Remove dead code for MLOG_UNDO_HDR_DISCARD

The redo log record MLOG_UNDO_HDR_DISCARD is never written, and
the function trx_undo_discard_latest_update_undo() is never called
outside crash recovery.

Starting with MariaDB 10.2.2, crash recovery from an older InnoDB
version is refused. Therefore, we can safely remove the code for
parsing this long-unused redo log record type.

The use of MLOG_UNDO_HDR_DISCARD was removed by Heikki Tuuri,
the original designer and author of InnoDB, more than 13 years ago:

  commit 3caab0f3
  Author: unknown <heikki@hundin.mysql.fi>
  Date:   Thu Mar 18 14:57:22 2004 +0200

    trx0undo.h, trx0undo.c, trx0trx.c:
      Update an obsolete comment about trx commit: we can no longer call
      trx_undo_update_cleanup_by_discard(), and actually, the idea to
      call it was erroneous, it cannot work

storage/innobase/include/mtr0types.h

@@ -109,9 +109,6 @@ enum mlog_id_t {
 	/** initialize a page in an undo log */
 	MLOG_UNDO_INIT = 22,
 
-	/** discard an update undo log header */
-	MLOG_UNDO_HDR_DISCARD = 23,
-
 	/** reuse an insert undo log header */
 	MLOG_UNDO_HDR_REUSE = 24,
 

storage/innobase/include/trx0undo.h

@@ -357,16 +357,6 @@ trx_undo_parse_page_header(
 	const byte*	end_ptr,
 	page_t*		page,
 	mtr_t*		mtr);
-/***********************************************************//**
-Parses the redo log entry of an undo log page header discard.
-@return end of log record or NULL */
-byte*
-trx_undo_parse_discard_latest(
-/*==========================*/
-	byte*	ptr,	/*!< in: buffer */
-	byte*	end_ptr,/*!< in: buffer end */
-	page_t*	page,	/*!< in: page or NULL */
-	mtr_t*	mtr);	/*!< in: mtr or NULL */
 /************************************************************************
 Frees an undo log memory copy. */
 void

storage/innobase/log/log0recv.cc

@@ -1334,10 +1334,6 @@ recv_parse_or_apply_log_rec_body(
 		/* Allow anything in page_type when creating a page. */
 		ptr = trx_undo_parse_page_init(ptr, end_ptr, page, mtr);
 		break;
-	case MLOG_UNDO_HDR_DISCARD:
-		ut_ad(!page || page_type == FIL_PAGE_UNDO_LOG);
-		ptr = trx_undo_parse_discard_latest(ptr, end_ptr, page, mtr);
-		break;
 	case MLOG_UNDO_HDR_CREATE:
 	case MLOG_UNDO_HDR_REUSE:
 		ut_ad(!page || page_type == FIL_PAGE_UNDO_LOG);
@@ -3564,9 +3560,6 @@ get_mlog_string(mlog_id_t type)
 	case MLOG_UNDO_INIT:
 		return("MLOG_UNDO_INIT");
 
-	case MLOG_UNDO_HDR_DISCARD:
-		return("MLOG_UNDO_HDR_DISCARD");
-
 	case MLOG_UNDO_HDR_REUSE:
 		return("MLOG_UNDO_HDR_REUSE");
 

storage/innobase/trx/trx0undo.cc

@@ -132,15 +132,6 @@ trx_undo_insert_header_reuse(
 					header page, x-latched */
 	trx_id_t	trx_id,		/*!< in: transaction id */
 	mtr_t*		mtr);		/*!< in: mtr */
-/**********************************************************************//**
-If an update undo log can be discarded immediately, this function frees the
-space, resetting the page to the proper state for caching. */
-static
-void
-trx_undo_discard_latest_update_undo(
-/*================================*/
-	page_t*	undo_page,	/*!< in: header page of an undo log of size 1 */
-	mtr_t*	mtr);		/*!< in: mtr */
 
 /***********************************************************************//**
 Gets the previous record in an undo log from the previous page.
@@ -783,80 +774,6 @@ trx_undo_insert_header_reuse(
 	return(free);
 }
 
-/**********************************************************************//**
-Writes the redo log entry of an update undo log header discard. */
-UNIV_INLINE
-void
-trx_undo_discard_latest_log(
-/*========================*/
-	page_t* undo_page,	/*!< in: undo log header page */
-	mtr_t*	mtr)		/*!< in: mtr */
-{
-	mlog_write_initial_log_record(undo_page, MLOG_UNDO_HDR_DISCARD, mtr);
-}
-
-/***********************************************************//**
-Parses the redo log entry of an undo log page header discard.
-@return end of log record or NULL */
-byte*
-trx_undo_parse_discard_latest(
-/*==========================*/
-	byte*	ptr,	/*!< in: buffer */
-	byte*	end_ptr MY_ATTRIBUTE((unused)), /*!< in: buffer end */
-	page_t*	page,	/*!< in: page or NULL */
-	mtr_t*	mtr)	/*!< in: mtr or NULL */
-{
-	ut_ad(end_ptr);
-
-	if (page) {
-		trx_undo_discard_latest_update_undo(page, mtr);
-	}
-
-	return(ptr);
-}
-
-/**********************************************************************//**
-If an update undo log can be discarded immediately, this function frees the
-space, resetting the page to the proper state for caching. */
-static
-void
-trx_undo_discard_latest_update_undo(
-/*================================*/
-	page_t*	undo_page,	/*!< in: header page of an undo log of size 1 */
-	mtr_t*	mtr)		/*!< in: mtr */
-{
-	trx_usegf_t*	seg_hdr;
-	trx_upagef_t*	page_hdr;
-	trx_ulogf_t*	log_hdr;
-	trx_ulogf_t*	prev_log_hdr;
-	ulint		free;
-	ulint		prev_hdr_offset;
-
-	seg_hdr = undo_page + TRX_UNDO_SEG_HDR;
-	page_hdr = undo_page + TRX_UNDO_PAGE_HDR;
-
-	free = mach_read_from_2(seg_hdr + TRX_UNDO_LAST_LOG);
-	log_hdr = undo_page + free;
-
-	prev_hdr_offset = mach_read_from_2(log_hdr + TRX_UNDO_PREV_LOG);
-
-	if (prev_hdr_offset != 0) {
-		prev_log_hdr = undo_page + prev_hdr_offset;
-
-		mach_write_to_2(page_hdr + TRX_UNDO_PAGE_START,
-				mach_read_from_2(prev_log_hdr
-						 + TRX_UNDO_LOG_START));
-		mach_write_to_2(prev_log_hdr + TRX_UNDO_NEXT_LOG, 0);
-	}
-
-	mach_write_to_2(page_hdr + TRX_UNDO_PAGE_FREE, free);
-
-	mach_write_to_2(seg_hdr + TRX_UNDO_STATE, TRX_UNDO_CACHED);
-	mach_write_to_2(seg_hdr + TRX_UNDO_LAST_LOG, prev_hdr_offset);
-
-	trx_undo_discard_latest_log(undo_page, mtr);
-}
-
 /** Allocate an undo log page.
 @param[in,out]	trx	transaction
 @param[in,out]	undo	undo log