MDEV-19022: InnoDB fails to cleanup useless B-tree pages

The test case for reproducing MDEV-14126 demonstrates that InnoDB can
end up with an index tree where a non-leaf page has only one child page.

The test case innodb.innodb_bug14676111 demonstrates that such pages
are sometimes unavoidable, because InnoDB does not implement any sort
of B-tree rotation.

But, there is no reason to allow a root page with only one child page.

btr_cur_node_ptr_delete(): Replaces btr_node_ptr_delete().

btr_page_get_father(): Declare globally.

btr_discard_only_page_on_level(): Declare with ATTRIBUTE_COLD.
It turns out that this function is not covered by the
innodb.innodb_bug14676111 test case after all.

btr_discard_page(): If the root page ends up having only one child
page, shrink the tree by invoking btr_lift_page_up().

mysql-test/suite/innodb/r/innodb_bug14676111.result

@@ -39,7 +39,7 @@ Table	Op	Msg_type	Msg_text
 test.t1	analyze	status	OK
 select CLUST_INDEX_SIZE from information_schema.INNODB_SYS_TABLESTATS where NAME = 'test/t1';
 CLUST_INDEX_SIZE
-5
+4
 set global innodb_limit_optimistic_insert_debug = 10000;
 connection con2;
 rollback;
@@ -50,7 +50,7 @@ Table	Op	Msg_type	Msg_text
 test.t1	analyze	status	OK
 select CLUST_INDEX_SIZE from information_schema.INNODB_SYS_TABLESTATS where NAME = 'test/t1';
 CLUST_INDEX_SIZE
-3
+2
 begin;
 insert into t1 values (2);
 rollback;
@@ -59,7 +59,7 @@ Table	Op	Msg_type	Msg_text
 test.t1	analyze	status	OK
 select CLUST_INDEX_SIZE from information_schema.INNODB_SYS_TABLESTATS where NAME = 'test/t1';
 CLUST_INDEX_SIZE
-2
+1
 begin;
 insert into t1 values (2);
 rollback;

storage/innobase/btr/btr0btr.cc

@@ -1062,18 +1062,13 @@ btr_page_get_father_block(
 	return(btr_page_get_father_node_ptr(offsets, heap, cursor, mtr));
 }
 
-/************************************************************//**
-Seeks to the upper level node pointer to a page.
-It is assumed that mtr holds an x-latch on the tree. */
-static
-void
-btr_page_get_father(
-/*================*/
-	dict_index_t*	index,	/*!< in: b-tree index */
-	buf_block_t*	block,	/*!< in: child page in the index */
-	mtr_t*		mtr,	/*!< in: mtr */
-	btr_cur_t*	cursor)	/*!< out: cursor on node pointer record,
-				its page x-latched */
+/** Seek to the parent page of a B-tree page.
+@param[in,out]	index	b-tree
+@param[in]	block	child page
+@param[in,out]	mtr	mini-transaction
+@param[out]	cursor	cursor pointing to the x-latched parent page */
+void btr_page_get_father(dict_index_t* index, buf_block_t* block, mtr_t* mtr,
+			 btr_cur_t* cursor)
 {
 	mem_heap_t*	heap;
 	rec_t*		rec
@@ -3463,33 +3458,6 @@ btr_set_min_rec_mark(
 	}
 }
 
