Refactor pool.py into a package

for better separation of concerns split out the
core pooling constructs from the implementations.  also
makes more room for new pool classes should we decide to
add them.  The DBProxy feature is never used so remove it
from documentation and split it out.

Change-Id: I04c9d7ae1882b17f3cf5c37ed05b781b05ef88fa
Fixes: #4255

4 files changed, 1566 insertions, 0 deletions

diff --git a/lib/sqlalchemy/pool/__init__.py b/lib/sqlalchemy/pool/__init__.py
new file mode 100644
index 000000000..f2f035051
--- /dev/null
+++ b/lib/sqlalchemy/pool/__init__.py
@@ -0,0 +1,30 @@
+# sqlalchemy/pool/__init__.py
+# Copyright (C) 2005-2018 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: http://www.opensource.org/licenses/mit-license.php
+
+
+"""Connection pooling for DB-API connections.
+
+Provides a number of connection pool implementations for a variety of
+usage scenarios and thread behavior requirements imposed by the
+application, DB-API or database itself.
+
+Also provides a DB-API 2.0 connection proxying mechanism allowing
+regular DB-API connect() methods to be transparently managed by a
+SQLAlchemy connection pool.
+"""
+
+from .base import _refs  # noqa
+from .base import Pool  # noqa
+from .impl import (  # noqa
+    QueuePool, StaticPool, NullPool, AssertionPool, SingletonThreadPool)
+from .dbapi_proxy import manage, clear_managers  # noqa
+
+from .base import reset_rollback, reset_commit, reset_none  # noqa
+
+# as these are likely to be used in various test suites, debugging
+# setups, keep them in the sqlalchemy.pool namespace
+from .base import _ConnectionFairy, _ConnectionRecord, _finalize_fairy  # noqa
diff --git a/lib/sqlalchemy/pool/base.py b/lib/sqlalchemy/pool/base.py
new file mode 100644
index 000000000..855110160
--- /dev/null
+++ b/lib/sqlalchemy/pool/base.py
@@ -0,0 +1,971 @@
+# sqlalchemy/pool.py
+# Copyright (C) 2005-2018 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: http://www.opensource.org/licenses/mit-license.php
+
+
+"""Base constructs for connection pools.
+
+"""
+
+from collections import deque
+import time
+import weakref
+
+from .. import exc, log, event, interfaces, util
+from ..util import threading
+
+
+reset_rollback = util.symbol('reset_rollback')
+reset_commit = util.symbol('reset_commit')
+reset_none = util.symbol('reset_none')
+
+
+class _ConnDialect(object):
+
+    """partial implementation of :class:`.Dialect`
+    which provides DBAPI connection methods.
+
+    When a :class:`.Pool` is combined with an :class:`.Engine`,
+    the :class:`.Engine` replaces this with its own
+    :class:`.Dialect`.
+
+    """
+
+    def do_rollback(self, dbapi_connection):
+        dbapi_connection.rollback()
+
+    def do_commit(self, dbapi_connection):
+        dbapi_connection.commit()
+
+    def do_close(self, dbapi_connection):
+        dbapi_connection.close()
+
+    def do_ping(self, dbapi_connection):
+        raise NotImplementedError(
+            "The ping feature requires that a dialect is "
+            "passed to the connection pool.")
+
+
+class Pool(log.Identified):
+
+    """Abstract base class for connection pools."""
+
+    _dialect = _ConnDialect()
+
+    def __init__(self,
+                 creator, recycle=-1, echo=None,
+                 use_threadlocal=False,
+                 logging_name=None,
+                 reset_on_return=True,
+                 listeners=None,
+                 events=None,
+                 dialect=None,
+                 pre_ping=False,
+                 _dispatch=None):
+        """
+        Construct a Pool.
+
+        :param creator: a callable function that returns a DB-API
+          connection object.  The function will be called with
+          parameters.
+
+        :param recycle: If set to non -1, number of seconds between
+          connection recycling, which means upon checkout, if this
+          timeout is surpassed the connection will be closed and
+          replaced with a newly opened connection. Defaults to -1.
+
+        :param logging_name:  String identifier which will be used within
+          the "name" field of logging records generated within the
+          "sqlalchemy.pool" logger. Defaults to a hexstring of the object's
+          id.
+
+        :param echo: If True, connections being pulled and retrieved
+          from the pool will be logged to the standard output, as well
+          as pool sizing information.  Echoing can also be achieved by
+          enabling logging for the "sqlalchemy.pool"
+          namespace. Defaults to False.
+
+        :param use_threadlocal: If set to True, repeated calls to
+          :meth:`connect` within the same application thread will be
+          guaranteed to return the same connection object, if one has
+          already been retrieved from the pool and has not been
+          returned yet.  Offers a slight performance advantage at the
+          cost of individual transactions by default.  The
+          :meth:`.Pool.unique_connection` method is provided to return
+          a consistently unique connection to bypass this behavior
