Support EndpointGroupName metadata in MVC ApiExplorer (#37264)

* Support EndpointGroupName metadata in MVC ApiExplorer

* Address feedback from peer review

* Update src/Mvc/Mvc.ApiExplorer/src/DefaultApiDescriptionProvider.cs

Co-authored-by: Pranav K <prkrishn@hotmail.com>

Co-authored-by: Pranav K <prkrishn@hotmail.com>

src/Mvc/Mvc.ApiExplorer/src/DefaultApiDescriptionProvider.cs

@@ -71,13 +71,17 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                     continue;
                 }
 
+                // ApiDescriptionActionData is only added to the ControllerActionDescriptor if
+                // the action is marked as `IsVisible` to the ApiExplorer. This null-check is
+                // effectively asserting if the endpoint should be generated into the final
+                // OpenAPI metadata.
                 var extensionData = action.GetProperty<ApiDescriptionActionData>();
                 if (extensionData != null)
                 {
                     var httpMethods = GetHttpMethods(action);
                     foreach (var httpMethod in httpMethods)
                     {
-                        context.Results.Add(CreateApiDescription(action, httpMethod, extensionData.GroupName));
+                        context.Results.Add(CreateApiDescription(action, httpMethod, GetGroupName(action, extensionData)));
                     }
                 }
             }
@@ -463,6 +467,19 @@ namespace Microsoft.AspNetCore.Mvc.ApiExplorer
                 .ToArray();
         }
 
+        private static string? GetGroupName(ControllerActionDescriptor action, ApiDescriptionActionData extensionData)
+        {
+            // The `GroupName` set in the `ApiDescriptionActionData` is either the
+            // group name set via [ApiExplorerSettings(GroupName = "foo")] on the
+            // action or controller. So, this lookup favors the following sequence:
+            // - EndpointGroupName on the action, if it is set
+            // - EndpointGroupName on the controller, if it is set
+            // - ApiExplorerSettings.GroupName on the action, if it is set
+            // - ApiExplorerSettings.GroupName on the controller, if it is set
+            var endpointGroupName = action.EndpointMetadata.OfType<IEndpointGroupNameMetadata>().LastOrDefault();
+            return endpointGroupName?.EndpointGroupName ?? extensionData.GroupName;
+        }
+
         private class ApiParameterDescriptionContext
         {
             public ModelMetadata ModelMetadata { get; }

src/Mvc/Mvc.ApiExplorer/test/DefaultApiDescriptionProviderTest.cs

@@ -88,6 +88,37 @@ namespace Microsoft.AspNetCore.Mvc.Description
             Assert.Equal("Customers", description.GroupName);
         }
 
+        [Fact]
+        public void GetApiDescription_PopulatesGroupName_FromMetadata()
+        {
+            // Arrange
+            var action = CreateActionDescriptor();
+            action.EndpointMetadata = new List<object>() { new EndpointGroupNameAttribute("Customers") };
+
+            // Act
+            var descriptions = GetApiDescriptions(action);
+
+            // Assert
+            var description = Assert.Single(descriptions);
+            Assert.Equal("Customers", description.GroupName);
+        }
+
+        [Fact]
+        public void GetApiDescription_PopulatesGroupName_FromMetadataOrExtensionData()
+        {
+            // Arrange
+            var action = CreateActionDescriptor();
+            action.EndpointMetadata = new List<object>() { new EndpointGroupNameAttribute("Customers") };
+            action.GetProperty<ApiDescriptionActionData>().GroupName = "NotUsedCustomers";
+
+            // Act
+            var descriptions = GetApiDescriptions(action);
+
+            // Assert
+            var description = Assert.Single(descriptions);
+            Assert.Equal("Customers", description.GroupName);
+        }
+
         [Fact]
         public void GetApiDescription_HttpMethodIsNullWithoutConstraint()
         {

src/Mvc/test/Mvc.FunctionalTests/ApiExplorerTest.cs

@@ -120,6 +120,34 @@ namespace Microsoft.AspNetCore.Mvc.FunctionalTests
             Assert.Equal("SetOnAction", description.GroupName);
         }
 
+        [Fact]
+        public async Task ApiExplorer_GroupName_SetByEndpointMetadataOnController()
+        {
+            // Arrange & Act
+            var response = await Client.GetAsync("http://localhost/ApiExplorerApiController/ActionWithIdParameter");
+
+            var body = await response.Content.ReadAsStringAsync();
+            var result = JsonConvert.DeserializeObject<List<ApiExplorerData>>(body);
+
+            // Assert
+            var description = Assert.Single(result);
+            Assert.Equal("GroupNameOnController", description.GroupName);
+        }
+
+        [Fact]
+        public async Task ApiExplorer_GroupName_SetByEndpointMetadataOnAction()
+        {
+            // Arrange & Act
+            var response = await Client.GetAsync("http://localhost/ApiExplorerApiController/ActionWithSomeParameters");
+
+            var body = await response.Content.ReadAsStringAsync();
+            var result = JsonConvert.DeserializeObject<List<ApiExplorerData>>(body);
+
+            // Assert
+            var description = Assert.Single(result);
+            Assert.Equal("GroupNameOnAction", description.GroupName);
+        }
+
         [Fact]
         public async Task ApiExplorer_RouteTemplate_DisplaysFixedRoute()
         {

src/Mvc/test/WebSites/ApiExplorerWebSite/Controllers/ApiExplorerApiController.cs

@@ -4,15 +4,18 @@
 using System.IO;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Routing;
 
 namespace ApiExplorerWebSite
 {
     [Route("ApiExplorerApiController/[action]")]
     [ApiController]
+    [EndpointGroupName("GroupNameOnController")]
     public class ApiExplorerApiController : Controller
     {
         public IActionResult ActionWithoutParameters() => Ok();
 
+        [EndpointGroupName("GroupNameOnAction")]
         public void ActionWithSomeParameters(object input)
         {
         }