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The crash scenario is as follows: (1) A non-empty table exists. (2) MDEV-15562 instant ADD/DROP/reorder has been invoked. (3) Some purgeable undo log exists for the table. (4) The table becomes empty, containing not even any delete-marked records, only containing the hidden metadata record that was added in (2). (5) An instant ADD/DROP/reorder column is executed, and the table is emptied and the (2) metadata removed. (6) Purge processes an undo log record from (3), which will refer to a non-existent clustered index field, because the metadata that was created in (2) was remoeved in (5). We fix this by adjusting step (5) so that we will never remove the MDEV-15562-style metadata record. Removing the MDEV-11369 metadata record (instant ADD COLUMN to the end of the table) is completely fine at any time when the table becomes empty, because dict_index_t::n_fields will remain unchanged. innobase_instant_try(): Never remove the MDEV-15562 metadata record. page_cur_delete_rec(): Do not reset FIL_PAGE_TYPE when the MDEV-15562 metadata record is being removed as part of btr_cur_pessimistic_update() invoked by innobase_instant_try().