model.py

# Copyright 2019 Canonical Ltd.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import datetime
import decimal
import ipaddress
import json
import os
import re
import shutil
import tempfile
import time
import typing
import weakref

from abc import ABC, abstractmethod
from collections.abc import Mapping, MutableMapping
from pathlib import Path
from subprocess import run, PIPE, CalledProcessError

import ops


class Model:
    """Represents the Juju Model as seen from this unit.

    Attributes:
        unit: A :class:`Unit` that represents the unit that is running this code (eg yourself)
        app: A :class:`Application` that represents the application this unit is a part of.
        relations: Mapping of endpoint to list of :class:`Relation` answering the question
            "what am I currently related to". See also :meth:`.get_relation`
        config: A dict of the config for the current application.
        resources: Access to resources for this charm. Use ``model.resources.fetch(resource_name)``
            to get the path on disk where the resource can be found.
        storages: Mapping of storage_name to :class:`Storage` for the storage points defined in
            metadata.yaml
        pod: Used to get access to ``model.pod.set_spec`` to set the container specification
            for Kubernetes charms.
    """

    def __init__(
            self, unit_name: str, meta: 'ops.charm.CharmMeta', backend: '_ModelBackend', *,
            model_name: str = None):
        self._cache = _ModelCache(backend)
        self._backend = backend
        self.unit = self.get_unit(unit_name)
        self.app = self.unit.app
        self.relations = RelationMapping(meta.relations, self.unit, self._backend, self._cache)
        self.config = ConfigData(self._backend)
        self.resources = Resources(list(meta.resources), self._backend)
        self.pod = Pod(self._backend)
        self.storages = StorageMapping(list(meta.storages), self._backend)
        self._bindings = BindingMapping(self._backend)
        self._model_name = model_name

    @property
    def name(self) -> str:
        """Return the name of the Model that this unit is running in.

        This is read from the environment variable ``JUJU_MODEL_NAME``.
        """
        return self._model_name

    def get_unit(self, unit_name: str) -> 'Unit':
        """Get an arbitrary unit by name.

        Internally this uses a cache, so asking for the same unit two times will
        return the same object.
        """
        return self._cache.get(Unit, unit_name)

    def get_app(self, app_name: str) -> 'Application':
        """Get an application by name.

        Internally this uses a cache, so asking for the same application two times will
        return the same object.
        """
        return self._cache.get(Application, app_name)

    def get_relation(
            self, relation_name: str,
            relation_id: typing.Optional[int] = None) -> 'Relation':
        """Get a specific Relation instance.

        If relation_id is not given, this will return the Relation instance if the
        relation is established only once or None if it is not established. If this
        same relation is established multiple times the error TooManyRelatedAppsError is raised.

        Args:
            relation_name: The name of the endpoint for this charm
            relation_id: An identifier for a specific relation. Used to disambiguate when a
                given application has more than one relation on a given endpoint.
        Raises:
            TooManyRelatedAppsError: is raised if there is more than one relation to the
                supplied relation_name and no relation_id was supplied
        """
        return self.relations._get_unique(relation_name, relation_id)

    def get_binding(self, binding_key: typing.Union[str, 'Relation']) -> 'Binding':
        """Get a network space binding.

        Args:
            binding_key: The relation name or instance to obtain bindings for.
        Returns:
            If ``binding_key`` is a relation name, the method returns the default binding
            for that relation. If a relation instance is provided, the method first looks
            up a more specific binding for that specific relation ID, and if none is found
            falls back to the default binding for the relation name.
        """
        return self._bindings.get(binding_key)


class _ModelCache:

    def __init__(self, backend):
        self._backend = backend
        self._weakrefs = weakref.WeakValueDictionary()

    def get(self, entity_type, *args):
        key = (entity_type,) + args
        entity = self._weakrefs.get(key)
        if entity is None:
            entity = entity_type(*args, backend=self._backend, cache=self)
            self._weakrefs[key] = entity
        return entity


class Application:
    """Represents a named application in the model.

    This might be your application, or might be an application that you are related to.
    Charmers should not instantiate Application objects directly, but should use
    :meth:`Model.get_app` if they need a reference to a given application.

