gcs_xcom_state_exchange.h

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/* Copyright (c) 2015, 2023, Oracle and/or its affiliates.
 
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
   as published by the Free Software Foundation.
 
   This program is also distributed with certain software (including
   but not limited to OpenSSL) that is licensed under separate terms,
   as designated in a particular file or component or in included license
   documentation.  The authors of MySQL hereby grant you an additional
   permission to link the program and your derivative works with the
   separately licensed software that they have included with MySQL.
 
   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, version 2.0, 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 GCS_XCOM_STATE_EXCHANGE_INCLUDED
#define GCS_XCOM_STATE_EXCHANGE_INCLUDED
 
#include <stdio.h>
#include <sys/types.h>
#include <map>
#include <memory>  // std::unique_ptr
#include <set>
#include <string>
#include <vector>
 
#include "plugin/group_replication/libmysqlgcs/include/mysql/gcs/gcs_communication_interface.h"
#include "plugin/group_replication/libmysqlgcs/include/mysql/gcs/gcs_message.h"
#include "plugin/group_replication/libmysqlgcs/include/mysql/gcs/gcs_types.h"
#include "plugin/group_replication/libmysqlgcs/include/mysql/gcs/gcs_view.h"
#include "plugin/group_replication/libmysqlgcs/include/mysql/gcs/xplatform/my_xp_cond.h"
#include "plugin/group_replication/libmysqlgcs/include/mysql/gcs/xplatform/my_xp_mutex.h"
#include "plugin/group_replication/libmysqlgcs/src/bindings/xcom/gcs_internal_message.h"
#include "plugin/group_replication/libmysqlgcs/src/bindings/xcom/gcs_xcom_group_member_information.h"
#include "plugin/group_replication/libmysqlgcs/src/bindings/xcom/gcs_xcom_view_identifier.h"
#include "plugin/group_replication/libmysqlgcs/xdr_gen/xcom_vp.h"
 
/**
  @class Xcom_member_state
 
  Class that conveys the state to be exchanged between members, which is not
  provided by XCom.
 
  In the original GCS protocol (version 1), this message had the following wire
  format:
 
      +--------+---------------------+
      | Header | Upper-layer payload |
      +--------+---------------------+
 
  Where the header has the following format:
 
      +---------+------------+
      | View ID | XCom synod |
      +---------+------------+
 
  With the introduction of the fragmentation stage into the message pipeline, it
  is possible that an original message is fragmented into several fragments, and
  each fragment is delivered by XCom individually.
 
  This means that when a node joins, it is possible for the transmission of a
  fragmented message to be ongoing, i.e. the existing group members have already
  received some, but not all, of the fragments.
  In this situation, the last fragment will arrive after the new node has joined
  the group.
  Since the original message is only delivered when all its fragments are
  received, this means that the original message will need to be delivered by
  the new node as well.
  But the problem is that new node does not have the fragments that were
  received before the new node joined.
 
  We solve this problem by augmenting the state exchange message with
  information about the ongoing transmission of fragmented messages.
  In particular, for every fragmented message whose transmission is ongoing, a
  node will attach the XCom synods of the fragments it has already received.
  The new node can use this information to fetch the fragments from XCom.
 
  As such, in GCS protocol version 2, this message has the following wire
  format:
 
      +--------+---------------------+----------------+
      | Header | Upper-layer payload | Recovery info. |
      +--------+---------------------+----------------+
 
  Where the recovery information has the following format:
 
      +---------------+-...-+----------------+------------+
      | XCom synod #1 |     | XCom synode #n | Nr. synods |
      +---------------+-...-+----------------+------------+
 
  Older GCS instances that only support protocol version 1 will deserialize the
  recovery information as part of the upper-layer payload.
  However, this works as long as the upper layer consumes only the portion it is
  expecting (upper-layer payload), even though it is fed its expected payload
  with the recovery information appended.
*/
class Xcom_member_state {
 public:
  /**
    Xcom_member_state constructor.
 
    @param[in] view_id the view identifier from the node
    @param[in] configuration_id Configuration identifier in use when the state
                                exchange message was created
    @param[in] version Protocol version used to represent the state
    @param[in] snapshot Snapshot information currently in use
    @param[in] data the generic data to be exchanged
    @param[in] data_size data's size
  */
 
  explicit Xcom_member_state(const Gcs_xcom_view_identifier &view_id,
                             synode_no configuration_id,
                             Gcs_protocol_version version,
                             const Gcs_xcom_synode_set &snapshot,
                             const uchar *data, uint64_t data_size);
 
  /**
    Xcom_member_state constructor.
 
