+output "sha" {
+  value = util_register.example.value  # "a1b2c3"
+}
+```
+
+## Argument Reference
+
+* `set` - set the register value (`""` or `null` values ignored)
+
+## Attribute Reference
+
+* `value` - computed value of the register

example/.gitignore

@@ -0,0 +1,3 @@
+.terraform
+*.tfstate*
+output

example/main.tf

@@ -0,0 +1,3 @@
+resource "util_register" "example" {
+  set = "a1b2c3"
+}

example/providers.tf

@@ -0,0 +1,9 @@
+terraform {
+  required_providers {
+    util = {
+      source = "poseidon/util"
+      #source = "terraform.localhost/poseidon/util"
+      version = "0.2.0"
+    }
+  }
+}

util/provider.go

@@ -4,9 +4,12 @@ import (
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
 )
 
-// Provider returns a config transpiler Provider.
+// Provider returns a util Provider.
 func Provider() *schema.Provider {
 	return &schema.Provider{
+		ResourcesMap: map[string]*schema.Resource{
+			"util_register": resourceRegister(),
+		},
 		DataSourcesMap: map[string]*schema.Resource{
 			"util_replace": datasourceReplace(),
 		},

util/resource_register.go

@@ -0,0 +1,64 @@
+package util
+
+import (
+	"context"
+	"strconv"
+
+	"github.com/hashicorp/terraform-plugin-sdk/helper/hashcode"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
+)
+
+func resourceRegister() *schema.Resource {
+	return &schema.Resource{
+		CreateContext: resourceCreate,
+		ReadContext:   registerRead,
+		UpdateContext: registerUpdate,
+		DeleteContext: registerDelete,
+		Schema: map[string]*schema.Schema{
+			"set": &schema.Schema{
+				Type:                  schema.TypeString,
+				Optional:              true,
+				ForceNew:              false,
+				DiffSuppressOnRefresh: true,
+				DiffSuppressFunc:      registerDiffSuppress,
+			},
+			"value": &schema.Schema{
+				Type:        schema.TypeString,
+				Computed:    true,
+				Description: "Last set register value",
+			},
+		},
+	}
+}
+
+func resourceCreate(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	var diags diag.Diagnostics
+
+	set := d.Get("set").(string)
+	d.Set("value", set)
+	d.SetId(strconv.Itoa(hashcode.String(set)))
+	return diags
+}
+
+func registerRead(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	return nil
+}
+
+func registerUpdate(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	var diags diag.Diagnostics
+	set := d.Get("set").(string)
+	if set != "" {
+		d.Set("value", set)
+	}
+	return diags
+}
+
+func registerDelete(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	d.SetId("")
+	return nil
+}
+
+func registerDiffSuppress(k, oldV, newV string, d *schema.ResourceData) bool {
+	return newV == ""
+}

util/resource_register_test.go

@@ -0,0 +1,62 @@
+package util
+
+import (
+	"testing"
+
+	r "github.com/hashicorp/terraform-plugin-sdk/v2/helper/resource"
+)
+
+const registerInitial = `
+resource "util_register" "example" {
+	set = "a1b2c3"
+}
+`
+
+const registerUnsetSHA = `
+resource "util_register" "example" {
+	set = ""
+}
+`
+
+const registerUpdateSHA = `
+resource "util_register" "example" {
+	set = "b2c3d4"
+}
+`
+
+func TestRegister(t *testing.T) {
+	r.UnitTest(t, r.TestCase{
+		Providers: testProviders,
+		Steps: []r.TestStep{
+			// set initial value
+			r.TestStep{
+				Config: registerInitial,
+				Check: r.ComposeTestCheckFunc(
+					r.TestCheckResourceAttr("util_register.example", "value", "a1b2c3"),
+				),
+			},
+			// set with empty value doesn't change value
+			r.TestStep{
+				Config: registerUnsetSHA,
+				Check: r.ComposeTestCheckFunc(
+					r.TestCheckResourceAttr("util_register.example", "value", "a1b2c3"),
+				),
+			},
+			// set with content updates values
+			r.TestStep{
+				Config: registerUpdateSHA,
+				Check: r.ComposeTestCheckFunc(
+					r.TestCheckResourceAttr("util_register.example", "value", "b2c3d4"),
+				),
+			},
+			// suppress noisy diffs that won't affect value
+			r.TestStep{
+				Config:   registerUnsetSHA,
+				PlanOnly: true,
+				Check: r.ComposeTestCheckFunc(
+					r.TestCheckResourceAttr("util_register.example", "value", "b2c3d4"),
+				),
+			},
+		},
+	})
+}