path: root/storage/innobase/srv/srv0srv.cc

/*****************************************************************************

Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
Copyright (c) 2008, 2009 Google Inc.
Copyright (c) 2009, Percona Inc.
Copyright (c) 2013, 2023, MariaDB Corporation.

Portions of this file contain modifications contributed and copyrighted by
Google, Inc. Those modifications are gratefully acknowledged and are described
briefly in the InnoDB documentation. The contributions by Google are
incorporated with their permission, and subject to the conditions contained in
the file COPYING.Google.

Portions of this file contain modifications contributed and copyrighted
by Percona Inc.. Those modifications are
gratefully acknowledged and are described briefly in the InnoDB
documentation. The contributions by Percona Inc. are incorporated with
their permission, and subject to the conditions contained in the file
COPYING.Percona.

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
Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA

*****************************************************************************/

/**************************************************//**
@file srv/srv0srv.cc
The database server main program

Created 10/8/1995 Heikki Tuuri
*******************************************************/

#include "my_global.h"
#include "mysql/psi/mysql_stage.h"
#include "mysql/psi/psi.h"

#include "btr0sea.h"
#include "buf0flu.h"
#include "buf0lru.h"
#include "dict0boot.h"
#include "dict0load.h"
#include "lock0lock.h"
#include "log0recv.h"
#include "mem0mem.h"
#include "pars0pars.h"
#include "que0que.h"
#include "row0mysql.h"
#include "row0log.h"
#include "srv0mon.h"
#include "srv0srv.h"
#include "srv0start.h"
#include "trx0i_s.h"
#include "trx0purge.h"
#include "ut0mem.h"
#include "fil0fil.h"
#include "fil0crypt.h"
#include "fil0pagecompress.h"
#include "trx0types.h"
#include <list>
#include "log.h"

#include "transactional_lock_guard.h"

#include <my_service_manager.h>
/* The following is the maximum allowed duration of a lock wait. */
ulong	srv_fatal_semaphore_wait_threshold =  DEFAULT_SRV_FATAL_SEMAPHORE_TIMEOUT;

/* How much data manipulation language (DML) statements need to be delayed,
in microseconds, in order to reduce the lagging of the purge thread. */
ulint	srv_dml_needed_delay;

const char*	srv_main_thread_op_info = "";

/** Prefix used by MySQL to indicate pre-5.1 table name encoding */
const char		srv_mysql50_table_name_prefix[10] = "#mysql50#";

/* Server parameters which are read from the initfile */

/* The following three are dir paths which are catenated before file
names, where the file name itself may also contain a path */

char*	srv_data_home;

/** Rollback files directory, can be absolute. */
char*	srv_undo_dir;

/** The number of tablespaces to use for rollback segments. */
uint	srv_undo_tablespaces;

/** The number of UNDO tablespaces that are open and ready to use. */
uint32_t srv_undo_tablespaces_open;

/** The number of UNDO tablespaces that are active (hosting some rollback
segment). It is quite possible that some of the tablespaces doesn't host
any of the rollback-segment based on configuration used. */
uint32_t srv_undo_tablespaces_active;

/** Enable or Disable Truncate of UNDO tablespace.
Note: If enabled then UNDO tablespace will be selected for truncate.
While Server waits for undo-tablespace to truncate if user disables
it, truncate action is completed but no new tablespace is marked
for truncate (action is never aborted). */
my_bool	srv_undo_log_truncate;

/** Maximum size of undo tablespace. */
unsigned long long	srv_max_undo_log_size;

/** Set if InnoDB must operate in read-only mode. We don't do any
recovery and open all tables in RO mode instead of RW mode. We don't
sync the max trx id to disk either. */
my_bool	srv_read_only_mode;
/** store to its own file each table created by an user; data
dictionary tables are in the system tablespace 0 */
my_bool	srv_file_per_table;
/** Set if innodb_read_only is set or innodb_force_recovery
is SRV_FORCE_NO_UNDO_LOG_SCAN or greater. */
bool high_level_read_only;

/** Sort buffer size in index creation */
ulong	srv_sort_buf_size;
/** Maximum modification log file size for online index creation */
unsigned long long	srv_online_max_size;

/* If this flag is TRUE, then we will use the native aio of the
OS (provided we compiled Innobase with it in), otherwise we will
use simulated aio we build below with threads.
Currently we support native aio on windows and linux */
my_bool	srv_use_native_aio;
my_bool	srv_numa_interleave;
/** copy of innodb_use_atomic_writes; @see innodb_init_params() */
my_bool	srv_use_atomic_writes;
/** innodb_compression_algorithm; used with page compression */
ulong	innodb_compression_algorithm;

/*------------------------- LOG FILES ------------------------ */
char*	srv_log_group_home_dir;

/** The InnoDB redo log file size, or 0 when changing the redo log format
at startup (while disallowing writes to the redo log). */
ulonglong	srv_log_file_size;
/** innodb_flush_log_at_trx_commit */
ulong		srv_flush_log_at_trx_commit;
/** innodb_flush_log_at_timeout */
uint		srv_flush_log_at_timeout;
/** innodb_page_size */
ulong		srv_page_size;
/** log2 of innodb_page_size; @see innodb_init_params() */
uint32_t	srv_page_size_shift;

/** innodb_adaptive_flushing; try to flush dirty pages so as to avoid
IO bursts at the checkpoints. */
my_bool	srv_adaptive_flushing;

/** innodb_flush_sync; whether to ignore io_capacity at log checkpoints */
my_bool	srv_flush_sync;

/** common thread pool*/
tpool::thread_pool* srv_thread_pool;

/** Maximum number of times allowed to conditionally acquire
mutex before switching to blocking wait on the mutex */
#define MAX_MUTEX_NOWAIT	2

/** Check whether the number of failed nonblocking mutex
acquisition attempts exceeds maximum allowed value. If so,
srv_printf_innodb_monitor() will request mutex acquisition
with mysql_mutex_lock(), which will wait until it gets the mutex. */
#define MUTEX_NOWAIT(mutex_skipped)	((mutex_skipped) < MAX_MUTEX_NOWAIT)

/** copy of innodb_buffer_pool_size */
ulint	srv_buf_pool_size;
/** Requested buffer pool chunk size */
size_t	srv_buf_pool_chunk_unit;
/** innodb_lru_scan_depth; number of blocks scanned in LRU flush batch */
ulong	srv_LRU_scan_depth;
/** innodb_flush_neighbors; whether or not to flush neighbors of a block */
ulong	srv_flush_neighbors;
/** Previously requested size */
ulint	srv_buf_pool_old_size;
/** Current size as scaling factor for the other components */
ulint	srv_buf_pool_base_size;
/** Current size in bytes */
ulint	srv_buf_pool_curr_size;
/** Dump this % of each buffer pool during BP dump */
ulong	srv_buf_pool_dump_pct;
/** Abort load after this amount of pages */
#ifdef UNIV_DEBUG
ulong srv_buf_pool_load_pages_abort = LONG_MAX;
#endif
/** Lock table size in bytes */
ulint	srv_lock_table_size	= ULINT_MAX;

