Prevent assigning socket id until opened at modem

Local modem ip stacks vary in their implementations and the way of
working. Some of the modems may not open a socket until an IP context is
assigned. That's why we came up with a container that stores addresses of
any CellularSocket instances created on-demand by the application. When
the application requests opening a socket we store allocate and store the
premitive in the container however actual socket creation at the modem
may happen at a later stage, e.g., a call to send_to() may result in
actual opening of a socket.

That's why we must not assign socket ids in the CellularSocket object
during construction. It must happen when actual socket is opened and is
alive.

Another implication of the previous model is that we may have multiple
sockets created in our container but the actual socket ids are not
assigned yet, so we cannot directly map the socket id to the container
indices which has been happening previously.

To solve this issue we have promoted the AT_CellularStac::find_socket_index(...) method
to be a protected method rather than being private so that the children
can use the method to determine if the given index in the container
corrsponds to the assigned socket id or not.

We have given up on the socket->created flag and the whole decision
making to actually open a socket on the modem happens on the basis of a
valid socket being assigned or not.

Author: Hasnain Virk

features/cellular/framework/AT/AT_CellularStack.cpp

@@ -52,7 +52,6 @@ int AT_CellularStack::find_socket_index(nsapi_socket_t handle)
     return -1;
 }
 
-
 /** NetworkStack
  */
 
@@ -131,10 +130,12 @@ nsapi_error_t AT_CellularStack::socket_open(nsapi_socket_t *handle, nsapi_protoc
 
     tr_info("Socket %d open", index);
     // create local socket structure, socket on modem is created when app calls sendto/recvfrom
+    // Do not assign a socket ID yet. Socket is not created at the Modem yet.
+    // create_socket_impl(handle) will assign the correct socket ID.
     _socket[index] = new CellularSocket;
     CellularSocket *psock = _socket[index];
     SocketAddress addr(0, get_dynamic_ip_port());
-    psock->id = index;
+
     psock->localAddress = addr;
     psock->proto = proto;
     *handle = psock;
@@ -153,7 +154,6 @@ nsapi_error_t AT_CellularStack::socket_close(nsapi_socket_t handle)
         return err;
     }
     int sock_id = socket->id;
-    bool sock_created = socket->created;
 
     int index = find_socket_index(handle);
     if (index == -1) {
@@ -165,14 +165,14 @@ nsapi_error_t AT_CellularStack::socket_close(nsapi_socket_t handle)
 
     // Close the socket on the modem if it was created
     _at.lock();
-    if (sock_created) {
+    if (sock_id > -1) {
         err = socket_close_impl(sock_id);
     }
 
     if (!err) {
         tr_info("Socket %d closed", index);
     } else {
-        tr_info("Socket %d close (id %d, created %d, started %d, error %d)", index, sock_id, socket->created, socket->started, err);
+        tr_info("Socket %d close (id %d, started %d, error %d)", index, sock_id, socket->started, err);
     }
 
     _socket[index] = NULL;
@@ -206,7 +206,7 @@ nsapi_error_t AT_CellularStack::socket_bind(nsapi_socket_t handle, const SocketA
         }
     }
 
-    if (!socket->created) {
+    if (socket->id == -1) {
         create_socket_impl(socket);
     }
 
@@ -266,7 +266,7 @@ nsapi_size_or_error_t AT_CellularStack::socket_sendto(nsapi_socket_t handle, con
 
     nsapi_size_or_error_t ret_val = NSAPI_ERROR_OK;
 
-    if (!socket->created) {
+    if (socket->id == -1) {
         _at.lock();
 
         ret_val = create_socket_impl(socket);
@@ -317,7 +317,7 @@ nsapi_size_or_error_t AT_CellularStack::socket_recvfrom(nsapi_socket_t handle, S
 
     nsapi_size_or_error_t ret_val = NSAPI_ERROR_OK;
 
-    if (!socket->created) {
+    if (socket->id == -1) {
         _at.lock();
 
         ret_val = create_socket_impl(socket);

features/cellular/framework/AT/AT_CellularStack.h

@@ -97,7 +97,6 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
             localAddress("", 0),
             _cb(NULL),
             _data(NULL),
-            created(false),
             closed(false),
             started(false),
             tx_ready(false),
@@ -115,7 +114,6 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
         SocketAddress localAddress;
         void (*_cb)(void *);
         void *_data;
-        bool created; // socket has been created on modem stack
         bool closed; // socket has been closed by a peer
         bool started; // socket has been opened on modem stack
         bool tx_ready; // socket is ready for sending on modem stack
@@ -176,12 +174,22 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
                                                        void *buffer, nsapi_size_t size) = 0;
 
     /**
-     *  Find the socket handle based on socket identifier
+     *  Find the socket handle based on the index of the socket construct
+     *  in the socket container. Please note that this index may or may not be
+     *  the socket id. The actual meaning of this index depends upon the modem
+     *  being used.
      *
-     *  @param sock_id  Socket identifier
+     *  @param index    Index of the socket construct in the container
      *  @return         Socket handle, NULL on error
      */
-    CellularSocket *find_socket(int sock_id);
+    CellularSocket *find_socket(int index);
+
+    /**
+     *  Find the index of the given CellularSocket handle. This index may or may
+     *  not be the socket id. The actual meaning of this index depends upon the modem
+     *  being used.
+     */
+    int find_socket_index(nsapi_socket_t handle);
 
     // socket container
     CellularSocket **_socket;
@@ -199,7 +207,6 @@ class AT_CellularStack : public NetworkStack, public AT_CellularBase {
     nsapi_ip_stack_t _stack_type;
 
 private:
-    int find_socket_index(nsapi_socket_t handle);
 
     int get_socket_index_by_port(uint16_t port);