RouteEndpoint metadata from convention builders are duplicated

[martincostello]

Describe the bug

When inspecting endpoints with EndpointDataSource, for example in an IApiDescriptionProvider, some of the metadata on RouteEndpoint instances is duplicated. As shown in the screenshot below, there are two duplicated HTTP method metadata items.

Tracing through the code, this appears to be caused by a number of factors that together contribute to the routing endpoint conventions for an endpoint builder being run multiple times. The screenshot below shows the HTTP method metadata being added to the endpoint builder a second time.

The contributing factors to this issue appear to be:

1. There are two CompositeEndpointDataSource instances created by the application:

   aspnetcore/src/Http/Routing/src/EndpointRoutingMiddleware.cs

   Line 44 in 5577c0d

   _endpointDataSource = new CompositeEndpointDataSource(endpointRouteBuilder.DataSources);

   aspnetcore/src/Http/Routing/src/DependencyInjection/RoutingServiceCollectionExtensions.cs

   Line 65 in 5577c0d

   return new CompositeEndpointDataSource(dataSources);

2. Each CompositeEndpointDataSource enumerates the Endpoints property of each underlying endpoint data source once:

   aspnetcore/src/Http/Routing/src/CompositeEndpointDataSource.cs

   Line 115 in 5577c0d

   _endpoints = _dataSources.SelectMany(d => d.Endpoints).ToArray();

3. ModelEndpointDataSource calls Build() on each convention builder each time the Endpoints property is accessed

   aspnetcore/src/Http/Routing/src/ModelEndpointDataSource.cs

   Line 35 in 5577c0d

   public override IReadOnlyList<Endpoint> Endpoints => _endpointConventionBuilders.Select(e => e.Build()).ToArray();

4. Each call to Build() runs the convention action(s):

   aspnetcore/src/Http/Routing/src/DefaultEndpointConventionBuilder.cs

   Lines 30 to 33 in 5577c0d

   	foreach (var convention in _conventions)
   	{
   	convention(EndpointBuilder);
   	}

5. The conventions may mutate the endpoint builder's state multiple times:

   aspnetcore/src/Http/Routing/src/Builder/RoutingEndpointConventionBuilderExtensions.cs

   Lines 111 to 117 in 5577c0d

   	builder.Add(b =>
   	{
   	foreach (var item in items)
   	{
   	b.Metadata.Add(item);
   	}
   	});

The net result is that every time the Endpoints property of an ModelEndpointDataSource is accessed, the number of Metadata items on a route endpoint increases by the number of conventions configured.

Below is an example of four duplicate metadata items in a sample application:

To Reproduce

To reproduce this issue, edit this test to have the following code:

app.MapGet("/json", (IApiDescriptionGroupCollectionProvider provider) =>
{
    if (provider.ApiDescriptionGroups is not null)
    {
        // Used to inspect and debug ApiDescriptionGroups
    }

    return Json(new Person("John", 42));
});

Then debug the JsonResult_Works() test with a breakpoint set here:

aspnetcore/src/Mvc/Mvc.ApiExplorer/src/EndpointMetadataApiDescriptionProvider.cs

Line 59 in b9efadc

foreach (var httpMethod in httpMethodMetadata.HttpMethods)

Inspecting routeEndpoint.Metadata with the debugger will show the duplicated metadata as shown in the preceding screenshots.

Further technical details

• ASP.NET Core version 6.0.0-preview.7.21357.1
• .NET Core SDK 6.0.100-preview.7.21357.57
• Visual Studio 2022 17.0.0 Preview 1.1

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We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. Because it's not immediately obvious that this is a bug in our framework, we would like to keep this around to collect more feedback, which can later help us determine the impact of it. We will re-evaluate this issue, during our next planning meeting(s).
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