# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket 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 synapse.util.async

from ._base import SQLBaseStore
from . import engines

from twisted.internet import defer

import ujson as json
import logging

logger = logging.getLogger(__name__)


class BackgroundUpdatePerformance(object):
    """Tracks the how long a background update is taking to update its items"""

    def __init__(self, name):
        self.name = name
        self.total_item_count = 0
        self.total_duration_ms = 0
        self.avg_item_count = 0
        self.avg_duration_ms = 0

    def update(self, item_count, duration_ms):
        """Update the stats after doing an update"""
        self.total_item_count += item_count
        self.total_duration_ms += duration_ms

        # Exponential moving averages for the number of items updated and
        # the duration.
        self.avg_item_count += 0.1 * (item_count - self.avg_item_count)
        self.avg_duration_ms += 0.1 * (duration_ms - self.avg_duration_ms)

    def average_items_per_ms(self):
        """An estimate of how long it takes to do a single update.
        Returns:
            A duration in ms as a float
        """
        if self.total_item_count == 0:
            return None
        else:
            # Use the exponential moving average so that we can adapt to
            # changes in how long the update process takes.
            return float(self.avg_item_count) / float(self.avg_duration_ms)

    def total_items_per_ms(self):
        """An estimate of how long it takes to do a single update.
        Returns:
            A duration in ms as a float
        """
        if self.total_item_count == 0:
            return None
        else:
            return float(self.total_item_count) / float(self.total_duration_ms)


class BackgroundUpdateStore(SQLBaseStore):
    """ Background updates are updates to the database that run in the
    background. Each update processes a batch of data at once. We attempt to
    limit the impact of each update by monitoring how long each batch takes to
    process and autotuning the batch size.
    """

    MINIMUM_BACKGROUND_BATCH_SIZE = 100
    DEFAULT_BACKGROUND_BATCH_SIZE = 100
    BACKGROUND_UPDATE_INTERVAL_MS = 1000
    BACKGROUND_UPDATE_DURATION_MS = 100

    def __init__(self, db_conn, hs):
        super(BackgroundUpdateStore, self).__init__(db_conn, hs)
        self._background_update_performance = {}
        self._background_update_queue = []
        self._background_update_handlers = {}

    @defer.inlineCallbacks
    def start_doing_background_updates(self):
        logger.info("Starting background schema updates")

        while True:
            yield synapse.util.async.sleep(
                self.BACKGROUND_UPDATE_INTERVAL_MS / 1000.)

            try:
                result = yield self.do_next_background_update(
                    self.BACKGROUND_UPDATE_DURATION_MS
                )
            except Exception:
                logger.exception("Error doing update")
            else:
                if result is None:
                    logger.info(
                        "No more background updates to do."
                        " Unscheduling background update task."
                    )
                    defer.returnValue(None)

    @defer.inlineCallbacks
    def do_next_background_update(self, desired_duration_ms):
        """Does some amount of work on the next queued background update

        Args:
            desired_duration_ms(float): How long we want to spend
                updating.
        Returns:
            A deferred that completes once some amount of work is done.
            The deferred will have a value of None if there is currently
            no more work to do.
        """
        if not self._background_update_queue:
            updates = yield self._simple_select_list(
                "background_updates",
                keyvalues=None,
                retcols=("update_name", "depends_on"),
            )
            in_flight = set(update["update_name"] for update in updates)
            for update in updates:
                if update["depends_on"] not in in_flight:
                    self._background_update_queue.append(update['update_name'])

        if not self._background_update_queue:
            # no work left to do
            defer.returnValue(None)

        # pop from the front, and add back to the back
        update_name = self._background_update_queue.pop(0)
        self._background_update_queue.append(update_name)

        res = yield self._do_background_update(update_name, desired_duration_ms)
        defer.returnValue(res)

    @defer.inlineCallbacks
    def _do_background_update(self, update_name, desired_duration_ms):
        logger.info("Starting update batch on background update '%s'",
                    update_name)

        update_handler = self._background_update_handlers[update_name]

        performance = self._background_update_performance.get(update_name)

        if performance is None:
            performance = BackgroundUpdatePerformance(update_name)
            self._background_update_performance[update_name] = performance

        items_per_ms = performance.average_items_per_ms()

        if items_per_ms is not None:
            batch_size = int(desired_duration_ms * items_per_ms)
            # Clamp the batch size so that we always make progress
            batch_size = max(batch_size, self.MINIMUM_BACKGROUND_BATCH_SIZE)
        else:
            batch_size = self.DEFAULT_BACKGROUND_BATCH_SIZE

        progress_json = yield self._simple_select_one_onecol(
            "background_updates",
            keyvalues={"update_name": update_name},
            retcol="progress_json"
        )

        progress = json.loads(progress_json)

        time_start = self._clock.time_msec()
        items_updated = yield update_handler(progress, batch_size)
        time_stop = self._clock.time_msec()

        duration_ms = time_stop - time_start

        logger.info(
            "Updating %r. Updated %r items in %rms."
            " (total_rate=%r/ms, current_rate=%r/ms, total_updated=%r, batch_size=%r)",
            update_name, items_updated, duration_ms,
            performance.total_items_per_ms(),
            performance.average_items_per_ms(),
            performance.total_item_count,
            batch_size,
        )

        performance.update(items_updated, duration_ms)

        defer.returnValue(len(self._background_update_performance))

    def register_background_update_handler(self, update_name, update_handler):
        """Register a handler for doing a background update.