/** the value of innodb_checksum_algorithm */
ulong	srv_checksum_algorithm;

/** innodb_read_io_threads */
uint	srv_n_read_io_threads;
/** innodb_write_io_threads */
uint	srv_n_write_io_threads;

/** innodb_random_read_ahead */
my_bool	srv_random_read_ahead;
/** innodb_read_ahead_threshold; the number of pages that must be present
in the buffer cache and accessed sequentially for InnoDB to trigger a
readahead request. */
ulong	srv_read_ahead_threshold;

/** copy of innodb_open_files; @see innodb_init_params() */
ulint	srv_max_n_open_files;

/** innodb_io_capacity */
ulong	srv_io_capacity;
/** innodb_io_capacity_max */
ulong	srv_max_io_capacity;

/* The InnoDB main thread tries to keep the ratio of modified pages
in the buffer pool to all database pages in the buffer pool smaller than
the following number. But it is not guaranteed that the value stays below
that during a time of heavy update/insert activity. */

/** innodb_max_dirty_pages_pct */
double	srv_max_buf_pool_modified_pct;
/** innodb_max_dirty_pages_pct_lwm */
double	srv_max_dirty_pages_pct_lwm;

/** innodb_adaptive_flushing_lwm; the percentage of log capacity at
which adaptive flushing, if enabled, will kick in. */
double	srv_adaptive_flushing_lwm;

/** innodb_flushing_avg_loops; number of iterations over which
adaptive flushing is averaged */
ulong	srv_flushing_avg_loops;

/** innodb_purge_threads; the number of purge tasks to use */
uint srv_n_purge_threads;

/** innodb_purge_batch_size, in pages */
ulong	srv_purge_batch_size;

/** innodb_stats_method decides how InnoDB treats
NULL value when collecting statistics. By default, it is set to
SRV_STATS_NULLS_EQUAL(0), ie. all NULL value are treated equal */
ulong srv_innodb_stats_method;

srv_stats_t	srv_stats;

/* structure to pass status variables to MySQL */
export_var_t export_vars;

/** Normally 0. When nonzero, skip some phases of crash recovery,
starting from SRV_FORCE_IGNORE_CORRUPT, so that data can be recovered
by SELECT or mysqldump. When this is nonzero, we do not allow any user
modifications to the data. */
ulong	srv_force_recovery;

/** innodb_print_all_deadlocks; whether to print all user-level
transactions deadlocks to the error log */
my_bool	srv_print_all_deadlocks;

/** innodb_cmp_per_index_enabled; enable
INFORMATION_SCHEMA.innodb_cmp_per_index */
my_bool	srv_cmp_per_index_enabled;

/** innodb_fast_shutdown=1 skips the purge of transaction history.
innodb_fast_shutdown=2 effectively crashes the server (no log checkpoint).
innodb_fast_shutdown=3 is a clean shutdown that skips the rollback
of active transaction (to be done on restart). */
uint	srv_fast_shutdown;

/** copy of innodb_status_file; generate a innodb_status.<pid> file */
ibool	srv_innodb_status;

/** innodb_stats_transient_sample_pages;
When estimating number of different key values in an index, sample
this many index pages, there are 2 ways to calculate statistics:
* persistent stats that are calculated by ANALYZE TABLE and saved
  in the innodb database.
* quick transient stats, that are used if persistent stats for the given
  table/index are not found in the innodb database */
unsigned long long	srv_stats_transient_sample_pages;
/** innodb_stats_persistent */
my_bool		srv_stats_persistent;
/** innodb_stats_include_delete_marked */
my_bool		srv_stats_include_delete_marked;
/** innodb_stats_persistent_sample_pages */
unsigned long long	srv_stats_persistent_sample_pages;
/** innodb_stats_auto_recalc */
my_bool		srv_stats_auto_recalc;

/** innodb_stats_modified_counter; The number of rows modified before
we calculate new statistics (default 0 = current limits) */
unsigned long long srv_stats_modified_counter;

/** innodb_stats_traditional; enable traditional statistic calculation
based on number of configured pages */
my_bool	srv_stats_sample_traditional;

/** innodb_sync_spin_loops */
ulong	srv_n_spin_wait_rounds;
/** innodb_spin_wait_delay */
uint	srv_spin_wait_delay;

/** Number of initialized rollback segments for persistent undo log */
ulong	srv_available_undo_logs;

/** Current mode of operation */
enum srv_operation_mode srv_operation;

/** whether this is the server's first start after mariabackup --prepare */
bool srv_start_after_restore;

/* Set the following to 0 if you want InnoDB to write messages on
stderr on startup/shutdown. Not enabled on the embedded server. */
ibool	srv_print_verbose_log;
my_bool	srv_print_innodb_monitor;
my_bool	srv_print_innodb_lock_monitor;
/** innodb_force_primary_key; whether to disallow CREATE TABLE without
PRIMARY KEY */
my_bool	srv_force_primary_key;

/** Key version to encrypt the temporary tablespace */
my_bool innodb_encrypt_temporary_tables;

my_bool srv_immediate_scrub_data_uncompressed;

static time_t	srv_last_monitor_time;

static mysql_mutex_t srv_innodb_monitor_mutex;

/** Mutex protecting page_zip_stat_per_index */
mysql_mutex_t page_zip_stat_per_index_mutex;

/** Mutex for locking srv_monitor_file */
mysql_mutex_t srv_monitor_file_mutex;

/** Temporary file for innodb monitor output */
FILE*	srv_monitor_file;
/** Mutex for locking srv_misc_tmpfile */
mysql_mutex_t srv_misc_tmpfile_mutex;
/** Temporary file for miscellanous diagnostic output */
FILE*	srv_misc_tmpfile;

/* The following counts are used by the srv_master_callback. */

/** Iterations of the loop bounded by 'srv_active' label. */
ulint		srv_main_active_loops;
/** Iterations of the loop bounded by the 'srv_idle' label. */
ulint		srv_main_idle_loops;
/** Log writes involving flush. */
ulint		srv_log_writes_and_flush;

/* This is only ever touched by the master thread. It records the
time when the last flush of log file has happened. The master
thread ensures that we flush the log files at least once per
second. */
static time_t	srv_last_log_flush_time;

/** Buffer pool dump status frequence in percentages */
ulong srv_buf_dump_status_frequency;

/*
	IMPLEMENTATION OF THE SERVER MAIN PROGRAM
	=========================================

There is the following analogue between this database
server and an operating system kernel:

DB concept			equivalent OS concept
----------			---------------------
transaction		--	process;

query thread		--	thread;

lock			--	semaphore;

kernel			--	kernel;

query thread execution:
(a) without lock_sys.latch
reserved		--	process executing in user mode;
(b) with lock_sys.latch reserved
			--	process executing in kernel mode;

The server has several background threads all running at the same
priority as user threads.