    @param[in] version Protocol version used to represent the state
    @param[in] data the generic data to be exchanged
    @param[in] data_size data's size
  */
 
  explicit Xcom_member_state(Gcs_protocol_version version, const uchar *data,
                             uint64_t data_size);
 
  /**
    Member state destructor.
  */
 
  ~Xcom_member_state();
 
  /**
    Encodes the Member State's header to be sent through the newtwork.
 
    @param[out] buffer where the header will be stored.
    @param[in,out] buffer_len pointer to the variable that will hold the
                   header's size and has the buffer's len as input.
 
    @return True if there is no space to store the header.
            Otherwise, false.
  */
  bool encode_header(uchar *buffer, uint64_t *buffer_len) const;
 
  /**
   Decodes the Member State's header that was sent through the network.
 
   @param[out] buffer Where the header was stored.
   @param[in,out] buffer_len Pointer to the variable that holds the
   header's size and has the buffer's len as input.
 
   @return true if nothing went wrong. Otherwise, false.
   */
  bool decode_header(const uchar *buffer, uint64_t buffer_len);
 
  /**
   Encodes the Member State's snapshot to be sent through the network.
 
   @param[out] buffer where the snapshot will be stored.
   @param[in,out] buffer_len pointer to the variable that will hold the
   snapshot's size and has the buffer's len as input.
 
   @return True if there is no space to store the snapshot.
   Otherwise, false.
   */
  bool encode_snapshot(uchar *buffer, uint64_t *buffer_len) const;
 
  /**
   Decodes the Member State's snapshot that was sent through the network.
 
   @param[out] buffer Where the snapshot was stored.
   @param[in,out] buffer_len It holds the snapshot's size.
 
   @return true if nothing went wrong. Otherwise, false.
   */
  bool decode_snapshot(const uchar *buffer, uint64_t buffer_len);
 
  /**
   Decodes Member State that was sent through the network.
 
   @param[out] data where the data was stored.
   @param[out] data_size pointer to the variable that holds the
   data's size.
 
   @return True if for any reason the data could not be exchanged.
           Otherwise, false.
   */
  bool decode(const uchar *data, uint64_t data_size);
 
  /**
    @return the size of the encoded payload when put on the wire.
  */
 
  uint64_t get_encode_payload_size() const;
 
  /**
    @return the size of the encoded header when put on the wire.
  */
 
  static constexpr uint64_t get_encode_header_size() {
    return WIRE_XCOM_VARIABLE_VIEW_ID_SIZE + WIRE_XCOM_VIEW_ID_SIZE +
           WIRE_XCOM_GROUP_ID_SIZE + WIRE_XCOM_MSG_ID_SIZE +
           WIRE_XCOM_NODE_ID_SIZE;
  }
 
  /**
   @return the size of the encoded snapshot when put on the wire.
   */
 
  uint64_t get_encode_snapshot_size() const;
 
  /**
    @return the view identifier
  */
 
  Gcs_xcom_view_identifier *get_view_id() { return m_view_id; }
 
  /**
    @return the configuration identifier
  */
 
  synode_no get_configuration_id() const { return m_configuration_id; }
 
  /**
    @return the generic exchangeable data
  */
 
  const uchar *get_data() const { return m_data; }
 
  /**
    @return the size of the generic exchangeable data
  */
 
  uint64_t get_data_size() const { return m_data_size; }
 
  const Gcs_xcom_synode_set &get_snapshot() const { return m_snapshot; }
 
 private:
  static constexpr auto WIRE_XCOM_VARIABLE_VIEW_ID_SIZE = 8;
  static constexpr auto WIRE_XCOM_VIEW_ID_SIZE = 4;
  static constexpr auto WIRE_XCOM_GROUP_ID_SIZE = 4;
  static constexpr auto WIRE_XCOM_MSG_ID_SIZE = 8;
  static constexpr auto WIRE_XCOM_NODE_ID_SIZE = 4;
  static constexpr auto WIRE_XCOM_SNAPSHOT_NR_ELEMS_SIZE = 8;
 
  static constexpr uint64_t get_encode_snapshot_elem_size() {
    return WIRE_XCOM_MSG_ID_SIZE + WIRE_XCOM_NODE_ID_SIZE;
  }
 
