# -*- coding: utf-8 -*-
# © Copyright EnterpriseDB UK Limited 2011-2023
#
# This file is part of Barman.
#
# Barman 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, either version 3 of the License, or
# (at your option) any later version.
#
# Barman 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 Barman.  If not, see <http://www.gnu.org/licenses/>.

"""
This module represents the interface towards a PostgreSQL server.
"""

import atexit
import datetime
import logging
import os
import signal
import threading
import time
from abc import ABCMeta
from multiprocessing import Process, Queue

import psycopg2
from psycopg2.errorcodes import DUPLICATE_OBJECT, OBJECT_IN_USE, UNDEFINED_OBJECT
from psycopg2.extensions import STATUS_IN_TRANSACTION, STATUS_READY
from psycopg2.extras import DictCursor, NamedTupleCursor

from barman.exceptions import (
    BackupFunctionsAccessRequired,
    ConninfoException,
    PostgresAppNameError,
    PostgresCheckpointPrivilegesRequired,
    PostgresConnectionError,
    PostgresConnectionLost,
    PostgresDuplicateReplicationSlot,
    PostgresException,
    PostgresInvalidReplicationSlot,
    PostgresIsInRecovery,
    PostgresObsoleteFeature,
    PostgresReplicationSlotInUse,
    PostgresReplicationSlotsFull,
    PostgresUnsupportedFeature,
)
from barman.infofile import Tablespace
from barman.postgres_plumbing import function_name_map
from barman.remote_status import RemoteStatusMixin
from barman.utils import force_str, simplify_version, with_metaclass

try:
    from queue import Empty
except ImportError:
    from Queue import Empty

# This is necessary because the CONFIGURATION_LIMIT_EXCEEDED constant
# has been added in psycopg2 2.5, but Barman supports version 2.4.2+ so
# in case of import error we declare a constant providing the correct value.
try:
    from psycopg2.errorcodes import CONFIGURATION_LIMIT_EXCEEDED
except ImportError:
    CONFIGURATION_LIMIT_EXCEEDED = "53400"


_logger = logging.getLogger(__name__)

_live_connections = []
"""
List of connections to be closed at the interpreter shutdown
"""


@atexit.register
def _atexit():
    """
    Ensure that all the connections are correctly closed
    at interpreter shutdown
    """
    # Take a copy of the list because the conn.close() method modify it
    for conn in list(_live_connections):
        _logger.warning(
            "Forcing %s cleanup during process shut down.", conn.__class__.__name__
        )
        conn.close()


class PostgreSQL(with_metaclass(ABCMeta, RemoteStatusMixin)):
    """
    This abstract class represents a generic interface to a PostgreSQL server.
    """

    CHECK_QUERY = "SELECT 1"
    MINIMAL_VERSION = 90600

    def __init__(self, conninfo):
        """
        Abstract base class constructor for PostgreSQL interface.

        :param str conninfo: Connection information (aka DSN)
        """
        super(PostgreSQL, self).__init__()
        self.conninfo = conninfo
        self._conn = None
        self.allow_reconnect = True
        # Build a dictionary with connection info parameters
        # This is mainly used to speed up search in conninfo
        try:
            self.conn_parameters = self.parse_dsn(conninfo)
        except (ValueError, TypeError) as e:
            _logger.debug(e)
            raise ConninfoException(
                'Cannot connect to postgres: "%s" '
                "is not a valid connection string" % conninfo
            )

    @staticmethod
    def parse_dsn(dsn):
        """
        Parse connection parameters from 'conninfo'

        :param str dsn: Connection information (aka DSN)
        :rtype: dict[str,str]
        """
        # TODO: this might be made more robust in the future
        return dict(x.split("=", 1) for x in dsn.split())

    @staticmethod
    def encode_dsn(parameters):
        """
        Build a connection string from a dictionary of connection
        parameters

        :param dict[str,str] parameters: Connection parameters
        :rtype: str
        """
        # TODO: this might be made more robust in the future
        return " ".join(["%s=%s" % (k, v) for k, v in sorted(parameters.items())])

    def get_connection_string(self, application_name=None):
        """
        Return the connection string, adding the application_name parameter
        if requested, unless already defined by user in the connection string

        :param str application_name: the application_name to add
        :return str: the connection string
        """
        conn_string = self.conninfo
        # check if the application name is already defined by user
        if application_name and "application_name" not in self.conn_parameters:
            # Then add the it to the connection string
            conn_string += " application_name=%s" % application_name
        # adopt a secure schema-usage pattern. See:
        # https://www.postgresql.org/docs/current/libpq-connect.html
        if "options" not in self.conn_parameters:
            conn_string += " options=-csearch_path="

        return conn_string

    def connect(self):
        """
        Generic function for Postgres connection (using psycopg2)
        """

        if not self._check_connection():
            try:
                self._conn = psycopg2.connect(self.conninfo)
                self._conn.autocommit = True
            # If psycopg2 fails to connect to the host,
            # raise the appropriate exception
            except psycopg2.DatabaseError as e:
                raise PostgresConnectionError(force_str(e).strip())
            # Register the connection to the list of live connections
            _live_connections.append(self)
        return self._conn

    def _check_connection(self):
        """
        Return false if the connection is broken

        :rtype: bool
        """
        # If the connection is not present return False
        if not self._conn:
            return False

        # Check if the connection works by running 'SELECT 1'
        cursor = None
        initial_status = None
        try:
            initial_status = self._conn.status
            cursor = self._conn.cursor()
            cursor.execute(self.CHECK_QUERY)
            # Rollback if initial status was IDLE because the CHECK QUERY
            # has started a new transaction.
            if initial_status == STATUS_READY:
                self._conn.rollback()
        except psycopg2.DatabaseError:
            # Connection is broken, so we need to reconnect
            self.close()
            # Raise an error if reconnect is not allowed
            if not self.allow_reconnect:
                raise PostgresConnectionError(
                    "Connection lost, reconnection not allowed"
                )
            return False
        finally:
            if cursor:
                cursor.close()

        return True

    def close(self):
        """
        Close the connection to PostgreSQL
        """
        if self._conn:
            # If the connection is still alive, rollback and close it
            if not self._conn.closed:
                if self._conn.status == STATUS_IN_TRANSACTION:
                    self._conn.rollback()
                self._conn.close()
            # Remove the connection from the live connections list
            self._conn = None
            _live_connections.remove(self)

    def _cursor(self, *args, **kwargs):
        """
        Return a cursor
        """
        conn = self.connect()
        return conn.cursor(*args, **kwargs)

    @property
    def server_version(self):
        """
        Version of PostgreSQL (returned by psycopg2)
        """
        conn = self.connect()
        return conn.server_version

    @property
    def server_txt_version(self):
        """
        Human readable version of PostgreSQL (calculated from server_version)

        :rtype: str|None
        """
        try:
            conn = self.connect()
            return self.int_version_to_string_version(conn.server_version)

        except PostgresConnectionError as e:
            _logger.debug(
                "Error retrieving PostgreSQL version: %s", force_str(e).strip()
            )
            return None

    @property
    def minimal_txt_version(self):
        """
        Human readable version of PostgreSQL (calculated from server_version)

        :rtype: str|None
        """
        return self.int_version_to_string_version(self.MINIMAL_VERSION)

