db/db.py

#! /usr/bin/python

#  Copyright 2014-2015 Linaro Limited
#  Authors: Dave Pigott <dave.pigott@linaro.org>,
#           Steve McIntyre <steve.mcintyre@linaro.org>
#
#  This program is free software; you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#  You should have received a copy of the GNU General Public License
#  along with this program; if not, write to the Free Software
#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
#  MA 02110-1301, USA.

import psycopg2
import psycopg2.extras
import datetime, os, sys
import logging

TRUNK_ID_NONE = -1

if __name__ == '__main__':
    vlandpath = os.path.abspath(os.path.normpath(os.path.dirname(sys.argv[0])))
    sys.path.insert(0, vlandpath)
    sys.path.insert(0, "%s/.." % vlandpath)

from errors import CriticalError, InputError

class VlanDB:
    def __init__(self, db_name="vland", username="vland"):
        try:
            self.connection = psycopg2.connect(database=db_name, user=username)
            self.cursor = self.connection.cursor(cursor_factory=psycopg2.extras.NamedTupleCursor)
        except Exception as e:
            logging.error("Failed to access database: %s", e)
            raise

    def __del__(self):
        self.cursor.close()
        self.connection.close()

    # Create a new switch in the database. Switches are really simple
    # devices - they're just containers for ports.
    #
    # Constraints:
    # Switches must be uniquely named
    def create_switch(self, name):

        switch_id = self.get_switch_id_by_name(name)
        if switch_id is not None:
            raise InputError("Switch name %s already exists" % name)

        try:
            sql = "INSERT INTO switch (name) VALUES (%s) RETURNING switch_id"
            data = (name, )
            self.cursor.execute(sql, data)
            switch_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise

        return switch_id

    # Create a new port in the database. Three of the fields are
    # created with default values (is_locked, is_trunk, trunk_id)
    # here, and should be updated separately if desired. For the
    # current_vlan_id and base_vlan_id fields, *BE CAREFUL* that you
    # have already looked up the correct VLAN_ID for each. This is
    # *NOT* the same as the VLAN tag (likely to be 1). You Have Been
    # Warned!
    #
    # Constraints:
    # 1. The switch referred to must already exist
    # 2. The VLANs mentioned here must already exist
    # 3. (Switch/name) must be unique
    # 4. (Switch/number) must be unique
    def create_port(self, switch_id, name, number, current_vlan_id, base_vlan_id):

        switch = self.get_switch_by_id(switch_id)
        if switch is None:
            raise InputError("Switch ID %d does not exist" % int(switch_id))

        for vlan_id in (current_vlan_id, base_vlan_id):
            vlan = self.get_vlan_by_id(vlan_id)
            if vlan is None:
                raise InputError("VLAN ID %d does not exist" % int(vlan_id))

        port_id = self.get_port_by_switch_and_name(switch_id, name)
        if port_id is not None:
            raise InputError("Already have a port %s on switch ID %d" % (name, int(switch_id)))

        port_id = self.get_port_by_switch_and_number(switch_id, int(number))
        if port_id is not None:
            raise InputError("Already have a port %d on switch ID %d" % (int(number), int(switch_id)))

        try:
            sql = "INSERT INTO port (name, number, switch_id, is_locked, is_trunk, current_vlan_id, base_vlan_id, trunk_id) VALUES (%s, %s, %s, %s, %s, %s, %s, %s) RETURNING port_id"
            data = (name, number, switch_id,
                    False, False,
                    current_vlan_id, base_vlan_id, TRUNK_ID_NONE)
            self.cursor.execute(sql, data)
            port_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise

        return port_id

    # Create a new vlan in the database. We locally add a creation
    # timestamp, for debug purposes. If vlans seems to be sticking
    # around, we'll be able to see when they were created.
    #
    # Constraints:
    # Names and tags must be unique
    # Tags must be in the range 1-4095 (802.1q spec)
    # Names can be any free-form text, length 1-32 characters
    def create_vlan(self, name, tag, is_base_vlan):

        if int(tag) < 1 or int(tag) > 4095:
            raise InputError("VLAN tag %d is outside of the valid range (1-4095)" % int(tag))

