netlify_site_domain_settings_changes fix update in place without changes (#121)

* fix: prevent drift in netlify_site_domain_settings resource

Fix constant drift detection in netlify_site_domain_settings by properly
handling null values from the API. The Read function now correctly sets
null values for empty optional fields instead of converting them to
empty strings, preventing Terraform from detecting false differences.

* test: prevent drift in netlify_site_domain_settings resource

Author: werwolfby

internal/provider/site_domain_settings_resource.go

@@ -133,13 +133,26 @@ func (r *siteDomainSettingsResource) Read(ctx context.Context, req resource.Read
 		return
 	}
 
-	state.CustomDomain = types.StringValue(site.CustomDomain)
+	if site.CustomDomain != "" {
+		state.CustomDomain = types.StringValue(site.CustomDomain)
+	} else {
+		state.CustomDomain = types.StringNull()
+	}
 	state.DomainAliases = make([]types.String, len(site.DomainAliases))
 	for i, domainAlias := range site.DomainAliases {
 		state.DomainAliases[i] = types.StringValue(domainAlias)
 	}
-	state.BranchDeployCustomDomain = types.StringValue(site.BranchDeployCustomDomain)
-	state.DeployPreviewCustomDomain = types.StringValue(site.DeployPreviewCustomDomain)
+	if site.BranchDeployCustomDomain != "" {
+		state.BranchDeployCustomDomain = types.StringValue(site.BranchDeployCustomDomain)
+	} else {
+		state.BranchDeployCustomDomain = types.StringNull()
+	}
+
+	if site.DeployPreviewCustomDomain != "" {
+		state.DeployPreviewCustomDomain = types.StringValue(site.DeployPreviewCustomDomain)
+	} else {
+		state.DeployPreviewCustomDomain = types.StringNull()
+	}
 
 	resp.Diagnostics.Append(resp.State.Set(ctx, &state)...)
 	if resp.Diagnostics.HasError() {

internal/provider/site_domain_settings_resource_test.go

@@ -54,3 +54,44 @@ func TestAccSiteDomainSettings(t *testing.T) {
 		},
 	}, func(s *terraform.State) error { return nil })
 }
+
+func TestAccSiteDomainSettingsNullFields(t *testing.T) {
+	// Test case for preventing drift when optional fields are null from API
+	accTest(t, []resource.TestStep{
+		{
+			Config: `resource "netlify_site_domain_settings" "null_fields" {
+  site_id       = "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"
+  custom_domain = "tf-test-null-fields.examplepetstore.com"
+}`,
+			Check: resource.ComposeAggregateTestCheckFunc(
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "site_id", "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "custom_domain", "tf-test-null-fields.examplepetstore.com"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "domain_aliases.#", "0"),
+				// These should be null/empty when not set
+				resource.TestCheckNoResourceAttr("netlify_site_domain_settings.null_fields", "branch_deploy_custom_domain"),
+				resource.TestCheckNoResourceAttr("netlify_site_domain_settings.null_fields", "deploy_preview_custom_domain"),
+			),
+		},
+		{
+			// Second step with same config should not detect changes (no drift)
+			Config: `resource "netlify_site_domain_settings" "null_fields" {
+  site_id       = "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"
+  custom_domain = "tf-test-null-fields.examplepetstore.com"
+}`,
+			PlanOnly: true,
+			Check: resource.ComposeAggregateTestCheckFunc(
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "site_id", "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "custom_domain", "tf-test-null-fields.examplepetstore.com"),
+				resource.TestCheckResourceAttr("netlify_site_domain_settings.null_fields", "domain_aliases.#", "0"),
+			),
+		},
+		{
+			ResourceName:                         "netlify_site_domain_settings.null_fields",
+			ImportState:                          true,
+			ImportStateId:                        "5b407d6d-9385-4e7a-a4c4-8efc11ea3c26",
+			ImportStateVerifyIdentifierAttribute: "site_id",
+			ImportStateVerify:                    true,
+			ImportStateVerifyIgnore:              []string{"last_updated"},
+		},
+	}, func(s *terraform.State) error { return nil })
+}