    @staticmethod
    def int_version_to_string_version(int_version):
        """
        takes an int version
        :param int_version: ex: 10.22 121200 or 130800
        :return: str  ex 10.22.00 12.12.00  13.8.00
        """
        major = int(int_version / 10000)
        minor = int(int_version / 100 % 100)
        patch = int(int_version % 100)
        if major < 10:
            return "%d.%d.%d" % (major, minor, patch)
        if minor != 0:
            _logger.warning(
                "Unexpected non zero minor version %s in %s",
                minor,
                int_version,
            )
        return "%d.%d" % (major, patch)

    @property
    def server_major_version(self):
        """
        PostgreSQL major version (calculated from server_txt_version)

        :rtype: str|None
        """
        result = self.server_txt_version
        if result is not None:
            return simplify_version(result)
        return None

    def is_minimal_postgres_version(self):
        """Checks if postgres version has at least minimal version"""
        return self.server_version >= self.MINIMAL_VERSION


class StreamingConnection(PostgreSQL):
    """
    This class represents a streaming connection to a PostgreSQL server.
    """

    CHECK_QUERY = "IDENTIFY_SYSTEM"

    def __init__(self, conninfo):
        """
        Streaming connection constructor

        :param str conninfo: Connection information (aka DSN)
        """
        super(StreamingConnection, self).__init__(conninfo)

        # Make sure we connect using the 'replication' option which
        # triggers streaming replication protocol communication
        self.conn_parameters["replication"] = "true"
        # ensure that the datestyle is set to iso, working around an
        # issue in some psycopg2 versions
        self.conn_parameters["options"] = "-cdatestyle=iso"
        # Override 'dbname' parameter. This operation is required to mimic
        # the behaviour of pg_receivexlog and pg_basebackup
        self.conn_parameters["dbname"] = "replication"
        # Rebuild the conninfo string from the modified parameter lists
        self.conninfo = self.encode_dsn(self.conn_parameters)

    def connect(self):
        """
        Connect to the PostgreSQL server. It reuses an existing connection.

        :returns: the connection to the server
        """
        if self._check_connection():
            return self._conn

        # Build a connection
        self._conn = super(StreamingConnection, self).connect()
        return self._conn

    def fetch_remote_status(self):
        """
        Returns the status of the connection to the PostgreSQL server.

        This method does not raise any exception in case of errors,
        but set the missing values to None in the resulting dictionary.

        :rtype: dict[str, None|str]
        """
        result = dict.fromkeys(
            (
                "connection_error",
                "streaming_supported",
                "streaming",
                "streaming_systemid",
                "timeline",
                "xlogpos",
                "version_supported",
            ),
            None,
        )
        try:
            # This needs to be protected by the try/except because
            # `self.is_minimal_postgres_version` can raise a PostgresConnectionError
            result["version_supported"] = self.is_minimal_postgres_version()
            if not self.is_minimal_postgres_version():
                return result
            # streaming is always supported
            result["streaming_supported"] = True
            # Execute a IDENTIFY_SYSTEM to check the connection
            cursor = self._cursor()
            cursor.execute("IDENTIFY_SYSTEM")
            row = cursor.fetchone()
            # If something has been returned, barman is connected
            # to a replication backend
            if row:
                result["streaming"] = True
                # IDENTIFY_SYSTEM always returns at least two values
                result["streaming_systemid"] = row[0]
                result["timeline"] = row[1]
                # PostgreSQL 9.1+ returns also the current xlog flush location
                if len(row) > 2:
                    result["xlogpos"] = row[2]
        except psycopg2.ProgrammingError:
            # This is not a streaming connection
            result["streaming"] = False
        except PostgresConnectionError as e:
            result["connection_error"] = force_str(e).strip()
            _logger.warning(
                "Error retrieving PostgreSQL status: %s", force_str(e).strip()
            )
        return result

    def create_physical_repslot(self, slot_name):
        """
        Create a physical replication slot using the streaming connection
        :param str slot_name: Replication slot name
        """
        cursor = self._cursor()
        try:
            # In the following query, the slot name is directly passed
            # to the CREATE_REPLICATION_SLOT command, without any
            # quoting. This is a characteristic of the streaming
            # connection, otherwise if will fail with a generic
            # "syntax error"
            cursor.execute("CREATE_REPLICATION_SLOT %s PHYSICAL" % slot_name)
            _logger.info("Replication slot '%s' successfully created", slot_name)
        except psycopg2.DatabaseError as exc:
            if exc.pgcode == DUPLICATE_OBJECT:
                # A replication slot with the same name exists
                raise PostgresDuplicateReplicationSlot()
            elif exc.pgcode == CONFIGURATION_LIMIT_EXCEEDED:
                # Unable to create a new physical replication slot.
                # All slots are full.
                raise PostgresReplicationSlotsFull()
            else:
                raise PostgresException(force_str(exc).strip())

    def drop_repslot(self, slot_name):
        """
        Drop a physical replication slot using the streaming connection
        :param str slot_name: Replication slot name
        """
        cursor = self._cursor()
        try:
            # In the following query, the slot name is directly passed
            # to the DROP_REPLICATION_SLOT command, without any
            # quoting. This is a characteristic of the streaming
            # connection, otherwise if will fail with a generic
            # "syntax error"
            cursor.execute("DROP_REPLICATION_SLOT %s" % slot_name)
            _logger.info("Replication slot '%s' successfully dropped", slot_name)
        except psycopg2.DatabaseError as exc:
            if exc.pgcode == UNDEFINED_OBJECT:
                # A replication slot with the that name does not exist
                raise PostgresInvalidReplicationSlot()
            if exc.pgcode == OBJECT_IN_USE:
                # The replication slot is still in use
                raise PostgresReplicationSlotInUse()
            else:
                raise PostgresException(force_str(exc).strip())


class PostgreSQLConnection(PostgreSQL):
    """
    This class represents a standard client connection to a PostgreSQL server.
    """

    # Streaming replication client types
    STANDBY = 1
    WALSTREAMER = 2
    ANY_STREAMING_CLIENT = (STANDBY, WALSTREAMER)

    HEARTBEAT_QUERY = "SELECT 1"

    def __init__(
        self,
        conninfo,
        immediate_checkpoint=False,
        slot_name=None,
        application_name="barman",
    ):
        """
        PostgreSQL connection constructor.

        :param str conninfo: Connection information (aka DSN)
        :param bool immediate_checkpoint: Whether to do an immediate checkpoint
           when start a backup
        :param str|None slot_name: Replication slot name
        """
        super(PostgreSQLConnection, self).__init__(conninfo)
        self.immediate_checkpoint = immediate_checkpoint
        self.slot_name = slot_name
        self.application_name = application_name
        self.configuration_files = None

    def connect(self):
        """
        Connect to the PostgreSQL server. It reuses an existing connection.
        """
        if self._check_connection():
            return self._conn

        self._conn = super(PostgreSQLConnection, self).connect()
        if "application_name" not in self.conn_parameters:
            try:
                cur = self._conn.cursor()
                # Do not use parameter substitution with SET
                cur.execute("SET application_name TO %s" % self.application_name)
                cur.close()
            # If psycopg2 fails to set the application name,
            # raise the appropriate exception
            except psycopg2.ProgrammingError as e:
                raise PostgresAppNameError(force_str(e).strip())
        return self._conn

    @property
    def has_connection(self):
        """Checks if the Postgres connection has already been set"""
        return True if self._conn is not None else False

    @property
    def server_txt_version(self):
        """
        Human readable version of PostgreSQL (returned by the server).