        if (len(name) < 1) or (len(name) > 32):
            raise InputError("VLAN name %s is invalid (must be 1-32 chars)" % name)

        vlan_id = self.get_vlan_id_by_name(name)
        if vlan_id is not None:
            raise InputError("VLAN name %s is already in use" % name)

        vlan_id = self.get_vlan_id_by_tag(tag)
        if vlan_id is not None:
            raise InputError("VLAN tag %d is already in use" % int(tag))

        try:
            dt = datetime.datetime.now()
            sql = "INSERT INTO vlan (name, tag, is_base_vlan, creation_time) VALUES (%s, %s, %s, %s) RETURNING vlan_id"
            data = (name, tag, is_base_vlan, dt)
            self.cursor.execute(sql, data)
            vlan_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise

        return vlan_id

    # Create a new trunk in the database, linking two ports. Trunks
    # are really simple objects for our use - they're just containers
    # for 2 ports.
    #
    # Constraints:
    # 1. Both ports listed must already exist.
    # 2. Both ports must be in trunk mode.
    # 3. Both must not be locked.
    # 4. Both must not already be in a trunk.
    def create_trunk(self, port_id1, port_id2):

        for port_id in (port_id1, port_id2):
            port = self.get_port_by_id(int(port_id))
            if port is None:
                raise InputError("Port ID %d does not exist" % int(port_id))
            if not port.is_trunk:
                raise InputError("Port ID %d is not in trunk mode" % int(port_id))
            if port.is_locked:
                raise InputError("Port ID %d is locked" % int(port_id))
            if port.trunk_id != TRUNK_ID_NONE:
                raise InputError("Port ID %d is already on trunk ID %d" % (int(port_id), int(port.trunk_id)))

        try:
            # Add the trunk itself
            dt = datetime.datetime.now()
            sql = "INSERT INTO trunk (creation_time) VALUES (%s) RETURNING trunk_id"
            data = (dt, )
            self.cursor.execute(sql, data)
            trunk_id = self.cursor.fetchone()[0]
            self.connection.commit()
            # And update the ports
            for port_id in (port_id1, port_id2):
                self._set_port_trunk(port_id, trunk_id)
        except:
            self.delete_trunk(trunk_id)
            raise

        return trunk_id

    # Internal helper function
    def _delete_row(self, table, field, value):
        try:
            sql = "DELETE FROM %s WHERE %s = %s" % (table, field, '%s')
            data = (value,)
            self.cursor.execute(sql, data)
            self.connection.commit()
        except:
            self.connection.rollback()
            raise

    # Delete the specified switch
    #
    # Constraints:
    # 1. The switch must exist
    # 2. The switch may not be referenced by any ports -
    #    delete them first!
    def delete_switch(self, switch_id):
        switch = self.get_switch_by_id(switch_id)
        if switch is None:
            raise InputError("Switch ID %d does not exist" % int(switch_id))
        ports = self.get_ports_by_switch(switch_id)
        if ports is not None:
            raise InputError("Cannot delete switch ID %d when it still has %d ports" %
                             (int(switch_id), len(ports)))
        self._delete_row("switch", "switch_id", switch_id)
        return switch_id

    # Delete the specified port
    #
    # Constraints:
    # 1. The port must exist
    # 2. The port must not be locked
    def delete_port(self, port_id):
        port = self.get_port_by_id(port_id)
        if port is None:
            raise InputError("Port ID %d does not exist" % int(port_id))
        if port.is_locked:
            raise InputError("Cannot delete port ID %d as it is locked" % int(port_id))
        self._delete_row("port", "port_id", port_id)
        return port_id

    # Delete the specified VLAN
    #
    # Constraints:
    # 1. The VLAN must exist
    # 2. The VLAN may not contain any ports - move or delete them first!
    def delete_vlan(self, vlan_id):
        vlan = self.get_vlan_by_id(vlan_id)
        if vlan is None:
            raise InputError("VLAN ID %d does not exist" % int(vlan_id))
        ports = self.get_ports_by_current_vlan(vlan_id)
        if ports is not None:
            raise InputError("Cannot delete VLAN ID %d when it still has %d ports" %
                             (int(vlan_id), len(ports)))
        ports = self.get_ports_by_base_vlan(vlan_id)
        if ports is not None:
            raise InputError("Cannot delete VLAN ID %d when it still has %d ports" %
                             (int(vlan_id), len(ports)))
        self._delete_row("vlan", "vlan_id", vlan_id)
        return vlan_id

