[ESP8266] socket_send() returns WOULD_BLOCK if server won't accept

Driver must return NSAPI_ERROR_WOULD_BLOCK if a server won't accept
data from the modem and the modem's buffers are full. In case that
socket is closed driver returns NSAPI_ERROR_CONNECTION_LOST.

Author: Veijo Pesonen

components/wifi/esp8266-driver/ESP8266/ESP8266.cpp

@@ -563,6 +563,7 @@ bool ESP8266::dns_lookup(const char *name, char *ip)
 
 nsapi_error_t ESP8266::send(int id, const void *data, uint32_t amount)
 {
+    nsapi_error_t ret = NSAPI_ERROR_DEVICE_ERROR;
     // +CIPSEND supports up to 2048 bytes at a time
     // Data stream can be truncated
     if (amount > 2048 && _sock_i[id].proto == NSAPI_TCP) {
@@ -576,31 +577,50 @@ nsapi_error_t ESP8266::send(int id, const void *data, uint32_t amount)
     _smutex.lock();
     set_timeout(ESP8266_SEND_TIMEOUT);
     _busy = false;
-    if (_parser.send("AT+CIPSEND=%d,%lu", id, amount)
-            && _parser.recv(">")
-            && _parser.write((char *)data, (int)amount) >= 0
-            && _parser.recv("SEND OK")) {
-        // No flow control, data overrun is possible
-        if (_serial_rts == NC) {
-            while (_parser.process_oob()); // Drain USART receive register
-        }
-        _smutex.unlock();
-        return NSAPI_ERROR_OK;
+    _error = false;
+    if (!_parser.send("AT+CIPSEND=%d,%lu", id, amount)) {
+        tr_debug("ESP8266::send(): AT+CIPSEND failed");
+        goto END;
     }
-    if (_error) {
-        _error = false;
+
+    if(!_parser.recv(">")) {
+        tr_debug("ESP8266::send(): didn't get \">\"");
+        ret = NSAPI_ERROR_WOULD_BLOCK;
+        goto END;
     }
+
+    if (_parser.write((char *)data, (int)amount) >= 0 && _parser.recv("SEND OK")) {
+        ret = NSAPI_ERROR_OK;
+    }
+
+END:
+    _process_oob(ESP8266_RECV_TIMEOUT, true); // Drain USART receive register to avoid data overrun
+
+    // error hierarchy, from low to high
     if (_busy) {
-        set_timeout();
-        _smutex.unlock();
-        tr_debug("returning WOULD_BLOCK");
-        return NSAPI_ERROR_WOULD_BLOCK;
+        ret = NSAPI_ERROR_WOULD_BLOCK;
+        tr_debug("ESP8266::send(): modem busy");
+    }
+
+    if (ret == NSAPI_ERROR_DEVICE_ERROR) {
+        ret = NSAPI_ERROR_WOULD_BLOCK;
+        tr_debug("ESP8266::send(): send failed");
+    }
+
+    if (_error) {
+        ret = NSAPI_ERROR_CONNECTION_LOST;
+        tr_debug("ESP8266::send(): connection disrupted");
+    }
+
+    if (!_sock_i[id].open && ret != NSAPI_ERROR_OK) {
+        ret = NSAPI_ERROR_CONNECTION_LOST;
+        tr_debug("ESP8266::send(): socket closed abruptly");
     }
 
     set_timeout();
     _smutex.unlock();
 
-    return NSAPI_ERROR_DEVICE_ERROR;
+    return ret;
 }
 
 void ESP8266::_oob_packet_hdlr()