Prevent unnecessary polling in passive receive mode

socket_recv calls are passed all the way to the ESP8266 regardless
of data availability.

With this change the data availability variable is checked
before sending the receive AT command. The return value is also
changed so that the caller will know to wait for an event before
calling again in non-blocking mode.

Author: Marcus Chang

ESP8266/ESP8266.cpp

@@ -35,6 +35,7 @@ ESP8266::ESP8266(PinName tx, PinName rx, bool debug, PinName rts, PinName cts)
     : _sdk_v(-1, -1, -1),
       _at_v(-1, -1, -1),
       _tcp_passive(false),
+      _callback(0),
       _serial(tx, rx, ESP8266_DEFAULT_BAUD_RATE),
       _serial_rts(rts),
       _serial_cts(cts),
@@ -560,7 +561,12 @@ void ESP8266::_oob_packet_hdlr()
 
     if(_tcp_passive && _sock_i[id].open == true && _sock_i[id].proto == NSAPI_TCP) {
         if (_parser.recv("%d\n", &amount)) {
-            _sock_i[id].tcp_data_avbl = amount; // Not used but stored for the sake of visibility
+            _sock_i[id].tcp_data_avbl = amount;
+
+            // notify data is available
+            if (_callback) {
+                _callback();
+            }
         }
         return;
     } else if (!_parser.recv("%d:", &amount)) {
@@ -617,6 +623,11 @@ int32_t ESP8266::_recv_tcp_passive(int id, void *data, uint32_t amount, uint32_t
 {
     int32_t ret;
 
+    // return immediately if no data is available
+    if (_sock_i[id].tcp_data_avbl == 0) {
+        return NSAPI_ERROR_WOULD_BLOCK;
+    }
+
     _smutex.lock();
 
     _sock_i[id].tcp_data = (char*)data;
@@ -823,6 +834,7 @@ bool ESP8266::writeable()
 void ESP8266::sigio(Callback<void()> func)
 {
     _serial.sigio(func);
+    _callback = func;
 }
 
 void ESP8266::attach(Callback<void()> status_cb)

ESP8266/ESP8266.h

@@ -372,6 +372,7 @@ class ESP8266 {
     // FW version specific settings and functionalities
     bool _tcp_passive;
     int32_t _recv_tcp_passive(int id, void *data, uint32_t amount, uint32_t timeout);
+    mbed::Callback<void()> _callback;
 
     // UART settings
     mbed::UARTSerial _serial;