The threads which we call user threads serve the queries of the MySQL
server. They run at normal priority.

When there is no activity in the system, also the master thread
suspends itself to wait for an event making the server totally silent.

There is still one complication in our server design. If a
background utility thread obtains a resource (e.g., mutex) needed by a user
thread, and there is also some other user activity in the system,
the user thread may have to wait indefinitely long for the
resource, as the OS does not schedule a background thread if
there is some other runnable user thread. This problem is called
priority inversion in real-time programming.

One solution to the priority inversion problem would be to keep record
of which thread owns which resource and in the above case boost the
priority of the background thread so that it will be scheduled and it
can release the resource.  This solution is called priority inheritance
in real-time programming.  A drawback of this solution is that the overhead
of acquiring a mutex increases slightly, maybe 0.2 microseconds on a 100
MHz Pentium, because the thread has to call pthread_self.  This may
be compared to 0.5 microsecond overhead for a mutex lock-unlock pair. Note
that the thread cannot store the information in the resource , say mutex,
itself, because competing threads could wipe out the information if it is
stored before acquiring the mutex, and if it stored afterwards, the
information is outdated for the time of one machine instruction, at least.
(To be precise, the information could be stored to lock_word in mutex if
the machine supports atomic swap.)

The above solution with priority inheritance may become actual in the
future, currently we do not implement any priority twiddling solution.
Our general aim is to reduce the contention of all mutexes by making
them more fine grained.

The thread table contains information of the current status of each
thread existing in the system, and also the event semaphores used in
suspending the master thread and utility threads when they have nothing
to do.  The thread table can be seen as an analogue to the process table
in a traditional Unix implementation. */

/** The server system struct */
struct srv_sys_t{
	mysql_mutex_t	tasks_mutex;		/*!< variable protecting the
						tasks queue */
	UT_LIST_BASE_NODE_T(que_thr_t)
			tasks;			/*!< task queue */

	srv_stats_t::ulint_ctr_1_t
			activity_count;		/*!< For tracking server
						activity */
};

static srv_sys_t	srv_sys;

/*
  Structure shared by timer and coordinator_callback.
  No protection necessary since timer and task never run
  in parallel (being in the same task group of size 1).
*/
struct purge_coordinator_state
{
  /** Snapshot of the last history length before the purge call.*/
  size_t history_size;
  Atomic_counter<int> m_running;
public:
  inline void do_purge();
};

static purge_coordinator_state purge_state;

/** threadpool timer for srv_monitor_task() */
std::unique_ptr<tpool::timer> srv_monitor_timer;


/** The buffer pool dump/load file name */
char*	srv_buf_dump_filename;

/** Boolean config knobs that tell InnoDB to dump the buffer pool at shutdown
and/or load it during startup. */
char	srv_buffer_pool_dump_at_shutdown = TRUE;
char	srv_buffer_pool_load_at_startup = TRUE;

#ifdef HAVE_PSI_STAGE_INTERFACE
/** Performance schema stage event for monitoring ALTER TABLE progress
in ha_innobase::commit_inplace_alter_table(). */
PSI_stage_info	srv_stage_alter_table_end
	= {0, "alter table (end)", PSI_FLAG_STAGE_PROGRESS};

/** Performance schema stage event for monitoring ALTER TABLE progress
row_merge_insert_index_tuples(). */
PSI_stage_info	srv_stage_alter_table_insert
	= {0, "alter table (insert)", PSI_FLAG_STAGE_PROGRESS};

/** Performance schema stage event for monitoring ALTER TABLE progress
row_log_apply(). */
PSI_stage_info	srv_stage_alter_table_log_index
	= {0, "alter table (log apply index)", PSI_FLAG_STAGE_PROGRESS};

/** Performance schema stage event for monitoring ALTER TABLE progress
row_log_table_apply(). */
PSI_stage_info	srv_stage_alter_table_log_table
	= {0, "alter table (log apply table)", PSI_FLAG_STAGE_PROGRESS};

/** Performance schema stage event for monitoring ALTER TABLE progress
row_merge_sort(). */
PSI_stage_info	srv_stage_alter_table_merge_sort
	= {0, "alter table (merge sort)", PSI_FLAG_STAGE_PROGRESS};

/** Performance schema stage event for monitoring ALTER TABLE progress
row_merge_read_clustered_index(). */
PSI_stage_info	srv_stage_alter_table_read_pk_internal_sort
	= {0, "alter table (read PK and internal sort)", PSI_FLAG_STAGE_PROGRESS};

/** Performance schema stage event for monitoring buffer pool load progress. */
PSI_stage_info	srv_stage_buffer_pool_load
	= {0, "buffer pool load", PSI_FLAG_STAGE_PROGRESS};
#endif /* HAVE_PSI_STAGE_INTERFACE */

/*********************************************************************//**
Prints counters for work done by srv_master_thread. */
static
void
srv_print_master_thread_info(
/*=========================*/
	FILE  *file)    /* in: output stream */
{
	fprintf(file, "srv_master_thread loops: " ULINTPF " srv_active, "
		ULINTPF " srv_idle\n"
		"srv_master_thread log flush and writes: " ULINTPF "\n",
		srv_main_active_loops,
		srv_main_idle_loops,
		srv_log_writes_and_flush);
}

static void thread_pool_thread_init()
{
	my_thread_init();
	pfs_register_thread(thread_pool_thread_key);
}
static void thread_pool_thread_end()
{
	pfs_delete_thread();
	my_thread_end();
}


void srv_thread_pool_init()
{
  DBUG_ASSERT(!srv_thread_pool);

#if defined (_WIN32)
  srv_thread_pool= tpool::create_thread_pool_win();
#else
  srv_thread_pool= tpool::create_thread_pool_generic();
#endif
  srv_thread_pool->set_thread_callbacks(thread_pool_thread_init,
                                        thread_pool_thread_end);
}


void srv_thread_pool_end()
{
  ut_ad(!srv_master_timer);
  delete srv_thread_pool;
  srv_thread_pool= nullptr;
}

static bool need_srv_free;

/** Initialize the server. */
static void srv_init()
{
	mysql_mutex_init(srv_innodb_monitor_mutex_key,
			 &srv_innodb_monitor_mutex, nullptr);
	mysql_mutex_init(srv_threads_mutex_key, &srv_sys.tasks_mutex, nullptr);
	UT_LIST_INIT(srv_sys.tasks, &que_thr_t::queue);

	need_srv_free = true;

	mysql_mutex_init(page_zip_stat_per_index_mutex_key,
			 &page_zip_stat_per_index_mutex, nullptr);

