# Copyright 2016 Google Inc. All rights reserved.
#
# 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.

"""Pools managing shared Session objects."""

import datetime

from six.moves import queue
from six.moves import xrange

from google.cloud.exceptions import NotFound


_NOW = datetime.datetime.utcnow  # unit tests may replace


class AbstractSessionPool(object):
    """Specifies required API for concrete session pool implementations."""

    _database = None

    def bind(self, database):
        """Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.

        Concrete implementations of this method may pre-fill the pool
        using the database.
        """
        raise NotImplementedError()

    def get(self):
        """Check a session out from the pool.

        Concrete implementations of this method are allowed to raise an
        error to signal that the pool is exhausted, or to block until a
        session is available.
        """
        raise NotImplementedError()

    def put(self, session):
        """Return a session to the pool.

        :type session: :class:`~google.cloud.spanner.session.Session`
        :param session: the session being returned.

        Concrete implementations of this method are allowed to raise an
        error to signal that the pool is full, or to block until it is
        not full.
        """
        raise NotImplementedError()

    def clear(self):
        """Delete all sessions in the pool.

        Concrete implementations of this method are allowed to raise an
        error to signal that the pool is full, or to block until it is
        not full.
        """
        raise NotImplementedError()

    def session(self, **kwargs):
        """Check out a session from the pool.

        :type kwargs: dict
        :param kwargs: (optional) keyword arguments, passed through to
                       the returned checkout.

        :rtype: :class:`~google.cloud.spanner.session.SessionCheckout`
        :returns: a checkout instance, to be used as a context manager for
                  accessing the session and returning it to the pool.
        """
        return SessionCheckout(self, **kwargs)


class FixedSizePool(AbstractSessionPool):
    """Concrete session pool implementation:

    - Pre-allocates / creates a fixed number of sessions.

    - "Pings" existing sessions via :meth:`session.exists` before returning
      them, and replaces expired sessions.

    - Blocks, with a timeout, when :meth:`get` is called on an empty pool.
      Raises after timing out.

    - Raises when :meth:`put` is called on a full pool.  That error is
      never expected in normal practice, as users should be calling
      :meth:`get` followed by :meth:`put` whenever in need of a session.

    :type size: int
    :param size: fixed pool size

    :type default_timeout: int
    :param default_timeout: default timeout, in seconds, to wait for
                                 a returned session.
    """
    DEFAULT_SIZE = 10
    DEFAULT_TIMEOUT = 10

    def __init__(self, size=DEFAULT_SIZE, default_timeout=DEFAULT_TIMEOUT):
        self.size = size
        self.default_timeout = default_timeout
        self._sessions = queue.Queue(size)

    def bind(self, database):
        """Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.
        """
        self._database = database

        while not self._sessions.full():
            session = database.session()
            session.create()
            self._sessions.put(session)

    def get(self, timeout=None):  # pylint: disable=arguments-differ
        """Check a session out from the pool.

        :type timeout: int
        :param timeout: seconds to block waiting for an available session

        :rtype: :class:`~google.cloud.spanner.session.Session`
        :returns: an existing session from the pool, or a newly-created
                  session.
        :raises: :exc:`six.moves.queue.Empty` if the queue is empty.
        """
        if timeout is None:
            timeout = self.default_timeout

        session = self._sessions.get(block=True, timeout=timeout)

        if not session.exists():
            session = self._database.session()
            session.create()

        return session

    def put(self, session):
        """Return a session to the pool.

        Never blocks:  if the pool is full, raises.

        :type session: :class:`~google.cloud.spanner.session.Session`
        :param session: the session being returned.

        :raises: :exc:`six.moves.queue.Full` if the queue is full.
        """
        self._sessions.put_nowait(session)

    def clear(self):
        """Delete all sessions in the pool."""

        while True:
            try:
                session = self._sessions.get(block=False)
            except queue.Empty:
                break
            else:
                session.delete()


class BurstyPool(AbstractSessionPool):
    """Concrete session pool implementation:

    - "Pings" existing sessions via :meth:`session.exists` before returning
      them.

    - Creates a new session, rather than blocking, when :meth:`get` is called
      on an empty pool.

    - Discards the returned session, rather than blocking, when :meth:`put`
      is called on a full pool.

    :type target_size: int
    :param target_size: max pool size
    """

    def __init__(self, target_size=10):
        self.target_size = target_size
        self._database = None
        self._sessions = queue.Queue(target_size)

    def bind(self, database):
        """Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.
        """
        self._database = database

    def get(self):
        """Check a session out from the pool.

        :rtype: :class:`~google.cloud.spanner.session.Session`
        :returns: an existing session from the pool, or a newly-created
                  session.
        """
        try:
            session = self._sessions.get_nowait()
        except queue.Empty:
            session = self._database.session()
            session.create()
        else:
            if not session.exists():
                session = self._database.session()
                session.create()
        return session

    def put(self, session):
        """Return a session to the pool.

        Never blocks:  if the pool is full, the returned session is
        discarded.

        :type session: :class:`~google.cloud.spanner.session.Session`
        :param session: the session being returned.
        """
        try:
            self._sessions.put_nowait(session)
        except queue.Full:
            try:
                session.delete()
            except NotFound:
                pass

    def clear(self):
        """Delete all sessions in the pool."""

        while True:
            try:
                session = self._sessions.get(block=False)
            except queue.Empty:
                break
            else:
                session.delete()


class PingingPool(AbstractSessionPool):
    """Concrete session pool implementation:

    - Pre-allocates / creates a fixed number of sessions.

    - Sessions are used in "round-robin" order (LRU first).