    Attributes:
        name: The name of this application (eg, 'mysql'). This name may differ from the name of
            the charm, if the user has deployed it to a different name.
    """

    def __init__(self, name, backend, cache):
        self.name = name
        self._backend = backend
        self._cache = cache
        self._is_our_app = self.name == self._backend.app_name
        self._status = None

    def _invalidate(self):
        self._status = None

    @property
    def status(self) -> 'StatusBase':
        """Used to report or read the status of the overall application.

        Can only be read and set by the lead unit of the application.

        The status of remote units is always Unknown.

        Raises:
            RuntimeError: if you try to set the status of another application, or if you try to
                set the status of this application as a unit that is not the leader.
            InvalidStatusError: if you try to set the status to something that is not a
                :class:`StatusBase`

        Example::

            self.model.app.status = BlockedStatus('I need a human to come help me')
        """
        if not self._is_our_app:
            return UnknownStatus()

        if not self._backend.is_leader():
            raise RuntimeError('cannot get application status as a non-leader unit')

        if self._status:
            return self._status

        s = self._backend.status_get(is_app=True)
        self._status = StatusBase.from_name(s['status'], s['message'])
        return self._status

    @status.setter
    def status(self, value: 'StatusBase'):
        if not isinstance(value, StatusBase):
            raise InvalidStatusError(
                'invalid value provided for application {} status: {}'.format(self, value)
            )

        if not self._is_our_app:
            raise RuntimeError('cannot to set status for a remote application {}'.format(self))

        if not self._backend.is_leader():
            raise RuntimeError('cannot set application status as a non-leader unit')

        self._backend.status_set(value.name, value.message, is_app=True)
        self._status = value

    def __repr__(self):
        return '<{}.{} {}>'.format(type(self).__module__, type(self).__name__, self.name)


class Unit:
    """Represents a named unit in the model.

    This might be your unit, another unit of your application, or a unit of another application
    that you are related to.

    Attributes:
        name: The name of the unit (eg, 'mysql/0')
        app: The Application the unit is a part of.
    """

    def __init__(self, name, backend, cache):
        self.name = name

        app_name = name.split('/')[0]
        self.app = cache.get(Application, app_name)

        self._backend = backend
        self._cache = cache
        self._is_our_unit = self.name == self._backend.unit_name
        self._status = None

    def _invalidate(self):
        self._status = None

    @property
    def status(self) -> 'StatusBase':
        """Used to report or read the status of a specific unit.

        The status of any unit other than yourself is always Unknown.

        Raises:
            RuntimeError: if you try to set the status of a unit other than yourself.
            InvalidStatusError: if you try to set the status to something other than
                a :class:`StatusBase`
        Example::

            self.model.unit.status = MaintenanceStatus('reconfiguring the frobnicators')
        """
        if not self._is_our_unit:
            return UnknownStatus()

        if self._status:
            return self._status

        s = self._backend.status_get(is_app=False)
        self._status = StatusBase.from_name(s['status'], s['message'])
        return self._status

    @status.setter
    def status(self, value: 'StatusBase'):
        if not isinstance(value, StatusBase):
            raise InvalidStatusError(
                'invalid value provided for unit {} status: {}'.format(self, value)
            )

        if not self._is_our_unit:
            raise RuntimeError('cannot set status for a remote unit {}'.format(self))

        self._backend.status_set(value.name, value.message, is_app=False)
        self._status = value

    def __repr__(self):
        return '<{}.{} {}>'.format(type(self).__module__, type(self).__name__, self.name)

    def is_leader(self) -> bool:
        """Return whether this unit is the leader of its application.

        This can only be called for your own unit.
        Returns:
            True if you are the leader, False otherwise
        Raises:
            RuntimeError: if called for a unit that is not yourself
        """
        if self._is_our_unit:
            # This value is not cached as it is not guaranteed to persist for the whole duration
            # of a hook execution.
            return self._backend.is_leader()
        else:
            raise RuntimeError(
                'leadership status of remote units ({}) is not visible to other'
                ' applications'.format(self)
            )

    def set_workload_version(self, version: str) -> None:
        """Record the version of the software running as the workload.