	/* Initialize some INFORMATION SCHEMA internal structures */
	trx_i_s_cache_init(trx_i_s_cache);
}

/*********************************************************************//**
Frees the data structures created in srv_init(). */
void
srv_free(void)
/*==========*/
{
	if (!need_srv_free) {
		return;
	}

	mysql_mutex_destroy(&srv_innodb_monitor_mutex);
	mysql_mutex_destroy(&page_zip_stat_per_index_mutex);
	mysql_mutex_destroy(&srv_sys.tasks_mutex);

	trx_i_s_cache_free(trx_i_s_cache);
	srv_thread_pool_end();
}

/*********************************************************************//**
Boots the InnoDB server. */
void srv_boot()
{
#ifndef NO_ELISION
  if (transactional_lock_enabled())
    sql_print_information("InnoDB: Using transactional memory");
#endif
  buf_dblwr.init();
  srv_thread_pool_init();
  trx_pool_init();
  srv_init();
}

/******************************************************************//**
Refreshes the values used to calculate per-second averages. */
static void srv_refresh_innodb_monitor_stats(time_t current_time)
{
	mysql_mutex_lock(&srv_innodb_monitor_mutex);

	if (difftime(current_time, srv_last_monitor_time) < 60) {
		/* We refresh InnoDB Monitor values so that averages are
		printed from at most 60 last seconds */
		mysql_mutex_unlock(&srv_innodb_monitor_mutex);
		return;
	}

	srv_last_monitor_time = current_time;

	os_aio_refresh_stats();

#ifdef BTR_CUR_HASH_ADAPT
	btr_cur_n_sea_old = btr_cur_n_sea;
	btr_cur_n_non_sea_old = btr_cur_n_non_sea;
#endif /* BTR_CUR_HASH_ADAPT */

	buf_refresh_io_stats();

	mysql_mutex_unlock(&srv_innodb_monitor_mutex);
}

/******************************************************************//**
Outputs to a file the output of the InnoDB Monitor.
@return FALSE if not all information printed
due to failure to obtain necessary mutex */
ibool
srv_printf_innodb_monitor(
/*======================*/
	FILE*	file,		/*!< in: output stream */
	ibool	nowait,		/*!< in: whether to wait for lock_sys.latch */
	ulint*	trx_start_pos,	/*!< out: file position of the start of
				the list of active transactions */
	ulint*	trx_end)	/*!< out: file position of the end of
				the list of active transactions */
{
	double	time_elapsed;
	time_t	current_time;
	ibool	ret;

	mysql_mutex_lock(&srv_innodb_monitor_mutex);

	current_time = time(NULL);

	/* We add 0.001 seconds to time_elapsed to prevent division
	by zero if two users happen to call SHOW ENGINE INNODB STATUS at the
	same time */

	time_elapsed = difftime(current_time, srv_last_monitor_time)
		+ 0.001;

	srv_last_monitor_time = time(NULL);

	fputs("\n=====================================\n", file);

	ut_print_timestamp(file);
	fprintf(file,
		" INNODB MONITOR OUTPUT\n"
		"=====================================\n"
		"Per second averages calculated from the last %lu seconds\n",
		(ulong) time_elapsed);

	fputs("-----------------\n"
	      "BACKGROUND THREAD\n"
	      "-----------------\n", file);
	srv_print_master_thread_info(file);

	/* This section is intentionally left blank, for tools like "innotop" */
	fputs("----------\n"
	      "SEMAPHORES\n"
	      "----------\n", file);
	/* End of intentionally blank section */

	/* Conceptually, srv_innodb_monitor_mutex has a very high latching
	order level, while dict_foreign_err_mutex has a very low level.
	Therefore we can reserve the latter mutex here without
	a danger of a deadlock of threads. */

	mysql_mutex_lock(&dict_foreign_err_mutex);

	if (!srv_read_only_mode && ftell(dict_foreign_err_file) != 0L) {
		fputs("------------------------\n"
		      "LATEST FOREIGN KEY ERROR\n"
		      "------------------------\n", file);
		ut_copy_file(file, dict_foreign_err_file);
	}

	mysql_mutex_unlock(&dict_foreign_err_mutex);

	/* Only if lock_print_info_summary proceeds correctly,
	before we call the lock_print_info_all_transactions
	to print all the lock information. IMPORTANT NOTE: This
	function acquires exclusive lock_sys.latch on success. */
	ret = lock_print_info_summary(file, nowait);

	if (ret) {
		if (trx_start_pos) {
			long	t = ftell(file);
			if (t < 0) {
				*trx_start_pos = ULINT_UNDEFINED;
			} else {
				*trx_start_pos = (ulint) t;
			}
		}

		/* NOTE: The following function will release the lock_sys.latch
		that lock_print_info_summary() acquired. */

		lock_print_info_all_transactions(file);

		if (trx_end) {
			long	t = ftell(file);
			if (t < 0) {
				*trx_end = ULINT_UNDEFINED;
			} else {
				*trx_end = (ulint) t;
			}
		}
	}

	fputs("--------\n"
	      "FILE I/O\n"
	      "--------\n", file);
	os_aio_print(file);

#ifdef BTR_CUR_HASH_ADAPT
	if (btr_search_enabled) {
		fputs("-------------------\n"
		      "ADAPTIVE HASH INDEX\n"
		      "-------------------\n", file);
		for (ulint i = 0; i < btr_ahi_parts; ++i) {
			const auto part= &btr_search_sys.parts[i];
			part->latch.rd_lock(SRW_LOCK_CALL);
			ut_ad(part->heap->type == MEM_HEAP_FOR_BTR_SEARCH);
			fprintf(file, "Hash table size " ULINTPF
				", node heap has " ULINTPF " buffer(s)\n",
				part->table.n_cells,
				part->heap->base.count
				- !part->heap->free_block);
			part->latch.rd_unlock();
		}

		const ulint with_ahi = btr_cur_n_sea;
		const ulint without_ahi = btr_cur_n_non_sea;
		fprintf(file,
			"%.2f hash searches/s, %.2f non-hash searches/s\n",
			static_cast<double>(with_ahi - btr_cur_n_sea_old)
			/ time_elapsed,
			static_cast<double>(without_ahi - btr_cur_n_non_sea_old)
			/ time_elapsed);
		btr_cur_n_sea_old = with_ahi;
		btr_cur_n_non_sea_old = without_ahi;
	}
#endif /* BTR_CUR_HASH_ADAPT */

	fputs("---\n"
	      "LOG\n"
	      "---\n", file);
	log_print(file);

