Kernel: Implement Nagle’s Algorithm

This is an initial implementation, about as basic as intended by the
RFC, and not configurable from userspace at the moment. It should reduce
the amount of low-sized packets sent, reducing overhead and thereby
network traffic.

Kernel/Net/TCPSocket.cpp

@@ -210,6 +210,18 @@ ErrorOr<size_t> TCPSocket::protocol_send(UserOrKernelBuffer const& data, size_t
     if (routing_decision.is_zero())
         return set_so_error(EHOSTUNREACH);
     size_t mss = routing_decision.adapter->mtu() - sizeof(IPv4Packet) - sizeof(TCPPacket);
+
+    // RFC 896 (Nagle’s algorithm): https://www.ietf.org/rfc/rfc0896
+    // "The solution is to inhibit the sending of new TCP  segments when
+    //  new  outgoing  data  arrives  from  the  user  if  any previously
+    //  transmitted data on the connection remains unacknowledged.   This
+    //  inhibition  is  to be unconditional; no timers, tests for size of
+    //  data received, or other conditions are required."
+    // FIXME: Make this configurable via TCP_NODELAY.
+    auto has_unacked_data = m_unacked_packets.with_shared([&](auto const& packets) { return packets.size > 0; });
+    if (has_unacked_data && data_length < mss)
+        return 0;
+
     data_length = min(data_length, mss);
     TRY(send_tcp_packet(TCPFlags::PSH | TCPFlags::ACK, &data, data_length, &routing_decision));
     return data_length;