init: Document the locking strategy in class ShutdownState

Let the compiler verify that shutdown_command_lock_ is held when
shutdown_command_ is accessed.

Bug: 266255006
Change-Id: Ibd05137ab65e20f247f35bbb2bb1865e05f51f41
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/init/init.cpp b/init/init.cpp
index f964c60..9312f26 100644
--- a/init/init.cpp
+++ b/init/init.cpp
@@ -51,6 +51,7 @@
 #include <android-base/properties.h>
 #include <android-base/stringprintf.h>
 #include <android-base/strings.h>
+#include <android-base/thread_annotations.h>
 #include <fs_avb/fs_avb.h>
 #include <fs_mgr_vendor_overlay.h>
 #include <keyutils.h>
@@ -259,7 +260,7 @@
 
   private:
     std::mutex shutdown_command_lock_;
-    std::string shutdown_command_;
+    std::string shutdown_command_ GUARDED_BY(shutdown_command_lock_);
     bool do_shutdown_ = false;
 } shutdown_state;