MDEV-14711 Assertion `mode == 16 || mode == 12 ||...

MDEV-14711 Assertion `mode == 16 || mode == 12 || !fix_block->page.file_page_was_freed' failed in buf_page_get_gen (rollback requesting a freed undo page)

Problem:
=======
In buf_cur_optimistic_latch_leaves(), requesting a left block with BTR_GET
after releasing current block. But there is no guarantee that left block
could be still available.

Fix:
====

(1) In btr_cur_optimistic_latch_leaves(), replace the BUF_GET with
BUF_GET_POSSIBLY_FREED for fetching left block.
(2) Once InnoDB acquires left block, it should check FIL_PAGE_NEXT with
current block page number. If not, release cursor->left_block and return
false.

storage/innobase/btr/btr0cur.cc

@@ -415,6 +415,7 @@ btr_cur_optimistic_latch_leaves(
 {
 	ulint		mode;
 	ulint		left_page_no;
+	ulint		curr_page_no;
 
 	switch (*latch_mode) {
 	case BTR_SEARCH_LEAF:
@@ -441,6 +442,8 @@ btr_cur_optimistic_latch_leaves(
 
 			goto unpin_failed;
 		}
+
+		curr_page_no = block->page.id.page_no();
 		left_page_no = btr_page_get_prev(
 			buf_block_get_frame(block));
 		rw_lock_s_unlock(&block->lock);
@@ -449,11 +452,26 @@ btr_cur_optimistic_latch_leaves(
 			const page_id_t	page_id(
 				dict_index_get_space(cursor->index),
 				left_page_no);
+			dberr_t	err = DB_SUCCESS;
 
-			cursor->left_block = btr_block_get(
+			cursor->left_block = buf_page_get_gen(
 				page_id,
 				dict_table_page_size(cursor->index->table),
-				mode, cursor->index, mtr);
+				mode, NULL, BUF_GET_POSSIBLY_FREED,
+				__FILE__, __LINE__, mtr, &err);
+
+			if (err == DB_DECRYPTION_FAILED) {
+				cursor->index->table->file_unreadable = true;
+			}
+
+			if (btr_page_get_next(buf_block_get_frame(
+					cursor->left_block))
+			    != curr_page_no) {
+				/* release the left block */
+				btr_leaf_page_release(
+					cursor->left_block, mode, mtr);
+				return false;
+			}
 		} else {
 			cursor->left_block = NULL;
 		}