        Note: The return value of this function is used when composing include
        patterns which are passed to rsync when copying tablespaces. If the
        value does not exactly match the PostgreSQL version then Barman may
        fail to copy tablespace files during a backup.
        """
        try:
            cur = self._cursor()
            cur.execute("SELECT version()")
            version_string = cur.fetchone()[0]
            platform, version = version_string.split()[:2]
            # EPAS <= 10 will return a version string which starts with
            # EnterpriseDB followed by the PostgreSQL version with an
            # additional version field. This additional field must be discarded
            # so that we return the exact PostgreSQL version. Later versions of
            # EPAS report the PostgreSQL version directly so do not need
            # special handling.
            if platform == "EnterpriseDB":
                return ".".join(version.split(".")[:-1])
            else:
                return version
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving PostgreSQL version: %s", force_str(e).strip()
            )
            return None

    @property
    def is_in_recovery(self):
        """
        Returns true if PostgreSQL server is in recovery mode (hot standby)
        """
        try:
            cur = self._cursor()
            cur.execute("SELECT pg_is_in_recovery()")
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error calling pg_is_in_recovery() function: %s", force_str(e).strip()
            )
            return None

    @property
    def is_superuser(self):
        """
        Returns true if current user has superuser privileges
        """
        try:
            cur = self._cursor()
            cur.execute("SELECT usesuper FROM pg_user WHERE usename = CURRENT_USER")
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error calling is_superuser() function: %s", force_str(e).strip()
            )
            return None

    @property
    def has_backup_privileges(self):
        """
        Returns true if current user is superuser or, for PostgreSQL 10
        or above, is a standard user that has grants to read server
        settings and to execute all the functions needed for
        exclusive/concurrent backup control and WAL control.
        """
        # pg_monitor / pg_read_all_settings only available from v10
        if self.server_version < 100000:
            return self.is_superuser

        stop_fun_check = ""
        if self.server_version < 150000:
            pg_backup_start_args = "text,bool,bool"
            pg_backup_stop_args = "bool,bool"
            stop_fun_check = (
                "has_function_privilege("
                "CURRENT_USER, '{pg_backup_stop}()', 'EXECUTE') OR "
            ).format(**self.name_map)
        else:
            pg_backup_start_args = "text,bool"
            pg_backup_stop_args = "bool"

        start_fun_check = (
            "has_function_privilege("
            "CURRENT_USER, '{pg_backup_start}({pg_backup_start_args})', 'EXECUTE')"
        ).format(pg_backup_start_args=pg_backup_start_args, **self.name_map)
        stop_fun_check += (
            "has_function_privilege(CURRENT_USER, "
            "'{pg_backup_stop}({pg_backup_stop_args})', 'EXECUTE')"
        ).format(pg_backup_stop_args=pg_backup_stop_args, **self.name_map)

        backup_check_query = """
        SELECT
          usesuper
          OR
          (
            (
              pg_has_role(CURRENT_USER, 'pg_monitor', 'USAGE')
              OR
              (
                pg_has_role(CURRENT_USER, 'pg_read_all_settings', 'USAGE')
                AND pg_has_role(CURRENT_USER, 'pg_read_all_stats', 'USAGE')
              )
            )
            AND
            (
                {start_fun_check}
            )
            AND
            (
                {stop_fun_check}
            )
            AND has_function_privilege(
              CURRENT_USER, 'pg_switch_wal()', 'EXECUTE')
            AND has_function_privilege(
              CURRENT_USER, 'pg_create_restore_point(text)', 'EXECUTE')
          )
        FROM
          pg_user
        WHERE
          usename = CURRENT_USER
        """.format(
            start_fun_check=start_fun_check,
            stop_fun_check=stop_fun_check,
            **self.name_map
        )
        try:
            cur = self._cursor()
            cur.execute(backup_check_query)
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error checking privileges for functions needed for backups: %s",
                force_str(e).strip(),
            )
            return None

    @property
    def has_checkpoint_privileges(self):
        """
        Returns true if the current user is a superuser or if,
        for PostgreSQL 14 and above, the user has the "pg_checkpoint" role.
        """

        if self.server_version < 140000:
            return self.is_superuser

        if self.is_superuser:
            return True
        else:
            role_check_query = (
                "select pg_has_role(CURRENT_USER ,'pg_checkpoint', 'USAGE');"
            )
            try:
                cur = self._cursor()
                cur.execute(role_check_query)
                return cur.fetchone()[0]
            except (PostgresConnectionError, psycopg2.Error) as e:
                _logger.warning(
                    "Error checking privileges for functions needed for creating checkpoints: %s",
                    force_str(e).strip(),
                )
                return None

    @property
    def has_monitoring_privileges(self):
        """
        Check whether the current user can access monitoring information.

        Returns ``True`` if the current user is a superuser or if the user has the
        necessary privileges to monitor system status.

        :rtype: bool
        :return: ``True`` if the current user can access monitoring information.
        """
        if self.is_superuser:
            return True
        else:
            monitoring_check_query = """
            SELECT
            (
                pg_has_role(CURRENT_USER, 'pg_monitor', 'USAGE')
                OR
                (
                    pg_has_role(CURRENT_USER, 'pg_read_all_settings', 'USAGE')
                    AND pg_has_role(CURRENT_USER, 'pg_read_all_stats', 'USAGE')
                )
            )
            """
            try:
                cur = self._cursor()
                cur.execute(monitoring_check_query)
                return cur.fetchone()[0]
            except (PostgresConnectionError, psycopg2.Error) as e:
                _logger.debug(
                    "Error checking privileges for functions needed for monitoring: %s",
                    force_str(e).strip(),
                )
                return None

    @property
    def current_xlog_info(self):
        """
        Get detailed information about the current WAL position in PostgreSQL.

        This method returns a dictionary containing the following data:

         * location
         * file_name
         * file_offset
         * timestamp

        When executed on a standby server file_name and file_offset are always
        None

        :rtype: psycopg2.extras.DictRow
        """
        try:
            cur = self._cursor(cursor_factory=DictCursor)
            if not self.is_in_recovery:
                cur.execute(
                    "SELECT location, "
                    "({pg_walfile_name_offset}(location)).*, "
                    "CURRENT_TIMESTAMP AS timestamp "
                    "FROM {pg_current_wal_lsn}() AS location".format(**self.name_map)
                )
                return cur.fetchone()
            else:
                cur.execute(
                    "SELECT location, "
                    "NULL AS file_name, "
                    "NULL AS file_offset, "
                    "CURRENT_TIMESTAMP AS timestamp "
                    "FROM {pg_last_wal_replay_lsn}() AS location".format(
                        **self.name_map
                    )
                )
                return cur.fetchone()
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving current xlog detailed information: %s",
                force_str(e).strip(),
            )
        return None

    @property
    def current_xlog_file_name(self):
        """
        Get current WAL file from PostgreSQL

        :return str: current WAL file in PostgreSQL
        """
        current_xlog_info = self.current_xlog_info
        if current_xlog_info is not None:
            return current_xlog_info["file_name"]
        return None

    @property
    def xlog_segment_size(self):
        """
        Retrieve the size of one WAL file.

