"""mysql connector."""
from contextlib import contextmanager
from functools import wraps
import time

from flask import g
from sqlalchemy import create_engine
from sqlalchemy import event
from sqlalchemy import exc
from sqlalchemy import pool
from sqlalchemy import select
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

from content_review import config
from content_review.constants import mysql as mysql_consts


def _create_engine(db_url):
    """Create engine based on configuration settings."""
    if config.POOL_CLASS == pool.QueuePool:
        db_engine = create_engine(
            db_url, pool_size=config.POOL_SIZE,
            max_overflow=config.POOL_MAX_OVERFLOW,
            pool_recycle=config.POOL_RECYCLE_MS)

        # SQLAlchemy 1.12b supports pessimistic disconnect handling out of box.
        # Once version 1.12 is out of beta, it is recommended to use
        # the `pool_pre_ping` param for `create_engine` and remove any code
        # in this file related to `_ping_connection`
        # See http://docs.sqlalchemy.org/en/latest/core/pooling.html
        if config.POOL_PRE_PING:
            @event.listens_for(db_engine, 'engine_connect')
            def ping_connection(connection, branch):
                _ping_connection(connection, branch)

        return db_engine
    else:
        return create_engine(db_url, poolclass=config.POOL_CLASS)


# Do not use these variables directly other than running unit tests
db_engine = _create_engine(config.DB_URL)

# please don't use sessions directly;
# instead use db_session
sessions = {
    mysql_consts.OWS_CONTENT_REVIEW: sessionmaker(bind=db_engine)
}

BaseModel = declarative_base()


@contextmanager
def db_session(db_name=mysql_consts.OWS_CONTENT_REVIEW):
    """Provide a transactional scope around a series of operations.

    Taken from http://docs.sqlalchemy.org/en/latest/orm/session_basics.html.
    This handles rollback and closing of session, so there is no need
    to do that throughout the code.

    Args:
        db_name (session): Name of database to connect to

    Usage:
        with db_session() as session:
            session.execute(query)
    """
    session = sessions[db_name]()
    try:
        yield session
        session.commit()
    except:
        session.rollback()
        raise
    finally:
        session.close()


def _ping_connection(connection, branch):
    """Ping database connection after engine_connect event.

    This function is copied verbatim from
    http://docs.sqlalchemy.org/en/latest/core/pooling.html
    """
    if branch:
        # "branch" refers to a sub-connection of a connection,
        # we don't want to bother pinging on these.
        return

    # turn off "close with result".  This flag is only used with
    # "connectionless" execution, otherwise will be False in any case
    save_should_close_with_result = connection.should_close_with_result
    connection.should_close_with_result = False

    try:
        # run a SELECT 1.   use a core select() so that
        # the SELECT of a scalar value without a table is
        # appropriately formatted for the backend
        connection.scalar(select([1]))
    except exc.DBAPIError as err:
        # catch SQLAlchemy's DBAPIError, which is a wrapper
        # for the DBAPI's exception.  It includes a .connection_invalidated
        # attribute which specifies if this connection is a "disconnect"
        # condition, which is based on inspection of the original exception
        # by the dialect in use.
        if err.connection_invalidated:
            # run the same SELECT again - the connection will re-validate
            # itself and establish a new connection.  The disconnect detection
            # here also causes the whole connection pool to be invalidated
            # so that all stale connections are discarded.
            connection.scalar(select([1]))
        else:
            raise
    finally:
        # restore "close with result"
        connection.should_close_with_result = save_should_close_with_result


def db_session_wrap(fn):
    """DB Session Wrappper.

    Creates a new session if one isn't passed in.
    """
    @wraps(fn)
    def wrapper(*args, **kwargs):
        session = kwargs.pop('session', None)
        if session:
            return fn(*args, session=session, **kwargs)
        else:
            with db_session() as session:
                return fn(*args, session=session, **kwargs)

    return wrapper


def db_session_retry_wrap(fn):
    """DB Session Wrappper with retry.

    Creates a new session if one isn't passed in, and does 1 retry if an
    internal error occurs while executing query. db_session_wrap is still
    preferred since this can rollback the session.
    """
    @wraps(fn)
    def wrapper(*args, **kwargs):
        session = kwargs.pop('session', None)
        try:
            if session:
                return fn(*args, session=session, **kwargs)
            else:
                with db_session() as session:
                    return fn(*args, session=session, **kwargs)
        except exc.InternalError as e:
            # Log warning and sleep for a bit
            g.log.warning(e)
            time.sleep(mysql_consts.INTERNAL_ERROR_SLEEP_TIME)

            if session:
                session.rollback()
                return fn(*args, session=session, **kwargs)
            else:
                with db_session() as session:
                    return fn(*args, session=session, **kwargs)

    return wrapper