-/*************************************************************//**
-Deletes on the upper level the node pointer to a page. */
-void
-btr_node_ptr_delete(
-/*================*/
-	dict_index_t*	index,	/*!< in: index tree */
-	buf_block_t*	block,	/*!< in: page whose node pointer is deleted */
-	mtr_t*		mtr)	/*!< in: mtr */
-{
-	btr_cur_t	cursor;
-	ibool		compressed;
-	dberr_t		err;
-
-	ut_ad(mtr_is_block_fix(mtr, block, MTR_MEMO_PAGE_X_FIX, index->table));
-
-	/* Delete node pointer on father page */
-	btr_page_get_father(index, block, mtr, &cursor);
-
-	compressed = btr_cur_pessimistic_delete(&err, TRUE, &cursor,
-						BTR_CREATE_FLAG, false, mtr);
-	ut_a(err == DB_SUCCESS);
-
-	if (!compressed) {
-		btr_cur_compress_if_useful(&cursor, FALSE, mtr);
-	}
-}
-
 /*************************************************************//**
 If page is the only on its level, this function moves its records to the
 father page, thus reducing the tree height.
@@ -3939,7 +3907,7 @@ btr_compress(
 			lock_rec_free_all_from_discard_page(block);
 			lock_mutex_exit();
 		} else {
-			btr_node_ptr_delete(index, block, mtr);
+			btr_cur_node_ptr_delete(&father_cursor, mtr);
 			if (!dict_table_is_locking_disabled(index->table)) {
 				lock_update_merge_left(
 					merge_block, orig_pred, block);
@@ -4217,8 +4185,9 @@ btr_compress(
 /*************************************************************//**
 Discards a page that is the only page on its level.  This will empty
 the whole B-tree, leaving just an empty root page.  This function
-should never be reached, because btr_compress(), which is invoked in
+should almost never be reached, because btr_compress(), which is invoked in
 delete operations, calls btr_lift_page_up() to flatten the B-tree. */
+ATTRIBUTE_COLD
 static
 void
 btr_discard_only_page_on_level(
@@ -4318,10 +4287,7 @@ btr_discard_page(
 	buf_block_t*	block;
 	page_t*		page;
 	rec_t*		node_ptr;
-#ifdef UNIV_DEBUG
 	btr_cur_t	parent_cursor;
-	bool		parent_is_different = false;
-#endif
 
 	block = btr_cur_get_block(cursor);
 	index = btr_cur_get_index(cursor);
@@ -4337,13 +4303,11 @@ btr_discard_page(
 
 	MONITOR_INC(MONITOR_INDEX_DISCARD);
 
-#ifdef UNIV_DEBUG
 	if (dict_index_is_spatial(index)) {
 		rtr_page_get_father(index, block, mtr, cursor, &parent_cursor);
 	} else {
 		btr_page_get_father(index, block, mtr, &parent_cursor);
 	}
-#endif
 
 	/* Decide the page which will inherit the locks */
 
@@ -4351,7 +4315,7 @@ btr_discard_page(
 	right_page_no = btr_page_get_next(buf_block_get_frame(block), mtr);
 
 	const page_size_t	page_size(dict_table_page_size(index->table));
-
+	ut_d(bool parent_is_different = false);
 	if (left_page_no != FIL_NULL) {
 		merge_block = btr_block_get(
 			page_id_t(space, left_page_no), page_size,
@@ -4407,15 +4371,9 @@ btr_discard_page(
 	}
 
 	if (dict_index_is_spatial(index)) {
-		btr_cur_t	father_cursor;
-
-		/* Since rtr_node_ptr_delete doesn't contain get father
-		node ptr, so, we need to get father node ptr first and then
-		delete it. */
-		rtr_page_get_father(index, block, mtr, cursor, &father_cursor);
-		rtr_node_ptr_delete(index, &father_cursor, block, mtr);
+		rtr_node_ptr_delete(index, &parent_cursor, block, mtr);
 	} else {
-		btr_node_ptr_delete(index, block, mtr);
+		btr_cur_node_ptr_delete(&parent_cursor, mtr);
 	}
 
 	/* Remove the page from the level list */
@@ -4453,6 +4411,12 @@ btr_discard_page(
 	we cannot use btr_check_node_ptr() */
 	ut_ad(parent_is_different
 	      || btr_check_node_ptr(index, merge_block, mtr));
+
+	if (btr_cur_get_block(&parent_cursor)->page.id.page_no() == index->page
+	    && !page_has_siblings(btr_cur_get_page(&parent_cursor))
+	    && page_get_n_recs(btr_cur_get_page(&parent_cursor)) == 1) {
+		btr_lift_page_up(index, merge_block, mtr);
+	}
 }
 
 #ifdef UNIV_BTR_PRINT

storage/innobase/btr/btr0cur.cc

@@ -5358,8 +5358,10 @@ btr_cur_pessimistic_delete(
 			on the page */
 			ulint level = btr_page_get_level(page, mtr);
 
-			btr_node_ptr_delete(index, block, mtr);
-
+			btr_cur_t cursor;
+			btr_page_get_father(index, block, mtr, &cursor);
+			btr_cur_node_ptr_delete(&cursor, mtr);
+			// FIXME: reuse the node_ptr from above
 			dtuple_t*	node_ptr = dict_index_build_node_ptr(
 				index, next_rec, block->page.id.page_no(),
 				heap, level);
@@ -5428,6 +5430,23 @@ btr_cur_pessimistic_delete(
 	return(ret);
 }
 
