model.py

# Copyright 2019 PrivateStorage.io, LLC
#
# 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.

"""
This module implements models (in the MVC sense) for the client side of
the storage plugin.
"""

from uuid import (
    uuid4,
)
from functools import (
    wraps,
)
from json import (
    loads,
    dumps,
)
from datetime import (
    datetime,
)
from zope.interface import (
    Interface,
    implementer,
)

from sqlite3 import (
    OperationalError,
    connect as _connect,
)

import attr

from aniso8601 import (
    parse_datetime,
)
from twisted.logger import (
    Logger,
)
from twisted.python.filepath import (
    FilePath,
)

from ._base64 import (
    urlsafe_b64decode,
)

from .validators import (
    is_base64_encoded,
    has_length,
    greater_than,
)

from .storage_common import (
    pass_value_attribute,
    get_configured_pass_value,
    required_passes,
)

from .schema import (
    get_schema_version,
    get_schema_upgrades,
    run_schema_upgrades,
)


class ILeaseMaintenanceObserver(Interface):
    """
    An object which is interested in receiving events related to the progress
    of lease maintenance activity.
    """
    def observe(sizes):
        """
        Observe some shares encountered during lease maintenance.

        :param list[int] sizes: The sizes of the shares encountered.
        """

    def finish():
        """
        Observe that a run of lease maintenance has completed.
        """


class StoreOpenError(Exception):
    """
    There was a problem opening the underlying data store.
    """
    def __init__(self, reason):
        self.reason = reason


class NotEnoughTokens(Exception):
    """
    An attempt to extract tokens failed because the store does not contain as
    many tokens as were requested.
    """


CONFIG_DB_NAME = u"privatestorageio-zkapauthz-v1.sqlite3"

def open_and_initialize(path, connect=None):
    """
    Open a SQLite3 database for use as a voucher store.

    Create the database and populate it with a schema, if it does not already
    exist.

    :param FilePath path: The location of the SQLite3 database file.

    :return: A SQLite3 connection object for the database at the given path.
    """
    if connect is None:
        connect = _connect
    try:
        path.parent().makedirs(ignoreExistingDirectory=True)
    except OSError as e:
        raise StoreOpenError(e)

    dbfile = path.asBytesMode().path
    try:
        conn = connect(
            dbfile,
            isolation_level="IMMEDIATE",
        )
    except OperationalError as e:
        raise StoreOpenError(e)

    # Enforcement of foreign key constraints is off by default.  It must be
    # enabled on a per-connection basis.  This is a helpful feature to ensure
    # consistency so we want it enforced and we use it in our schema.
    conn.execute("PRAGMA foreign_keys = ON")

    with conn:
        cursor = conn.cursor()
        actual_version = get_schema_version(cursor)
        schema_upgrades = list(get_schema_upgrades(actual_version))
        run_schema_upgrades(schema_upgrades, cursor)

    conn.execute("""
    -- It might already exist if there is still another connection to this
    -- database.  It goes away once all connections have been closed, though.
    CREATE TABLE IF NOT EXISTS [temp.in-use] (
        [unblinded-token] text, -- The base64 encoded unblinded token.
        [operation-id] text,    -- A unique identifier for a group of tokens in-use together.

        PRIMARY KEY([unblinded-token])
        FOREIGN KEY([unblinded-token]) REFERENCES [unblinded-tokens]([token])
    )
    """)

    return conn



def with_cursor(f):
    @wraps(f)
    def with_cursor(self, *a, **kw):
        with self._connection:
            cursor = self._connection.cursor()
            cursor.execute("BEGIN IMMEDIATE TRANSACTION")
            return f(self, cursor, *a, **kw)
    return with_cursor


def memory_connect(path, *a, **kw):
    """
    Always connect to an in-memory SQLite3 database.
    """
    return _connect(":memory:", *a, **kw)


# The largest integer SQLite3 can represent in an integer column.  Larger than
# this an the representation loses precision as a floating point.
_SQLITE3_INTEGER_MAX = 2 ** 63 - 1


@attr.s(frozen=True)
class VoucherStore(object):
    """
    This class implements persistence for vouchers.

    :ivar allmydata.node._Config node_config: The Tahoe-LAFS node configuration object for
        the node that owns the persisted vouchers.