        This shouldn't be confused with the revision of the charm. This is informative only;
        shown in the output of 'juju status'.
        """
        if not isinstance(version, str):
            raise TypeError("workload version must be a str, not {}: {!r}".format(
                type(version).__name__, version))
        self._backend.application_version_set(version)


class LazyMapping(Mapping, ABC):
    """Represents a dict that isn't populated until it is accessed.

    Charm authors should generally never need to use this directly, but it forms
    the basis for many of the dicts that the framework tracks.
    """

    _lazy_data = None

    @abstractmethod
    def _load(self):
        raise NotImplementedError()

    @property
    def _data(self):
        data = self._lazy_data
        if data is None:
            data = self._lazy_data = self._load()
        return data

    def _invalidate(self):
        self._lazy_data = None

    def __contains__(self, key):
        return key in self._data

    def __len__(self):
        return len(self._data)

    def __iter__(self):
        return iter(self._data)

    def __getitem__(self, key):
        return self._data[key]


class RelationMapping(Mapping):
    """Map of relation names to lists of :class:`Relation` instances."""

    def __init__(self, relations_meta, our_unit, backend, cache):
        self._peers = set()
        for name, relation_meta in relations_meta.items():
            if relation_meta.role.is_peer():
                self._peers.add(name)
        self._our_unit = our_unit
        self._backend = backend
        self._cache = cache
        self._data = {relation_name: None for relation_name in relations_meta}

    def __contains__(self, key):
        return key in self._data

    def __len__(self):
        return len(self._data)

    def __iter__(self):
        return iter(self._data)

    def __getitem__(self, relation_name):
        is_peer = relation_name in self._peers
        relation_list = self._data[relation_name]
        if relation_list is None:
            relation_list = self._data[relation_name] = []
            for rid in self._backend.relation_ids(relation_name):
                relation = Relation(relation_name, rid, is_peer,
                                    self._our_unit, self._backend, self._cache)
                relation_list.append(relation)
        return relation_list

    def _invalidate(self, relation_name):
        """Used to wipe the cache of a given relation_name.

        Not meant to be used by Charm authors. The content of relation data is
        static for the lifetime of a hook, so it is safe to cache in memory once
        accessed.
        """
        self._data[relation_name] = None

    def _get_unique(self, relation_name, relation_id=None):
        if relation_id is not None:
            if not isinstance(relation_id, int):
                raise ModelError('relation id {} must be int or None not {}'.format(
                    relation_id,
                    type(relation_id).__name__))
            for relation in self[relation_name]:
                if relation.id == relation_id:
                    return relation
            else:
                # The relation may be dead, but it is not forgotten.
                is_peer = relation_name in self._peers
                return Relation(relation_name, relation_id, is_peer,
                                self._our_unit, self._backend, self._cache)
        num_related = len(self[relation_name])
        if num_related == 0:
            return None
        elif num_related == 1:
            return self[relation_name][0]
        else:
            # TODO: We need something in the framework to catch and gracefully handle
            # errors, ideally integrating the error catching with Juju's mechanisms.
            raise TooManyRelatedAppsError(relation_name, num_related, 1)


class BindingMapping:
    """Mapping of endpoints to network bindings.

    Charm authors should not instantiate this directly, but access it via
    :meth:`Model.get_binding`
    """

    def __init__(self, backend):
        self._backend = backend
        self._data = {}

    def get(self, binding_key: typing.Union[str, 'Relation']) -> 'Binding':
        """Get a specific Binding for an endpoint/relation.

        Not used directly by Charm authors. See :meth:`Model.get_binding`
        """
        if isinstance(binding_key, Relation):
            binding_name = binding_key.name
            relation_id = binding_key.id
        elif isinstance(binding_key, str):
            binding_name = binding_key
            relation_id = None
        else:
            raise ModelError('binding key must be str or relation instance, not {}'
                             ''.format(type(binding_key).__name__))
        binding = self._data.get(binding_key)
        if binding is None:
            binding = Binding(binding_name, relation_id, self._backend)
            self._data[binding_key] = binding
        return binding


class Binding:
    """Binding to a network space.

