Http3 test

src/libraries/Common/tests/System/Net/Http/Http3LoopbackConnection.cs

@@ -118,7 +118,7 @@ public override Task InitializeConnectionAsync()
             throw new NotImplementedException();
         }
 
-        private Task EnsureControlStreamAcceptedAsync()
+        public Task EnsureControlStreamAcceptedAsync()
         {
             if (_inboundControlStream != null)
             {

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.MaxResponseHeadersLength.cs

@@ -22,6 +22,50 @@ public abstract class HttpClientHandler_MaxResponseHeadersLength_Test : HttpClie
     {
         public HttpClientHandler_MaxResponseHeadersLength_Test(ITestOutputHelper output) : base(output) { }
 
+        [Theory]
+        [InlineData(null)]
+        [InlineData(1)]
+        [InlineData(16)]
+        [InlineData(64)]
+        [InlineData(256)]
+        [InlineData(1024)]
+        [InlineData(10240)]
+        public async Task Http3Test(int? maxResponseHeadersLength)
+        {
+            if (UseVersion.Major != 3) return;
+
+            var requestCts = new CancellationTokenSource();
+            var controlStreamEstablishedTcs = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+
+            await LoopbackServerFactory.CreateClientAndServerAsync(async uri =>
+            {
+                using HttpClientHandler handler = CreateHttpClientHandler();
+                using HttpClient client = CreateHttpClient(handler);
+
+                if (maxResponseHeadersLength.HasValue)
+                {
+                    handler.MaxResponseHeadersLength = maxResponseHeadersLength.Value;
+                }
+
+                await Assert.ThrowsAnyAsync<Exception>(() => client.GetAsync(uri, requestCts.Token));
+
+                await controlStreamEstablishedTcs.Task.WaitAsync(TestHelper.PassingTestTimeout);
+            },
+            async server =>
+            {
+                await server.AcceptConnectionAsync(async connection =>
+                {
+                    requestCts.Cancel();
+
+                    var http3Connection = (Http3LoopbackConnection)connection;
+
+                    await http3Connection.EnsureControlStreamAcceptedAsync().WaitAsync(TestHelper.PassingTestTimeout);
+
+                    controlStreamEstablishedTcs.SetResult();
+                });
+            });
+        }
+
         [Theory]
         [InlineData(0)]
         [InlineData(-1)]