        In PostgreSQL 11, users will be able to change the WAL size
        at runtime. Up to PostgreSQL 10, included, the WAL size can be changed
        at compile time

        :return: The wal size (In bytes)
        """

        try:
            cur = self._cursor(cursor_factory=DictCursor)
            # We can't use the `get_setting` method here, because it
            # use `SHOW`, returning an human readable value such as "16MB",
            # while we prefer a raw value such as 16777216.
            cur.execute("SELECT setting FROM pg_settings WHERE name='wal_segment_size'")
            result = cur.fetchone()
            wal_segment_size = int(result[0])

            # Prior to PostgreSQL 11, the wal segment size is returned in
            # blocks
            if self.server_version < 110000:
                cur.execute(
                    "SELECT setting FROM pg_settings WHERE name='wal_block_size'"
                )
                result = cur.fetchone()
                wal_block_size = int(result[0])

                wal_segment_size *= wal_block_size

            return wal_segment_size
        except ValueError as e:
            _logger.error(
                "Error retrieving current xlog segment size: %s",
                force_str(e).strip(),
            )
            return None

    @property
    def current_xlog_location(self):
        """
        Get current WAL location from PostgreSQL

        :return str: current WAL location in PostgreSQL
        """
        current_xlog_info = self.current_xlog_info
        if current_xlog_info is not None:
            return current_xlog_info["location"]
        return None

    @property
    def current_size(self):
        """
        Returns the total size of the PostgreSQL server
        (requires superuser or pg_read_all_stats)
        """
        if not self.has_backup_privileges:
            return None

        try:
            cur = self._cursor()
            cur.execute("SELECT sum(pg_tablespace_size(oid)) FROM pg_tablespace")
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving PostgreSQL total size: %s", force_str(e).strip()
            )
            return None

    @property
    def archive_timeout(self):
        """
        Retrieve the archive_timeout setting in PostgreSQL

        :return: The archive timeout (in seconds)
        """
        try:
            cur = self._cursor(cursor_factory=DictCursor)
            # We can't use the `get_setting` method here, because it
            # uses `SHOW`, returning an human readable value such as "5min",
            # while we prefer a raw value such as 300.
            cur.execute("SELECT setting FROM pg_settings WHERE name='archive_timeout'")
            result = cur.fetchone()
            archive_timeout = int(result[0])

            return archive_timeout
        except ValueError as e:
            _logger.error("Error retrieving archive_timeout: %s", force_str(e).strip())
            return None

    @property
    def checkpoint_timeout(self):
        """
        Retrieve the checkpoint_timeout setting in PostgreSQL

        :return: The checkpoint timeout (in seconds)
        """
        try:
            cur = self._cursor(cursor_factory=DictCursor)
            # We can't use the `get_setting` method here, because it
            # uses `SHOW`, returning an human readable value such as "5min",
            # while we prefer a raw value such as 300.
            cur.execute(
                "SELECT setting FROM pg_settings WHERE name='checkpoint_timeout'"
            )
            result = cur.fetchone()
            checkpoint_timeout = int(result[0])

            return checkpoint_timeout
        except ValueError as e:
            _logger.error(
                "Error retrieving checkpoint_timeout: %s", force_str(e).strip()
            )
            return None

    def get_archiver_stats(self):
        """
        This method gathers statistics from pg_stat_archiver.
        Only for Postgres 9.4+ or greater. If not available, returns None.

        :return dict|None: a dictionary containing Postgres statistics from
            pg_stat_archiver or None
        """
        try:
            cur = self._cursor(cursor_factory=DictCursor)
            # Select from pg_stat_archiver statistics view,
            # retrieving statistics about WAL archiver process activity,
            # also evaluating if the server is archiving without issues
            # and the archived WALs per second rate.
            #
            # We are using current_settings to check for archive_mode=always.
            # current_setting does normalise its output so we can just
            # check for 'always' settings using a direct string
            # comparison
            cur.execute(
                "SELECT *, "
                "current_setting('archive_mode') IN ('on', 'always') "
                "AND (last_failed_wal IS NULL "
                "OR last_failed_wal LIKE '%.history' "
                "AND substring(last_failed_wal from 1 for 8) "
                "<= substring(last_archived_wal from 1 for 8) "
                "OR last_failed_time <= last_archived_time) "
                "AS is_archiving, "
                "CAST (archived_count AS NUMERIC) "
                "/ EXTRACT (EPOCH FROM age(now(), stats_reset)) "
                "AS current_archived_wals_per_second "
                "FROM pg_stat_archiver"
            )
            return cur.fetchone()
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving pg_stat_archive data: %s", force_str(e).strip()
            )
            return None

    def fetch_remote_status(self):
        """
        Get the status of the PostgreSQL server

        This method does not raise any exception in case of errors,
        but set the missing values to None in the resulting dictionary.

        :rtype: dict[str, None|str]
        """
        # PostgreSQL settings to get from the server (requiring superuser)
        pg_superuser_settings = ["data_directory"]
        # PostgreSQL settings to get from the server
        pg_settings = []
        pg_query_keys = [
            "server_txt_version",
            "is_superuser",
            "is_in_recovery",
            "current_xlog",
            "replication_slot_support",
            "replication_slot",
            "synchronous_standby_names",
            "postgres_systemid",
            "version_supported",
        ]
        # Initialise the result dictionary setting all the values to None
        result = dict.fromkeys(
            pg_superuser_settings + pg_settings + pg_query_keys, None
        )
        try:
            # Retrieve wal_level, hot_standby and max_wal_senders
            # only if version is >= 9.0
            pg_settings.extend(
                [
                    "wal_level",
                    "hot_standby",
                    "max_wal_senders",
                    "data_checksums",
                    "max_replication_slots",
                    "wal_compression",
                ]
            )
            # Retrieve wal_keep_segments from version 9.0 onwards, until
            # version 13.0, where it was renamed to wal_keep_size
            if self.server_version < 130000:
                pg_settings.append("wal_keep_segments")
            else:
                pg_settings.append("wal_keep_size")

            # retrieves superuser settings
            if self.has_backup_privileges:
                for name in pg_superuser_settings:
                    result[name] = self.get_setting(name)

            # retrieves standard settings
            for name in pg_settings:
                result[name] = self.get_setting(name)

            result["is_superuser"] = self.is_superuser
            result["has_backup_privileges"] = self.has_backup_privileges
            result["has_monitoring_privileges"] = self.has_monitoring_privileges
            result["is_in_recovery"] = self.is_in_recovery
            result["server_txt_version"] = self.server_txt_version
            result["version_supported"] = self.is_minimal_postgres_version()
            current_xlog_info = self.current_xlog_info
            if current_xlog_info:
                result["current_lsn"] = current_xlog_info["location"]
                result["current_xlog"] = current_xlog_info["file_name"]
            else:
                result["current_lsn"] = None
                result["current_xlog"] = None
            result["current_size"] = self.current_size
            result["archive_timeout"] = self.archive_timeout
            result["checkpoint_timeout"] = self.checkpoint_timeout
            result["xlog_segment_size"] = self.xlog_segment_size

            result.update(self.get_configuration_files())