	fputs("----------------------\n"
	      "BUFFER POOL AND MEMORY\n"
	      "----------------------\n", file);
	fprintf(file,
		"Total large memory allocated " ULINTPF "\n"
		"Dictionary memory allocated " ULINTPF "\n",
		ulint{os_total_large_mem_allocated},
		dict_sys.rough_size());

	buf_print_io(file);

	fputs("--------------\n"
	      "ROW OPERATIONS\n"
	      "--------------\n", file);
	fprintf(file, ULINTPF " read views open inside InnoDB\n",
		trx_sys.view_count());

	if (ulint n_reserved = fil_system.sys_space->n_reserved_extents) {
		fprintf(file,
			ULINTPF " tablespace extents now reserved for"
			" B-tree split operations\n",
			n_reserved);
	}

	fprintf(file, "state: %s\n", srv_main_thread_op_info);

	fputs("----------------------------\n"
	      "END OF INNODB MONITOR OUTPUT\n"
	      "============================\n", file);
	mysql_mutex_unlock(&srv_innodb_monitor_mutex);
	fflush(file);

	return(ret);
}

/******************************************************************//**
Function to pass InnoDB status variables to MySQL */
void
srv_export_innodb_status(void)
/*==========================*/
{
	fil_crypt_stat_t	crypt_stat;

	if (!srv_read_only_mode) {
		fil_crypt_total_stat(&crypt_stat);
	}

#ifdef BTR_CUR_HASH_ADAPT
	export_vars.innodb_ahi_hit = btr_cur_n_sea;
	export_vars.innodb_ahi_miss = btr_cur_n_non_sea;

	ulint mem_adaptive_hash = 0;
	for (ulong i = 0; i < btr_ahi_parts; i++) {
		const auto part= &btr_search_sys.parts[i];
		part->latch.rd_lock(SRW_LOCK_CALL);
		if (part->heap) {
			ut_ad(part->heap->type == MEM_HEAP_FOR_BTR_SEARCH);

			mem_adaptive_hash += mem_heap_get_size(part->heap)
				+ part->table.n_cells * sizeof(hash_cell_t);
		}
		part->latch.rd_unlock();
	}
	export_vars.innodb_mem_adaptive_hash = mem_adaptive_hash;
#endif

	export_vars.innodb_mem_dictionary = dict_sys.rough_size();

	mysql_mutex_lock(&srv_innodb_monitor_mutex);

	export_vars.innodb_data_pending_reads =
		ulint(MONITOR_VALUE(MONITOR_OS_PENDING_READS));

	export_vars.innodb_data_pending_writes =
		ulint(MONITOR_VALUE(MONITOR_OS_PENDING_WRITES));

	export_vars.innodb_data_read = srv_stats.data_read;

	export_vars.innodb_data_reads = os_n_file_reads;

	export_vars.innodb_data_writes = os_n_file_writes;

	buf_dblwr.lock();
	ulint dblwr = buf_dblwr.written();
	export_vars.innodb_dblwr_pages_written = dblwr;
	export_vars.innodb_dblwr_writes = buf_dblwr.batches();
	buf_dblwr.unlock();

	export_vars.innodb_data_written = srv_stats.data_written
		+ (dblwr << srv_page_size_shift);

	export_vars.innodb_buffer_pool_read_requests
		= buf_pool.stat.n_page_gets;

	export_vars.innodb_buffer_pool_bytes_data =
		buf_pool.stat.LRU_bytes
		+ (UT_LIST_GET_LEN(buf_pool.unzip_LRU)
		   << srv_page_size_shift);

#ifdef UNIV_DEBUG
	export_vars.innodb_buffer_pool_pages_latched =
		buf_get_latched_pages_number();
#endif /* UNIV_DEBUG */
	export_vars.innodb_buffer_pool_pages_total = buf_pool.get_n_pages();

	export_vars.innodb_buffer_pool_pages_misc =
		buf_pool.get_n_pages()
		- UT_LIST_GET_LEN(buf_pool.LRU)
		- UT_LIST_GET_LEN(buf_pool.free);

	export_vars.innodb_max_trx_id = trx_sys.get_max_trx_id();
	export_vars.innodb_history_list_length = trx_sys.history_size_approx();

	mysql_mutex_lock(&lock_sys.wait_mutex);
	export_vars.innodb_row_lock_waits = lock_sys.get_wait_cumulative();

	export_vars.innodb_row_lock_current_waits= lock_sys.get_wait_pending();

	export_vars.innodb_row_lock_time = lock_sys.get_wait_time_cumulative();
	export_vars.innodb_row_lock_time_max = lock_sys.get_wait_time_max();

	mysql_mutex_unlock(&lock_sys.wait_mutex);

	export_vars.innodb_row_lock_time_avg= export_vars.innodb_row_lock_waits
		? static_cast<ulint>(export_vars.innodb_row_lock_time
				     / export_vars.innodb_row_lock_waits)
		: 0;

	export_vars.innodb_page_compression_saved = srv_stats.page_compression_saved;
	export_vars.innodb_pages_page_compressed = srv_stats.pages_page_compressed;
	export_vars.innodb_page_compressed_trim_op = srv_stats.page_compressed_trim_op;
	export_vars.innodb_pages_page_decompressed = srv_stats.pages_page_decompressed;
	export_vars.innodb_pages_page_compression_error = srv_stats.pages_page_compression_error;
	export_vars.innodb_pages_decrypted = srv_stats.pages_decrypted;
	export_vars.innodb_pages_encrypted = srv_stats.pages_encrypted;
	export_vars.innodb_n_merge_blocks_encrypted = srv_stats.n_merge_blocks_encrypted;
	export_vars.innodb_n_merge_blocks_decrypted = srv_stats.n_merge_blocks_decrypted;
	export_vars.innodb_n_rowlog_blocks_encrypted = srv_stats.n_rowlog_blocks_encrypted;
	export_vars.innodb_n_rowlog_blocks_decrypted = srv_stats.n_rowlog_blocks_decrypted;

	export_vars.innodb_n_temp_blocks_encrypted =
		srv_stats.n_temp_blocks_encrypted;

	export_vars.innodb_n_temp_blocks_decrypted =
		srv_stats.n_temp_blocks_decrypted;

	export_vars.innodb_onlineddl_rowlog_rows = onlineddl_rowlog_rows;
	export_vars.innodb_onlineddl_rowlog_pct_used = onlineddl_rowlog_pct_used;
	export_vars.innodb_onlineddl_pct_progress = onlineddl_pct_progress;

