fix: support in-place updates for repository custom property values

The github_repository_custom_property resource declared property_value
as ForceNew with no Update function, causing every value change to
plan as a destroy+create. The destroy step calls the upsert API with
a nil value, which unsets the property on GitHub before the create
step rewrites it.

Wire an Update function that calls the same upsert as Create, and
remove ForceNew from property_value. Identity fields (repository,
property_name, property_type) remain ForceNew.

Author: milan-dewilde_liantis

github/resource_github_repository_custom_property.go

@@ -13,6 +13,7 @@ func resourceGithubRepositoryCustomProperty() *schema.Resource {
 	return &schema.Resource{
 		Create: resourceGithubRepositoryCustomPropertyCreate,
 		Read:   resourceGithubRepositoryCustomPropertyRead,
+		Update: resourceGithubRepositoryCustomPropertyUpdate,
 		Delete: resourceGithubRepositoryCustomPropertyDelete,
 		Importer: &schema.ResourceImporter{
 			StateContext: schema.ImportStatePassthroughContext,
@@ -46,7 +47,6 @@ func resourceGithubRepositoryCustomProperty() *schema.Resource {
 				Elem: &schema.Schema{
 					Type: schema.TypeString,
 				},
-				ForceNew: true,
 			},
 		},
 	}
@@ -86,6 +86,13 @@ func resourceGithubRepositoryCustomPropertyCreate(d *schema.ResourceData, meta a
 	return resourceGithubRepositoryCustomPropertyRead(d, meta)
 }
 
+func resourceGithubRepositoryCustomPropertyUpdate(d *schema.ResourceData, meta any) error {
+	if err := resourceGithubRepositoryCustomPropertyCreate(d, meta); err != nil {
+		return err
+	}
+	return resourceGithubRepositoryCustomPropertyRead(d, meta)
+}
+
 func resourceGithubRepositoryCustomPropertyRead(d *schema.ResourceData, meta any) error {
 	client := meta.(*Owner).v3client
 	ctx := context.Background()

github/resource_github_repository_custom_property_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/hashicorp/terraform-plugin-testing/helper/acctest"
 	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
+	"github.com/hashicorp/terraform-plugin-testing/terraform"
 )
 
 func TestAccGithubRepositoryCustomProperty(t *testing.T) {
@@ -135,6 +136,79 @@ func TestAccGithubRepositoryCustomProperty(t *testing.T) {
 		})
 	})
 
+	t.Run("updates custom property value in place without replacement", func(t *testing.T) {
+		randomID := acctest.RandStringFromCharSet(5, acctest.CharSetAlphaNum)
+		repoName := fmt.Sprintf("%srepo-custom-prop-%s", testResourcePrefix, randomID)
+		propertyName := fmt.Sprintf("tf-acc-test-property-%s", randomID)
+
+		configTemplate := `
+			resource "github_organization_custom_properties" "test" {
+				allowed_values = ["alpha", "beta"]
+				description    = "Test Description"
+				property_name  = "%s"
+				value_type     = "single_select"
+			}
+			resource "github_repository" "test" {
+				name = "%s"
+				auto_init = true
+			}
+			resource "github_repository_custom_property" "test" {
+				repository    = github_repository.test.name
+				property_name = github_organization_custom_properties.test.property_name
+				property_type = github_organization_custom_properties.test.value_type
+				property_value = ["%s"]
+			}
+		`
+
+		configAlpha := fmt.Sprintf(configTemplate, propertyName, repoName, "alpha")
+		configBeta := fmt.Sprintf(configTemplate, propertyName, repoName, "beta")
+
+		var firstID string
+
+		resource.Test(t, resource.TestCase{
+			PreCheck:          func() { skipUnlessHasOrgs(t) },
+			ProviderFactories: providerFactories,
+			Steps: []resource.TestStep{
+				{
+					Config: configAlpha,
+					Check: resource.ComposeTestCheckFunc(
+						resource.TestCheckResourceAttr("github_repository_custom_property.test", "property_value.#", "1"),
+						resource.TestCheckResourceAttr("github_repository_custom_property.test", "property_value.0", "alpha"),
+						func(s *terraform.State) error {
+							rs, ok := s.RootModule().Resources["github_repository_custom_property.test"]
+							if !ok {
+								return fmt.Errorf("resource not found in state")
+							}
+							firstID = rs.Primary.ID
+							return nil
+						},
+					),
+				},
+				{
+					Config: configBeta,
+					Check: resource.ComposeTestCheckFunc(
+						resource.TestCheckResourceAttr("github_repository_custom_property.test", "property_value.#", "1"),
+						resource.TestCheckResourceAttr("github_repository_custom_property.test", "property_value.0", "beta"),
+						func(s *terraform.State) error {
+							rs, ok := s.RootModule().Resources["github_repository_custom_property.test"]
+							if !ok {
+								return fmt.Errorf("resource not found in state")
+							}
+							if rs.Primary.ID != firstID {
+								return fmt.Errorf("resource ID changed across update: %q -> %q (expected in-place update)", firstID, rs.Primary.ID)
+							}
+							return nil
+						},
+					),
+				},
+				{
+					Config:   configBeta,
+					PlanOnly: true,
+				},
+			},
+		})
+	})
+
 	t.Run("creates custom property of type string without error", func(t *testing.T) {
 		randomID := acctest.RandStringFromCharSet(5, acctest.CharSetAlphaNum)
 		repoName := fmt.Sprintf("%srepo-custom-prop-%s", testResourcePrefix, randomID)