+/** Delete the node pointer in a parent page.
+@param[in,out]	parent	cursor pointing to parent record
+@param[in,out]	mtr	mini-transaction */
+void btr_cur_node_ptr_delete(btr_cur_t* parent, mtr_t* mtr)
+{
+	ut_ad(mtr_memo_contains(mtr, btr_cur_get_block(parent),
+				MTR_MEMO_PAGE_X_FIX));
+	dberr_t err;
+	ibool compressed = btr_cur_pessimistic_delete(&err, TRUE, parent,
+						      BTR_CREATE_FLAG, false,
+						      mtr);
+	ut_a(err == DB_SUCCESS);
+	if (!compressed) {
+		btr_cur_compress_if_useful(parent, FALSE, mtr);
+	}
+}
+
 /*******************************************************************//**
 Adds path information to the cursor for the current page, for which
 the binary search has been performed. */

storage/innobase/btr/btr0defragment.cc

@@ -484,6 +484,7 @@ btr_defragment_merge_pages(
 				ULINT_UNDEFINED);
 		}
 	}
+	btr_cur_t parent;
 	if (n_recs_to_move == n_recs) {
 		/* The whole page is merged with the previous page,
 		free it. */
@@ -491,7 +492,8 @@ btr_defragment_merge_pages(
 				       from_block);
 		btr_search_drop_page_hash_index(from_block);
 		btr_level_list_remove(space, page_size, (page_t*)from_page, index, mtr);
-		btr_node_ptr_delete(index, from_block, mtr);
+		btr_page_get_father(index, from_block, mtr, &parent);
+		btr_cur_node_ptr_delete(&parent, mtr);
 		/* btr_blob_dbg_remove(from_page, index,
 		"btr_defragment_n_pages"); */
 		btr_page_free(index, from_block, mtr);
@@ -509,7 +511,9 @@ btr_defragment_merge_pages(
 			lock_update_split_and_merge(to_block,
 						    orig_pred,
 						    from_block);
-			btr_node_ptr_delete(index, from_block, mtr);
+			// FIXME: reuse the node_ptr!
+			btr_page_get_father(index, from_block, mtr, &parent);
+			btr_cur_node_ptr_delete(&parent, mtr);
 			rec = page_rec_get_next(
 				page_get_infimum_rec(from_page));
 			node_ptr = dict_index_build_node_ptr(

storage/innobase/include/btr0btr.h

@@ -2,7 +2,7 @@
 
 Copyright (c) 1994, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2014, 2018, MariaDB Corporation.
+Copyright (c) 2014, 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -545,14 +545,13 @@ btr_set_min_rec_mark(
 	rec_t*	rec,	/*!< in/out: record */
 	mtr_t*	mtr)	/*!< in: mtr */
 	MY_ATTRIBUTE((nonnull));
-/*************************************************************//**
-Deletes on the upper level the node pointer to a page. */
-void
-btr_node_ptr_delete(
-/*================*/
-	dict_index_t*	index,	/*!< in: index tree */
-	buf_block_t*	block,	/*!< in: page whose node pointer is deleted */
-	mtr_t*		mtr)	/*!< in: mtr */
+/** Seek to the parent page of a B-tree page.
+@param[in,out]	index	b-tree
+@param[in]	block	child page
+@param[in,out]	mtr	mini-transaction
+@param[out]	cursor	cursor pointing to the x-latched parent page */
+void btr_page_get_father(dict_index_t* index, buf_block_t* block, mtr_t* mtr,
+			 btr_cur_t* cursor)
 	MY_ATTRIBUTE((nonnull));
 #ifdef UNIV_DEBUG
 /************************************************************//**

storage/innobase/include/btr0cur.h

 /*****************************************************************************
 
 Copyright (c) 1994, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, MariaDB Corporation.
+Copyright (c) 2017, 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -529,6 +529,11 @@ btr_cur_pessimistic_delete(
 	bool		rollback,/*!< in: performing rollback? */
 	mtr_t*		mtr)	/*!< in: mtr */
 	MY_ATTRIBUTE((nonnull));
+/** Delete the node pointer in a parent page.
+@param[in,out]	parent	cursor pointing to parent record
+@param[in,out]	mtr	mini-transaction */
+void btr_cur_node_ptr_delete(btr_cur_t* parent, mtr_t* mtr)
+	MY_ATTRIBUTE((nonnull));
 /***********************************************************//**
 Parses a redo log record of updating a record in-place.
 @return end of log record or NULL */