        The handler should take two arguments:

        * A dict of the current progress
        * An integer count of the number of items to update in this batch.

        The handler should return a deferred integer count of items updated.
        The hander is responsible for updating the progress of the update.

        Args:
            update_name(str): The name of the update that this code handles.
            update_handler(function): The function that does the update.
        """
        self._background_update_handlers[update_name] = update_handler

    def register_background_index_update(self, update_name, index_name,
                                         table, columns, where_clause=None,
                                         unique=False,
                                         psql_only=False):
        """Helper for store classes to do a background index addition

        To use:

        1. use a schema delta file to add a background update. Example:
            INSERT INTO background_updates (update_name, progress_json) VALUES
                ('my_new_index', '{}');

        2. In the Store constructor, call this method

        Args:
            update_name (str): update_name to register for
            index_name (str): name of index to add
            table (str): table to add index to
            columns (list[str]): columns/expressions to include in index
            unique (bool): true to make a UNIQUE index
            psql_only: true to only create this index on psql databases (useful
                for virtual sqlite tables)
        """

        def create_index_psql(conn):
            conn.rollback()
            # postgres insists on autocommit for the index
            conn.set_session(autocommit=True)

            try:
                c = conn.cursor()

                # If a previous attempt to create the index was interrupted,
                # we may already have a half-built index. Let's just drop it
                # before trying to create it again.

                sql = "DROP INDEX IF EXISTS %s" % (index_name,)
                logger.debug("[SQL] %s", sql)
                c.execute(sql)

                sql = (
                    "CREATE %(unique)s INDEX CONCURRENTLY %(name)s"
                    " ON %(table)s"
                    " (%(columns)s) %(where_clause)s"
                ) % {
                    "unique": "UNIQUE" if unique else "",
                    "name": index_name,
                    "table": table,
                    "columns": ", ".join(columns),
                    "where_clause": "WHERE " + where_clause if where_clause else ""
                }
                logger.debug("[SQL] %s", sql)
                c.execute(sql)
            finally:
                conn.set_session(autocommit=False)

        def create_index_sqlite(conn):
            # Sqlite doesn't support concurrent creation of indexes.
            #
            # We don't use partial indices on SQLite as it wasn't introduced
            # until 3.8, and wheezy has 3.7
            #
            # We assume that sqlite doesn't give us invalid indices; however
            # we may still end up with the index existing but the
            # background_updates not having been recorded if synapse got shut
            # down at the wrong moment - hance we use IF NOT EXISTS. (SQLite
            # has supported CREATE TABLE|INDEX IF NOT EXISTS since 3.3.0.)
            sql = (
                "CREATE %(unique)s INDEX IF NOT EXISTS %(name)s ON %(table)s"
                " (%(columns)s)"
            ) % {
                "unique": "UNIQUE" if unique else "",
                "name": index_name,
                "table": table,
                "columns": ", ".join(columns),
            }

            c = conn.cursor()
            logger.debug("[SQL] %s", sql)
            c.execute(sql)

        if isinstance(self.database_engine, engines.PostgresEngine):
            runner = create_index_psql
        elif psql_only:
            runner = None
        else:
            runner = create_index_sqlite

        @defer.inlineCallbacks
        def updater(progress, batch_size):
            if runner is not None:
                logger.info("Adding index %s to %s", index_name, table)
                yield self.runWithConnection(runner)
            yield self._end_background_update(update_name)
            defer.returnValue(1)

        self.register_background_update_handler(update_name, updater)

    def start_background_update(self, update_name, progress):
        """Starts a background update running.

        Args:
            update_name: The update to set running.
            progress: The initial state of the progress of the update.

        Returns:
            A deferred that completes once the task has been added to the
            queue.
        """
        # Clear the background update queue so that we will pick up the new
        # task on the next iteration of do_background_update.
        self._background_update_queue = []
        progress_json = json.dumps(progress)

        return self._simple_insert(
            "background_updates",
            {"update_name": update_name, "progress_json": progress_json}
        )

    def _end_background_update(self, update_name):
        """Removes a completed background update task from the queue.

        Args:
            update_name(str): The name of the completed task to remove
        Returns:
            A deferred that completes once the task is removed.
        """
        self._background_update_queue = [
            name for name in self._background_update_queue if name != update_name
        ]
        return self._simple_delete_one(
            "background_updates", keyvalues={"update_name": update_name}
        )

    def _background_update_progress_txn(self, txn, update_name, progress):
        """Update the progress of a background update

        Args:
            txn(cursor): The transaction.
            update_name(str): The name of the background update task
            progress(dict): The progress of the update.
        """

        progress_json = json.dumps(progress)

        self._simple_update_one_txn(
            txn,
            "background_updates",
            keyvalues={"update_name": update_name},
            updatevalues={"progress_json": progress_json},
        )
