"""DB Connector. Manages interactions with Snowflake."""

from contextlib import contextmanager
from functools import wraps

from oto import response
import sentry_sdk
from snowflake.sqlalchemy import URL
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 conflict_manager import config


BaseModel = declarative_base()


def _create_engine():
    """Create engine based on configuration settings.

    Args:
        db_url (str): database URL

    Returns:
        obj: db_engine
    """
    db_url = URL(
        account=config.SNOWFLAKE_ACCOUNT,
        user=config.SNOWFLAKE_USER,
        role=config.SNOWFLAKE_ROLE,
        warehouse=config.SNOWFLAKE_WAREHOUSE)

    if config.POOL_CLASS != pool.QueuePool:
        return create_engine(
            db_url,
            connect_args=config.SNOWFLAKE_CONNECT_ARGS,
            poolclass=config.POOL_CLASS)

    db_engine = create_engine(
        db_url,
        connect_args=config.SNOWFLAKE_CONNECT_ARGS,
        pool_size=config.POOL_SIZE,
        max_overflow=config.POOL_MAX_OVERFLOW,
        pool_recycle=config.POOL_RECYCLE)

    # SQLAlchemy 1.2b supports pessimistic disconnect handling out of box.
    # Once version 1.2 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.listen(db_engine, 'engine_connect', _ping_connection)

    return db_engine


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:
        connection.should_close_with_result = save_should_close_with_result


@contextmanager
def db_session():
    """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)
    """
    _db_engine = _create_engine()
    _db_session = sessionmaker(bind=_db_engine)
    session = _db_session()
    try:
        yield session
        session.commit()
    except:  # noqa: E722
        session.rollback()
        raise
    finally:
        session.close()


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

    Creates a new session if one isn't passed in.
    """
    @wraps(func)
    def wrapper(*args, **kwargs):
        session = kwargs.pop('session', None)
        try:
            if session:
                result = func(*args, session=session, **kwargs)
            else:
                with db_session() as session:
                    result = func(*args, session=session, **kwargs)
            return result
        except Exception as e:
            if config.SENTRY_DSN:
                # Capture exceptions using Sentry when available
                sentry_sdk.capture_exception(e)
                return response.create_fatal_response(str(e))

            raise

    return wrapper