TestServer ignores stream position passed through StreamContent

[afkos]

Describe the bug

When using TestServer, HttpClient sends the whole stream from the start, the actual position in the stream is ignored.
Test below shows passing stream with length = 3 and position = 1
I expect that only 2 bytes would be passed to the controller
The same problem with FileStream and with any stream that support Seek operation.

To Reproduce

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.TestHost;
using Microsoft.Extensions.DependencyInjection;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;
using Xunit;

namespace TestProject1
{
    public class UnitTest1
    { 
        [Fact]
        public async void Test1()
        {
            var server = new TestServer(new WebHostBuilder().UseStartup<Startup>());

            using var client = server.CreateClient();

            using var mem = new MemoryStream(new byte[] { 1, 2, 3 });

            // set stream offset 
            mem.Position = 1;

            using var msg = new HttpRequestMessage(HttpMethod.Put, "Upload") { Content = new StreamContent(mem) };
            using var resp = await client.SendAsync(msg);
            
            var actualLength = await resp.EnsureSuccessStatusCode().Content.ReadAsStringAsync();

            Assert.Equal("2", actualLength);
        }
    }

    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllers();
        }

        public void Configure(IApplicationBuilder app)
        {
            app.UseRouting();
            app.UseEndpoints(endpoints => endpoints.MapControllers());
        }
    }

    [ApiController]
    [Route("[controller]")]
    public class UploadController : ControllerBase
    {
        [HttpPut]
        public async Task<IActionResult> Upload()
        {
            using var mem = new MemoryStream();
            await Request.Body.CopyToAsync(mem);
            return Content(mem.Length.ToString());
        }
    }
}

Further technical details

• ASP.NET Core version - 5.0.7
• Microsoft.AspNetCore.Mvc.Testing 5.0.7
• Microsoft Visual Studio Community 2019 Version 16.10.2
• The output of dotnet --info:

Пакет SDK для .NET (отражающий любой global.json):
 Version:   5.0.301
 Commit:    ef17233f86

Среда выполнения:
 OS Name:     Windows
 OS Version:  10.0.19043
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.301\

Host (useful for support):
  Version: 5.0.7
  Commit:  556582d964

.NET SDKs installed:
  3.1.405 [C:\Program Files\dotnet\sdk]
  5.0.100 [C:\Program Files\dotnet\sdk]
  5.0.102 [C:\Program Files\dotnet\sdk]
  5.0.202 [C:\Program Files\dotnet\sdk]
  5.0.301 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.0.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.0.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[BrennanConroy]

Triage: This behavior has been here since the beginning and would be a major breaking change if we changed it. We wont be making any changes here.

As a workaround you can copy the partial stream to a new MemoryStream.

[afkos]

The behavior is different for TestServer and a real server.
It took a lot of time to investigate, that the problem is not with our code.

Maybe it's better to document the difference.

[BrennanConroy]

Filed dotnet/AspNetCore.Docs#22618 for docs