Sockets: do 0 byte reads to avoid allocating memory for idle connections

- Do not do 0 byte reads on OSX due to potential sockets bug

src/Kestrel.Transport.Sockets/Internal/SocketConnection.cs

@@ -21,6 +21,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
     {
         private static readonly int MinAllocBufferSize = KestrelMemoryPool.MinimumSegmentSize / 2;
         private static readonly bool IsWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
+        private static readonly bool IsMacOS = RuntimeInformation.IsOSPlatform(OSPlatform.OSX);
 
         private readonly Socket _socket;
         private readonly PipeScheduler _scheduler;
@@ -164,6 +165,13 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
         {
             while (true)
             {
+                // MacOS blocked on https://github.com/dotnet/corefx/issues/31766
+                if (!IsMacOS)
+                {
+                    // Wait for data before allocating a buffer.
+                    await _receiver.WaitForDataAsync();
+                }
+
                 // Ensure we have some reasonable amount of buffer space
                 var buffer = Input.GetMemory(MinAllocBufferSize);
 

src/Kestrel.Transport.Sockets/Internal/SocketReceiver.cs

@@ -13,6 +13,18 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal
         {
         }
 
+        public SocketAwaitableEventArgs WaitForDataAsync()
+        {
+            _awaitableEventArgs.SetBuffer(Array.Empty<byte>(), 0, 0);
+
+            if (!_socket.ReceiveAsync(_awaitableEventArgs))
+            {
+                _awaitableEventArgs.Complete();
+            }
+
+            return _awaitableEventArgs;
+        }
+
         public SocketAwaitableEventArgs ReceiveAsync(Memory<byte> buffer)
         {
 #if NETCOREAPP2_1