shell: flush RX buffer on resume to prevent unexpected commands

Introduced an RX buffer flush in the backend when calling shell_start.
Previously, after shell_stop, the backend continued buffering data and
commands. When shell_start was called, all buffered data was processed,
leading to the potential execution of commands issued while the shell
was inactive. This update ensures the RX buffer is flushed upon
resuming the shell, preventing unexpected command execution.

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 +548,18 @@ 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 = (void)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