    # Delete the specified trunk
    #
    # Constraints:
    # 1. The trunk must exist
    #
    # Any ports attached will be detached (i.e. moved to trunk TRUNK_ID_NONE)
    def delete_trunk(self, trunk_id):
        trunk = self.get_trunk_by_id(trunk_id)
        if trunk is None:
            raise InputError("Trunk ID %d does not exist" % int(trunk_id))
        ports = self.get_ports_by_trunk(trunk_id)
        for port_id in ports:
            self._set_port_trunk(port_id, TRUNK_ID_NONE)
        self._delete_row("trunk", "trunk_id", trunk_id)
        return trunk_id

    # Find the lowest unused VLAN tag and return it
    #
    # Constraints:
    # None
    def find_lowest_unused_vlan_tag(self):
        sql = "SELECT tag FROM vlan ORDER BY tag ASC"
        self.cursor.execute(sql,)

        # Walk through the list, looking for gaps
        last = 1
        result = None

        for record in self.cursor:
            if (record[0] - last) > 1:
                result = last + 1
                break
            last = record[0]

        if result is None:
            result = last + 1

        if result > 4093:
            raise CriticalError("Can't find any VLAN tags remaining for allocation!")

        return result

    # Grab one column from one row of a query on one column; useful as
    # a quick wrapper
    def _get_element(self, select_field, table, compare_field, value):

        # We really want to use psycopg's type handling deal with the
        # (potentially) user-supplied data in the value field, so we
        # have to pass (sql,data) through to cursor.execute. However,
        # we can't have psycopg do all the argument substitution here
        # as it will quote all the params like the table name. That
        # doesn't work. So, we substitute a "%s" for "%s" here so we
        # keep it after python's own string substitution.
        sql = "SELECT %s FROM %s WHERE %s = %s" % (select_field, table, compare_field, "%s")

        # Now, the next icky thing: we need to make sure that we're
        # passing a dict so that psycopg2 can pick it apart properly
        # for its own substitution code. We force this with the
        # trailing comma here
        data = (value, )
        self.cursor.execute(sql, data)

        if self.cursor.rowcount > 0:
            return self.cursor.fetchone()[0]
        else:
            return None

    # Grab one column from one row of a query on 2 columns; useful as
    # a quick wrapper
    def _get_element2(self, select_field, table, compare_field1, value1, compare_field2, value2):

        # We really want to use psycopg's type handling deal with the
        # (potentially) user-supplied data in the value field, so we
        # have to pass (sql,data) through to cursor.execute. However,
        # we can't have psycopg do all the argument substitution here
        # as it will quote all the params like the table name. That
        # doesn't work. So, we substitute a "%s" for "%s" here so we
        # keep it after python's own string substitution.
        sql = "SELECT %s FROM %s WHERE %s = %s AND %s = %s" % (select_field, table, compare_field1, "%s", compare_field2, "%s")

        data = (value1, value2)
        self.cursor.execute(sql, data)

        if self.cursor.rowcount > 0:
            return self.cursor.fetchone()[0]
        else:
            return None

    # Grab one column from multiple rows of a query; useful as a quick
    # wrapper
    def _get_multi_elements(self, select_field, table, compare_field, value, sort_field):

        # We really want to use psycopg's type handling deal with the
        # (potentially) user-supplied data in the value field, so we
        # have to pass (sql,data) through to cursor.execute. However,
        # we can't have psycopg do all the argument substitution here
        # as it will quote all the params like the table name. That
        # doesn't work. So, we substitute a "%s" for "%s" here so we
        # keep it after python's own string substitution.
        sql = "SELECT %s FROM %s WHERE %s = %s ORDER BY %s ASC" % (select_field, table, compare_field, "%s", sort_field)

