Summary
The WT_READING_CHECKPOINT checks if the current session dhandle is not NULL and if the dhandle references a checkpoint.

#define WT_READING_CHECKPOINT(s) ((s)->dhandle != NULL && WT_DHANDLE_IS_CHECKPOINT((s)->dhandle))

This definition is primarily used to for checkpoint cursors to look at the content of a checkpoint. This ticket should add a check that the session dhandle must be open, this can be done through the WT_DHANDLE_OPEN flag. After the ticket has been implemented, run a patch build to make sure that there are no fallouts.