            # Retrieve the replication_slot status
            result["replication_slot_support"] = True
            if self.slot_name is not None:
                result["replication_slot"] = self.get_replication_slot(self.slot_name)

            # Retrieve the list of synchronous standby names
            result["synchronous_standby_names"] = self.get_synchronous_standby_names()
            result["postgres_systemid"] = self.get_systemid()
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.warning(
                "Error retrieving PostgreSQL status: %s", force_str(e).strip()
            )
        return result

    def get_systemid(self):
        """
        Get a Postgres instance systemid
        """
        try:
            cur = self._cursor()
            cur.execute("SELECT system_identifier::text FROM pg_control_system()")
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving PostgreSQL system Id: %s", force_str(e).strip()
            )
            return None

    def get_setting(self, name):
        """
        Get a Postgres setting with a given name

        :param name: a parameter name
        """
        try:
            cur = self._cursor()
            cur.execute('SHOW "%s"' % name.replace('"', '""'))
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving PostgreSQL setting '%s': %s",
                name.replace('"', '""'),
                force_str(e).strip(),
            )
            return None

    def get_tablespaces(self):
        """
        Returns a list of tablespaces or None if not present
        """
        try:
            cur = self._cursor()

            cur.execute(
                "SELECT spcname, oid, "
                "pg_tablespace_location(oid) AS spclocation "
                "FROM pg_tablespace "
                "WHERE pg_tablespace_location(oid) != ''"
            )

            # Generate a list of tablespace objects
            return [Tablespace._make(item) for item in cur.fetchall()]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving PostgreSQL tablespaces: %s", force_str(e).strip()
            )
            return None

    def get_configuration_files(self):
        """
        Get postgres configuration files or an empty dictionary
        in case of error

        :rtype: dict
        """
        if self.configuration_files:
            return self.configuration_files
        try:
            self.configuration_files = {}
            cur = self._cursor()
            cur.execute(
                "SELECT name, setting FROM pg_settings "
                "WHERE name IN ('config_file', 'hba_file', 'ident_file')"
            )
            for cname, cpath in cur.fetchall():
                self.configuration_files[cname] = cpath

            # Retrieve additional configuration files

            cur.execute(
                "SELECT DISTINCT sourcefile AS included_file "
                "FROM pg_settings "
                "WHERE sourcefile IS NOT NULL "
                "AND sourcefile NOT IN "
                "(SELECT setting FROM pg_settings "
                "WHERE name = 'config_file') "
                "ORDER BY 1"
            )
            # Extract the values from the containing single element tuples
            included_files = [included_file for included_file, in cur.fetchall()]
            if len(included_files) > 0:
                self.configuration_files["included_files"] = included_files

        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving PostgreSQL configuration files location: %s",
                force_str(e).strip(),
            )
            self.configuration_files = {}

        return self.configuration_files

    def create_restore_point(self, target_name):
        """
        Create a restore point with the given target name

        The method executes the pg_create_restore_point() function through
        a PostgreSQL connection. Only for Postgres versions >= 9.1 when not
        in replication.

        If requirements are not met, the operation is skipped.

        :param str target_name: name of the restore point

        :returns: the restore point LSN
        :rtype: str|None
        """
        # Not possible if on a standby
        # Called inside the pg_connect context to reuse the connection
        if self.is_in_recovery:
            return None

        try:
            cur = self._cursor()
            cur.execute("SELECT pg_create_restore_point(%s)", [target_name])
            _logger.info("Restore point '%s' successfully created", target_name)
            return cur.fetchone()[0]
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error issuing pg_create_restore_point() command: %s",
                force_str(e).strip(),
            )
            return None

    def start_exclusive_backup(self, label):
        """
        Calls pg_backup_start() on the PostgreSQL server

        This method returns a dictionary containing the following data:

         * location
         * file_name
         * file_offset
         * timestamp

        :param str label: descriptive string to identify the backup
        :rtype: psycopg2.extras.DictRow
        """
        try:
            conn = self.connect()

            # Rollback to release the transaction, as the pg_backup_start
            # invocation can last up to PostgreSQL's checkpoint_timeout
            conn.rollback()

            # Start an exclusive backup
            cur = conn.cursor(cursor_factory=DictCursor)
            if self.server_version >= 150000:
                raise PostgresObsoleteFeature("15")
            else:
                cur.execute(
                    "SELECT location, "
                    "({pg_walfile_name_offset}(location)).*, "
                    "now() AS timestamp "
                    "FROM {pg_backup_start}(%s,%s) AS location".format(**self.name_map),
                    (label, self.immediate_checkpoint),
                )

            start_row = cur.fetchone()

            # Rollback to release the transaction, as the connection
            # is to be retained until the end of backup
            conn.rollback()

            return start_row
        except (PostgresConnectionError, psycopg2.Error) as e:
            msg = (
                "{pg_backup_start}(): %s".format(**self.name_map) % force_str(e).strip()
            )
            _logger.debug(msg)
            raise PostgresException(msg)

    def start_concurrent_backup(self, label):
        """
        Calls pg_backup_start on the PostgreSQL server using the
        API introduced with version 9.6

        This method returns a dictionary containing the following data:

         * location
         * timeline
         * timestamp

        :param str label: descriptive string to identify the backup
        :rtype: psycopg2.extras.DictRow
        """
        try:
            conn = self.connect()

            # Rollback to release the transaction, as the pg_backup_start
            # invocation can last up to PostgreSQL's checkpoint_timeout
            conn.rollback()

            # Start the backup using the api introduced in postgres 9.6
            cur = conn.cursor(cursor_factory=DictCursor)
            if self.server_version >= 150000:
                pg_backup_args = "%s, %s"
            else:
                # PostgreSQLs below 15 have a boolean parameter to specify
                # not to use exclusive backup
                pg_backup_args = "%s, %s, FALSE"

            # pg_backup_start and pg_backup_stop need to be run in the
            # same session when taking concurrent backups, so we disable
            # idle_session_timeout to avoid failures when stopping the
            # backup if copy takes more than idle_session_timeout to complete
            if self.server_version >= 140000:
                cur.execute("SET idle_session_timeout TO 0")

            cur.execute(
                "SELECT location, "
                "(SELECT timeline_id "
                "FROM pg_control_checkpoint()) AS timeline, "
                "now() AS timestamp "
                "FROM {pg_backup_start}({pg_backup_args}) AS location".format(
                    pg_backup_args=pg_backup_args, **self.name_map
                ),
                (label, self.immediate_checkpoint),
            )
            start_row = cur.fetchone()

            # Rollback to release the transaction, as the connection
            # is to be retained until the end of backup
            conn.rollback()

            return start_row
        except (PostgresConnectionError, psycopg2.Error) as e:
            msg = "{pg_backup_start} command: %s".format(**self.name_map) % (
                force_str(e).strip(),
            )
            _logger.debug(msg)
            raise PostgresException(msg)

    def stop_exclusive_backup(self):
        """
        Calls pg_backup_stop() on the PostgreSQL server

        This method returns a dictionary containing the following data:

         * location
         * file_name
         * file_offset
         * timestamp

        :rtype: psycopg2.extras.DictRow
        """
        try:
            conn = self.connect()

            # Rollback to release the transaction, as the pg_backup_stop
            # invocation could will wait until the current WAL file is shipped
            conn.rollback()