    :ivar now: A no-argument callable that returns the time of the call as a
        ``datetime`` instance.
    """
    _log = Logger()

    pass_value = pass_value_attribute()

    database_path = attr.ib(validator=attr.validators.instance_of(FilePath))
    now = attr.ib()

    _connection = attr.ib()

    @classmethod
    def from_node_config(cls, node_config, now, connect=None):
        """
        Create or open the ``VoucherStore`` for a given node.

        :param allmydata.node._Config node_config: The Tahoe-LAFS
            configuration object for the node for which we want to open a
            store.

        :param now: See ``VoucherStore.now``.

        :param connect: An alternate database connection function.  This is
            primarily for the purposes of the test suite.
        """
        db_path = FilePath(node_config.get_private_path(CONFIG_DB_NAME))
        conn = open_and_initialize(
            db_path,
            connect=connect,
        )
        return cls(
            get_configured_pass_value(node_config),
            db_path,
            now,
            conn,
        )

    @with_cursor
    def get(self, cursor, voucher):
        """
        :param unicode voucher: The text value of a voucher to retrieve.

        :return Voucher: The voucher object that matches the given value.
        """
        cursor.execute(
            """
            SELECT
                [number], [created], [expected-tokens], [state], [finished], [token-count], [public-key], [counter]
            FROM
                [vouchers]
            WHERE
                [number] = ?
            """,
            (voucher,),
        )
        refs = cursor.fetchall()
        if len(refs) == 0:
            raise KeyError(voucher)
        return Voucher.from_row(refs[0])

    @with_cursor
    def add(self, cursor, voucher, expected_tokens, counter, get_tokens):
        """
        Add random tokens associated with a voucher (possibly new, possibly
        existing) to the database.  If the (voucher, counter) pair is already
        present, do nothing.

        :param unicode voucher: The text value of a voucher with which to
            associate the tokens.

        :param int expected_tokens: The total number of tokens for which this
            voucher is expected to be redeemed.  This is only respected the
            first time a voucher is added.  Subsequent calls with the same
            voucher but a different count ignore the value because it is
            already known (and the database knows better than the caller what
            it should be).

            This probably means ``add`` is a broken interface for doing these
            two things.  Maybe it should be fixed someday.

        :param int counter: The redemption counter for the given voucher with
            which to associate the tokens.

        :param list[RandomToken]: The tokens to add alongside the voucher.
        """
        now = self.now()
        if not isinstance(now, datetime):
            raise TypeError("{} returned {}, expected datetime".format(self.now, now))

        cursor.execute(
            """
            SELECT [text]
            FROM [tokens]
            WHERE [voucher] = ? AND [counter] = ?
            """,
            (voucher, counter),
        )
        rows = cursor.fetchall()
        if len(rows) > 0:
            self._log.info(
                "Loaded {count} random tokens for a voucher ({voucher}[{counter}]).",
                count=len(rows),
                voucher=voucher,
                counter=counter,
            )
            tokens = list(
                RandomToken(token_value)
                for (token_value,)
                in rows
            )
        else:
            tokens = get_tokens()
            self._log.info(
                "Persisting {count} random tokens for a voucher ({voucher}[{counter}]).",
                count=len(tokens),
                voucher=voucher,
                counter=counter,
            )
            cursor.execute(
                """
                INSERT OR IGNORE INTO [vouchers] ([number], [expected-tokens], [created]) VALUES (?, ?, ?)
                """,
                (voucher, expected_tokens, self.now())
            )
            cursor.executemany(
                """
                INSERT INTO [tokens] ([voucher], [counter], [text]) VALUES (?, ?, ?)
                """,
                list(
                    (voucher, counter, token.token_value)
                    for token
                    in tokens
                ),
            )
        return tokens

    @with_cursor
    def list(self, cursor):
        """
        Get all known vouchers.