        # Now, the next icky thing: we need to make sure that we're
        # passing a dict so that psycopg2 can pick it apart properly
        # for its own substitution code. We force this with the
        # trailing comma here
        data = (value, )
        self.cursor.execute(sql, data)

        if self.cursor.rowcount > 0:
            results = []
            for record in self.cursor:
                results.append(record[0])
            return results
        else:
            return None

    # Grab one column from multiple rows of a 2-part query; useful as
    # a wrapper
    def _get_multi_elements2(self, select_field, table, compare_field1, value1, compare_field2, value2, sort_field):

        # We really want to use psycopg's type handling deal with the
        # (potentially) user-supplied data in the value field, so we
        # have to pass (sql,data) through to cursor.execute. However,
        # we can't have psycopg do all the argument substitution here
        # as it will quote all the params like the table name. That
        # doesn't work. So, we substitute a "%s" for "%s" here so we
        # keep it after python's own string substitution.
        sql = "SELECT %s FROM %s WHERE %s = %s AND %s = %s ORDER by %s ASC" % (select_field, table, compare_field1, "%s", compare_field2, "%s", sort_field)

        data = (value1, value2)
        self.cursor.execute(sql, data)

        if self.cursor.rowcount > 0:
            results = []
            for record in self.cursor:
                results.append(record[0])
            return results
        else:
            return None

    # Simple lookup: look up a switch by ID, and return all the
    # details of that switch.
    #
    # Returns None on failure.
    def get_switch_by_id(self, switch_id):
        return self._get_row("switch", "switch_id", int(switch_id))

    # Simple lookup: look up a switch by name, and return the ID of
    # that switch.
    #
    # Returns None on failure.
    def get_switch_id_by_name(self, name):
        return self._get_element("switch_id", "switch", "name", name)

    # Simple lookup: look up a switch by ID, and return the name of
    # that switch.
    #
    # Returns None on failure.
    def get_switch_name_by_id(self, switch_id):
        return self._get_element("name", "switch", "switch_id", int(switch_id))

    # Simple lookup: look up a port by ID, and return all the details
    # of that port.
    #
    # Returns None on failure.
    def get_port_by_id(self, port_id):
        return self._get_row("port", "port_id", int(port_id))

    # Simple lookup: look up a switch by ID, and return the IDs of all
    # the ports on that switch.
    #
    # Returns None on failure.
    def get_ports_by_switch(self, switch_id):
        return self._get_multi_elements("port_id", "port", "switch_id", int(switch_id), "port_id")

    # More complex lookup: look up all the trunk ports on a switch by
    # ID
    #
    # Returns None on failure.
    def get_trunk_port_names_by_switch(self, switch_id):
        return self._get_multi_elements2("name", "port", "switch_id", int(switch_id), "is_trunk", True, "port_id")

    # Simple lookup: look up a port by its name and its parent switch
    # by ID, and return the ID of the port.
    #
    # Returns None on failure.
    def get_port_by_switch_and_name(self, switch_id, name):
        return self._get_element2("port_id", "port", "switch_id", int(switch_id), "name", name)

    # Simple lookup: look up a port by its external name and its
    # parent switch by ID, and return the ID of the port.
    #
    # Returns None on failure.
    def get_port_by_switch_and_number(self, switch_id, number):
        return self._get_element2("port_id", "port", "switch_id", int(switch_id), "number", int(number))

    # Simple lookup: look up a port by ID, and return the current VLAN
    # id of that port.
    #
    # Returns None on failure.
    def get_current_vlan_id_by_port(self, port_id):
        return self._get_element("current_vlan_id", "port", "port_id", int(port_id))

    # Simple lookup: look up a port by ID, and return the base VLAN
    # id of that port.
    #
    # Returns None on failure.
    def get_base_vlan_id_by_port(self, port_id):
        return self._get_element("base_vlan_id", "port", "port_id", int(port_id))

    # Simple lookup: look up a current VLAN by ID, and return the IDs
    # of all the ports on that VLAN.
    #
    # Returns None on failure.
    def get_ports_by_current_vlan(self, vlan_id):
        return self._get_multi_elements("port_id", "port", "current_vlan_id", int(vlan_id), "port_id")

