Workaround for when USB CDC is unplugged (#7583)

* Workaround for when USB CDC is unplugged

* Considers default TX timeout

Sets back default TX timeout whenever USB is plugged, otherwise it is kept as zero.

* fixed left over code

Author: SuGlider

cores/esp32/HWCDC.cpp

@@ -32,7 +32,11 @@ static uint8_t rx_data_buf[64];
 static intr_handle_t intr_handle = NULL;
 static volatile bool initial_empty = false;
 static xSemaphoreHandle tx_lock = NULL;
-static uint32_t tx_timeout_ms = 200;
+
+// workaround for when USB CDC is not connected
+static uint32_t tx_timeout_ms = 0;
+static bool tx_timeout_change_request = false;
+
 static esp_event_loop_handle_t arduino_hw_cdc_event_loop_handle = NULL;
 
 static esp_err_t arduino_hw_cdc_event_post(esp_event_base_t event_base, int32_t event_id, void *event_data, size_t event_data_size, BaseType_t *task_unblocked){
@@ -72,9 +76,14 @@ static void hw_cdc_isr_handler(void *arg) {
         if (usb_serial_jtag_ll_txfifo_writable() == 1) {
             // We disable the interrupt here so that the interrupt won't be triggered if there is no data to send.
             usb_serial_jtag_ll_disable_intr_mask(USB_SERIAL_JTAG_INTR_SERIAL_IN_EMPTY);
-
             if(!initial_empty){
                 initial_empty = true;
+                // First time USB is plugged and the application has not explicitly set TX Timeout, set it to default 100ms.
+                // Otherwise, USB is still unplugged and the timeout will be kept as Zero in order to avoid any delay in the
+                // application whenever it uses write() and the TX Queue gets full.
+                if (!tx_timeout_change_request) {
+                    tx_timeout_ms = 100;
+                }
                 //send event?
                 //ets_printf("CONNECTED\n");
                 arduino_hw_cdc_event_post(ARDUINO_HW_CDC_EVENTS, ARDUINO_HW_CDC_CONNECTED_EVENT, &event, sizeof(arduino_hw_cdc_event_data_t), &xTaskWoken);
@@ -197,6 +206,9 @@ void HWCDC::end()
 
 void HWCDC::setTxTimeoutMs(uint32_t timeout){
     tx_timeout_ms = timeout;
+    // it registers that the user has explicitly requested to use a value as TX timeout
+    // used for the workaround with unplugged USB and TX Queue Full that causes a delay on every write()
+    tx_timeout_change_request = true;
 }
 
 /*