	if (!srv_read_only_mode) {
		export_vars.innodb_encryption_rotation_pages_read_from_cache =
			crypt_stat.pages_read_from_cache;
		export_vars.innodb_encryption_rotation_pages_read_from_disk =
			crypt_stat.pages_read_from_disk;
		export_vars.innodb_encryption_rotation_pages_modified =
			crypt_stat.pages_modified;
		export_vars.innodb_encryption_rotation_pages_flushed =
			crypt_stat.pages_flushed;
		export_vars.innodb_encryption_rotation_estimated_iops =
			crypt_stat.estimated_iops;
		export_vars.innodb_encryption_key_requests =
			srv_stats.n_key_requests;
	}

	mysql_mutex_unlock(&srv_innodb_monitor_mutex);

	log_sys.latch.rd_lock(SRW_LOCK_CALL);
	export_vars.innodb_lsn_current = log_sys.get_lsn();
	export_vars.innodb_lsn_flushed = log_sys.get_flushed_lsn();
	export_vars.innodb_lsn_last_checkpoint = log_sys.last_checkpoint_lsn;
	export_vars.innodb_checkpoint_max_age = static_cast<ulint>(
		log_sys.max_checkpoint_age);
	log_sys.latch.rd_unlock();
	export_vars.innodb_os_log_written = export_vars.innodb_lsn_current
		- recv_sys.lsn;

	export_vars.innodb_checkpoint_age = static_cast<ulint>(
		export_vars.innodb_lsn_current
		- export_vars.innodb_lsn_last_checkpoint);
}

struct srv_monitor_state_t
{
  time_t last_monitor_time;
  ulint mutex_skipped;
  bool last_srv_print_monitor;
  srv_monitor_state_t() : mutex_skipped(0), last_srv_print_monitor(false)
  {
    srv_last_monitor_time = time(NULL);
    last_monitor_time= srv_last_monitor_time;
  }
};

static srv_monitor_state_t monitor_state;

/** A task which prints the info output by various InnoDB monitors.*/
static void srv_monitor()
{
	time_t current_time = time(NULL);

	if (difftime(current_time, monitor_state.last_monitor_time) >= 15) {
		monitor_state.last_monitor_time = current_time;

		if (srv_print_innodb_monitor) {
			/* Reset mutex_skipped counter everytime
			srv_print_innodb_monitor changes. This is to
			ensure we will not be blocked by lock_sys.latch
			for short duration information printing */
			if (!monitor_state.last_srv_print_monitor) {
				monitor_state.mutex_skipped = 0;
				monitor_state.last_srv_print_monitor = true;
			}

			if (!srv_printf_innodb_monitor(stderr,
						MUTEX_NOWAIT(monitor_state.mutex_skipped),
						NULL, NULL)) {
				monitor_state.mutex_skipped++;
			} else {
				/* Reset the counter */
				monitor_state.mutex_skipped = 0;
			}
		} else {
			monitor_state.last_monitor_time = 0;
		}


		/* We don't create the temp files or associated
		mutexes in read-only-mode */

		if (!srv_read_only_mode && srv_innodb_status) {
			mysql_mutex_lock(&srv_monitor_file_mutex);
			rewind(srv_monitor_file);
			if (!srv_printf_innodb_monitor(srv_monitor_file,
						MUTEX_NOWAIT(monitor_state.mutex_skipped),
						NULL, NULL)) {
				monitor_state.mutex_skipped++;
			} else {
				monitor_state.mutex_skipped = 0;
			}

			os_file_set_eof(srv_monitor_file);
			mysql_mutex_unlock(&srv_monitor_file_mutex);
		}
	}

	srv_refresh_innodb_monitor_stats(current_time);
}

/** Periodic task which prints the info output by various InnoDB monitors.*/
void srv_monitor_task(void*)
{
	/* number of successive fatal timeouts observed */
	static lsn_t		old_lsn = recv_sys.lsn;

	ut_ad(!srv_read_only_mode);

	/* Try to track a strange bug reported by Harald Fuchs and others,
	where the lsn seems to decrease at times */

	lsn_t new_lsn = log_sys.get_lsn();
	ut_a(new_lsn >= old_lsn);
	old_lsn = new_lsn;

	/* Update the statistics collected for deciding LRU
	eviction policy. */
	buf_LRU_stat_update();

	ulonglong now = my_hrtime_coarse().val;
	const ulong threshold = srv_fatal_semaphore_wait_threshold;

	if (ulonglong start = dict_sys.oldest_wait()) {
		if (now >= start) {
			now -= start;
			ulong waited = static_cast<ulong>(now / 1000000);
			if (waited >= threshold) {
				ib::fatal() << dict_sys.fatal_msg;
			}

			if (waited == threshold / 4
			    || waited == threshold / 2
			    || waited == threshold / 4 * 3) {
				ib::warn() << "Long wait (" << waited
					   << " seconds) for dict_sys.latch";
			}
		}
	}

	srv_monitor();
}

/******************************************************************//**
Increment the server activity count. */
void
srv_inc_activity_count(void)
/*========================*/
{
	srv_sys.activity_count.inc();
}

#ifdef UNIV_DEBUG
/** @return whether purge or master task is active */
bool srv_any_background_activity()
{
  if (purge_sys.enabled() || srv_master_timer.get())
  {
    ut_ad(!srv_read_only_mode);
    return true;
  }
  return false;
}
#endif /* UNIV_DEBUG */

static void purge_worker_callback(void*);
static void purge_coordinator_callback(void*);
static void purge_truncation_callback(void*)
{
  purge_sys.latch.rd_lock(SRW_LOCK_CALL);
  const purge_sys_t::iterator head= purge_sys.head;
  purge_sys.latch.rd_unlock();
  head.free_history();
}

static tpool::task_group purge_task_group;
tpool::waitable_task purge_worker_task(purge_worker_callback, nullptr,
                                       &purge_task_group);
static tpool::task_group purge_coordinator_task_group(1);
static tpool::waitable_task purge_coordinator_task
  (purge_coordinator_callback, nullptr, &purge_coordinator_task_group);
static tpool::task_group purge_truncation_task_group(1);
static tpool::waitable_task purge_truncation_task
  (purge_truncation_callback, nullptr, &purge_truncation_task_group);

/** Wake up the purge threads if there is work to do. */
void purge_sys_t::wake_if_not_active()
{
  if (enabled() && !paused() && !purge_state.m_running &&
      (srv_undo_log_truncate || trx_sys.history_exists()) &&
      ++purge_state.m_running == 1)
    srv_thread_pool->submit_task(&purge_coordinator_task);
}

/** @return whether the purge tasks are active */
bool purge_sys_t::running()
{
  return purge_coordinator_task.is_running();
}

void purge_sys_t::stop_FTS()
{
  latch.rd_lock(SRW_LOCK_CALL);
  m_FTS_paused++;
  latch.rd_unlock();
  while (m_active)
    std::this_thread::sleep_for(std::chrono::seconds(1));
}