    # Simple lookup: look up a base VLAN by ID, and return the IDs
    # of all the ports on that VLAN.
    #
    # Returns None on failure.
    def get_ports_by_base_vlan(self, vlan_id):
        return self._get_multi_elements("port_id", "port", "base_vlan_id", int(vlan_id), "port_id")

    # Simple lookup: look up a trunk by ID, and return the IDs of the
    # ports on both ends of that trunk.
    #
    # Returns None on failure.
    def get_ports_by_trunk(self, trunk_id):
        return self._get_multi_elements("port_id", "port", "trunk_id", int(trunk_id), "port_id")

    # Simple lookup: look up a VLAN by ID, and return all the details
    # of that VLAN.
    #
    # Returns None on failure.
    def get_vlan_by_id(self, vlan_id):
        return self._get_row("vlan", "vlan_id", int(vlan_id))

    # Simple lookup: look up a VLAN by name, and return the ID of that
    # VLAN.
    #
    # Returns None on failure.
    def get_vlan_id_by_name(self, name):
        return self._get_element("vlan_id", "vlan", "name", name)

    # Simple lookup: look up a VLAN by tag, and return the ID of that
    # VLAN.
    #
    # Returns None on failure.
    def get_vlan_id_by_tag(self, tag):
        return self._get_element("vlan_id", "vlan", "tag", int(tag))

    # Simple lookup: look up a VLAN by ID, and return the name of that
    # VLAN.
    #
    # Returns None on failure.
    def get_vlan_name_by_id(self, vlan_id):
        return self._get_element("name", "vlan", "vlan_id", int(vlan_id))

    # Simple lookup: look up a VLAN by ID, and return the tag of that
    # VLAN.
    #
    # Returns None on failure.
    def get_vlan_tag_by_id(self, vlan_id):
        return self._get_element("tag", "vlan", "vlan_id", int(vlan_id))

    # Simple lookup: look up a trunk by ID, and return all the details
    # of that trunk.
    #
    # Returns None on failure.
    def get_trunk_by_id(self, trunk_id):
        return self._get_row("trunk", "trunk_id", int(trunk_id))

    # Grab one row of a query on one column; useful as a quick wrapper
    def _get_row(self, table, field, value):

        # We really want to use psycopg's type handling deal with the
        # (potentially) user-supplied data in the value field, so we
        # have to pass (sql,data) through to cursor.execute. However,
        # we can't have psycopg do all the argument substitution here
        # as it will quote all the params like the table name. That
        # doesn't work. So, we substitute a "%s" for "%s" here so we
        # keep it after python's own string substitution.
        sql = "SELECT * FROM %s WHERE %s = %s" % (table, field, "%s")

        # Now, the next icky thing: we need to make sure that we're
        # passing a dict so that psycopg2 can pick it apart properly
        # for its own substitution code. We force this with the
        # trailing comma here
        data = (value, )
        self.cursor.execute(sql, data)
        return self.cursor.fetchone()

    # (Un)Lock a port in the database. This can only be done through
    # the admin interface, and will stop API users from modifying
    # settings on the port. Use this to lock down ports that are used
    # for PDUs and other core infrastructure
    def set_port_is_locked(self, port_id, is_locked):
        port = self.get_port_by_id(port_id)
        if port is None:
            raise InputError("Port ID %d does not exist" % int(port_id))
        try:
            sql = "UPDATE port SET is_locked=%s WHERE port_id=%s RETURNING port_id"
            data = (is_locked, port_id)
            self.cursor.execute(sql, data)
            port_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise
        return port_id

    # Set the mode of a port in the database. Valid values for mode
    # are "trunk" and "access"
    def set_port_mode(self, port_id, mode):
        port = self.get_port_by_id(port_id)
        if port is None:
            raise InputError("Port ID %d does not exist" % int(port_id))
        if mode == "access":
            is_trunk = False
        elif mode == "trunk":
            is_trunk = True
        else:
            raise InputError("Port mode %s is not valid" % mode)
        try:
            sql = "UPDATE port SET is_trunk=%s WHERE port_id=%s RETURNING port_id"
            data = (is_trunk, port_id)
            self.cursor.execute(sql, data)
            port_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise
        return port_id