            # Stop the backup
            cur = conn.cursor(cursor_factory=DictCursor)

            if self.server_version >= 150000:
                raise PostgresObsoleteFeature("15")

            cur.execute(
                "SELECT location, "
                "({pg_walfile_name_offset}(location)).*, "
                "now() AS timestamp "
                "FROM {pg_backup_stop}() AS location".format(**self.name_map)
            )

            return cur.fetchone()
        except (PostgresConnectionError, psycopg2.Error) as e:
            msg = "Error issuing {pg_backup_stop} command: %s" % force_str(e).strip()
            _logger.debug(msg)
            raise PostgresException(
                "Cannot terminate exclusive backup. "
                "You might have to manually execute {pg_backup_stop} "
                "on your PostgreSQL server".format(**self.name_map)
            )

    def stop_concurrent_backup(self):
        """
        Calls pg_backup_stop on the PostgreSQL server using the
        API introduced with version 9.6

        This method returns a dictionary containing the following data:

         * location
         * timeline
         * backup_label
         * timestamp

        :rtype: psycopg2.extras.DictRow
        """
        try:
            conn = self.connect()

            # Rollback to release the transaction, as the pg_backup_stop
            # invocation could will wait until the current WAL file is shipped
            conn.rollback()

            if self.server_version >= 150000:
                # The pg_backup_stop function accepts one argument, a boolean
                # wait_for_archive indicating whether PostgreSQL should wait
                # until all required WALs are archived. This is not set so that
                # we get the default behaviour which is to wait for the wals.
                pg_backup_args = ""
            else:
                # For PostgreSQLs below 15 the function accepts two arguments -
                # a boolean to indicate exclusive or concurrent backup and the
                # wait_for_archive boolean. We set exclusive to FALSE and leave
                # wait_for_archive unset as with PG >= 15.
                pg_backup_args = "FALSE"

            # Stop the backup  using the api introduced with version 9.6
            cur = conn.cursor(cursor_factory=DictCursor)

            # As we are about to run pg_backup_stop we can now reset
            # idle_session_timeout to whatever the user had
            # originally configured in PostgreSQL
            if self.server_version >= 140000:
                cur.execute("RESET idle_session_timeout")

            cur.execute(
                "SELECT end_row.lsn AS location, "
                "(SELECT CASE WHEN pg_is_in_recovery() "
                "THEN min_recovery_end_timeline ELSE timeline_id END "
                "FROM pg_control_checkpoint(), pg_control_recovery()"
                ") AS timeline, "
                "end_row.labelfile AS backup_label, "
                "now() AS timestamp FROM {pg_backup_stop}({pg_backup_args}) AS end_row".format(
                    pg_backup_args=pg_backup_args, **self.name_map
                )
            )

            return cur.fetchone()
        except (PostgresConnectionError, psycopg2.Error) as e:
            msg = (
                "Error issuing {pg_backup_stop} command: %s".format(**self.name_map)
                % force_str(e).strip()
            )
            _logger.debug(msg)
            raise PostgresException(msg)

    def switch_wal(self):
        """
        Execute a pg_switch_wal()

        To be SURE of the switch of a xlog, we collect the xlogfile name
        before and after the switch.
        The method returns the just closed xlog file name if the current xlog
        file has changed, it returns an empty string otherwise.

        The method returns None if something went wrong during the execution
        of the pg_switch_wal command.

        :rtype: str|None
        """
        try:
            conn = self.connect()
            if not self.has_backup_privileges:
                raise BackupFunctionsAccessRequired(
                    "Postgres user '%s' is missing required privileges "
                    '(see "Preliminary steps" in the Barman manual)'
                    % self.conn_parameters.get("user")
                )

            # If this server is in recovery there is nothing to do
            if self.is_in_recovery:
                raise PostgresIsInRecovery()

            cur = conn.cursor()
            # Collect the xlog file name before the switch
            cur.execute(
                "SELECT {pg_walfile_name}("
                "{pg_current_wal_insert_lsn}())".format(**self.name_map)
            )
            pre_switch = cur.fetchone()[0]
            # Switch
            cur.execute(
                "SELECT {pg_walfile_name}({pg_switch_wal}())".format(**self.name_map)
            )
            # Collect the xlog file name after the switch
            cur.execute(
                "SELECT {pg_walfile_name}("
                "{pg_current_wal_insert_lsn}())".format(**self.name_map)
            )
            post_switch = cur.fetchone()[0]
            if pre_switch < post_switch:
                return pre_switch
            else:
                return ""
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error issuing {pg_switch_wal}() command: %s".format(**self.name_map),
                force_str(e).strip(),
            )
            return None

    def checkpoint(self):
        """
        Execute a checkpoint
        """
        try:
            conn = self.connect()

            # Requires superuser privilege
            if not self.has_checkpoint_privileges:
                raise PostgresCheckpointPrivilegesRequired()

            cur = conn.cursor()
            cur.execute("CHECKPOINT")
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug("Error issuing CHECKPOINT: %s", force_str(e).strip())

    def get_replication_stats(self, client_type=STANDBY):
        """
        Returns streaming replication information
        """
        try:
            cur = self._cursor(cursor_factory=NamedTupleCursor)

            if not self.has_monitoring_privileges:
                raise BackupFunctionsAccessRequired(
                    "Postgres user '%s' is missing required privileges "
                    '(see "Preliminary steps" in the Barman manual)'
                    % self.conn_parameters.get("user")
                )

            # pg_stat_replication is a system view that contains one
            # row per WAL sender process with information about the
            # replication status of a standby server. It has been
            # introduced in PostgreSQL 9.1. Current fields are:
            #
            # - pid (procpid in 9.1)
            # - usesysid
            # - usename
            # - application_name
            # - client_addr
            # - client_hostname
            # - client_port
            # - backend_start
            # - backend_xmin (9.4+)
            # - state
            # - sent_lsn (sent_location before 10)
            # - write_lsn (write_location before 10)
            # - flush_lsn (flush_location before 10)
            # - replay_lsn (replay_location before 10)
            # - sync_priority
            # - sync_state
            #

            from_repslot = ""
            where_clauses = []
            if self.server_version >= 100000:
                # Current implementation (10+)
                what = "r.*, rs.slot_name"
                # Look for replication slot name
                from_repslot = (
                    "LEFT JOIN pg_replication_slots rs ON (r.pid = rs.active_pid) "
                )
                where_clauses += ["(rs.slot_type IS NULL OR rs.slot_type = 'physical')"]
            else:
                # PostgreSQL 9.5/9.6
                what = (
                    "pid, "
                    "usesysid, "
                    "usename, "
                    "application_name, "
                    "client_addr, "
                    "client_hostname, "
                    "client_port, "
                    "backend_start, "
                    "backend_xmin, "
                    "state, "
                    "sent_location AS sent_lsn, "
                    "write_location AS write_lsn, "
                    "flush_location AS flush_lsn, "
                    "replay_location AS replay_lsn, "
                    "sync_priority, "
                    "sync_state, "
                    "rs.slot_name"
                )
                # Look for replication slot name
                from_repslot = (
                    "LEFT JOIN pg_replication_slots rs ON (r.pid = rs.active_pid) "
                )
                where_clauses += ["(rs.slot_type IS NULL OR rs.slot_type = 'physical')"]