    Attributes:
        name: The name of the endpoint this binding represents (eg, 'db')
    """

    def __init__(self, name, relation_id, backend):
        self.name = name
        self._relation_id = relation_id
        self._backend = backend
        self._network = None

    @property
    def network(self) -> 'Network':
        """The network information for this binding."""
        if self._network is None:
            try:
                self._network = Network(self._backend.network_get(self.name, self._relation_id))
            except RelationNotFoundError:
                if self._relation_id is None:
                    raise
                # If a relation is dead, we can still get network info associated with an
                # endpoint itself
                self._network = Network(self._backend.network_get(self.name))
        return self._network


class Network:
    """Network space details.

    Charm authors should not instantiate this directly, but should get access to the Network
    definition from :meth:`Model.get_binding` and its ``network`` attribute.

    Attributes:
        interfaces: A list of :class:`NetworkInterface` details. This includes the
            information about how your application should be configured (eg, what
            IP addresses should you bind to.)
            Note that multiple addresses for a single interface are represented as multiple
            interfaces. (eg, ``[NetworKInfo('ens1', '10.1.1.1/32'),
            NetworkInfo('ens1', '10.1.2.1/32'])``)
        ingress_addresses: A list of :class:`ipaddress.ip_address` objects representing the IP
            addresses that other units should use to get in touch with you.
        egress_subnets: A list of :class:`ipaddress.ip_network` representing the subnets that
            other units will see you connecting from. Due to things like NAT it isn't always
            possible to narrow it down to a single address, but when it is clear, the CIDRs
            will be constrained to a single address. (eg, 10.0.0.1/32)
    Args:
        network_info: A dict of network information as returned by ``network-get``.
    """

    def __init__(self, network_info: dict):
        self.interfaces = []
        # Treat multiple addresses on an interface as multiple logical
        # interfaces with the same name.
        for interface_info in network_info['bind-addresses']:
            interface_name = interface_info['interface-name']
            for address_info in interface_info['addresses']:
                self.interfaces.append(NetworkInterface(interface_name, address_info))
        self.ingress_addresses = []
        for address in network_info['ingress-addresses']:
            self.ingress_addresses.append(ipaddress.ip_address(address))
        self.egress_subnets = []
        for subnet in network_info['egress-subnets']:
            self.egress_subnets.append(ipaddress.ip_network(subnet))

    @property
    def bind_address(self):
        """A single address that your application should bind() to.

        For the common case where there is a single answer. This represents a single
        address from :attr:`.interfaces` that can be used to configure where your
        application should bind() and listen().
        """
        return self.interfaces[0].address

    @property
    def ingress_address(self):
        """The address other applications should use to connect to your unit.

        Due to things like public/private addresses, NAT and tunneling, the address you bind()
        to is not always the address other people can use to connect() to you.
        This is just the first address from :attr:`.ingress_addresses`.
        """
        return self.ingress_addresses[0]


class NetworkInterface:
    """Represents a single network interface that the charm needs to know about.

    Charmers should not instantiate this type directly. Instead use :meth:`Model.get_binding`
    to get the network information for a given endpoint.

    Attributes:
        name: The name of the interface (eg. 'eth0', or 'ens1')
        subnet: An :class:`ipaddress.ip_network` representation of the IP for the network
            interface. This may be a single address (eg '10.0.1.2/32')
    """

    def __init__(self, name: str, address_info: dict):
        self.name = name
        # TODO: expose a hardware address here, see LP: #1864070.
        self.address = ipaddress.ip_address(address_info['value'])
        cidr = address_info['cidr']
        if not cidr:
            # The cidr field may be empty, see LP: #1864102.
            # In this case, make it a /32 or /128 IP network.
            self.subnet = ipaddress.ip_network(address_info['value'])
        else:
            self.subnet = ipaddress.ip_network(cidr)
        # TODO: expose a hostname/canonical name for the address here, see LP: #1864086.


class Relation:
    """Represents an established relation between this application and another application.

    This class should not be instantiated directly, instead use :meth:`Model.get_relation`
    or :attr:`RelationEvent.relation`.

