libcamera: log: Document LOG() restrictions

The LOG() macro uses the global Logger and LogCategory objects
internally. This can result in crashes or other undefined behaviour when
logging from destructors of global objects, as the Logger and/or the
LogCategory instances may have been destroyed.

This isn't ideal and should eventually be fixed. For the time being,
document the restriction, and add a todo list item to fix the problem.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/log.cpp

@@ -980,6 +980,12 @@ LogMessage _log(const LogCategory *category, LogSeverity severity,
  *
  * If the severity is set to Fatal, execution is aborted and the program
  * terminates immediately after printing the message.
+ *
+ * \warning Logging from the destructor of a global object, either directly or
+ * indirectly, results in undefined behaviour.
+ *
+ * \todo Allow logging from destructors of global objects to the largest
+ * possible extent
  */
 
 /**