    - "Pings" existing sessions in the background after a specified interval
      via an API call (``session.exists()``).

    - Blocks, with a timeout, when :meth:`get` is called on an empty pool.
      Raises after timing out.

    - Raises when :meth:`put` is called on a full pool.  That error is
      never expected in normal practice, as users should be calling
      :meth:`get` followed by :meth:`put` whenever in need of a session.

    The application is responsible for calling :meth:`ping` at appropriate
    times, e.g. from a background thread.

    :type size: int
    :param size: fixed pool size

    :type default_timeout: int
    :param default_timeout: default timeout, in seconds, to wait for
                            a returned session.

    :type ping_interval: int
    :param ping_interval: interval at which to ping sessions.
    """

    def __init__(self, size=10, default_timeout=10, ping_interval=3000):
        self.size = size
        self.default_timeout = default_timeout
        self._delta = datetime.timedelta(seconds=ping_interval)
        self._sessions = queue.PriorityQueue(size)

    def bind(self, database):
        """Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.
        """
        self._database = database

        for _ in xrange(self.size):
            session = database.session()
            session.create()
            self.put(session)

    def get(self, timeout=None):  # pylint: disable=arguments-differ
        """Check a session out from the pool.

        :type timeout: int
        :param timeout: seconds to block waiting for an available session

        :rtype: :class:`~google.cloud.spanner.session.Session`
        :returns: an existing session from the pool, or a newly-created
                  session.
        :raises: :exc:`six.moves.queue.Empty` if the queue is empty.
        """
        if timeout is None:
            timeout = self.default_timeout

        ping_after, session = self._sessions.get(block=True, timeout=timeout)

        if _NOW() > ping_after:
            if not session.exists():
                session = self._database.session()
                session.create()

        return session

    def put(self, session):
        """Return a session to the pool.

        Never blocks:  if the pool is full, raises.

        :type session: :class:`~google.cloud.spanner.session.Session`
        :param session: the session being returned.

        :raises: :exc:`six.moves.queue.Full` if the queue is full.
        """
        self._sessions.put_nowait((_NOW() + self._delta, session))

    def clear(self):
        """Delete all sessions in the pool."""
        while True:
            try:
                _, session = self._sessions.get(block=False)
            except queue.Empty:
                break
            else:
                session.delete()

    def ping(self):
        """Refresh maybe-expired sessions in the pool.

        This method is designed to be called from a background thread,
        or during the "idle" phase of an event loop.
        """
        while True:
            try:
                ping_after, session = self._sessions.get(block=False)
            except queue.Empty:  # all sessions in use
                break
            if ping_after > _NOW():  # oldest session is fresh
                # Re-add to queue with existing expiration
                self._sessions.put((ping_after, session))
                break
            if not session.exists():  # stale
                session = self._database.session()
                session.create()
            # Re-add to queue with new expiration
            self.put(session)


class TransactionPingingPool(PingingPool):
    """Concrete session pool implementation:

    In addition to the features of :class:`PingingPool`, this class
    creates and begins a transaction for each of its sessions at startup.

    When a session is returned to the pool, if its transaction has been
    committed or rolled back, the pool creates a new transaction for the
    session and pushes the transaction onto a separate queue of "transactions
    to begin."  The application is responsible for flushing this queue
    as appropriate via the pool's :meth:`begin_pending_transactions` method.

    :type size: int
    :param size: fixed pool size

    :type default_timeout: int
    :param default_timeout: default timeout, in seconds, to wait for
                            a returned session.

    :type ping_interval: int
    :param ping_interval: interval at which to ping sessions.
    """

    def __init__(self, size=10, default_timeout=10, ping_interval=3000):
        self._pending_sessions = queue.Queue()

        super(TransactionPingingPool, self).__init__(
            size, default_timeout, ping_interval)

        self.begin_pending_transactions()

    def bind(self, database):
        """Associate the pool with a database.

        :type database: :class:`~google.cloud.spanner.database.Database`
        :param database: database used by the pool:  used to create sessions
                         when needed.
        """
        super(TransactionPingingPool, self).bind(database)
        self.begin_pending_transactions()

    def put(self, session):
        """Return a session to the pool.

        Never blocks:  if the pool is full, raises.

        :type session: :class:`~google.cloud.spanner.session.Session`
        :param session: the session being returned.

        :raises: :exc:`six.moves.queue.Full` if the queue is full.
        """
        if self._sessions.full():
            raise queue.Full

        txn = session._transaction
        if txn is None or txn.committed() or txn._rolled_back:
            session.transaction()
            self._pending_sessions.put(session)
        else:
            super(TransactionPingingPool, self).put(session)

    def begin_pending_transactions(self):
        """Begin all transactions for sessions added to the pool."""
        while not self._pending_sessions.empty():
            session = self._pending_sessions.get()
            session._transaction.begin()
            super(TransactionPingingPool, self).put(session)


class SessionCheckout(object):
    """Context manager: hold session checked out from a pool.

    :type pool: concrete subclass of
        :class:`~google.cloud.spanner.session.AbstractSessionPool`
    :param pool: Pool from which to check out a session.

    :type kwargs: dict
    :param kwargs: extra keyword arguments to be passed to :meth:`pool.get`.
    """
    _session = None  # Not checked out until '__enter__'.

    def __init__(self, pool, **kwargs):
        self._pool = pool
        self._kwargs = kwargs.copy()

    def __enter__(self):
        self._session = self._pool.get(**self._kwargs)
        return self._session

    def __exit__(self, *ignored):
        self._pool.put(self._session)