    Attributes:
        name: The name of the local endpoint of the relation (eg 'db')
        id: The identifier for a particular relation (integer)
        app: An :class:`Application` representing the remote application of this relation.
            For peer relations this will be the local application.
        units: A set of :class:`Unit` for units that have started and joined this relation.
        data: A :class:`RelationData` holding the data buckets for each entity
            of a relation. Accessed via eg Relation.data[unit]['foo']
    """

    def __init__(
            self, relation_name: str, relation_id: int, is_peer: bool, our_unit: Unit,
            backend: '_ModelBackend', cache: '_ModelCache'):
        self.name = relation_name
        self.id = relation_id
        self.app = None
        self.units = set()

        # For peer relations, both the remote and the local app are the same.
        if is_peer:
            self.app = our_unit.app
        try:
            for unit_name in backend.relation_list(self.id):
                unit = cache.get(Unit, unit_name)
                self.units.add(unit)
                if self.app is None:
                    self.app = unit.app
        except RelationNotFoundError:
            # If the relation is dead, just treat it as if it has no remote units.
            pass
        self.data = RelationData(self, our_unit, backend)

    def __repr__(self):
        return '<{}.{} {}:{}>'.format(type(self).__module__,
                                      type(self).__name__,
                                      self.name,
                                      self.id)


class RelationData(Mapping):
    """Represents the various data buckets of a given relation.

    Each unit and application involved in a relation has their own data bucket.
    Eg: ``{entity: RelationDataContent}``
    where entity can be either a :class:`Unit` or a :class:`Application`.

    Units can read and write their own data, and if they are the leader,
    they can read and write their application data. They are allowed to read
    remote unit and application data.

    This class should not be created directly. It should be accessed via
    :attr:`Relation.data`
    """

    def __init__(self, relation: Relation, our_unit: Unit, backend: '_ModelBackend'):
        self.relation = weakref.proxy(relation)
        self._data = {
            our_unit: RelationDataContent(self.relation, our_unit, backend),
            our_unit.app: RelationDataContent(self.relation, our_unit.app, backend),
        }
        self._data.update({
            unit: RelationDataContent(self.relation, unit, backend)
            for unit in self.relation.units})
        # The relation might be dead so avoid a None key here.
        if self.relation.app is not None:
            self._data.update({
                self.relation.app: RelationDataContent(self.relation, self.relation.app, backend),
            })

    def __contains__(self, key):
        return key in self._data

    def __len__(self):
        return len(self._data)

    def __iter__(self):
        return iter(self._data)

    def __getitem__(self, key):
        return self._data[key]


# We mix in MutableMapping here to get some convenience implementations, but whether it's actually
# mutable or not is controlled by the flag.
class RelationDataContent(LazyMapping, MutableMapping):

    def __init__(self, relation, entity, backend):
        self.relation = relation
        self._entity = entity
        self._backend = backend
        self._is_app = isinstance(entity, Application)

    def _load(self):
        try:
            return self._backend.relation_get(self.relation.id, self._entity.name, self._is_app)
        except RelationNotFoundError:
            # Dead relations tell no tales (and have no data).
            return {}

    def _is_mutable(self):
        if self._is_app:
            is_our_app = self._backend.app_name == self._entity.name
            if not is_our_app:
                return False
            # Whether the application data bag is mutable or not depends on
            # whether this unit is a leader or not, but this is not guaranteed
            # to be always true during the same hook execution.
            return self._backend.is_leader()
        else:
            is_our_unit = self._backend.unit_name == self._entity.name
            if is_our_unit:
                return True
        return False

    def __setitem__(self, key, value):
        if not self._is_mutable():
            raise RelationDataError('cannot set relation data for {}'.format(self._entity.name))
        if not isinstance(value, str):
            raise RelationDataError('relation data values must be strings')

        self._backend.relation_set(self.relation.id, key, value, self._is_app)

        # Don't load data unnecessarily if we're only updating.
        if self._lazy_data is not None:
            if value == '':
                # Match the behavior of Juju, which is that setting the value to an
                # empty string will remove the key entirely from the relation data.
                del self._data[key]
            else:
                self._data[key] = value

    def __delitem__(self, key):
        # Match the behavior of Juju, which is that setting the value to an empty
        # string will remove the key entirely from the relation data.
        self.__setitem__(key, '')


class ConfigData(LazyMapping):

    def __init__(self, backend):
        self._backend = backend

    def _load(self):
        return self._backend.config_get()


class StatusBase:
    """Status values specific to applications and units.

