TCP: add writable check during poll

When a poll requesting POLLOUT happens, the poll should return
immediately if a write will not block.  This change adds that, as
opposed to the old behaviour of blocking until a timer from the
Ethernet driver eventually triggers the poll to complete.

This is only implemented for buffered TCP.  Unbuffered TCP should
behave as before.

net/tcp/tcp.h

@@ -1,7 +1,7 @@
 /****************************************************************************
  * net/tcp/tcp.h
  *
- *   Copyright (C) 2014-2015 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2014-2016 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -1143,6 +1143,35 @@ struct socket;
 ssize_t psock_tcp_send(FAR struct socket *psock, FAR const void *buf,
                        size_t len);
 
+/****************************************************************************
+ * Function: psock_tcp_cansend
+ *
+ * Description:
+ *   psock_tcp_cansend() returns a value indicating if a write to the socket
+ *   would block.  No space in the buffer is actually reserved, so it is
+ *   possible that the write may still block if the buffer is filled by
+ *   another means.
+ *
+ * Parameters:
+ *   psock    An instance of the internal socket structure.
+ *
+ * Returned Value:
+ *   OK
+ *     At least one byte of data could be succesfully written.
+ *   -EWOULDBLOCK
+ *     There is no room in the output buffer.
+ *   -EBADF
+ *     An invalid descriptor was specified.
+ *   -ENOTCONN
+ *     The socket is not connected.
+ *
+ * Assumptions:
+ *   Not running at the interrupt level
+ *
+ ****************************************************************************/
+
+int psock_tcp_cansend(FAR struct socket *psock);
+
 /****************************************************************************
  * Function: tcp_wrbuffer_initialize
  *
@@ -1197,6 +1226,21 @@ FAR struct tcp_wrbuffer_s *tcp_wrbuffer_alloc(void);
 void tcp_wrbuffer_release(FAR struct tcp_wrbuffer_s *wrb);
 #endif /* CONFIG_NET_TCP_WRITE_BUFFERS */
 
+/****************************************************************************
+ * Function: tcp_wrbuffer_test
+ *
+ * Description:
+ *   Check if there is room in the write buffer.  Does not reserve any space.
+ *
+ * Assumptions:
+ *   None.
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_NET_TCP_WRITE_BUFFERS
+int tcp_wrbuffer_test(void);
+#endif /* CONFIG_NET_TCP_WRITE_BUFFERS */
+
 /****************************************************************************
  * Function: tcp_wrbuffer_dump
  *

net/tcp/tcp_netpoll.c

@@ -1,7 +1,7 @@
 /****************************************************************************
  * net/tcp/tcp_netpoll.c
  *
- *   Copyright (C) 2008-2009, 2011-2015 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2008-2009, 2011-2016 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -282,6 +282,10 @@ int tcp_pollsetup(FAR struct socket *psock, FAR struct pollfd *fds)
 
       fds->revents |= (POLLERR | POLLHUP);
     }
+  else if (_SS_ISCONNECTED(psock->s_flags) && psock_tcp_cansend(psock) >= 0)
+    {
+      fds->revents |= (POLLWRNORM & fds->events);
+    }
 
   /* Check if any requested events are already in effect */
 

net/tcp/tcp_send_buffered.c

@@ -1,7 +1,7 @@
 /****************************************************************************
  * net/tcp/tcp_send_buffered.c
  *
- *   Copyright (C) 2007-2014 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2007-2014, 2016 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *           Jason Jiang  <jasonj@live.cn>
  *
@@ -1104,4 +1104,53 @@ errout:
   return ERROR;
 }
 
+/****************************************************************************
+ * Function: psock_tcp_cansend
+ *
+ * Description:
+ *   psock_tcp_cansend() returns a value indicating if a write to the socket
+ *   would block.  No space in the buffer is actually reserved, so it is
+ *   possible that the write may still block if the buffer is filled by
+ *   another means.
+ *
+ * Parameters:
+ *   psock    An instance of the internal socket structure.
+ *
+ * Returned Value:
+ *   OK
+ *     At least one byte of data could be succesfully written.
+ *   -EWOULDBLOCK
+ *     There is no room in the output buffer.
+ *   -EBADF
+ *     An invalid descriptor was specified.
+ *   -ENOTCONN
+ *     The socket is not connected.
+ *
+ * Assumptions:
+ *   Not running at the interrupt level
+ *
+ ****************************************************************************/
+
+int psock_tcp_cansend(FAR struct socket *psock)
+{
+  if (!psock || psock->s_crefs <= 0)
+    {
+      ndbg("ERROR: Invalid socket\n");
+      return -EBADF;
+    }
+
+  if (psock->s_type != SOCK_STREAM || !_SS_ISCONNECTED(psock->s_flags))
+    {
+      ndbg("ERROR: Not connected\n");
+      return -ENOTCONN;
+    }
+
+  if (tcp_wrbuffer_test())
+    {
+      return -EWOULDBLOCK;
+    }
+
+  return OK;
+}
+
 #endif /* CONFIG_NET && CONFIG_NET_TCP && CONFIG_NET_TCP_WRITE_BUFFERS */

net/tcp/tcp_send_unbuffered.c

@@ -880,4 +880,30 @@ errout:
   return ERROR;
 }
 
+/****************************************************************************
+ * Function: psock_tcp_cansend
+ *
+ * Description:
+ *   psock_tcp_cansend() returns a value indicating if a write to the socket
+ *   would block.  It is still possible that the write may block if another
+ *   write occurs first.
+ *
+ * Parameters:
+ *   psock    An instance of the internal socket structure.
+ *
+ * Returned Value:
+ *   OK (Function not implemented).
+ *
+ * Assumptions:
+ *   None
+ *
+ ****************************************************************************/
+
+int psock_tcp_cansend(FAR struct socket *psock)
+{
+  /* TODO: return OK unless someone is waiting for a packet to send */
+
+  return OK;
+}
+
 #endif /* CONFIG_NET && CONFIG_NET_TCP && !CONFIG_NET_TCP_WRITE_BUFFERS */

net/tcp/tcp_wrbuffer.c

@@ -201,4 +201,22 @@ void tcp_wrbuffer_release(FAR struct tcp_wrbuffer_s *wrb)
   sem_post(&g_wrbuffer.sem);
 }
 
+/****************************************************************************
+ * Function: tcp_wrbuffer_test
+ *
+ * Description:
+ *   Check if there is room in the write buffer.  Does not reserve any space.
+ *
+ * Assumptions:
+ *   None.
+ *
+ ****************************************************************************/
+
+int tcp_wrbuffer_test(void)
+{
+  int val = 0;
+  sem_getvalue(&g_wrbuffer.sem, &val);
+  return val > 0 ? OK : ERROR;
+}
+
 #endif /* CONFIG_NET && CONFIG_NET_TCP && CONFIG_NET_TCP_WRITE_BUFFERS */