        :return list[Voucher]: All vouchers known to the store.
        """
        cursor.execute(
            """
            SELECT
                [number], [created], [expected-tokens], [state], [finished], [token-count], [public-key], [counter]
            FROM
                [vouchers]
            """,
        )
        refs = cursor.fetchall()

        return list(
            Voucher.from_row(row)
            for row
            in refs
        )

    def _insert_unblinded_tokens(self, cursor, unblinded_tokens):
        """
        Helper function to really insert unblinded tokens into the database.
        """
        cursor.executemany(
            """
            INSERT INTO [unblinded-tokens] VALUES (?)
            """,
            list(
                (token,)
                for token
                in unblinded_tokens
            ),
        )

    @with_cursor
    def insert_unblinded_tokens(self, cursor, unblinded_tokens):
        """
        Store some unblinded tokens, for example as part of a backup-restore
        process.

        :param list[unicode] unblinded_tokens: The unblinded tokens to store.
        """
        self._insert_unblinded_tokens(cursor, unblinded_tokens)

    @with_cursor
    def insert_unblinded_tokens_for_voucher(self, cursor, voucher, public_key, unblinded_tokens, completed):
        """
        Store some unblinded tokens received from redemption of a voucher.

        :param unicode voucher: The voucher associated with the unblinded
            tokens.  This voucher will be marked as redeemed to indicate it
            has fulfilled its purpose and has no further use for us.

        :param unicode public_key: The encoded public key for the private key
            which was used to sign these tokens.

        :param list[UnblindedToken] unblinded_tokens: The unblinded tokens to
            store.

        :param bool completed: ``True`` if redemption of this voucher is now
            complete, ``False`` otherwise.
        """
        if  completed:
            voucher_state = u"redeemed"
        else:
            voucher_state = u"pending"

        cursor.execute(
            """
            UPDATE [vouchers]
            SET [state] = ?
              , [token-count] = COALESCE([token-count], 0) + ?
              , [finished] = ?
              , [public-key] = ?
              , [counter] = [counter] + 1
            WHERE [number] = ?
            """,
            (
                voucher_state,
                len(unblinded_tokens),
                self.now(),
                public_key,
                voucher,
            ),
        )
        if cursor.rowcount == 0:
            raise ValueError("Cannot insert tokens for unknown voucher; add voucher first")
        self._insert_unblinded_tokens(
            cursor,
            list(
                t.unblinded_token
                for t
                in unblinded_tokens
            ),
        )

    @with_cursor
    def mark_voucher_double_spent(self, cursor, voucher):
        """
        Mark a voucher as having failed redemption because it has already been
        spent.
        """
        cursor.execute(
            """
            UPDATE [vouchers]
            SET [state] = "double-spend"
              , [finished] = ?
            WHERE [number] = ?
              AND [state] = "pending"
            """,
            (self.now(), voucher),
        )
        if cursor.rowcount == 0:
            # Was there no matching voucher or was it in the wrong state?
            cursor.execute(
                """
                SELECT [state]
                FROM [vouchers]
                WHERE [number] = ?
                """,
                (voucher,)
            )
            rows = cursor.fetchall()
            if len(rows) == 0:
                raise ValueError("Voucher {} not found".format(voucher))
            else:
                raise ValueError(
                    "Voucher {} in state {} cannot transition to double-spend".format(
                        voucher,
                        rows[0][0],
                    ),
                )

    @with_cursor
    def get_unblinded_tokens(self, cursor, count):
        """
        Get some unblinded tokens.

        These tokens are not removed from the store but they will not be
        returned from a future call to ``get_unblinded_tokens`` *on this
        ``VoucherStore`` instance* unless ``reset_unblinded_tokens`` is used
        to reset their state.

        If the underlying storage is access via another ``VoucherStore``
        instance then the behavior of this method will be as if all tokens
        which have not had their state changed to invalid or spent have been
        reset.

        :return list[UnblindedTokens]: The removed unblinded tokens.
        """
        if count > _SQLITE3_INTEGER_MAX:
            # An unreasonable number of tokens and also large enough to
            # provoke undesirable behavior from the database.
            raise NotEnoughTokens()

        operation_id = unicode(uuid4())
        cursor.execute(
            """
            INSERT INTO [temp.in-use]
            SELECT [token], ?
            FROM [unblinded-tokens]
            WHERE [token] NOT IN (SELECT [unblinded-token] FROM [temp.in-use])
            LIMIT ?
            """,
            (operation_id, count),
        )
        if cursor.rowcount < count:
            raise NotEnoughTokens()

        cursor.execute(
            """
            SELECT [unblinded-token] FROM [temp.in-use] WHERE [operation-id] = ?
            """,
            (operation_id,),
        )
        texts = cursor.fetchall()
        return list(
            UnblindedToken(t)
            for (t,)
            in texts
        )

    @with_cursor
    def discard_unblinded_tokens(self, cursor, unblinded_tokens):
        """
        Get rid of some unblinded tokens.  The tokens will be completely removed
        from the system.  This is useful when the tokens have been
        successfully spent.

