Replace purge_iter_t with purge_sys_t::iterator

Also, remove the field undo_rseg_space.
Apparently its purpose was to avoid problems with
temporary undo logs, which MySQL 5.7 unnecessarily adds to
the purge system. (Temporary undo log records are not purged.)
MariaDB 10.2 fixed this in MDEV-12289 or earlier.

storage/innobase/include/trx0purge.h

@@ -219,36 +219,6 @@ struct TrxUndoRsegsIterator {
 	static const TrxUndoRsegs	NullElement;
 };
 
-/** This is the purge pointer/iterator. We need both the undo no and the
-transaction no up to which purge has parsed and applied the records. */
-struct purge_iter_t {
-	purge_iter_t()
-		:
-		trx_no(),
-		undo_no(),
-		undo_rseg_space(ULINT_UNDEFINED)
-	{
-		// Do nothing
-	}
-
-	bool operator<=(const purge_iter_t& other) const
-	{
-		if (trx_no < other.trx_no) return true;
-		if (trx_no > other.trx_no) return false;
-		return undo_no <= other.undo_no;
-	}
-
-	trx_id_t	trx_no;		/*!< Purge has advanced past all
-					transactions whose number is less
-					than this */
-	undo_no_t	undo_no;	/*!< Purge has advanced past all records
-					whose undo number is less than this */
-	ulint		undo_rseg_space;
-					/*!< Last undo record resided in this
-					space id. */
-};
-
-
 /* Namespace to hold all the related functions and variables need for truncate
 of undo tablespace. */
 namespace undo {
@@ -522,16 +492,29 @@ class purge_sys_t
 					to the task queue */
 	ulint	n_completed;	/*!< Count of total tasks completed */
 
-	/*------------------------------*/
-	/* The following two fields form the 'purge pointer' which advances
-	during a purge, and which is used in history list truncation */
+	/** Iterator to the undo log records of committed transactions */
+	struct iterator
+	{
+		bool operator<=(const iterator& other) const
+		{
+			if (trx_no < other.trx_no) return true;
+			if (trx_no > other.trx_no) return false;
+			return undo_no <= other.undo_no;
+		}
+
+		/** The trx_t::no of the committed transaction */
+		trx_id_t	trx_no;
+		/** The record number within the committed transaction's undo
+		log, increasing, purged from from 0 onwards */
+		undo_no_t	undo_no;
+	};
 
 	/** The tail of the purge queue; the last parsed undo log of a
 	committed transaction. */
-	purge_iter_t	tail;
+	iterator	tail;
 	/** The head of the purge queue; any older undo logs of committed
 	transactions may be discarded (history list truncation). */
-	purge_iter_t	head;
+	iterator	head;
 	/*-----------------------------*/
 	bool		next_stored;	/*!< whether rseg holds the next record
 					to purge */

storage/innobase/trx/trx0purge.cc

@@ -455,10 +455,12 @@ trx_purge_free_segment(trx_rseg_t* rseg, fil_addr_t hdr_addr)
 
 /** Remove unnecessary history data from a rollback segment.
 @param[in,out]	rseg		rollback segment
-@param[in]	limit		truncate offset */
+@param[in]	limit		truncate anything before this */
 static
 void
-trx_purge_truncate_rseg_history(trx_rseg_t* rseg, const purge_iter_t* limit)
+trx_purge_truncate_rseg_history(
+	trx_rseg_t&			rseg,
+	const purge_sys_t::iterator&	limit)
 {
 	fil_addr_t	hdr_addr;
 	fil_addr_t	prev_hdr_addr;
@@ -469,48 +471,37 @@ trx_purge_truncate_rseg_history(trx_rseg_t* rseg, const purge_iter_t* limit)
 	mtr_t		mtr;
 	trx_id_t	undo_trx_no;
 
-	mtr_start(&mtr);
-	ut_ad(rseg->is_persistent());
-	mutex_enter(&(rseg->mutex));
+	mtr.start();
+	ut_ad(rseg.is_persistent());
+	mutex_enter(&rseg.mutex);
 
