shell: add generic RX buffer flush function

Added a generic function `z_shell_backend_rx_buffer_flush` to clear
the RX buffer when resuming the shell. The function repeatedly calls
the backend's `read` API until the buffer is empty or a maximum of
1000 iterations is reached. This prevents unintended command
execution after `shell_start`.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>

subsys/shell/shell.c

@@ -1452,6 +1452,14 @@ int shell_start(const struct shell *sh)
 	z_cursor_next_line_move(sh);
 	state_set(sh, SHELL_STATE_ACTIVE);
 
+	/*
+	 * If the shell is stopped with the shell_stop function, its backend remains active
+	 * and continues to buffer incoming data. As a result, when the shell is resumed,
+	 * all buffered data is processed, which may lead to the execution of commands
+	 * received while the shell was stopped.
+	 */
+	z_shell_backend_rx_buffer_flush(sh);
+
 	k_mutex_unlock(&sh->ctx->wr_mtx);
 
 	return 0;

subsys/shell/shell_ops.c

@@ -548,3 +548,17 @@ void z_shell_fprintf(const struct shell *sh,
 	z_shell_vfprintf(sh, color, fmt, args);
 	va_end(args);
 }
+
+void z_shell_backend_rx_buffer_flush(const struct shell *sh)
+{
+	__ASSERT_NO_MSG(sh);
+
+	int32_t max_iterations = 1000;
+	uint8_t buf[64];
+	size_t count = 0;
+	int err;
+
+	do {
+		err = sh->iface->api->read(sh->iface, buf, sizeof(buf), &count);
+	} while (count != 0 && err == 0 && --max_iterations > 0);
+}

subsys/shell/shell_ops.h

@@ -372,6 +372,15 @@ void z_shell_fprintf(const struct shell *sh, enum shell_vt100_color color,
 void z_shell_vfprintf(const struct shell *sh, enum shell_vt100_color color,
 		      const char *fmt, va_list args);
 
+/**
+ * @brief Flushes the shell backend receive buffer.
+ *
+ * This function repeatedly reads from the shell interface's receive buffer
+ * until it is empty or a maximum number of iterations is reached.
+ * It ensures that no additional data is left in the buffer.
+ */
+void z_shell_backend_rx_buffer_flush(const struct shell *sh);
+
 #ifdef __cplusplus
 }
 #endif