Cellular: don't allow ATHandler read/write if filehandle not usable

For example after going to ppp mode we must block at write and
read as filehandle is not usable.

features/cellular/framework/AT/ATHandler.cpp

@@ -408,6 +408,11 @@ void ATHandler::skip_param(uint32_t count)
         return;
     }
 
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return;
+    }
+
     for (uint32_t i = 0; (i < count && !_stop_tag->found); i++) {
         size_t match_pos = 0;
         while (true) {
@@ -436,6 +441,10 @@ void ATHandler::skip_param(ssize_t len, uint32_t count)
     if (_last_err || !_stop_tag || _stop_tag->found) {
         return;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return;
+    }
 
     for (uint32_t i = 0; i < count; i++) {
         ssize_t read_len = 0;
@@ -456,6 +465,10 @@ ssize_t ATHandler::read_bytes(uint8_t *buf, size_t len)
     if (_last_err) {
         return -1;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return -1;
+    }
 
     bool debug_on = _debug_on;
     size_t read_len = 0;
@@ -481,6 +494,10 @@ ssize_t ATHandler::read_string(char *buf, size_t size, bool read_even_stop_tag)
     if (_last_err || !_stop_tag || (_stop_tag->found && read_even_stop_tag == false)) {
         return -1;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return -1;
+    }
 
     unsigned int len = 0;
     size_t match_pos = 0;
@@ -547,6 +564,10 @@ ssize_t ATHandler::read_hex_string(char *buf, size_t size)
     if (_last_err || !_stop_tag || _stop_tag->found) {
         return -1;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return -1;
+    }
 
     size_t match_pos = 0;
 
@@ -620,6 +641,10 @@ int32_t ATHandler::read_int()
     if (_last_err || !_stop_tag || _stop_tag->found) {
         return -1;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return -1;
+    }
 
     char buff[BUFF_SIZE];
     if (read_string(buff, sizeof(buff)) == 0) {
@@ -879,6 +904,10 @@ void ATHandler::resp_start(const char *prefix, bool stop)
     if (_last_err) {
         return;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return;
+    }
 
     set_scope(NotSet);
     // Try get as much data as possible
@@ -905,6 +934,10 @@ bool ATHandler::info_resp()
     if (_last_err || _resp_stop.found) {
         return false;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return false;
+    }
 
     if (_prefix_matched) {
         _prefix_matched = false;
@@ -935,6 +968,10 @@ bool ATHandler::info_elem(char start_tag)
     if (_last_err) {
         return false;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return false;
+    }
 
     // If coming here after another info response element was started(looping), stop the previous one.
     // Trying to handle stopping in this level instead of doing it in upper level.
@@ -1005,6 +1042,11 @@ bool ATHandler::consume_to_stop_tag()
         return true;
     }
 
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return true;
+    }
+
     if (consume_to_tag((const char *)_stop_tag->tag, true)) {
         return true;
     }
@@ -1081,15 +1123,18 @@ const char *ATHandler::mem_str(const char *dest, size_t dest_len, const char *sr
 
 void ATHandler::cmd_start(const char *cmd)
 {
+    if (_last_err != NSAPI_ERROR_OK) {
+        return;
+    }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return;
+    }
 
     if (_at_send_delay) {
         rtos::ThisThread::sleep_until(_last_response_stop + _at_send_delay);
     }
 
-    if (_last_err != NSAPI_ERROR_OK) {
-        return;
-    }
-
     (void)write(cmd, strlen(cmd));
 
     _cmd_start = true;
@@ -1136,6 +1181,10 @@ void ATHandler::cmd_stop()
     if (_last_err != NSAPI_ERROR_OK) {
         return;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return;
+    }
     // Finish with CR
     (void)write(_output_delimiter, strlen(_output_delimiter));
 }
@@ -1152,6 +1201,10 @@ size_t ATHandler::write_bytes(const uint8_t *data, size_t len)
     if (_last_err != NSAPI_ERROR_OK) {
         return 0;
     }
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return 0;
+    }
 
     return write(data, len);
 }
@@ -1198,6 +1251,11 @@ bool ATHandler::check_cmd_send()
         return false;
     }
 
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return false;
+    }
+
     // Don't write delimiter if flag was set so
     if (!_use_delimiter) {
         return true;
@@ -1218,6 +1276,10 @@ bool ATHandler::check_cmd_send()
 
 void ATHandler::flush()
 {
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return;
+    }
     tr_debug("AT flush");
     reset_buffer();
     while (fill_buffer(false)) {
@@ -1255,6 +1317,10 @@ void ATHandler::debug_print(const char *p, int len)
 
 bool ATHandler::sync(int timeout_ms)
 {
+    if (!_is_fh_usable) {
+        _last_err = NSAPI_ERROR_BUSY;
+        return false;
+    }
     tr_debug("AT sync");
     lock();
     uint32_t timeout = _at_timeout;