        :param list[UnblindedToken] unblinded_tokens: The tokens to discard.

        :return: ``None``
        """
        cursor.execute(
            """
            CREATE TEMPORARY TABLE [to-discard] (
              [unblinded-token] text
            )
        """)
        cursor.executemany(
            """
            INSERT INTO [to-discard] VALUES (?)
            """,
            list((token.unblinded_token,) for token in unblinded_tokens),
        )
        cursor.execute(
            """
            DELETE FROM [temp.in-use]
            WHERE [unblinded-token] IN (SELECT [unblinded-token] FROM [to-discard])
            """,
        )
        cursor.execute(
            """
            DELETE FROM [unblinded-tokens]
            WHERE [token] IN (SELECT [unblinded-token] FROM [to-discard])
            """,
        )
        cursor.execute(
            """
            DROP TABLE [to-discard]
            """,
        )

    @with_cursor
    def invalidate_unblinded_tokens(self, cursor, reason, unblinded_tokens):
        """
        Mark some unblinded tokens as invalid and unusable.  Some record of the
        tokens may be retained for future inspection.  These tokens will not
        be returned by any future ``get_unblinded_tokens`` call.  This is
        useful when an attempt to spend a token has met with rejection by the
        validator.

        :param list[UnblindedToken] unblinded_tokens: The tokens to mark.

        :return: ``None``
        """
        cursor.executemany(
            """
            INSERT INTO [invalid-unblinded-tokens] VALUES (?, ?)
            """,
            list(
                (token.unblinded_token, reason)
                for token
                in unblinded_tokens
            ),
        )
        cursor.execute(
            """
            DELETE FROM [temp.in-use]
            WHERE [unblinded-token] IN (SELECT [token] FROM [invalid-unblinded-tokens])
            """,
        )
        cursor.execute(
            """
            DELETE FROM [unblinded-tokens]
            WHERE [token] IN (SELECT [token] FROM [invalid-unblinded-tokens])
            """,
        )

    @with_cursor
    def reset_unblinded_tokens(self, cursor, unblinded_tokens):
        """
        Make some unblinded tokens available to be retrieved from the store again.
        This is useful if a spending operation has failed with a transient
        error.
        """
        cursor.execute(
            """
            CREATE TEMPORARY TABLE [to-reset] (
              [unblinded-token] text
            )
            """,
        )
        cursor.executemany(
            """
            INSERT INTO [to-reset] VALUES (?)
            """,
            list((token.unblinded_token,) for token in unblinded_tokens),
        )
        cursor.execute(
            """
            DELETE FROM [temp.in-use]
            WHERE [unblinded-token] IN (SELECT [unblinded-token] FROM [to-reset])
            """,
        )
        cursor.execute(
            """
            DROP TABLE [to-reset]
            """,
        )

    @with_cursor
    def extract_unblinded_tokens(self, cursor, count):
        """
        Remove and return some unblinded tokens.

        :param int count: The maximum number of unblinded tokens to remove and
            return.  If fewer than this are available, only as many as are
            available are returned.

        :return list[UnblindedTokens]: The removed unblinded tokens.
        """
        cursor.execute(
            """
            SELECT COUNT(token)
            FROM [unblinded-tokens]
            """,
        )
        [(existing_tokens,)] = cursor.fetchall()
        if existing_tokens < count:
            raise NotEnoughTokens()

        cursor.execute(
            """
            CREATE TEMPORARY TABLE [extracting]
            AS
            SELECT [token] FROM [unblinded-tokens] LIMIT ?
            """,
            (count,),
        )
        cursor.execute(
            """
            DELETE FROM [unblinded-tokens] WHERE [token] IN [extracting]
            """,
        )
        cursor.execute(
            """
            SELECT [token] FROM [extracting]
            """,
        )
        texts = cursor.fetchall()
        cursor.execute(
            """
            DROP TABLE [extracting]
            """,
        )
        return list(
            UnblindedToken(t)
            for (t,)
            in texts
        )

    @with_cursor
    def backup(self, cursor):
        """
        Read out all state necessary to recreate this database in the event it is
        lost.
        """
        cursor.execute(
            """
            SELECT [token] FROM [unblinded-tokens]
            """,
        )
        tokens = cursor.fetchall()
        return {
            u"unblinded-tokens": list(token for (token,) in tokens),
        }

    def start_lease_maintenance(self):
        """
        Get an object which can track a newly started round of lease maintenance
        activity.

