fix(modem): Fixed clang-tidy warnings

* private field 'netif' is not used [clang-diagnostic-unused-private-field]
* private field 'instance' is not used [clang-diagnostic-unused-private-field]
* Call to virtual method 'FdTerminal::stop' during destruction bypasses virtual dispatch [clang-analyzer-optin.cplusplus.VirtualCall]
* unused variable 'TAG' [clang-diagnostic-unused-const-variable]
* Null pointer passed as 2nd argument to memory copy function [clang-analyzer-unix.cstring.NullArg]
* Array access (from variable 'data') results in a null pointer dereference [clang-analyzer-core.NullDereference]

components/esp_modem/include/cxx_include/esp_modem_cmux.hpp

@@ -142,7 +142,6 @@ private:
     size_t frame_header_offset;
     uint8_t *payload_start;
     size_t total_payload_size;
-    int instance;
     int sabm_ack;
 
     /**

components/esp_modem/include/cxx_include/esp_modem_netif.hpp

@@ -65,7 +65,6 @@ private:
     static void on_ppp_changed(void *arg, esp_event_base_t event_base, int32_t event_id, void *event_data);
 
     std::shared_ptr<DTE> ppp_dte;
-    esp_netif_t *netif;
     struct ppp_netif_driver driver {};
     SignalGroup signal;
     static const size_t PPP_STARTED = SignalGroup::bit0;

components/esp_modem/src/esp_modem_cmux.cpp

@@ -150,7 +150,7 @@ bool CMux::data_available(uint8_t *data, size_t len)
             return false;
         }
     } else if ((type & FT_UIH) == FT_UIH && dlci == 0) { // notify the internal DISC command
-        if ((len > 0 && (data[0] & 0xE1) == 0xE1) || (data == nullptr)) {
+        if ((data == nullptr) || (len > 0 && (data[0] & 0xE1) == 0xE1)) {
             // Not a DISC, ignore (MSC frame)
             return true;
         }
@@ -346,6 +346,9 @@ bool CMux::on_cmux_data(uint8_t *data, size_t actual_len)
         actual_len = term->read(data, buffer.size);
 #endif
     }
+    if (data == nullptr) {
+        return false;
+    }
     ESP_LOG_BUFFER_HEXDUMP("CMUX Received", data, actual_len, ESP_LOG_VERBOSE);
     CMuxFrame frame = { .ptr = data, .len = actual_len };
     while (frame.len > 0) {

components/esp_modem/src/esp_modem_netif.cpp

@@ -69,7 +69,7 @@ void Netif::receive(uint8_t *data, size_t len)
 }
 
 Netif::Netif(std::shared_ptr<DTE> e, esp_netif_t *ppp_netif) :
-    ppp_dte(std::move(e)), netif(ppp_netif)
+    ppp_dte(std::move(e))
 {
     driver.base.netif = ppp_netif;
     driver.ppp = this;

components/esp_modem/src/esp_modem_netif_linux.cpp

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2021-2022 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2021-2024 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -29,11 +29,15 @@ esp_err_t Netif::esp_modem_post_attach(esp_netif_t *esp_netif, void *args)
 
 void Netif::receive(uint8_t *data, size_t len)
 {
-    esp_netif_receive(netif, data, len);
+    esp_netif_receive(driver.base.netif, data, len);
 }
 
 Netif::Netif(std::shared_ptr<DTE> e, esp_netif_t *ppp_netif) :
-    ppp_dte(std::move(e)), netif(ppp_netif) {}
+    ppp_dte(std::move(e))
+{
+    driver.base.netif = ppp_netif;
+    driver.ppp = this;
+}
 
 void Netif::start()
 {
@@ -41,8 +45,8 @@ void Netif::start()
         receive(data, len);
         return true;
     });
-    netif->transmit = esp_modem_dte_transmit;
-    netif->ctx = (void *)this;
+    driver.base.netif->transmit = esp_modem_dte_transmit;
+    driver.base.netif->ctx = (void *)this;
     signal.set(PPP_STARTED);
 }
 

components/esp_modem/src/esp_modem_term_fs.cpp

@@ -155,7 +155,7 @@ int FdTerminal::write(uint8_t *data, size_t len)
 
 FdTerminal::~FdTerminal()
 {
-    stop();
+    FdTerminal::stop();
 }
 
 } // namespace esp_modem

components/esp_modem/src/esp_modem_vfs_uart_creator.cpp

@@ -14,7 +14,7 @@
 #include "uart_resource.hpp"
 #include "vfs_resource/vfs_create.hpp"
 
-constexpr const char *TAG = "vfs_uart_creator";
+[[maybe_unused]] constexpr const char *TAG = "vfs_uart_creator";
 
 
 struct esp_modem_vfs_resource {