[BUG:] azurerm_cdn_frontdoor_rule - allow empty query_string in url_redirect_action

internal/services/cdn/cdn_frontdoor_rule_resource.go

@@ -123,7 +123,7 @@ func resourceCdnFrontDoorRule() *pluginsdk.Resource {
 									},
 
 									// NOTE: it is valid for the query string to be an empty string.
-									// Leave blank to preserve the incoming query string. Issue #18249
+									// Leave blank to preserve the incoming query string. Issue #18249 & #19682
 									"query_string": {
 										Type:     pluginsdk.TypeString,
 										Optional: true,

internal/services/cdn/cdn_frontdoor_rule_resource_test.go

@@ -330,6 +330,21 @@ func TestAccCdnFrontDoorRule_honorOrigin(t *testing.T) {
 	})
 }
 
+func TestAccCdnFrontDoorRule_allowEmptyQueryString(t *testing.T) {
+	// NOTE: Regression test case for issue #19682
+	data := acceptance.BuildTestData(t, "azurerm_cdn_frontdoor_rule", "test")
+	r := CdnFrontDoorRuleResource{}
+	data.ResourceTest(t, r, []acceptance.TestStep{
+		{
+			Config: r.allowEmptyQueryString(data),
+			Check: acceptance.ComposeTestCheckFunc(
+				check.That(data.ResourceName).ExistsInAzure(r),
+			),
+		},
+		data.ImportStep(),
+	})
+}
+
 func (r CdnFrontDoorRuleResource) Exists(ctx context.Context, clients *clients.Client, state *pluginsdk.InstanceState) (*bool, error) {
 	id, err := parse.FrontDoorRuleID(state.ID)
 	if err != nil {
@@ -1119,3 +1134,41 @@ resource "azurerm_cdn_frontdoor_rule" "test" {
 }
 `, template, data.RandomInteger)
 }
+
+func (r CdnFrontDoorRuleResource) allowEmptyQueryString(data acceptance.TestData) string {
+	template := r.template(data)
+	return fmt.Sprintf(`
+  provider "azurerm" {
+    features {}
+  }
+
+  %s
+
+  resource "azurerm_cdn_frontdoor_rule" "test" {
+    depends_on = [azurerm_cdn_frontdoor_origin_group.test, azurerm_cdn_frontdoor_origin.test]
+
+    name                      = "accTestRule%d"
+    cdn_frontdoor_rule_set_id = azurerm_cdn_frontdoor_rule_set.test.id
+
+    order = 0
+
+    conditions {
+      request_uri_condition {
+        match_values     = ["contoso"]
+        negate_condition = false
+        operator         = "Contains"
+      }
+    }
+
+    actions {
+      url_redirect_action {
+        redirect_type        = "PermanentRedirect"
+        redirect_protocol    = "MatchRequest"
+        query_string         = ""
+        destination_hostname = "contoso.com"
+        destination_path     = "/test/page"
+      }
+    }
+  }
+`, template, data.RandomInteger)
+}

internal/services/cdn/validate/front_door_validation_helpers.go

@@ -141,8 +141,6 @@ func CdnFrontDoorUrlRedirectActionQueryString(i interface{}, k string) (_ []stri
 		if len(v) > 2048 {
 			return nil, []error{fmt.Errorf("'url_redirect_action' is invalid: %q cannot be longer than 2048 characters in length, got %d", k, len(v))}
 		}
-	} else {
-		return nil, []error{fmt.Errorf("'url_redirect_action' is invalid: %q must not be empty, got %q", k, v)}
 	}
 
 	return nil, nil