/** Stop purge during FLUSH TABLES FOR EXPORT */
void purge_sys_t::stop()
{
  latch.wr_lock(SRW_LOCK_CALL);

  if (!enabled())
  {
    /* Shutdown must have been initiated during FLUSH TABLES FOR EXPORT. */
    ut_ad(!srv_undo_sources);
    latch.wr_unlock();
    return;
  }

  ut_ad(srv_n_purge_threads > 0);

  const auto paused= m_paused++;

  latch.wr_unlock();

  if (!paused)
  {
    ib::info() << "Stopping purge";
    MONITOR_ATOMIC_INC(MONITOR_PURGE_STOP_COUNT);
    purge_coordinator_task.disable();
  }
}

/** Resume purge in data dictionary tables */
void purge_sys_t::resume_SYS(void *)
{
  ut_d(auto paused=) purge_sys.m_SYS_paused--;
  ut_ad(paused);
}

/** Resume purge at UNLOCK TABLES after FLUSH TABLES FOR EXPORT */
void purge_sys_t::resume()
{
   if (!enabled())
   {
     /* Shutdown must have been initiated during FLUSH TABLES FOR EXPORT. */
     ut_ad(!srv_undo_sources);
     return;
   }
   ut_ad(!srv_read_only_mode);
   ut_ad(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);
   purge_coordinator_task.enable();
   latch.wr_lock(SRW_LOCK_CALL);
   int32_t paused= m_paused--;
   ut_a(paused);

   if (paused == 1)
   {
     ib::info() << "Resuming purge";
     purge_state.m_running= 1;
     srv_thread_pool->submit_task(&purge_coordinator_task);
     MONITOR_ATOMIC_INC(MONITOR_PURGE_RESUME_COUNT);
   }
   latch.wr_unlock();
}

/*******************************************************************//**
Get current server activity count.
@return activity count. */
ulint
srv_get_activity_count(void)
/*========================*/
{
	return(srv_sys.activity_count);
}

/** Check if srv_inc_activity_count() has been called.
@param activity_count   copy of srv_sys.activity_count
@return whether the activity_count had changed */
static bool srv_check_activity(ulint *activity_count)
{
  ulint new_activity_count= srv_sys.activity_count;
  if (new_activity_count != *activity_count)
  {
    *activity_count= new_activity_count;
    return true;
  }

  return false;
}

/********************************************************************//**
The master thread is tasked to ensure that flush of log file happens
once every second in the background. This is to ensure that not more
than one second of trxs are lost in case of crash when
innodb_flush_logs_at_trx_commit != 1 */
static void srv_sync_log_buffer_in_background()
{
	time_t	current_time = time(NULL);

	srv_main_thread_op_info = "flushing log";
	if (difftime(current_time, srv_last_log_flush_time)
	    >= srv_flush_log_at_timeout) {
		log_buffer_flush_to_disk();
		srv_last_log_flush_time = current_time;
		srv_log_writes_and_flush++;
	}
}

/** Perform periodic tasks whenever the server is active.
@param counter_time  microsecond_interval_timer() */
static void srv_master_do_active_tasks(ulonglong counter_time)
{
	++srv_main_active_loops;

	MONITOR_INC(MONITOR_MASTER_ACTIVE_LOOPS);

	if (!(counter_time % (47 * 1000000ULL))) {
		srv_main_thread_op_info = "enforcing dict cache limit";
		if (ulint n_evicted = dict_sys.evict_table_LRU(true)) {
			MONITOR_INC_VALUE(
				MONITOR_SRV_DICT_LRU_EVICT_COUNT_ACTIVE,
				n_evicted);
		}
		MONITOR_INC_TIME_IN_MICRO_SECS(
			MONITOR_SRV_DICT_LRU_MICROSECOND, counter_time);
	}
}

/** Perform periodic tasks whenever the server is idle.
@param counter_time  microsecond_interval_timer() */
static void srv_master_do_idle_tasks(ulonglong counter_time)
{
	++srv_main_idle_loops;

	MONITOR_INC(MONITOR_MASTER_IDLE_LOOPS);

	srv_main_thread_op_info = "enforcing dict cache limit";
	if (ulint n_evicted = dict_sys.evict_table_LRU(false)) {
		MONITOR_INC_VALUE(
			MONITOR_SRV_DICT_LRU_EVICT_COUNT_IDLE, n_evicted);
	}
	MONITOR_INC_TIME_IN_MICRO_SECS(
		MONITOR_SRV_DICT_LRU_MICROSECOND, counter_time);
}

/** The periodic master task controlling the server. */
void srv_master_callback(void*)
{
  static ulint old_activity_count;

  ut_a(srv_shutdown_state <= SRV_SHUTDOWN_INITIATED);

  MONITOR_INC(MONITOR_MASTER_THREAD_SLEEP);
  purge_sys.wake_if_not_active();
  ulonglong counter_time= microsecond_interval_timer();
  srv_sync_log_buffer_in_background();
  MONITOR_INC_TIME_IN_MICRO_SECS(MONITOR_SRV_LOG_FLUSH_MICROSECOND,
				 counter_time);

  if (srv_check_activity(&old_activity_count))
    srv_master_do_active_tasks(counter_time);
  else
    srv_master_do_idle_tasks(counter_time);

  srv_main_thread_op_info= "sleeping";
}

/** @return whether purge should exit due to shutdown */
static bool srv_purge_should_exit(size_t old_history_size)
{
  ut_ad(srv_shutdown_state <= SRV_SHUTDOWN_CLEANUP);

  if (srv_undo_sources)
    return false;

  if (srv_fast_shutdown)
    return true;

  /* Slow shutdown was requested. */
  size_t prepared, active= trx_sys.any_active_transactions(&prepared);
  const size_t history_size= trx_sys.history_size();

  if (!history_size);
  else if (!active && history_size == old_history_size && prepared);
  else
  {
    static time_t progress_time;
    time_t now= time(NULL);
    if (now - progress_time >= 15)
    {
      progress_time= now;
#if defined HAVE_SYSTEMD && !defined EMBEDDED_LIBRARY
      service_manager_extend_timeout(INNODB_EXTEND_TIMEOUT_INTERVAL,
				     "InnoDB: to purge %zu transactions",
				     history_size);
      sql_print_information("InnoDB: to purge %zu transactions", history_size);
#endif
    }
    return false;
  }

  return !active;
}

/*********************************************************************//**
Fetch and execute a task from the work queue.
@param [in,out]	slot	purge worker thread slot
@return true if a task was executed */
static bool srv_task_execute()
{
	ut_ad(!srv_read_only_mode);
	ut_ad(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);

	mysql_mutex_lock(&srv_sys.tasks_mutex);

	if (que_thr_t* thr = UT_LIST_GET_FIRST(srv_sys.tasks)) {
		ut_a(que_node_get_type(thr->child) == QUE_NODE_PURGE);
		UT_LIST_REMOVE(srv_sys.tasks, thr);
		mysql_mutex_unlock(&srv_sys.tasks_mutex);
		que_run_threads(thr);
		return true;
	}

	ut_ad(UT_LIST_GET_LEN(srv_sys.tasks) == 0);
	mysql_mutex_unlock(&srv_sys.tasks_mutex);
	return false;
}

static void purge_create_background_thds(int );

