MDEV-20864 Introduce debug option innodb_change_buffer_dump

To diagnose a hang in slow shutdown (innodb_fast_shutdown=0),
let us introduce a Boolean startup option in debug builds
that will cause the contents of the InnoDB change buffer
to be dumped to the server error log at startup.

mysql-test/suite/innodb/t/ibuf_not_empty.test

@@ -41,7 +41,7 @@ INSERT INTO t1 SELECT 0,b,c FROM t1;
 INSERT INTO t1 SELECT 0,b,c FROM t1;
 INSERT INTO t1 SELECT 0,b,c FROM t1;
 
---let $restart_parameters= --innodb-force-recovery=6
+--let $restart_parameters= --innodb-force-recovery=6 --innodb-change-buffer-dump
 --source include/restart_mysqld.inc
 
 --replace_regex /contains \d+ entries/contains #### entries/

mysql-test/suite/sys_vars/r/sysvars_innodb.result

@@ -354,6 +354,18 @@ NUMERIC_BLOCK_SIZE	0
 ENUM_VALUE_LIST	NULL
 READ_ONLY	NO
 COMMAND_LINE_ARGUMENT	REQUIRED
+VARIABLE_NAME	INNODB_CHANGE_BUFFER_DUMP
+SESSION_VALUE	NULL
+DEFAULT_VALUE	OFF
+VARIABLE_SCOPE	GLOBAL
+VARIABLE_TYPE	BOOLEAN
+VARIABLE_COMMENT	Dump the change buffer at startup.
+NUMERIC_MIN_VALUE	NULL
+NUMERIC_MAX_VALUE	NULL
+NUMERIC_BLOCK_SIZE	NULL
+ENUM_VALUE_LIST	OFF,ON
+READ_ONLY	YES
+COMMAND_LINE_ARGUMENT	NONE
 VARIABLE_NAME	INNODB_CHANGE_BUFFER_MAX_SIZE
 SESSION_VALUE	NULL
 DEFAULT_VALUE	25

storage/innobase/handler/ha_innodb.cc

@@ -20642,6 +20642,11 @@ static MYSQL_SYSVAR_ENUM(stats_method, srv_innodb_stats_method,
    NULL, NULL, SRV_STATS_NULLS_EQUAL, &innodb_stats_method_typelib);
 
 #if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
+static MYSQL_SYSVAR_BOOL(change_buffer_dump, ibuf_dump,
+  PLUGIN_VAR_NOCMDARG | PLUGIN_VAR_READONLY,
+  "Dump the change buffer at startup.",
+  NULL, NULL, FALSE);
+
 static MYSQL_SYSVAR_UINT(change_buffering_debug, ibuf_debug,
   PLUGIN_VAR_RQCMDARG,
   "Debug flags for InnoDB change buffering (0=none, 1=try to buffer)",
@@ -21158,6 +21163,7 @@ static struct st_mysql_sys_var* innobase_system_variables[]= {
   MYSQL_SYSVAR(change_buffering),
   MYSQL_SYSVAR(change_buffer_max_size),
 #if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
+  MYSQL_SYSVAR(change_buffer_dump),
   MYSQL_SYSVAR(change_buffering_debug),
   MYSQL_SYSVAR(disable_background_merge),
 #endif /* UNIV_DEBUG || UNIV_IBUF_DEBUG */

storage/innobase/ibuf/ibuf0ibuf.cc

@@ -188,6 +188,8 @@ access order rules. */
 ibuf_use_t	ibuf_use		= IBUF_USE_ALL;
 
 #if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
+/** Dump the change buffer at startup */
+my_bool	ibuf_dump;
 /** Flag to control insert buffer debugging. */
 uint	ibuf_debug;
 #endif /* UNIV_DEBUG || UNIV_IBUF_DEBUG */
@@ -506,6 +508,25 @@ ibuf_init_at_db_start(void)
 #endif /* BTR_CUR_ADAPT */
 	ibuf->index->page = FSP_IBUF_TREE_ROOT_PAGE_NO;
 	ut_d(ibuf->index->cached = TRUE);
+
+#if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
+	if (!ibuf_dump) {
+		return error;
+	}
+	ib::info() << "Dumping the change buffer";
+	ibuf_mtr_start(&mtr);
+	btr_pcur_t pcur;
+	if (DB_SUCCESS == btr_pcur_open_at_index_side(
+		    true, ibuf->index, BTR_SEARCH_LEAF, &pcur,
+		    true, 0, &mtr)) {
+		while (btr_pcur_move_to_next_user_rec(&pcur, &mtr)) {
+			rec_print_old(stderr, btr_pcur_get_rec(&pcur));
+		}
+	}
+	ibuf_mtr_commit(&mtr);
+	ib::info() << "Dumped the change buffer";
+#endif
+
 	return (error);
 }
 

storage/innobase/include/dict0types.h

 /*****************************************************************************
 
 Copyright (c) 1996, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 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
@@ -96,6 +96,8 @@ typedef ib_mutex_t DictSysMutex;
 #define TEMP_TABLE_PATH_PREFIX           "/" TEMP_TABLE_PREFIX
 
 #if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
+/** Dump the change buffer at startup */
+extern my_bool		ibuf_dump;
 /** Flag to control insert buffer debugging. */
 extern uint		ibuf_debug;
 #endif /* UNIV_DEBUG || UNIV_IBUF_DEBUG */