Fix return value of partially successful Socket.Send (#24005)

* Fix return value of partially successful Socket.Send

* Fix Assert.InRange range

* Fix SocketError return

* move successfulSend outside loop

* Test: buffer size 100M -> 5M

* Disable test on Windows because all data gets sent

Author: tmds

src/System.Net.Sockets/src/System/Net/Sockets/SocketPal.Unix.cs

@@ -721,6 +721,7 @@ public static bool TryCompleteSendTo(SafeCloseSocket socket, IList<ArraySegment<
 
         public static bool TryCompleteSendTo(SafeCloseSocket socket, byte[] buffer, IList<ArraySegment<byte>> buffers, ref int bufferIndex, ref int offset, ref int count, SocketFlags flags, byte[] socketAddress, int socketAddressLen, ref int bytesSent, out SocketError errorCode)
         {
+            bool successfulSend = false;
             for (;;)
             {
                 int sent;
@@ -745,16 +746,17 @@ public static bool TryCompleteSendTo(SafeCloseSocket socket, byte[] buffer, ILis
 
                 if (sent == -1)
                 {
-                    if (errno != Interop.Error.EAGAIN && errno != Interop.Error.EWOULDBLOCK)
+                    if (!successfulSend && errno != Interop.Error.EAGAIN && errno != Interop.Error.EWOULDBLOCK)
                     {
                         errorCode = GetSocketErrorForErrorCode(errno);
                         return true;
                     }
 
-                    errorCode = SocketError.Success;
+                    errorCode = successfulSend ? SocketError.Success : SocketError.WouldBlock;
                     return false;
                 }
 
+                successfulSend = true;
                 bytesSent += sent;
 
                 bool isComplete = sent == 0 ||
@@ -902,8 +904,8 @@ public static SocketError Send(SafeCloseSocket handle, IList<ArraySegment<byte>>
             int bufferIndex = 0;
             int offset = 0;
             SocketError errorCode;
-            bool completed = TryCompleteSendTo(handle, bufferList, ref bufferIndex, ref offset, socketFlags, null, 0, ref bytesTransferred, out errorCode);
-            return completed ? errorCode : SocketError.WouldBlock;
+            TryCompleteSendTo(handle, bufferList, ref bufferIndex, ref offset, socketFlags, null, 0, ref bytesTransferred, out errorCode);
+            return errorCode;
         }
 
         public static SocketError Send(SafeCloseSocket handle, byte[] buffer, int offset, int count, SocketFlags socketFlags, out int bytesTransferred)
@@ -915,8 +917,8 @@ public static SocketError Send(SafeCloseSocket handle, byte[] buffer, int offset
 
             bytesTransferred = 0;
             SocketError errorCode;
-            bool completed = TryCompleteSendTo(handle, buffer, ref offset, ref count, socketFlags, null, 0, ref bytesTransferred, out errorCode);
-            return completed ? errorCode : SocketError.WouldBlock;
+            TryCompleteSendTo(handle, buffer, ref offset, ref count, socketFlags, null, 0, ref bytesTransferred, out errorCode);
+            return errorCode;
         }
 
         public static SocketError SendFile(SafeCloseSocket handle, FileStream fileStream)
@@ -947,8 +949,8 @@ public static SocketError SendTo(SafeCloseSocket handle, byte[] buffer, int offs
 
             bytesTransferred = 0;
             SocketError errorCode;
-            bool completed = TryCompleteSendTo(handle, buffer, ref offset, ref count, socketFlags, socketAddress, socketAddressLen, ref bytesTransferred, out errorCode);
-            return completed ? errorCode : SocketError.WouldBlock;
+            TryCompleteSendTo(handle, buffer, ref offset, ref count, socketFlags, socketAddress, socketAddressLen, ref bytesTransferred, out errorCode);
+            return errorCode;
         }
 
         public static SocketError Receive(SafeCloseSocket handle, IList<ArraySegment<byte>> buffers, ref SocketFlags socketFlags, out int bytesTransferred)

src/System.Net.Sockets/tests/FunctionalTests/SendReceive.cs

@@ -1028,6 +1028,35 @@ public async Task ReceiveIovMaxUdp_SuccessOrMessageSize()
             Assert.True(receiveTask.IsCompleted);
             await receiveTask;
         }
+
+        [Fact]
+        [PlatformSpecific(~TestPlatforms.Windows)] // All data is sent, even when very large (100M).
+        public void SocketSendWouldBlock_ReturnsBytesSent()
+        {
+            using (var server = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
+            using (var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
+            {
+                // listen
+                server.BindToAnonymousPort(IPAddress.Loopback);
+                server.Listen(1);
+                // connect
+                client.Connect(server.LocalEndPoint);
+                // accept
+                using (Socket socket = server.Accept())
+                {
+                    // We send a large amount of data but don't read it.
+                    // A chunck will be sent, attempts to send more will return SocketError.WouldBlock.
+                    // Socket.Send must return the success of the partial send.
+                    socket.Blocking = false;
+                    var data = new byte[5_000_000];
+                    SocketError error;
+                    int bytesSent = socket.Send(data, 0, data.Length, SocketFlags.None, out error);
+
+                    Assert.Equal(SocketError.Success, error);
+                    Assert.InRange(bytesSent, 1, data.Length - 1);
+                }
+            }
+        }
     }
 
     public sealed class SendReceiveUdpClient : MemberDatas