drivers: serial: stm32: fix race condition when both APIs enabled

Fix a bug where if CONFIG_UART_ASYNC_API was enabled in the config, the
interrupt flags would get cleared after the interrupt driven API callback
finished executing, causing data loss if data happened to arrive after the
callback but before the flags were cleared.

Signed-off-by: Kamil Krzyżanowski <kamnxt@kamnxt.com>

Author: kamnxt

drivers/serial/uart_stm32.c

@@ -1359,6 +1359,16 @@ static void uart_stm32_isr(const struct device *dev)
 #endif
 
 #ifdef CONFIG_UART_ASYNC_API
+#ifdef CONFIG_UART_INTERRUPT_DRIVEN
+	/* If both ASYNC and INTERRUPT modes are supported in this build,
+	 * check whether this instance is currently being used via the
+	 * interrupt-driven API. If it is, do not process interrupt flags
+	 * as the user callback invoked earlier is responsible for that.
+	 */
+	if (data->user_cb) {
+		return;
+	}
+#endif /* CONFIG_UART_INTERRUPT_DRIVEN */
 	if (LL_USART_IsEnabledIT_IDLE(usart) &&
 			LL_USART_IsActiveFlag_IDLE(usart)) {