    To access a status by name, see :meth:`StatusBase.from_name`, most use cases will just
    directly use the child class to indicate their status.
    """

    _statuses = {}
    name = None

    def __init__(self, message: str):
        self.message = message

    def __new__(cls, *args, **kwargs):
        if cls is StatusBase:
            raise TypeError("cannot instantiate a base class")
        return super().__new__(cls)

    def __eq__(self, other):
        if not isinstance(self, type(other)):
            return False
        return self.message == other.message

    def __repr__(self):
        return "{.__class__.__name__}({!r})".format(self, self.message)

    @classmethod
    def from_name(cls, name: str, message: str):
        if name == 'unknown':
            # unknown is special
            return UnknownStatus()
        else:
            return cls._statuses[name](message)

    @classmethod
    def register(cls, child):
        if child.name is None:
            raise AttributeError('cannot register a Status which has no name')
        cls._statuses[child.name] = child
        return child


@StatusBase.register
class UnknownStatus(StatusBase):
    """The unit status is unknown.

    A unit-agent has finished calling install, config-changed and start, but the
    charm has not called status-set yet.

    """
    name = 'unknown'

    def __init__(self):
        # Unknown status cannot be set and does not have a message associated with it.
        super().__init__('')

    def __repr__(self):
        return "UnknownStatus()"


@StatusBase.register
class ActiveStatus(StatusBase):
    """The unit is ready.

    The unit believes it is correctly offering all the services it has been asked to offer.
    """
    name = 'active'

    def __init__(self, message: str = ''):
        super().__init__(message)


@StatusBase.register
class BlockedStatus(StatusBase):
    """The unit requires manual intervention.

    An operator has to manually intervene to unblock the unit and let it proceed.
    """
    name = 'blocked'


@StatusBase.register
class MaintenanceStatus(StatusBase):
    """The unit is performing maintenance tasks.

    The unit is not yet providing services, but is actively doing work in preparation
    for providing those services.  This is a "spinning" state, not an error state. It
    reflects activity on the unit itself, not on peers or related units.

    """
    name = 'maintenance'


@StatusBase.register
class WaitingStatus(StatusBase):
    """A unit is unable to progress.

    The unit is unable to progress to an active state because an application to which
    it is related is not running.

    """
    name = 'waiting'


class Resources:
    """Object representing resources for the charm.
    """

    def __init__(self, names: typing.Iterable[str], backend: '_ModelBackend'):
        self._backend = backend
        self._paths = {name: None for name in names}

    def fetch(self, name: str) -> Path:
        """Fetch the resource from the controller or store.

        If successfully fetched, this returns a Path object to where the resource is stored
        on disk, otherwise it raises a ModelError.
        """
        if name not in self._paths:
            raise RuntimeError('invalid resource name: {}'.format(name))
        if self._paths[name] is None:
            self._paths[name] = Path(self._backend.resource_get(name))
        return self._paths[name]


class Pod:
    """Represents the definition of a pod spec in Kubernetes models.

    Currently only supports simple access to setting the Juju pod spec via :attr:`.set_spec`.
    """

    def __init__(self, backend: '_ModelBackend'):
        self._backend = backend

    def set_spec(self, spec: typing.Mapping, k8s_resources: typing.Mapping = None):
        """Set the specification for pods that Juju should start in kubernetes.

        See `juju help-tool pod-spec-set` for details of what should be passed.
        Args:
            spec: The mapping defining the pod specification
            k8s_resources: Additional kubernetes specific specification.