            # Streaming client
            if client_type == self.STANDBY:
                # Standby server
                where_clauses += ["{replay_lsn} IS NOT NULL".format(**self.name_map)]
            elif client_type == self.WALSTREAMER:
                # WAL streamer
                where_clauses += ["{replay_lsn} IS NULL".format(**self.name_map)]

            if where_clauses:
                where = "WHERE %s " % " AND ".join(where_clauses)
            else:
                where = ""

            # Execute the query
            cur.execute(
                "SELECT %s, "
                "pg_is_in_recovery() AS is_in_recovery, "
                "CASE WHEN pg_is_in_recovery() "
                "  THEN {pg_last_wal_receive_lsn}() "
                "  ELSE {pg_current_wal_lsn}() "
                "END AS current_lsn "
                "FROM pg_stat_replication r "
                "%s"
                "%s"
                "ORDER BY sync_state DESC, sync_priority".format(**self.name_map)
                % (what, from_repslot, where)
            )

            # Generate a list of standby objects
            return cur.fetchall()
        except (PostgresConnectionError, psycopg2.Error) as e:
            _logger.debug(
                "Error retrieving status of standby servers: %s", force_str(e).strip()
            )
            return None

    def get_replication_slot(self, slot_name):
        """
        Retrieve from the PostgreSQL server a physical replication slot
        with a specific slot_name.

        This method returns a dictionary containing the following data:

         * slot_name
         * active
         * restart_lsn

        :param str slot_name: the replication slot name
        :rtype: psycopg2.extras.DictRow
        """
        if self.server_version < 90400:
            # Raise exception if replication slot are not supported
            # by PostgreSQL version
            raise PostgresUnsupportedFeature("9.4")
        else:
            cur = self._cursor(cursor_factory=NamedTupleCursor)
            try:
                cur.execute(
                    "SELECT slot_name, "
                    "active, "
                    "restart_lsn "
                    "FROM pg_replication_slots "
                    "WHERE slot_type = 'physical' "
                    "AND slot_name = '%s'" % slot_name
                )
                # Retrieve the replication slot information
                return cur.fetchone()
            except (PostgresConnectionError, psycopg2.Error) as e:
                _logger.debug(
                    "Error retrieving replication_slots: %s", force_str(e).strip()
                )
                raise

    def get_synchronous_standby_names(self):
        """
        Retrieve the list of named synchronous standby servers from PostgreSQL

        This method returns a list of names

        :return list: synchronous standby names
        """
        if self.server_version < 90100:
            # Raise exception if synchronous replication is not supported
            raise PostgresUnsupportedFeature("9.1")
        else:
            synchronous_standby_names = self.get_setting("synchronous_standby_names")
            # Return empty list if not defined
            if synchronous_standby_names is None:
                return []
            # Normalise the list of sync standby names
            # On PostgreSQL 9.6 it is possible to specify the number of
            # required synchronous standby using this format:
            # n (name1, name2, ... nameN).
            # We only need the name list, so we discard everything else.

            # The name list starts after the first parenthesis or at pos 0
            names_start = synchronous_standby_names.find("(") + 1
            names_end = synchronous_standby_names.rfind(")")
            if names_end < 0:
                names_end = len(synchronous_standby_names)
            names_list = synchronous_standby_names[names_start:names_end]
            # We can blindly strip double quotes because PostgreSQL enforces
            # the format of the synchronous_standby_names content
            return [x.strip().strip('"') for x in names_list.split(",")]

    def send_heartbeat_query(self):
        """
        Sends a heartbeat query to the server with the already opened connection.

        :returns tuple[bool, Exception|None]: A tuple where the first value is a boolean
            indicating if the query executed successfully or not and the second is the
            exception raised by ``psycopg2`` in case it did not succeed.
        """
        try:
            with self._conn.cursor() as cursor:
                cursor.execute(self.HEARTBEAT_QUERY)
                _logger.debug("Sent heartbeat query to maintain the current connection")
            return True, None
        except psycopg2.Error as ex:
            _logger.debug(
                "Failed to execute heartbeat query on the current connection: %s"
                % force_str(ex)
            )
            return False, ex

    @property
    def name_map(self):
        """
        Return a map with function and directory names according to the current
        PostgreSQL version.

        Each entry has the `current` name as key and the name for the specific
        version as value.

        :rtype: dict[str]
        """

        # Avoid raising an error if the connection is not available
        try:
            server_version = self.server_version
        except PostgresConnectionError:
            _logger.debug(
                "Impossible to detect the PostgreSQL version, "
                "name_map will return names from latest version"
            )
            server_version = None

        return function_name_map(server_version)


class StandbyPostgreSQLConnection(PostgreSQLConnection):
    """
    A specialised PostgreSQLConnection for standby servers.

    Works almost exactly like a regular PostgreSQLConnection except it requires a
    primary_conninfo option at creation time which is used to create a connection
    to the primary for the purposes of forcing a WAL switch during the stop backup
    process.

    This increases the likelihood that backups against standbys with
    `archive_mode = always` and low traffic on the primary are able to complete.
    """

    def __init__(
        self,
        conninfo,
        primary_conninfo,
        immediate_checkpoint=False,
        slot_name=None,
        primary_checkpoint_timeout=0,
        application_name="barman",
    ):
        """
        Standby PostgreSQL connection constructor.

        :param str conninfo: Connection information (aka DSN) for the standby.
        :param str primary_conninfo: Connection information (aka DSN) for the
            primary.
        :param bool immediate_checkpoint: Whether to do an immediate checkpoint
            when a backup is started.
        :param str|None slot_name: Replication slot name.
        :param str: The application_name to use for this connection.
        """
        super(StandbyPostgreSQLConnection, self).__init__(
            conninfo,
            immediate_checkpoint=immediate_checkpoint,
            slot_name=slot_name,
            application_name=application_name,
        )
        # The standby connection has its own connection object used to talk to the
        # primary when switching WALs.
        self.primary_conninfo = primary_conninfo
        # The standby needs a connection to the primary so that it can
        # perform WAL switches itself when calling pg_backup_stop.
        self.primary = PostgreSQLConnection(self.primary_conninfo)
        self.primary_checkpoint_timeout = primary_checkpoint_timeout

    def close(self):
        """Close the connection to PostgreSQL."""
        super(StandbyPostgreSQLConnection, self).close()
        return self.primary.close()

    def switch_wal(self):
        """Perform a WAL switch on the primary PostgreSQL instance."""
        # Instead of calling the superclass switch_wal, which would invoke
        # pg_switch_wal on the standby, we use our connection to the primary to
        # switch the WAL directly.
        return self.primary.switch_wal()

    def switch_wal_in_background(self, done_q, times=10, wait=10):
        """
        Perform a pg_switch_wal in a background process.

        This function runs in a child process and is intended to keep calling
        pg_switch_wal() until it is told to stop or until `times` is exceeded.
        The parent process will use `done_q` to tell this process to stop.