  /*
    View identifier installed by the current member if there
    is any.
  */
  Gcs_xcom_view_identifier *m_view_id;
 
  /*
    Configuration identifier in use when the state exchange message
    was created.
  */
  synode_no m_configuration_id;
 
  /*
    Data to be disseminated by the state exchange phase.
  */
  uchar *m_data;
 
  /*
    Data's size disseminated by the state exchange phase.
  */
  uint64_t m_data_size;
 
  /**
   Recovery information which is currently a list of the XCom synods of the
   fragments that are in buffers when a node is joining.
 
   This is currently used only to transfer information on slice packets that
   need to be fetched by the joining node before it may become ready to serve
   requests.
   */
  Gcs_xcom_synode_set m_snapshot;
 
  /**
   GCS communication version number in use.
   */
  Gcs_protocol_version m_version;
 
  /*
    Disabling the copy constructor and assignment operator.
  */
  Xcom_member_state(Xcom_member_state const &) = delete;
  Xcom_member_state &operator=(Xcom_member_state const &) = delete;
};
 
/**
  @class gcs_xcom_state_exchange_interface
 
  Interface that defines the operations that state exchange will provide.
  In what follows, we describe how the state exchange algorithm works and
  the view change process where it is inserted and which is an essential
  part of our system.
 
  The view change process is comprised of two major parts:
    - Adding or removing a node from the system, accomplished in the
      XCom/Paxos layer: "The SMART Way to Migrate Replicated Stateful
      Services"
 
    - A state exchange phase in which all members distribute data among
     themselves.
 
  Whenever a node wants to add or remove itself from the group, or after
  a failure when a healthy member expels the faulty node from the group, a
  reconfiguration request is sent in the form of an add_node or remove_node
  message.
 
  After the success of the request, XCOM sends a global view message
  that contains information on all nodes tagging them as alive or
  faulty to all non-faulty members. MySQL GCS looks at this information
  and computes who has joined and who has left the group. The computation
  is trivially simple and compares the set of nodes received in the
  current view with the set of nodes in the previous view:
 
    . left nodes = (alive_members in old_set) - (alive_members in new_set)
 
    . joined nodes = (alive_members in new_set) - (alive_members in old_set)
 
  However, the new view is only delivered to an upper layer after all
  members exchange what we call a state message. While the view is being
  processed and the state exchange is ongoing, all incoming data messages
  are not delivered to the application and are put into a buffer. So after
  getting state messages from all members, the view change is delivered
  to the upper layer along with the content of the state messages and any
  buffered message is delivered afterwards.
 
  Why blocking the delivery of data messages and why these state
  messages?
 
  Recall that all messages are atomically delivered and we can guarantee
  that all nodes will have the same state which encompasses messages
  (e.g. transactions) in queues and in the storage (e.g. binary log)
  because all new data messages are buffered while the state messages
  are being exchanged.
 
  Blocking the delivery of new data messages give us a synchronization
  point.
 
  But if all nodes have the same state why gathering a state message
  from all members?
 
  The power of choice. Let us use MySQL Group Replication as a concrete
  example of an upper layer to understand why. This is done because having
  information on all members allow the new node to choose a member that is
  not lagging behind (i.e. has a small queue) as a donor in a recovery
  phase. Besides, the state message also carries information on IPs and
  Ports used to access the MySQL Instances. This information is necessary
  to start the recovery which will be asynchronously started and will dump
  the missing data from a donor.
 
  Note that the content of the state message is opaque to the MySQL GCS
  layer which only provides a synchronization point.
*/
class Gcs_xcom_state_exchange_interface {
 public:
  virtual ~Gcs_xcom_state_exchange_interface() = default;
 
  /**
    Accomplishes all necessary initialization steps.
  */
 
  virtual void init() = 0;
 
  /**
    If messages were buffered during its processing, they are discarded
    and internal structures needed are cleaned up.
  */
 
  virtual void reset() = 0;
 
  /**
    Has the same behavior as the reset but additionally flushes buffered
    messages.
  */
 
  virtual void reset_with_flush() = 0;
 
  /**
    If messages were buffered during its processing, they are delivered
    to upper layers and internal structures needed are cleaned up.
  */
 
  virtual void end() = 0;
 
  /**
    Signals the module to start a State Exchange.
 