-	rseg_hdr = trx_rsegf_get(rseg->space, rseg->page_no, &mtr);
+	rseg_hdr = trx_rsegf_get(rseg.space, rseg.page_no, &mtr);
 
 	hdr_addr = trx_purge_get_log_from_hist(
 		flst_get_last(rseg_hdr + TRX_RSEG_HISTORY, &mtr));
 loop:
 	if (hdr_addr.page == FIL_NULL) {
-
-		mutex_exit(&(rseg->mutex));
-
-		mtr_commit(&mtr);
-
+func_exit:
+		mutex_exit(&rseg.mutex);
+		mtr.commit();
 		return;
 	}
 
-	undo_page = trx_undo_page_get(page_id_t(rseg->space, hdr_addr.page),
+	undo_page = trx_undo_page_get(page_id_t(rseg.space, hdr_addr.page),
 				      &mtr);
 
 	log_hdr = undo_page + hdr_addr.boffset;
 
 	undo_trx_no = mach_read_from_8(log_hdr + TRX_UNDO_TRX_NO);
 
-	if (undo_trx_no >= limit->trx_no) {
-
-		/* limit space_id should match the rollback segment
-		space id to avoid freeing of the page belongs to
-		different rollback segment for the same trx_no. */
-		if (undo_trx_no == limit->trx_no
-		    && rseg->space == limit->undo_rseg_space) {
-
+	if (undo_trx_no >= limit.trx_no) {
+		if (undo_trx_no == limit.trx_no) {
 			trx_undo_truncate_start(
-				rseg, hdr_addr.page,
-				hdr_addr.boffset, limit->undo_no);
+				&rseg, hdr_addr.page,
+				hdr_addr.boffset, limit.undo_no);
 		}
 
-		mutex_exit(&(rseg->mutex));
-		mtr_commit(&mtr);
-
-		return;
+		goto func_exit;
 	}
 
 	prev_hdr_addr = trx_purge_get_log_from_hist(
@@ -523,24 +514,24 @@ trx_purge_truncate_rseg_history(trx_rseg_t* rseg, const purge_iter_t* limit)
 
 		/* We can free the whole log segment */
 
-		mutex_exit(&(rseg->mutex));
-		mtr_commit(&mtr);
+		mutex_exit(&rseg.mutex);
+		mtr.commit();
 
 		/* calls the trx_purge_remove_log_hdr()
 		inside trx_purge_free_segment(). */
-		trx_purge_free_segment(rseg, hdr_addr);
+		trx_purge_free_segment(&rseg, hdr_addr);
 	} else {
 		/* Remove the log hdr from the rseg history. */
 		trx_purge_remove_log_hdr(rseg_hdr, log_hdr, &mtr);
 
-		mutex_exit(&(rseg->mutex));
-		mtr_commit(&mtr);
+		mutex_exit(&rseg.mutex);
+		mtr.commit();
 	}
 
-	mtr_start(&mtr);
-	mutex_enter(&(rseg->mutex));
+	mtr.start();
+	mutex_enter(&rseg.mutex);
 
-	rseg_hdr = trx_rsegf_get(rseg->space, rseg->page_no, &mtr);
+	rseg_hdr = trx_rsegf_get(rseg.space, rseg.page_no, &mtr);
 
 	hdr_addr = prev_hdr_addr;
 
@@ -929,7 +920,7 @@ that resides in the tablespace are free.
 static
 void
 trx_purge_initiate_truncate(
-	purge_iter_t*	limit,
+	const purge_sys_t::iterator& limit,
 	undo::Truncate*	undo_trunc)
 {
 	/* Step-1: Early check to findout if any of the the UNDO tablespace
@@ -977,7 +968,7 @@ trx_purge_initiate_truncate(
 			     undo != NULL && all_free;
 			     undo = UT_LIST_GET_NEXT(undo_list, undo)) {
 
-				if (limit->trx_no < undo->trx_id) {
+				if (limit.trx_no < undo->trx_id) {
 					all_free = false;
 				} else {
 					cached_undo_size += undo->size;
@@ -1080,30 +1071,23 @@ trx_purge_initiate_truncate(
 /**
 Removes unnecessary history data from rollback segments. NOTE that when this
 function is called, the caller must not have any latches on undo log pages!
-
-@param[in] limit  truncate limit
 */
-static void trx_purge_truncate_history(purge_iter_t *limit)
+static void trx_purge_truncate_history()
 {
 	ut_ad(purge_sys->head <= purge_sys->tail);
+	purge_sys_t::iterator& head = purge_sys->head.trx_no
+		? purge_sys->head : purge_sys->tail;
 