    # Set the current vlan of a port in the database. The VLAN is
    # passed by ID.
    #
    # Constraints:
    # 1. The port must already exist
    # 2. The port must not be a trunk port
    # 3. The port must not be locked
    # 1. The VLAN must already exist in the database
    def set_current_vlan(self, port_id, vlan_id):
        port = self.get_port_by_id(port_id)
        if port is None:
            raise InputError("Port ID %d does not exist" % int(port_id))

        if port.is_trunk or port.is_locked:
            raise CriticalError("The port is locked")

        vlan = self.get_vlan_by_id(vlan_id)
        if vlan is None:
            raise InputError("VLAN ID %d does not exist" % int(vlan_id))

        try:
            sql = "UPDATE port SET current_vlan_id=%s WHERE port_id=%s RETURNING port_id"
            data = (vlan_id, port_id)
            self.cursor.execute(sql, data)
            port_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise
        return port_id

    # Set the base vlan of a port in the database. The VLAN is
    # passed by ID.
    #
    # Constraints:
    # 1. The port must already exist
    # 2. The port must not be a trunk port
    # 3. The port must not be locked
    # 4. The VLAN must already exist in the database
    def set_base_vlan(self, port_id, vlan_id):
        port = self.get_port_by_id(port_id)
        if port is None:
            raise InputError("Port ID %d does not exist" % int(port_id))

        if port.is_trunk or port.is_locked:
            raise CriticalError("The port is locked")

        vlan = self.get_vlan_by_id(vlan_id)
        if vlan is None:
            raise InputError("VLAN ID %d does not exist" % int(vlan_id))
        if not vlan.is_base_vlan:
            raise InputError("VLAN ID %d is not a base VLAN" % int(vlan_id))

        try:
            sql = "UPDATE port SET base_vlan_id=%s WHERE port_id=%s RETURNING port_id"
            data = (vlan_id, port_id)
            self.cursor.execute(sql, data)
            port_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise
        return port_id

    # Internal function: Attach a port to a trunk in the database.
    #
    # Constraints:
    # 1. The port must already exist
    # 2. The port must not be locked
    def _set_port_trunk(self, port_id, trunk_id):
        port = self.get_port_by_id(port_id)
        if port is None:
            raise InputError("Port ID %d does not exist" % int(port_id))
        if port.is_locked:
            raise CriticalError("The port is locked")
        try:
            sql = "UPDATE port SET trunk_id=%s WHERE port_id=%s RETURNING port_id"
            data = (int(trunk_id), int(port_id))
            self.cursor.execute(sql, data)
            port_id = self.cursor.fetchone()[0]
            self.connection.commit()
        except:
            self.connection.rollback()
            raise
        return port_id

    # Trivial helper function to return all the rows in a given table
    def _dump_table(self, table, order):
        result = []
        self.cursor.execute("SELECT * FROM %s ORDER by %s ASC" % (table, order))
        record = self.cursor.fetchone()
        while record != None:
            result.append(record)
            record = self.cursor.fetchone()
        return result

    def all_switches(self):
        return self._dump_table("switch", "switch_id")

    def all_ports(self):
        return self._dump_table("port", "port_id")

    def all_vlans(self):
        return self._dump_table("vlan", "vlan_id")

    def all_trunks(self):
        return self._dump_table("trunk", "trunk_id")

if __name__ == '__main__':
    db = VlanDB()
    s = db.all_switches()
    print 'The DB knows about %d switch(es)' % len(s)
    print s
    p = db.all_ports()
    print 'The DB knows about %d port(s)' % len(p)
    print p
    v = db.all_vlans()
    print 'The DB knows about %d vlan(s)' % len(v)
    print v
    t = db.all_trunks()
    print 'The DB knows about %d trunks(s)' % len(t)
    print t

    print 'First free VLAN tag is %d' % db.find_lowest_unused_vlan_tag()