net/tcp/sendfile: NET_TCP_WRITE_BUFFERS and NET_SENDFILE were inconsistent with each other:

tcp_sendfile() reads data directly from a file and does not use NET_TCP_WRITE_BUFFERS data flow
even if CONFIG_NET_TCP_WRITE_BUFFERS option is enabled.
Despite this, tcp_sendfile relied on NET_TCP_WRITE_BUFFERS specific flow control variables that
were idle during sendfile operation. Thus it was a total inconsistency.

E.g. because of the issue, TCP socket used by sendfile() operation never issued
FIN packet on close() command, and the TCP connection hung up.

As a result of the fix, simultaneously enabled CONFIG_NET_TCP_WRITE_BUFFERS and
CONFIG_NET_SENDFILE options can coexist.

net/tcp/tcp.h

@@ -287,6 +287,10 @@ struct tcp_conn_s
   uint8_t    keepretries; /* Number of retries attempted */
 #endif
 
+#if defined(CONFIG_NET_SENDFILE) && defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+  bool       sendfile;    /* True if sendfile operation is in progress */
+#endif
+
   /* connevents is a list of callbacks for each socket the uses this
    * connection (there can be more that one in the event that the the socket
    * was dup'ed).  It is used with the network monitor to handle

net/tcp/tcp_appsend.c

@@ -226,9 +226,19 @@ void tcp_appsend(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 
   else
     {
-#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+      /* The application cannot send more than what is allowed by the
+       * MSS (the minimum of the MSS and the available window).
+       */
+
       DEBUGASSERT(dev->d_sndlen <= conn->mss);
-#else
+
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS) || defined(CONFIG_NET_SENDFILE)
+
+#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+      if (conn->sendfile)
+        {
+#endif
+
       /* If d_sndlen > 0, the application has data to be sent. */
 
       if (dev->d_sndlen > 0)
@@ -241,15 +251,14 @@ void tcp_appsend(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
            */
 
           conn->tx_unacked += dev->d_sndlen;
-
-          /* The application cannot send more than what is allowed by the
-           * MSS (the minimum of the MSS and the available window).
-           */
-
-          DEBUGASSERT(dev->d_sndlen <= conn->mss);
         }
 
       conn->nrtx = 0;
+
+#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+        }
+#endif
+
 #endif
 
       /* Then handle the rest of the operation just as for the rexmit case */

net/tcp/tcp_sendfile.c

@@ -495,6 +495,9 @@ ssize_t tcp_sendfile(FAR struct socket *psock, FAR struct file *infile,
    */
 
   net_lock();
+#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+  conn->sendfile = true;
+#endif
   memset(&state, 0, sizeof(struct sendfile_s));
 
   /* This semaphore is used for signaling and, hence, should not have
@@ -574,6 +577,9 @@ errout_datacb:
 
 errout_locked:
   nxsem_destroy(&state.snd_sem);
+#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+  conn->sendfile = false;
+#endif
   net_unlock();
 
   /* Return the current file position */

net/tcp/tcp_timer.c

@@ -269,7 +269,12 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 
               else if (
 #ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+#  ifdef CONFIG_NET_SENDFILE
+                  (!conn->sendfile && conn->expired > 0) ||
+                  (conn->sendfile && conn->nrtx >= TCP_MAXRTX) ||
+#  else
                   conn->expired > 0 ||
+#  endif
 #else
                   conn->nrtx >= TCP_MAXRTX ||
 #endif