        Returns:
        """
        if not self._backend.is_leader():
            raise ModelError('cannot set a pod spec as this unit is not a leader')
        self._backend.pod_spec_set(spec, k8s_resources)


class StorageMapping(Mapping):
    """Map of storage names to lists of Storage instances."""

    def __init__(self, storage_names: typing.Iterable[str], backend: '_ModelBackend'):
        self._backend = backend
        self._storage_map = {storage_name: None for storage_name in storage_names}

    def __contains__(self, key: str):
        return key in self._storage_map

    def __len__(self):
        return len(self._storage_map)

    def __iter__(self):
        return iter(self._storage_map)

    def __getitem__(self, storage_name: str) -> typing.List['Storage']:
        storage_list = self._storage_map[storage_name]
        if storage_list is None:
            storage_list = self._storage_map[storage_name] = []
            for storage_id in self._backend.storage_list(storage_name):
                storage_list.append(Storage(storage_name, storage_id, self._backend))
        return storage_list

    def request(self, storage_name: str, count: int = 1):
        """Requests new storage instances of a given name.

        Uses storage-add tool to request additional storage. Juju will notify the unit
        via <storage-name>-storage-attached events when it becomes available.
        """
        if storage_name not in self._storage_map:
            raise ModelError(('cannot add storage {!r}:'
                              ' it is not present in the charm metadata').format(storage_name))
        self._backend.storage_add(storage_name, count)


class Storage:
    """"Represents a storage as defined in metadata.yaml

    Attributes:
        name: Simple string name of the storage
        id: The provider id for storage
    """

    def __init__(self, storage_name, storage_id, backend):
        self.name = storage_name
        self.id = storage_id
        self._backend = backend
        self._location = None

    @property
    def location(self):
        if self._location is None:
            raw = self._backend.storage_get('{}/{}'.format(self.name, self.id), "location")
            self._location = Path(raw)
        return self._location


class ModelError(Exception):
    """Base class for exceptions raised when interacting with the Model."""
    pass


class TooManyRelatedAppsError(ModelError):
    """Raised by :meth:`Model.get_relation` if there is more than one related application."""

    def __init__(self, relation_name, num_related, max_supported):
        super().__init__('Too many remote applications on {} ({} > {})'.format(
            relation_name, num_related, max_supported))
        self.relation_name = relation_name
        self.num_related = num_related
        self.max_supported = max_supported


class RelationDataError(ModelError):
    """Raised by ``Relation.data[entity][key] = 'foo'`` if the data is invalid.

    This is raised if you're either trying to set a value to something that isn't a string,
    or if you are trying to set a value in a bucket that you don't have access to. (eg,
    another application/unit or setting your application data but you aren't the leader.)
    """


class RelationNotFoundError(ModelError):
    """Backend error when querying juju for a given relation and that relation doesn't exist."""


class InvalidStatusError(ModelError):
    """Raised if trying to set an Application or Unit status to something invalid."""


class _ModelBackend:
    """Represents the connection between the Model representation and talking to Juju.

    Charm authors should not directly interact with the ModelBackend, it is a private
    implementation of Model.
    """

    LEASE_RENEWAL_PERIOD = datetime.timedelta(seconds=30)

    def __init__(self):
        self.unit_name = os.environ['JUJU_UNIT_NAME']
        self.app_name = self.unit_name.split('/')[0]

        self._is_leader = None
        self._leader_check_time = None

    def _run(self, *args, return_output=False, use_json=False):
        kwargs = dict(stdout=PIPE, stderr=PIPE)
        if use_json:
            args += ('--format=json',)
        try:
            result = run(args, check=True, **kwargs)
        except CalledProcessError as e:
            raise ModelError(e.stderr)
        if return_output:
            if result.stdout is None:
                return ''
            else:
                text = result.stdout.decode('utf8')
                if use_json:
                    return json.loads(text)
                else:
                    return text

    def relation_ids(self, relation_name):
        relation_ids = self._run('relation-ids', relation_name, return_output=True, use_json=True)
        return [int(relation_id.split(':')[-1]) for relation_id in relation_ids]

    def relation_list(self, relation_id):
        try:
            return self._run('relation-list', '-r', str(relation_id),
                             return_output=True, use_json=True)
        except ModelError as e:
            if 'relation not found' in str(e):
                raise RelationNotFoundError() from e
            raise

    def relation_get(self, relation_id, member_name, is_app):
        if not isinstance(is_app, bool):
            raise TypeError('is_app parameter to relation_get must be a boolean')

        try:
            return self._run('relation-get', '-r', str(relation_id),