-	/* We play safe and set the truncate limit at most to the purge view
-	low_limit number, though this is not necessary */
-
-	if (limit->trx_no >= purge_sys->view.low_limit_no()) {
-		limit->trx_no = purge_sys->view.low_limit_no();
-		limit->undo_no = 0;
-		limit->undo_rseg_space = ULINT_UNDEFINED;
+	if (head.trx_no >= purge_sys->view.low_limit_no()) {
+		/* This is sometimes necessary. TODO: find out why. */
+		head.trx_no = purge_sys->view.low_limit_no();
+		head.undo_no = 0;
 	}
 
-	ut_ad(limit->trx_no <= purge_sys->view.low_limit_no());
-
 	for (ulint i = 0; i < TRX_SYS_N_RSEGS; ++i) {
-		trx_rseg_t*	rseg = trx_sys.rseg_array[i];
-
-		if (rseg != NULL) {
-			ut_a(rseg->id == i);
-			trx_purge_truncate_rseg_history(rseg, limit);
+		if (trx_rseg_t* rseg = trx_sys.rseg_array[i]) {
+			ut_ad(rseg->id == i);
+			trx_purge_truncate_rseg_history(*rseg, head);
 		}
 	}
 
@@ -1112,7 +1096,7 @@ static void trx_purge_truncate_history(purge_iter_t *limit)
 	try and truncate all the UNDO tablespaces. */
 	for (ulint i = srv_undo_tablespaces_active; i--; ) {
 		trx_purge_mark_undo_for_truncate(&purge_sys->undo_trunc);
-		trx_purge_initiate_truncate(limit, &purge_sys->undo_trunc);
+		trx_purge_initiate_truncate(head, &purge_sys->undo_trunc);
 	}
 }
 
@@ -1139,7 +1123,6 @@ trx_purge_rseg_get_next_history_log(
 
 	purge_sys->tail.trx_no = rseg->last_trx_no + 1;
 	purge_sys->tail.undo_no = 0;
-	purge_sys->tail.undo_rseg_space = ULINT_UNDEFINED;
 	purge_sys->next_stored = false;
 
 	mtr_start(&mtr);
@@ -1216,7 +1199,6 @@ trx_purge_read_undo_rec()
 	ulint		offset;
 	ulint		page_no;
 	ib_uint64_t	undo_no;
-	ulint		undo_rseg_space;
 
 	purge_sys->hdr_offset = purge_sys->rseg->last_offset;
 	page_no = purge_sys->hdr_page_no = purge_sys->rseg->last_page_no;
@@ -1230,25 +1212,21 @@ trx_purge_read_undo_rec()
 
 			offset = page_offset(undo_rec);
 			undo_no = trx_undo_rec_get_undo_no(undo_rec);
-			undo_rseg_space = purge_sys->rseg->space;
 			page_no = page_get_page_no(page_align(undo_rec));
 		} else {
 			offset = 0;
 			undo_no = 0;
-			undo_rseg_space = ULINT_UNDEFINED;
 		}
 
 		mtr.commit();
 	} else {
 		offset = 0;
 		undo_no = 0;
-		undo_rseg_space = ULINT_UNDEFINED;
 	}
 
 	purge_sys->offset = offset;
 	purge_sys->page_no = page_no;
 	purge_sys->tail.undo_no = undo_no;
-	purge_sys->tail.undo_rseg_space = undo_rseg_space;
 
 	purge_sys->next_stored = true;
 }
@@ -1352,7 +1330,6 @@ trx_purge_get_next_rec(
 		purge_sys->offset = rec2 - page;
 		purge_sys->page_no = page_get_page_no(page);
 		purge_sys->tail.undo_no = trx_undo_rec_get_undo_no(rec2);
-		purge_sys->tail.undo_rseg_space = space;
 
 		if (undo_page != page) {
 			/* We advance to a new page of the undo log: */
@@ -1667,10 +1644,7 @@ trx_purge(
 	ut_a(purge_sys->n_submitted == purge_sys->n_completed);
 
 	if (truncate) {
-		trx_purge_truncate_history(
-			purge_sys->head.trx_no
-			? &purge_sys->head
-			: &purge_sys->tail);
+		trx_purge_truncate_history();
 	}
 
 	MONITOR_INC_VALUE(MONITOR_PURGE_INVOKED, 1);