        :param multiprocessing.Queue done_q: A Queue used by the parent process to
            communicate with the WAL switching process. A value of `True` on this
            queue indicates that this function should stop.
        :param int times: The maximum number of times a WAL switch should be
            performed.
        :param int wait: The number of seconds to wait between WAL switches.
        """
        # Use a new connection to prevent undefined behaviour
        self.primary = PostgreSQLConnection(self.primary_conninfo)

        # The stop backup call on the standby may have already completed by this
        # point so check whether we have been told to stop.
        try:
            if done_q.get(timeout=1):
                return
        except Empty:
            pass
        try:
            # Start calling pg_switch_wal on the primary until we either read something
            # from the done queue or we exceed the number of WAL switches we are allowed.
            for _ in range(0, times):
                self.switch_wal()
                # See if we have been told to stop. We use the wait value as our timeout
                # so that we can exit immediately if we receive a stop message or proceed
                # to another WAL switch if the wait time is exceeded.
                try:
                    if done_q.get(timeout=wait):
                        return
                except Empty:
                    # An empty queue just means we haven't yet been told to stop
                    pass
            if self.primary_checkpoint_timeout:
                _logger.warning(
                    "Barman attempted to switch WALs %s times on the primary "
                    "server, but the backup has not yet completed. "
                    "A checkpoint will be forced on the primary server "
                    "in %s seconds to ensure the backup can complete."
                    % (times, self.primary_checkpoint_timeout)
                )
                sleep_time = datetime.datetime.now() + datetime.timedelta(
                    seconds=self.primary_checkpoint_timeout
                )
                while True:
                    try:
                        # Always check if the queue is empty, so we know to stop
                        # before the checkpoint execution
                        if done_q.get(timeout=wait):
                            return
                    except Empty:
                        # If the queue is empty, we can proceed to the checkpoint
                        # if enough time has passed
                        if sleep_time < datetime.datetime.now():
                            self.primary.checkpoint()
                            self.primary.switch_wal()
                            break
                            # break out of the loop after the checkpoint and wal switch
                            # execution. The connection will be closed in the finally statement
        finally:
            # Close the connection since only this subprocess will ever use it
            self.primary.close()

    def _start_wal_switch(self):
        """Start switching WALs in a child process."""
        # The child process will stop if it reads a value of `True` from this queue.
        self.done_q = Queue()

        # Create and start the child process before we stop the backup.
        self.switch_wal_proc = Process(
            target=self.switch_wal_in_background, args=(self.done_q,)
        )
        self.switch_wal_proc.start()

    def _stop_wal_switch(self):
        """Stop the WAL switching process."""
        # Stop the child process by adding a `True` to its queue
        self.done_q.put(True)

        # Make sure the child process closes before we return.
        self.switch_wal_proc.join()

    def _stop_backup(self, stop_backup_fun):
        """
        Stop a backup while also calling pg_switch_wal().

        Starts a child process to call pg_switch_wal() on the primary before attempting
        to stop the backup on the standby. The WAL switch is intended to allow the
        pg_backup_stop call to complete when running against a standby with
        `archive_mode = always`. Once the call to `stop_concurrent_backup` completes
        the child process is stopped as no further WAL switches are required.

        :param function stop_backup_fun: The function which should be called to stop
            the backup. This will be a reference to one of the superclass methods
            stop_concurrent_backup or stop_exclusive_backup.
        :rtype: psycopg2.extras.DictRow
        """
        self._start_wal_switch()
        stop_info = stop_backup_fun()
        self._stop_wal_switch()
        return stop_info

    def stop_concurrent_backup(self):
        """
        Stop a concurrent backup on a standby PostgreSQL instance.

        :rtype: psycopg2.extras.DictRow
        """
        return self._stop_backup(
            super(StandbyPostgreSQLConnection, self).stop_concurrent_backup
        )

    def stop_exclusive_backup(self):
        """
        Stop an exclusive backup on a standby PostgreSQL instance.

        :rtype: psycopg2.extras.DictRow
        """
        return self._stop_backup(
            super(StandbyPostgreSQLConnection, self).stop_exclusive_backup
        )


class PostgresKeepAlive:
    """
    Context manager to maintain a Postgres connection alive.

    A child thread is spawned to execute heartbeat queries in the background
    at a specified interval during its living context.

    It does not open or close any connections on its own. Instead, it waits for the
    specified connection to be opened on the main thread before start sending any query.

    :cvar THREAD_NAME: The name identifying the keep-alive thread.
    """

    THREAD_NAME = "barman_keepalive_thread"

    def __init__(self, postgres, interval, raise_exception=False):
        """
        Constructor.

        :param barman.postgres.PostgreSQLConnection postgres: The
            Postgres connection to keep alive.
        :param int interval: An interval in seconds at which a
            heartbeat query will be sent to keep the connection alive.
            A value <= ``0`` won't start the keepalive.
        :param bool raise_exception: A boolean indicating if an exception
            should be raised in case the connection is lost. If ``True``, a
            ``PostgresConnectionLost`` exception will be raised as soon as
            it's noticed a connection failure. If ``False``, it will keep executing
            normally until the context exits.
        """
        self.postgres = postgres
        self.interval = interval
        self.raise_exception = raise_exception
        self._stop_thread = threading.Event()
        self._thread = threading.Thread(
            target=self._run_keep_alive,
            name=self.THREAD_NAME,
        )

    def _prepare_signal_handler(self):
        """
        Set up a signal handler to raise an exception on the main thread when
        the keep-alive thread wishes to interrupt it. This method listens for a
        ``SIGUSR1`` signal and, when received, raises a ``PostgresConnectionLost``
        exception.

        .. note::
            This code is, and only works if, executed while on the main thread. We
            are not able to set a signal listener on a child thread, therefore this
            method must be executed before the keep-alive thread starts.
        """

        def raise_exception(signum, frame):
            raise PostgresConnectionLost("Connection to Postgres server was lost.")

        signal.signal(signal.SIGUSR1, raise_exception)

    def _raise_exception_on_main(self):
        """
        Trigger an exception on the main thread at whatever frame is being executed
        at the moment. This is done by sending a ``SIGUSR1`` signal to the process,
        which will be caught by the signal handler set previously in this class.

        .. note::
            This is an alternative way of interrupting the main thread's work, since
            there is no direct way of killing or raising exceptions on the main thread
            from a child thread in Python. A handler for this signal has been set
            beforehand by the ``_prepare_signal_handler`` method in this class.
        """
        os.kill(os.getpid(), signal.SIGUSR1)

    def _run_keep_alive(self):
        """Runs the keepalive until a stop-thread event is set"""
        while not self._stop_thread.is_set():
            if not self.postgres.has_connection:
                # Wait for the connection to be opened on the main thread
                time.sleep(1)
                continue

            success, ex = self.postgres.send_heartbeat_query()

            if not success and self.raise_exception:
                # If one of the below exeptions was raised by psycopg2, it most likely
                # means that the connection (and consequently, the session) was lost. In
                # such cases, we can stop the keep-alive exection and raise the exception
                if isinstance(ex, (psycopg2.InterfaceError, psycopg2.OperationalError)):
                    self._stop_thread.set()
                    self._raise_exception_on_main()

            self._stop_thread.wait(self.interval)

    def __enter__(self):
        """Enters context. Starts the thread"""
        if self.interval > 0:
            if self.raise_exception:
                self._prepare_signal_handler()
            self._thread.start()

    def __exit__(self, exc_type, exc_val, exc_tb):
        """Exits context. Makes sure the thread is terminated"""
        if self.interval > 0:
            self._stop_thread.set()
            self._thread.join()