/** Flag which is set, whenever innodb_purge_threads changes. */
static Atomic_relaxed<bool> srv_purge_thread_count_changed;

static std::mutex purge_thread_count_mtx;
void srv_update_purge_thread_count(uint n)
{
	std::lock_guard<std::mutex> lk(purge_thread_count_mtx);
	ut_ad(n > 0);
	ut_ad(n <= innodb_purge_threads_MAX);
	srv_n_purge_threads = n;
	srv_purge_thread_count_changed = true;
}

inline void purge_coordinator_state::do_purge()
{
  ut_ad(!srv_read_only_mode);

  if (!purge_sys.enabled() || purge_sys.paused())
    return;

  uint n_threads;

  {
    std::lock_guard<std::mutex> lk(purge_thread_count_mtx);
    n_threads= srv_n_purge_threads;
    srv_purge_thread_count_changed= false;
    goto first_loop;
  }

  do
  {
    if (UNIV_UNLIKELY(srv_purge_thread_count_changed))
    {
      /* Read the fresh value of srv_n_purge_threads, reset
      the changed flag. Both are protected by purge_thread_count_mtx. */
      {
        std::lock_guard<std::mutex> lk(purge_thread_count_mtx);
        n_threads= srv_n_purge_threads;
        srv_purge_thread_count_changed= false;
      }
    }
  first_loop:
    ut_ad(n_threads);

    history_size= trx_sys.history_size();

    if (!history_size)
    {
    no_history:
      srv_dml_needed_delay= 0;
      purge_truncation_task.wait();
      trx_purge_truncate_history();
      break;
    }

    ulint n_pages_handled= trx_purge(n_threads, history_size);
    if (!trx_sys.history_exists())
      goto no_history;
    if (purge_sys.truncating_tablespace() ||
        srv_shutdown_state != SRV_SHUTDOWN_NONE)
    {
      purge_truncation_task.wait();
      trx_purge_truncate_history();
    }
    else
      srv_thread_pool->submit_task(&purge_truncation_task);
    if (!n_pages_handled)
      break;
  }
  while (purge_sys.enabled() && !purge_sys.paused() &&
         !srv_purge_should_exit(history_size));

  m_running= 0;
}

static std::list<THD*> purge_thds;
static std::mutex purge_thd_mutex;
extern void* thd_attach_thd(THD*);
extern void thd_detach_thd(void *);
static int n_purge_thds;

/* Ensure  that we have at least n background THDs for purge */
static void purge_create_background_thds(int n)
{
	THD *thd= current_thd;
	std::unique_lock<std::mutex> lk(purge_thd_mutex);
	while (n_purge_thds < n)
	{
		purge_thds.push_back(innobase_create_background_thd("InnoDB purge worker"));
		n_purge_thds++;
	}
	set_current_thd(thd);
}

static THD *acquire_thd(void **ctx)
{
	std::unique_lock<std::mutex> lk(purge_thd_mutex);
	ut_a(!purge_thds.empty());
	THD* thd = purge_thds.front();
	purge_thds.pop_front();
	lk.unlock();

	/* Set current thd, and thd->mysys_var as well,
	it might be used by something in the server.*/
	*ctx = thd_attach_thd(thd);
	return thd;
}

static void release_thd(THD *thd, void *ctx)
{
	thd_detach_thd(ctx);
	std::unique_lock<std::mutex> lk(purge_thd_mutex);
	purge_thds.push_back(thd);
	lk.unlock();
	set_current_thd(0);
}

static void purge_worker_callback(void*)
{
  ut_ad(!current_thd);
  ut_ad(!srv_read_only_mode);
  ut_ad(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);
  void *ctx;
  THD *thd= acquire_thd(&ctx);
  while (srv_task_execute())
    ut_ad(purge_sys.running());
  release_thd(thd,ctx);
}

static void purge_coordinator_callback(void*)
{
  void *ctx;
  THD *thd= acquire_thd(&ctx);
  purge_state.do_purge();
  release_thd(thd, ctx);
}

void srv_init_purge_tasks()
{
  purge_create_background_thds(innodb_purge_threads_MAX);
  purge_sys.coordinator_startup();
}

static void srv_shutdown_purge_tasks()
{
  purge_coordinator_task.disable();
  purge_worker_task.wait();
  std::unique_lock<std::mutex> lk(purge_thd_mutex);
  while (!purge_thds.empty())
  {
    destroy_background_thd(purge_thds.front());
    purge_thds.pop_front();
  }
  n_purge_thds= 0;
  purge_truncation_task.wait();
}

/**********************************************************************//**
Enqueues a task to server task queue and releases a worker thread, if there
is a suspended one. */
void
srv_que_task_enqueue_low(
/*=====================*/
	que_thr_t*	thr)	/*!< in: query thread */
{
	ut_ad(!srv_read_only_mode);
	mysql_mutex_lock(&srv_sys.tasks_mutex);

	UT_LIST_ADD_LAST(srv_sys.tasks, thr);

	mysql_mutex_unlock(&srv_sys.tasks_mutex);
}

#ifdef UNIV_DEBUG
/** @return number of tasks in queue */
ulint srv_get_task_queue_length()
{
	ulint	n_tasks;

	ut_ad(!srv_read_only_mode);

	mysql_mutex_lock(&srv_sys.tasks_mutex);

	n_tasks = UT_LIST_GET_LEN(srv_sys.tasks);

	mysql_mutex_unlock(&srv_sys.tasks_mutex);

	return(n_tasks);
}
#endif

/** Shut down the purge threads. */
void srv_purge_shutdown()
{
  if (purge_sys.enabled())
  {
    if (!srv_fast_shutdown && !opt_bootstrap)
    {
      srv_purge_batch_size= innodb_purge_batch_size_MAX;
      srv_update_purge_thread_count(innodb_purge_threads_MAX);
    }
    size_t history_size= trx_sys.history_size();
    while (!srv_purge_should_exit(history_size))
    {
      history_size= trx_sys.history_size();
      ut_a(!purge_sys.paused());
      srv_thread_pool->submit_task(&purge_coordinator_task);
      purge_coordinator_task.wait();
    }
    purge_sys.coordinator_shutdown();
    srv_shutdown_purge_tasks();
  }
}