    @param[in] configuration_id Configuration identifier in use when the state
                                exchange phase started
    @param[in] total          xcom total members in the new view
    @param[in] left           xcom members that left in the new view
    @param[in] joined         xcom members that joined in the new view
    @param[in] exchangeable_data generic exchanged data
    @param[in] current_view   the currently installed view
    @param[in] group          group name
    @param[in] local_info     the local GCS member identifier
    @param[in] xcom_nodes     list of nodes
 
    @return true if the member is leaving
  */
 
  virtual bool state_exchange(
      synode_no configuration_id, std::vector<Gcs_member_identifier *> &total,
      std::vector<Gcs_member_identifier *> &left,
      std::vector<Gcs_member_identifier *> &joined,
      std::vector<std::unique_ptr<Gcs_message_data>> &exchangeable_data,
      Gcs_view *current_view, std::string *group,
      const Gcs_member_identifier &local_info,
      const Gcs_xcom_nodes &xcom_nodes) = 0;
 
  /**
    Processes a member state message on an ongoing State Exchange round.
 
    @param[in] ms_info received Member State
    @param[in] p_id the node that the Member State pertains
    @param[in] maximum_supported_protocol_version maximum supported protocol
    version
    @param[in] used_protocol_version protocol version in use by a member during
    the state exchange phase
 
    @return true if State Exchanged is to be finished and the view can be
                 installed
  */
 
  virtual bool process_member_state(
      Xcom_member_state *ms_info, const Gcs_member_identifier &p_id,
      Gcs_protocol_version maximum_supported_protocol_version,
      Gcs_protocol_version used_protocol_version) = 0;
 
  /**
   Compute the set of incompatible members after the state exchange has
   finished.
   A member M is incompatible if it is attempting to join a group that is
   using protocol X, but M is using protocol Y s.t. X != Y.
   @returns the set of incompatible members
   */
  virtual std::vector<Gcs_xcom_node_information>
  compute_incompatible_members() = 0;
 
  /**
   Recovers any missing packets required for the member to join the group.
   @retval true if successful
   @retval false otherwise
   */
  virtual bool process_recovery_state() = 0;
 
  /**
    Retrieves the new view identifier after a State Exchange.
 
    @return the new view identifier
  */
 
  virtual Gcs_xcom_view_identifier *get_new_view_id() = 0;
 
  /**
    @return the members that joined in this State Exchange round
  */
 
  virtual std::set<Gcs_member_identifier *> *get_joined() = 0;
 
  /**
    @return the members that left in this State Exchange round
  */
 
  virtual std::set<Gcs_member_identifier *> *get_left() = 0;
 
  /**
    @return All the members in this State Exchange round
  */
 
  virtual std::set<Gcs_member_identifier *> *get_total() = 0;
 
  /**
    @return the group in which this State Exchange is occurring
  */
 
  virtual std::string *get_group() = 0;
 
  /**
    @return the saved states
  */
 
  virtual std::map<Gcs_member_identifier, Xcom_member_state *>
      *get_member_states() = 0;
 
  /**
   Computes the maximum protocol version supported by the group.
   */
  virtual void compute_maximum_supported_protocol_version() = 0;
};
 
/**
  Implementation of the gcs_xcom_state_exchange_interface.
*/
class Gcs_xcom_state_exchange : public Gcs_xcom_state_exchange_interface {
 public:
  /**
    State Exchange constructor.
 
    @param[in] comm Communication interface reference to allow broadcasting of
                    member states
  */
 
  explicit Gcs_xcom_state_exchange(Gcs_communication_interface *comm);
 
  ~Gcs_xcom_state_exchange() override;
 
  // Implementation of gcs_xcom_state_exchange_interface
  void init() override;
 
  void reset() override;
 
  void reset_with_flush() override;
 
  void end() override;
 
  bool state_exchange(
      synode_no configuration_id, std::vector<Gcs_member_identifier *> &total,
      std::vector<Gcs_member_identifier *> &left,
      std::vector<Gcs_member_identifier *> &joined,
      std::vector<std::unique_ptr<Gcs_message_data>> &exchangeable_data,
      Gcs_view *current_view, std::string *group,
      const Gcs_member_identifier &local_info,
      const Gcs_xcom_nodes &xcom_nodes) override;
 
  bool process_member_state(
      Xcom_member_state *ms_info, const Gcs_member_identifier &p_id,
      Gcs_protocol_version maximum_supported_protocol_version,
      Gcs_protocol_version used_protocol_version) override;
 
  std::vector<Gcs_xcom_node_information> compute_incompatible_members()
      override;
 
  bool process_recovery_state() override;
 
