mysql5/mysql-5.7.27/sql/mysqld_thd_manager.h

/* Copyright (c) 2013, 2015, Oracle and/or its affiliates. All rights reserved.

   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 St, Fifth Floor, Boston, MA  02110-1301  USA */

#ifndef MYSQLD_THD_MANAGER_INCLUDED
#define MYSQLD_THD_MANAGER_INCLUDED

#include "my_global.h"
#include "my_atomic.h"         // my_atomic_add32
#include "my_thread_local.h"   // my_thread_id
#include "prealloced_array.h"

class THD;

#ifdef __cplusplus
extern "C" {
#endif
void thd_lock_thread_count(THD *thd);
void thd_unlock_thread_count(THD *thd);
#ifdef __cplusplus
}
#endif


/**
  Base class to perform actions on all thds in the thd list.
  Users of do_for_all_thd() need to subclass this and override operator().
*/

class Do_THD_Impl
{
public:
  virtual ~Do_THD_Impl() {}
  virtual void operator()(THD*) = 0;
};


/**
  Base class to find specific thd from the thd list.
  Users of find_thd() need to subclass this and override operator()
  to provide implementation to find thd from thd list.
*/

class Find_THD_Impl
{
public:
  virtual ~Find_THD_Impl() {}
  /**
    Override this operator to provide implementation to find specific thd.

    @param thd THD of one element in global thread list

    @returns bool
      @retval true  for matching thd
              false otherwise
  */
  virtual bool operator()(THD* thd) = 0;
};


/**
  This class maintains THD object of all registered threads.
  It provides interface to perform functions such as find, count,
  perform some action for each THD object in the list.

  It also provide mutators for inserting, and removing an element:
  add_thd() inserts a THD into the set, and increments the counter.
  remove_thd() removes a THD from the set, and decrements the counter.
  Method remove_thd() also broadcasts COND_thd_list.
*/

class Global_THD_manager
{
public:
  /**
    Value for thread_id reserved for THDs which does not have an
    assigned value yet. get_new_thread_id() will never return this
    value.
  */
  static const my_thread_id reserved_thread_id;

  /**
    Retrieves singleton instance
  */
  static Global_THD_manager* get_instance()
  {
    DBUG_ASSERT(thd_manager != NULL);
    return thd_manager;
  }

  /**
    Initializes the thd manager.
    Must be called before get_instance() can be used.

    @return true if initialization failed, false otherwise.
  */
  static bool create_instance();

  /**
    Destroys the singleton instance.
  */
  static void destroy_instance();

  /**
    Internally used to bypass code.
    It enables unit test scripts to create dummy THD object for testing.
  */
  void set_unit_test() { unit_test= true; }

  /**
    Adds THD to global THD list.

    @param thd THD object
  */
  void add_thd(THD *thd);

  /**
    Removes THD from global THD list.

    @param thd THD object
  */
  void remove_thd(THD *thd);

  /**
    Retrieves thread running statistic variable.
    @return int Returns the total number of threads currently running
    @note       This is a dirty read.
  */
  int get_num_thread_running() const { return num_thread_running; }

  /**
    Increments thread running statistic variable.
  */
  void inc_thread_running()
  {
    my_atomic_add32(&num_thread_running, 1);
  }

  /**
    Decrements thread running statistic variable.
  */
  void dec_thread_running()
  {
    my_atomic_add32(&num_thread_running, -1);
  }

  /**
    Retrieves thread created statistic variable.
    @return ulonglong Returns the total number of threads created
                      after server start
    @note             This is a dirty read.
  */
  ulonglong get_num_thread_created() const
  {
    return static_cast<ulonglong>(thread_created);
  }

  /**
    Increments thread created statistic variable.
  */
  void inc_thread_created()
  {
    my_atomic_add64(&thread_created, 1);
  }

  /**
    Returns an unused thread id.
  */
  my_thread_id get_new_thread_id();

  /**
    Releases a thread id so that it can be reused.
    Note that this is done automatically by remove_thd().
  */
  void release_thread_id(my_thread_id thread_id);

  /**
    Retrieves thread id counter value.
    @return my_thread_id Returns the thread id counter value
    @note                This is a dirty read.
  */
  my_thread_id get_thread_id() const { return thread_id_counter; }

  /**
    Sets thread id counter value. Only used in testing for now.
    @param new_id  The next ID to hand out (if it's unused).
  */
  void set_thread_id_counter(my_thread_id new_id);

  /**
    Retrieves total number of items in global THD list.
    @return uint Returns the count of items in global THD list
    @note        This is a dirty read.
  */
  uint get_thd_count() const { return global_thd_count; }

  /**
    Waits until all thd are removed from global THD list. In other words,
    get_thd_count to become zero.
  */
  void wait_till_no_thd();

  /**
    This function calls func() for all thds in thd list after
    taking local copy of thd list. It acquires LOCK_thd_remove
    to prevent removal from thd list.
    @param func Object of class which overrides operator()
  */
  void do_for_all_thd_copy(Do_THD_Impl *func);

  /**
    This function calls func() for all thds in thd list.
    @param func Object of class which overrides operator()
  */
  void do_for_all_thd(Do_THD_Impl *func);

  /**
    Returns a pointer to the first THD for which operator() returns true.
    @param func Object of class which overrides operator()
    @return THD
      @retval THD* Matching THD
      @retval NULL When THD is not found in the list
  */
  THD* find_thd(Find_THD_Impl *func);

  // Declared static as it is referenced in handle_fatal_signal()
  static int global_thd_count;

private:
  Global_THD_manager();
  ~Global_THD_manager();

  // Singleton instance.
  static Global_THD_manager *thd_manager;

  // Array of current THDs. Protected by LOCK_thd_list.
  typedef Prealloced_array<THD*, 500, true> THD_array;
  THD_array thd_list;

  // Array of thread ID in current use. Protected by LOCK_thread_ids.
  typedef Prealloced_array<my_thread_id, 1000, true> Thread_id_array;
  Thread_id_array thread_ids;

  mysql_cond_t COND_thd_list;

  // Mutex that guards thd_list
  mysql_mutex_t LOCK_thd_list;
  // Mutex used to guard removal of elements from thd list.
  mysql_mutex_t LOCK_thd_remove;
  // Mutex protecting thread_ids
  mysql_mutex_t LOCK_thread_ids;

  // Count of active threads which are running queries in the system.
  volatile int32 num_thread_running;

  // Cumulative number of threads created by mysqld daemon.
  volatile int64 thread_created;

  // Counter to assign thread id.
  my_thread_id thread_id_counter;

  // Used during unit test to bypass creating real THD object.
  bool unit_test;

  friend void thd_lock_thread_count(THD *);
  friend void thd_unlock_thread_count(THD *);
};

#endif /* MYSQLD_INCLUDED */