Add register CustomizeDiff to propagate changed values

* Ensure that (non-empty) changes to content mark `value` as
computed so that the new value is propagated to resources that
use the value attribute (locals, outputs, other resources, etc)
* Add a test case to enforce a Terraform output get updated

util/resource_register.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/hashicorp/terraform-plugin-sdk/helper/hashcode"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
+	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/customdiff"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
 )
 
@@ -17,48 +18,66 @@ func resourceRegister() *schema.Resource {
 		DeleteContext: registerDelete,
 		Schema: map[string]*schema.Schema{
 			"set": &schema.Schema{
-				Type:                  schema.TypeString,
-				Optional:              true,
-				ForceNew:              false,
+				Type: schema.TypeString,
+				// Allow content to be null
+				Optional: true,
+				// ForceNew would create a new resource on any change,
+				// but this resource should ignore empty string or null
+				// changes
+				ForceNew: false,
+				// Suppress plan diffs setting content to an empty string
+				// or null. Empty changes are ignored / NoOps
 				DiffSuppressOnRefresh: true,
 				DiffSuppressFunc:      registerDiffSuppress,
 			},
 			"value": &schema.Schema{
 				Type:        schema.TypeString,
 				Computed:    true,
-				Description: "Last set register value",
+				Description: "Computed register value",
 			},
 		},
+		// Changes to set (that are non-empty) mark value as computed
+		CustomizeDiff: customdiff.ComputedIf("value", func(ctx context.Context, d *schema.ResourceDiff, meta interface{}) bool {
+			return d.HasChange("set") && d.Get("set").(string) != ""
+		}),
 	}
 }
 
+// registerCreate stores content as the 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
 }
 
+// registerRead sets register attributes
 func registerRead(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
+	// register has no remote API equivalent to check or attributes to set
 	return nil
 }
 
+// registerUpdate applies non-empty content changes to the value attribute.
 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
 }
 
+// registerDelete removes the resource from state.
 func registerDelete(ctx context.Context, d *schema.ResourceData, meta interface{}) diag.Diagnostics {
 	d.SetId("")
 	return nil
 }
 
+// registerDiffSuppress supresses plan diffs setting content to an empty
+// string or null (converts to empty string in ResourceData). Empty content
+// does not alter the register value.
 func registerDiffSuppress(k, oldV, newV string, d *schema.ResourceData) bool {
 	return newV == ""
 }

util/resource_register_test.go

@@ -10,20 +10,38 @@ const registerInitial = `
 resource "util_register" "example" {
 	set = "a1b2c3"
 }
+
+output "out" {
+	value = util_register.example.value
+}
 `
 
 const registerUnsetSHA = `
 resource "util_register" "example" {
 	set = ""
 }
+
+output "out" {
+	value = util_register.example.value
+}
 `
 
 const registerUpdateSHA = `
 resource "util_register" "example" {
 	set = "b2c3d4"
 }
+
+output "out" {
+	value = util_register.example.value
+}
 `
 
+// register expected values
+const (
+	registerInitialExpected = "a1b2c3"
+	registerUpdateExpected  = "b2c3d4"
+)
+
 func TestRegister(t *testing.T) {
 	r.UnitTest(t, r.TestCase{
 		Providers: testProviders,
@@ -32,29 +50,33 @@ func TestRegister(t *testing.T) {
 			r.TestStep{
 				Config: registerInitial,
 				Check: r.ComposeTestCheckFunc(
-					r.TestCheckResourceAttr("util_register.example", "value", "a1b2c3"),
+					r.TestCheckResourceAttr("util_register.example", "value", registerInitialExpected),
+					r.TestCheckOutput("out", registerInitialExpected),
 				),
 			},
-			// set with empty value doesn't change value
+			// Empty content does NOT change value
 			r.TestStep{
 				Config: registerUnsetSHA,
 				Check: r.ComposeTestCheckFunc(
-					r.TestCheckResourceAttr("util_register.example", "value", "a1b2c3"),
+					r.TestCheckResourceAttr("util_register.example", "value", registerInitialExpected),
+					r.TestCheckOutput("out", registerInitialExpected),
 				),
 			},
-			// set with content updates values
+			// Non-empty content DOES change value
 			r.TestStep{
 				Config: registerUpdateSHA,
 				Check: r.ComposeTestCheckFunc(
-					r.TestCheckResourceAttr("util_register.example", "value", "b2c3d4"),
+					r.TestCheckResourceAttr("util_register.example", "value", registerUpdateExpected),
+					r.TestCheckOutput("out", registerUpdateExpected),
 				),
 			},
 			// suppress noisy diffs that won't affect value
 			r.TestStep{
 				Config:   registerUnsetSHA,
 				PlanOnly: true,
 				Check: r.ComposeTestCheckFunc(
-					r.TestCheckResourceAttr("util_register.example", "value", "b2c3d4"),
+					r.TestCheckResourceAttr("util_register.example", "value", registerUpdateExpected),
+					r.TestCheckOutput("out", registerUpdateExpected),
 				),
 			},
 		},