        :return LeaseMaintenance: A new, started lease maintenance object.
        """
        m = LeaseMaintenance(self.pass_value, self.now, self._connection)
        m.start()
        return m

    @with_cursor
    def get_latest_lease_maintenance_activity(self, cursor):
        """
        Get a description of the most recently completed lease maintenance
        activity.

        :return LeaseMaintenanceActivity|None: If any lease maintenance has
            completed, an object describing its results.  Otherwise, None.
        """
        cursor.execute(
            """
            SELECT [started], [count], [finished]
            FROM [lease-maintenance-spending]
            WHERE [finished] IS NOT NULL
            ORDER BY [finished] DESC
            LIMIT 1
            """,
        )
        activity = cursor.fetchall()
        if len(activity) == 0:
            return None
        [(started, count, finished)] = activity
        return LeaseMaintenanceActivity(
            parse_datetime(started, delimiter=u" "),
            count,
            parse_datetime(finished, delimiter=u" "),
        )


@implementer(ILeaseMaintenanceObserver)
@attr.s
class LeaseMaintenance(object):
    """
    A state-updating helper for recording pass usage during a lease
    maintenance run.

    Get one of these from ``VoucherStore.start_lease_maintenance``.  Then use
    the ``observe`` and ``finish`` methods to persist state about a lease
    maintenance run.

    :ivar int _pass_value: The value of a single ZKAP in byte-months.

    :ivar _now: A no-argument callable which returns a datetime giving a time
        to use as current.

    :ivar _connection: A SQLite3 connection object to use to persist observed
        information.

    :ivar _rowid: None for unstarted lease maintenance objects.  For started
        objects, the database row id that corresponds to the started run.
        This is used to make sure future updates go to the right row.
    """
    _pass_value = pass_value_attribute()
    _now = attr.ib()
    _connection = attr.ib()
    _rowid = attr.ib(default=None)

    @with_cursor
    def start(self, cursor):
        """
        Record the start of a lease maintenance run.
        """
        if self._rowid is not None:
            raise Exception("Cannot re-start a particular _LeaseMaintenance.")

        cursor.execute(
            """
            INSERT INTO [lease-maintenance-spending] ([started], [finished], [count])
            VALUES (?, ?, ?)
            """,
            (self._now(), None, 0),
        )
        self._rowid = cursor.lastrowid

    @with_cursor
    def observe(self, cursor, sizes):
        """
        Record a storage shares of the given sizes.
        """
        count = required_passes(self._pass_value, sizes)
        cursor.execute(
            """
            UPDATE [lease-maintenance-spending]
            SET [count] = [count] + ?
            WHERE [id] = ?
            """,
            (count, self._rowid),
        )

    @with_cursor
    def finish(self, cursor):
        """
        Record the completion of this lease maintenance run.
        """
        cursor.execute(
            """
            UPDATE [lease-maintenance-spending]
            SET [finished] = ?
            WHERE [id] = ?
            """,
            (self._now(), self._rowid),
        )
        self._rowid = None


@attr.s
class LeaseMaintenanceActivity(object):
    started = attr.ib()
    passes_required = attr.ib()
    finished = attr.ib()


# store = ...
# x = store.start_lease_maintenance()
# x.observe(size=123)
# x.observe(size=456)
# ...
# x.finish()
#
# x = store.get_latest_lease_maintenance_activity()
# xs.started, xs.passes_required, xs.finished

@attr.s(frozen=True)
class UnblindedToken(object):
    """
    An ``UnblindedToken`` instance represents cryptographic proof of a voucher
    redemption.  It is an intermediate artifact in the PrivacyPass protocol
    and can be used to construct a privacy-preserving pass which can be
    exchanged for service.

