Robust discarding of stale data on new TCP connection

os/src/userspace/data_server.hpp

@@ -15,6 +15,7 @@
 #include <iomanip>
 #include <stdexcept>
 #include <system_error>
+#include <thread>
 
 #include <boost/asio.hpp>
 #include <boost/system/error_code.hpp>
@@ -202,6 +203,30 @@ public:
     }
 
 private:
+    /**
+     * Re-initialize DMA stream and discard stale data from the DMA buffer.
+     *
+     * Do not call this while an async_send() operation is in progress.
+     */
+    void discard_stale_data()
+    {
+        // Re-init DMA and clear buffer.
+        m_dma_stream.init();
+
+        // The DMA buffer and the internal RAM buffer in the FPGA are now empty.
+        // But the front-end logic in the FPGA may still contain some stale
+        // data as well as an overflow record. These will flow into the RAM
+        // buffer within a microsecond. Let's wait for that.
+        // Sleeping in an Asio handler is bad, but 1 microsecond is so short
+        // that it won't hurt.
+        std::this_thread::sleep_for(std::chrono::microseconds(1));
+
+        // Assuming no new data is being produced, the front-end buffer is
+        // now clean and overflow cleared. A little data may have moved into
+        // the FPGA RAM buffer. Re-init DMA again to discard that.
+        m_dma_stream.init();
+    }
+
     /** Accept completion handler. */
     void handle_accept(const boost::system::error_code& error,
                        asio::ip::tcp::socket conn)
@@ -281,10 +306,10 @@ private:
         }
 
         // Clear buffer, then enable DMA stream.
-        m_dma_stream.init();
+        discard_stale_data();
         m_dma_stream.set_enabled(true);
 
-        // Try to send some data.
+        // Prepare to send data.
         transmit_data(false);
     }