  Gcs_xcom_view_identifier *get_new_view_id() override;
 
  std::set<Gcs_member_identifier *> *get_joined() override {
    return &m_ms_joined;
  }
 
  std::set<Gcs_member_identifier *> *get_left() override { return &m_ms_left; }
 
  std::set<Gcs_member_identifier *> *get_total() override {
    return &m_ms_total;
  }
 
  std::map<Gcs_member_identifier, Xcom_member_state *> *get_member_states()
      override {
    return &m_member_states;
  }
 
  std::string *get_group() override { return m_group_name; }
 
  void compute_maximum_supported_protocol_version() override;
 
 private:
  /**
    Computes if the local member is leaving.
 
    @return true in case of the local member is leaving
  */
 
  bool is_leaving();
 
  /**
    Computes if the local member is joining.
 
    @return true in case of the local member is joining
  */
 
  bool is_joining();
 
  /**
   Update the communication system with information on membership.
 
   @param xcom_nodes List of nodes that belong to the current membership.
   */
  void update_communication_channel(const Gcs_xcom_nodes &xcom_nodes);
 
  /**
    Broadcasts the local state to all nodes in the Cluster.
 
    @param[in] proposed_view proposed view to broadcast
    @param[in] exchangeable_data List with exchangeable messages
  */
 
  enum_gcs_error broadcast_state(
      const Gcs_xcom_view_identifier &proposed_view,
      std::vector<std::unique_ptr<Gcs_message_data>> &exchangeable_data);
 
  /**
    Updates the structure that waits for State Exchanges.
  */
 
  void update_awaited_vector();
 
  /**
    Converts xcom data to a set of internal representation.
 
    @param[in] in xcom list
    @param[in] pset Set where the converted member ids will be written
  */
 
  void fill_member_set(std::vector<Gcs_member_identifier *> &in,
                       std::set<Gcs_member_identifier *> &pset);
 
  /**
   Stores the member's state and protocol version.
 
   @param ms_info state
   @param p_id member
   @param[in] maximum_supported_protocol_version maximum supported protocol
    version
   @param used_protocol_version protocol version
   */
  void save_member_state(
      Xcom_member_state *ms_info, const Gcs_member_identifier &p_id,
      Gcs_protocol_version maximum_supported_protocol_version,
      Gcs_protocol_version used_protocol_version);
 
  /**
   Auxiliary method that checks whether @c snapshot_to_recover contains all
   the synodes required.
   */
  bool snapshot_is_enough(Gcs_xcom_synode_set const &snapshot_to_recover) const;
 
  /**
   Checks whether all the existing group members, myself excluded, announce the
   same protocol version.
   @retval {false, _} if they do not all announce the same protocol version
   @retval {true, Gcs_protocol_version} if they all announced the same protocol
   version of the return value
   */
  std::pair<bool, Gcs_protocol_version> members_announce_same_version() const;
 
  /**
   Checks whether this server is incompatible with the group.
   @retval true If it is incompatible
   @retval false If it is compatible
   */
  bool incompatible_with_group() const;
 
  /**
   Computes the set of incompatible nodes that are trying to join the group.
   @returns the set of incompatible joiners
   */
  std::vector<Gcs_xcom_node_information> compute_incompatible_joiners();
 
  Gcs_communication_interface *m_broadcaster;
 
  std::map<Gcs_member_identifier, uint> m_awaited_vector;
 
  std::map<Gcs_member_identifier, uint> m_recover_vector;
 
  /* Set of ids in GCS native format as reported by View-change handler. */
  std::set<Gcs_member_identifier *> m_ms_total, m_ms_left, m_ms_joined;
 
  /* Collection of State Message contents to facilitate view installation. */
  std::map<Gcs_member_identifier, Xcom_member_state *> m_member_states;
 
  /* Collection of protocol version in use per member. */
  std::map<Gcs_member_identifier, Gcs_protocol_version> m_member_versions;
 
  /* Collection of maximum protocol version supported per member. */
  std::map<Gcs_member_identifier, Gcs_protocol_version> m_member_max_versions;
 
