[release/6.0] Support zero-byte reads on raw response streams in SocketsHttpHandler

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnection.cs

@@ -1749,7 +1749,7 @@ private ValueTask<int> ReadBufferedAsync(Memory<byte> destination)
             // If the caller provided buffer, and thus the amount of data desired to be read,
             // is larger than the internal buffer, there's no point going through the internal
             // buffer, so just do an unbuffered read.
-            return destination.Length >= _readBuffer.Length ?
+            return destination.Length == 0 || destination.Length >= _readBuffer.Length ?
                 ReadAsync(destination) :
                 ReadBufferedAsyncCore(destination);
         }

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/RawConnectionStream.cs

@@ -45,9 +45,9 @@ public override async ValueTask<int> ReadAsync(Memory<byte> buffer, Cancellation
                 CancellationHelper.ThrowIfCancellationRequested(cancellationToken);
 
                 HttpConnection? connection = _connection;
-                if (connection == null || buffer.Length == 0)
+                if (connection == null)
                 {
-                    // Response body fully consumed or the caller didn't ask for any data
+                    // Response body fully consumed
                     return 0;
                 }
 
@@ -74,7 +74,7 @@ public override async ValueTask<int> ReadAsync(Memory<byte> buffer, Cancellation
                     }
                 }
 
-                if (bytesRead == 0)
+                if (bytesRead == 0 && buffer.Length != 0)
                 {
                     // A cancellation request may have caused the EOF.
                     CancellationHelper.ThrowIfCancellationRequested(cancellationToken);

src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs

@@ -1329,6 +1329,39 @@ await server.AcceptConnectionAsync(async (LoopbackServer.Connection connection)
                 }
             });
         }
+
+        [Fact]
+        public async Task UpgradeConnection_ResponseStreamSupportsZeroByteReads()
+        {
+            var zeroByteReadIssued = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+
+            await LoopbackServer.CreateClientAndServerAsync(async uri =>
+            {
+                using HttpClient client = CreateHttpClient();
+
+                using HttpResponseMessage response = await client.GetAsync(uri, HttpCompletionOption.ResponseHeadersRead);
+                using Stream responseStream = await response.Content.ReadAsStreamAsync();
+
+                ValueTask<int> zeroByteReadTask = responseStream.ReadAsync(Memory<byte>.Empty);
+
+                Assert.False(zeroByteReadTask.IsCompleted);
+                await Task.Delay(10);
+                Assert.False(zeroByteReadTask.IsCompleted);
+
+                zeroByteReadIssued.SetResult();
+
+                Assert.Equal(0, await zeroByteReadTask);
+                Assert.Equal(1, await responseStream.ReadAsync(new byte[2]));
+            },
+            server => server.AcceptConnectionAsync(async connection =>
+            {
+                await connection.ReadRequestHeaderAndSendCustomResponseAsync($"HTTP/1.1 101 Switching Protocols\r\nDate: {DateTimeOffset.UtcNow:R}\r\n\r\n");
+
+                await zeroByteReadIssued.Task;
+
+                await connection.Stream.WriteAsync(new byte[] { (byte)'A' });
+            }));
+        }
     }
 
     [ConditionalClass(typeof(PlatformDetection), nameof(PlatformDetection.IsNotBrowser))]