net_startmonitor.c always returned zero. In the case where a socket has already been closed, it correctly handled the disconnetion event but still returned OK. Returning OK causes the callers of net_startmonitor to assume that the connection was okay, undoing the good things that net_startmonitor did and causing the socket to be marked as connected. This behavior was noted by Pelle Windestam.

net/socket/accept.c

@@ -299,7 +299,19 @@ int accept(int sockfd, FAR struct sockaddr *addr, FAR socklen_t *addrlen)
        * socket
        */
 
-      net_startmonitor(pnewsock);
+      ret = net_startmonitor(pnewsock);
+      if (ret < 0)
+        {
+          /* net_startmonitor() can only fail on certain race conditions
+           * where the connection was lost just before this function was
+           * called.  Undo everything we have done and return a failure.
+           */
+
+          net_unlock(state);
+          err = -ret;
+          goto errout_after_accept;
+        }
+
       net_unlock(state);
     }
 #endif /* CONFIG_NET_TCP */
@@ -310,6 +322,9 @@ int accept(int sockfd, FAR struct sockaddr *addr, FAR socklen_t *addrlen)
   pnewsock->s_flags &= ~_SF_CLOSED;
   return newfd;
 
+errout_after_accept:
+  psock_close(pnewsock);
+
 errout_with_socket:
   sockfd_release(newfd);
 

net/socket/connect.c

@@ -81,8 +81,8 @@ struct tcp_connect_s
 #ifdef CONFIG_NET_TCP
 static inline int psock_setup_callbacks(FAR struct socket *psock,
                                         FAR struct tcp_connect_s *pstate);
-static inline void psock_teardown_callbacks(FAR struct tcp_connect_s *pstate,
-                                          int status);
+static void psock_teardown_callbacks(FAR struct tcp_connect_s *pstate,
+                                     int status);
 static uint16_t psock_connect_interrupt(FAR struct net_driver_s *dev,
                                         FAR void *pvconn, FAR void *pvpriv,
                                         uint16_t flags);
@@ -124,9 +124,18 @@ static inline int psock_setup_callbacks(FAR struct socket *psock,
 
       /* Set up the connection event monitor */
 
-      net_startmonitor(psock);
-      ret = OK;
+      ret = net_startmonitor(psock);
+      if (ret < 0)
+        {
+          /* net_startmonitor() can only fail on certain race conditions
+           * where the connection was lost just before this function was
+           * called.  Undo everything we have done and return a failure.
+           */
+
+          psock_teardown_callbacks(pstate, ret);
+        }
     }
+
   return ret;
 }
 #endif /* CONFIG_NET_TCP */
@@ -136,15 +145,14 @@ static inline int psock_setup_callbacks(FAR struct socket *psock,
  ****************************************************************************/
 
 #ifdef CONFIG_NET_TCP
-static inline void psock_teardown_callbacks(FAR struct tcp_connect_s *pstate,
-                                            int status)
+static void psock_teardown_callbacks(FAR struct tcp_connect_s *pstate,
+                                     int status)
 {
   FAR struct tcp_conn_s *conn = pstate->tc_conn;
 
   /* Make sure that no further interrupts are processed */
 
   tcp_callback_free(conn, pstate->tc_cb);
-
   pstate->tc_cb = NULL;
 
   /* If we successfully connected, we will continue to monitor the connection

net/socket/net_close.c

@@ -523,8 +523,9 @@ int psock_close(FAR struct socket *psock)
       goto errout;
     }
 
-  /* We perform the uIP close operation only if this is the last count on the socket.
-   * (actually, I think the socket crefs only takes the values 0 and 1 right now).
+  /* We perform the uIP close operation only if this is the last count on
+   * the socket. (actually, I think the socket crefs only takes the values
+   * 0 and 1 right now).
    */
 
   if (psock->s_crefs <= 1)

net/socket/net_monitor.c

@@ -121,7 +121,13 @@ static void connection_event(FAR struct tcp_conn_s *conn, uint16_t flags)
  *   psock - The socket of interest
  *
  * Returned Value:
- *   For now, this function always returns OK.
+ *   On success, net_startmonitor returns OK; On any failure,
+ *   net_startmonitor will return a negated errno value.  The only failure
+ *   that can occur is if the socket has already been closed and, in this
+ *   case, -ENOTCONN is returned.
+ *
+ * Assumptions:
+ *   The caller holds the network lock.
  *
  ****************************************************************************/
 
@@ -131,22 +137,35 @@ int net_startmonitor(FAR struct socket *psock)
 
   DEBUGASSERT(psock && conn);
 
-  /* Set up to receive callbacks on connection-related events */
-
-  conn->connection_private = (void*)psock;
-  conn->connection_event   = connection_event;
-
-  /* Check if the connection has already been closed before any callbacks have
-   * been registered. (Maybe the connection is lost before accept has registered
-   * the monitoring callback.)
+  /* Check if the connection has already been closed before any callbacks
+   * have been registered. (Maybe the connection is lost before accept has
+   * registered the monitoring callback.)
    */
 
   if (!(conn->tcpstateflags == TCP_ESTABLISHED ||
         conn->tcpstateflags == TCP_SYN_RCVD))
     {
+      /* Invoke the TCP_CLOSE connection event now */
+
       connection_event(conn, TCP_CLOSE);
+
+      /* Make sure that the monitor is stopped */
+
+      conn->connection_private = NULL;
+      conn->connection_event   = NULL;
+
+      /* And return -ENOTCONN to indicate the the monitor was not started
+       * because the socket was already disconnected.
+       */
+
+      return -ENOTCONN;
     }
 
+  /* Set up to receive callbacks on connection-related events */
+
+  conn->connection_private = (void*)psock;
+  conn->connection_event   = connection_event;
+
   return OK;
 }
 

net/socket/socket.h

@@ -231,7 +231,13 @@ FAR struct socket *sockfd_socket(int sockfd);
  *   psock - The socket of interest
  *
  * Returned Value:
- *   For now, this function always returns OK.
+ *   On success, net_startmonitor returns OK; On any failure,
+ *   net_startmonitor will return a negated errno value.  The only failure
+ *   that can occur is if the socket has already been closed and, in this
+ *   case, -ENOTCONN is returned.
+ *
+ * Assumptions:
+ *   The caller holds the network lock.
  *
  ****************************************************************************/
 

net/tcp/tcp_accept.c

@@ -219,7 +219,8 @@ static int accept_interrupt(FAR struct tcp_conn_s *listener,
  * Parameters:
  *   psock    The listening TCP socket structure
  *   addr     Receives the address of the connecting client
- *   addrlen  Input: allocated size of 'addr', Return: returned size of 'addr'
+ *   addrlen  Input: allocated size of 'addr', Return: returned size of
+ *            'addr'
  *   newconn  The new, accepted TCP connection structure
  *
  * Returned Value:
@@ -259,9 +260,9 @@ int psock_tcp_accept(FAR struct socket *psock, FAR struct sockaddr *addr,
     }
 
   /* In general, this uIP-based implementation will not support non-blocking
-   * socket operations... except in a few cases:  Here for TCP accept with backlog
-   * enabled.  If this socket is configured as non-blocking then return EAGAIN
-   * if there is no pending connection in the backlog.
+   * socket operations... except in a few cases:  Here for TCP accept with
+   * backlog enabled.  If this socket is configured as non-blocking then
+   * return EAGAIN if there is no pending connection in the backlog.
    */
 
   else if (_SS_ISNONBLOCK(psock->s_flags))