  // Group name to exchange state
  std::string *m_group_name;
 
  // Local GCS member identification
  Gcs_member_identifier m_local_information;
 
  /* Configuration identifier in use when the state exchange phase started */
  synode_no m_configuration_id;
 
  std::vector<synode_no> cached_ids;
 
  /* XCom identifiers of the members in m_ms_total */
  Gcs_xcom_nodes m_ms_xcom_nodes;
 
  /*
    Disabling the copy constructor and assignment operator.
  */
  Gcs_xcom_state_exchange(Gcs_xcom_state_exchange const &);
  Gcs_xcom_state_exchange &operator=(Gcs_xcom_state_exchange const &);
};
 
/*
  @interface gcs_xcom_view_change_control_interface
 
  This interface will serve as a synchronization point to all those that are
  interested in maintaining view safety. This will guarantee that no actions are
  accomplished while a view change procedure is ongoing.
 
  The promoters of view change will indicate via start_view_exchange() and
  end_view_exchange() the boundaries of the process. Those that want to wait
  for the end, will synchronize on wait_for_view_change_end().
*/
class Gcs_xcom_view_change_control_interface {
 public:
  virtual ~Gcs_xcom_view_change_control_interface() = default;
 
  virtual void start_view_exchange() = 0;
  virtual void end_view_exchange() = 0;
  virtual void wait_for_view_change_end() = 0;
  virtual bool is_view_changing() = 0;
 
  // Leave related information
  virtual bool start_leave() = 0;
  virtual void end_leave() = 0;
  virtual bool is_leaving() = 0;
 
  // Join related information
  virtual bool start_join() = 0;
  virtual void end_join() = 0;
  virtual bool is_joining() = 0;
 
  // Keep track of delivered views
  virtual void set_current_view(Gcs_view *current_view) = 0;
  virtual Gcs_view *get_current_view() = 0;
  virtual bool belongs_to_group() = 0;
  virtual void set_belongs_to_group(bool belong) = 0;
  virtual void set_unsafe_current_view(Gcs_view *current_view) = 0;
  virtual Gcs_view *get_unsafe_current_view() = 0;
 
  // Keep track if GCS as a whole has been ordered to finalize;
  virtual void finalize() = 0;
  virtual bool is_finalized() = 0;
};
 
/*
  @class gcs_xcom_view_change_control
 
  Implementation of gcs_xcom_view_change_control_interface.
*/
class Gcs_xcom_view_change_control
    : public Gcs_xcom_view_change_control_interface {
 public:
  explicit Gcs_xcom_view_change_control();
  ~Gcs_xcom_view_change_control() override;
 
  void start_view_exchange() override;
  void end_view_exchange() override;
  void wait_for_view_change_end() override;
  bool is_view_changing() override;
 
  bool start_leave() override;
  void end_leave() override;
  bool is_leaving() override;
 
  bool start_join() override;
  void end_join() override;
  bool is_joining() override;
 
  void set_current_view(Gcs_view *current_view) override;
  Gcs_view *get_current_view() override;
  bool belongs_to_group() override;
  void set_belongs_to_group(bool belong) override;
  void set_unsafe_current_view(Gcs_view *current_view) override;
  Gcs_view *get_unsafe_current_view() override;
 
  void finalize() override;
  bool is_finalized() override;
 
 private:
  bool m_view_changing;
  bool m_leaving;
  bool m_joining;
 
  My_xp_cond_impl m_wait_for_view_cond;
  My_xp_mutex_impl m_wait_for_view_mutex;
  My_xp_mutex_impl m_joining_leaving_mutex;
 
  /*
    Reference to the currently installed view.
  */
  Gcs_view *m_current_view;
 
  /*
    Protect access to the current view so that it can be
    copied and returned.
  */
  My_xp_mutex_impl m_current_view_mutex;
 
  /*
    Whether the current node belongs to a group or not.
  */
  bool m_belongs_to_group;
 
  /*
   */
  std::atomic<bool> m_finalized;
 
  /*
    Disabling the copy constructor and assignment operator.
  */
  Gcs_xcom_view_change_control(Gcs_xcom_view_change_control const &);
  Gcs_xcom_view_change_control &operator=(Gcs_xcom_view_change_control const &);
};
#endif /* GCS_XCOM_STATE_EXCHANGE_INCLUDED */