    :ivar unicode unblinded_token: The base64 encoded serialized form of the
        unblinded token.  This can be used to reconstruct a
        ``challenge_bypass_ristretto.UnblindedToken`` using that class's
        ``decode_base64`` method.
    """
    unblinded_token = attr.ib(
        validator=attr.validators.and_(
            attr.validators.instance_of(unicode),
            is_base64_encoded(),
            has_length(128),
        ),
    )


@attr.s(frozen=True)
class Pass(object):
    """
    A ``Pass`` instance completely represents a single Zero-Knowledge Access Pass.

    :ivar unicode pass_text: The text value of the pass.  This can be sent to
        a service provider one time to anonymously prove a prior voucher
        redemption.  If it is sent more than once the service provider may
        choose to reject it and the anonymity property is compromised.  Pass
        text should be kept secret.  If pass text is divulged to third-parties
        the anonymity property may be compromised.
    """
    preimage = attr.ib(
        validator=attr.validators.and_(
            attr.validators.instance_of(unicode),
            is_base64_encoded(),
            has_length(88),
        ),
    )

    signature = attr.ib(
        validator=attr.validators.and_(
            attr.validators.instance_of(unicode),
            is_base64_encoded(),
            has_length(88),
        ),
    )

    @property
    def pass_text(self):
        return u"{} {}".format(self.preimage, self.signature)


@attr.s(frozen=True)
class RandomToken(object):
    """
    :ivar unicode token_value: The base64-encoded representation of the random
        token.
    """
    token_value = attr.ib(
        validator=attr.validators.and_(
            attr.validators.instance_of(unicode),
            is_base64_encoded(),
            has_length(128),
        ),
    )


def _counter_attribute():
    return attr.ib(
        validator=attr.validators.and_(
            attr.validators.instance_of((int, long)),
            greater_than(-1),
        ),
    )


@attr.s(frozen=True)
class Pending(object):
    """
    The voucher has not yet been completely redeemed for ZKAPs.

    :ivar int counter: The number of partial redemptions which have been
        successfully performed for the voucher.
    """
    counter = _counter_attribute()

    def should_start_redemption(self):
        return True

    def to_json_v1(self):
        return {
            u"name": u"pending",
            u"counter": self.counter,
        }


@attr.s(frozen=True)
class Redeeming(object):
    """
    This is a non-persistent state in which a voucher exists when the database
    state is **pending** but for which there is a redemption operation in
    progress.
    """
    started = attr.ib(validator=attr.validators.instance_of(datetime))
    counter = _counter_attribute()

    def should_start_redemption(self):
        return False

    def to_json_v1(self):
        return {
            u"name": u"redeeming",
            u"started": self.started.isoformat(),
            u"counter": self.counter,
        }


@attr.s(frozen=True)
class Redeemed(object):
    """
    The voucher was successfully redeemed.  Associated tokens were retrieved
    and stored locally.

    :ivar datetime finished: The time when the redemption finished.

    :ivar int token_count: The number of tokens the voucher was redeemed for.

    :ivar unicode public_key: The public part of the key used to sign the
        tokens for this voucher.
    """
    finished = attr.ib(validator=attr.validators.instance_of(datetime))
    token_count = attr.ib(validator=attr.validators.instance_of((int, long)))
    public_key = attr.ib(validator=attr.validators.optional(attr.validators.instance_of(unicode)))

    def should_start_redemption(self):
        return False

    def to_json_v1(self):
        return {
            u"name": u"redeemed",
            u"finished": self.finished.isoformat(),
            u"token-count": self.token_count,
            u"public-key": self.public_key,
        }


@attr.s(frozen=True)
class DoubleSpend(object):
    finished = attr.ib(validator=attr.validators.instance_of(datetime))

    def should_start_redemption(self):
        return False

    def to_json_v1(self):
        return {
            u"name": u"double-spend",
            u"finished": self.finished.isoformat(),
        }


@attr.s(frozen=True)
class Unpaid(object):
    """
    This is a non-persistent state in which a voucher exists when the database
    state is **pending** but the most recent redemption attempt has failed due
    to lack of payment.
    """
    finished = attr.ib(validator=attr.validators.instance_of(datetime))