+          when the flag is set.
+
+          .. warning::  The :paramref:`.Pool.use_threadlocal` flag
+             **does not affect the behavior** of :meth:`.Engine.connect`.
+             :meth:`.Engine.connect` makes use of the
+             :meth:`.Pool.unique_connection` method which **does not use thread
+             local context**.  To produce a :class:`.Connection` which refers
+             to the :meth:`.Pool.connect` method, use
+             :meth:`.Engine.contextual_connect`.
+
+             Note that other SQLAlchemy connectivity systems such as
+             :meth:`.Engine.execute` as well as the orm
+             :class:`.Session` make use of
+             :meth:`.Engine.contextual_connect` internally, so these functions
+             are compatible with the :paramref:`.Pool.use_threadlocal` setting.
+
+          .. seealso::
+
+            :ref:`threadlocal_strategy` - contains detail on the
+            "threadlocal" engine strategy, which provides a more comprehensive
+            approach to "threadlocal" connectivity for the specific
+            use case of using :class:`.Engine` and :class:`.Connection` objects
+            directly.
+
+        :param reset_on_return: Determine steps to take on
+          connections as they are returned to the pool.
+          reset_on_return can have any of these values:
+
+          * ``"rollback"`` - call rollback() on the connection,
+            to release locks and transaction resources.
+            This is the default value.  The vast majority
+            of use cases should leave this value set.
+          * ``True`` - same as 'rollback', this is here for
+            backwards compatibility.
+          * ``"commit"`` - call commit() on the connection,
+            to release locks and transaction resources.
+            A commit here may be desirable for databases that
+            cache query plans if a commit is emitted,
+            such as Microsoft SQL Server.  However, this
+            value is more dangerous than 'rollback' because
+            any data changes present on the transaction
+            are committed unconditionally.
+          * ``None`` - don't do anything on the connection.
+            This setting should only be made on a database
+            that has no transaction support at all,
+            namely MySQL MyISAM.   By not doing anything,
+            performance can be improved.   This
+            setting should **never be selected** for a
+            database that supports transactions,
+            as it will lead to deadlocks and stale
+            state.
+          * ``"none"`` - same as ``None``
+
+            .. versionadded:: 0.9.10
+
+          * ``False`` - same as None, this is here for
+            backwards compatibility.
+
+          .. versionchanged:: 0.7.6
+              :paramref:`.Pool.reset_on_return` accepts ``"rollback"``
+              and ``"commit"`` arguments.
+
+        :param events: a list of 2-tuples, each of the form
+         ``(callable, target)`` which will be passed to :func:`.event.listen`
+         upon construction.   Provided here so that event listeners
+         can be assigned via :func:`.create_engine` before dialect-level
+         listeners are applied.
+
+        :param listeners: Deprecated.  A list of
+          :class:`~sqlalchemy.interfaces.PoolListener`-like objects or
+          dictionaries of callables that receive events when DB-API
+          connections are created, checked out and checked in to the
+          pool.  This has been superseded by
+          :func:`~sqlalchemy.event.listen`.
+
+        :param dialect: a :class:`.Dialect` that will handle the job
+         of calling rollback(), close(), or commit() on DBAPI connections.
+         If omitted, a built-in "stub" dialect is used.   Applications that
+         make use of :func:`~.create_engine` should not use this parameter
+         as it is handled by the engine creation strategy.
+
+         .. versionadded:: 1.1 - ``dialect`` is now a public parameter
+            to the :class:`.Pool`.
+
+        :param pre_ping: if True, the pool will emit a "ping" (typically
+         "SELECT 1", but is dialect-specific) on the connection
+         upon checkout, to test if the connection is alive or not.   If not,
+         the connection is transparently re-connected and upon success, all
+         other pooled connections established prior to that timestamp are
+         invalidated.     Requires that a dialect is passed as well to
+         interpret the disconnection error.
+
+         .. versionadded:: 1.2
+
+        """
+        if logging_name:
+            self.logging_name = self._orig_logging_name = logging_name
+        else:
+            self._orig_logging_name = None
+
+        log.instance_logger(self, echoflag=echo)
+        self._threadconns = threading.local()
+        self._creator = creator
+        self._recycle = recycle
+        self._invalidate_time = 0
+        self._use_threadlocal = use_threadlocal
+        self._pre_ping = pre_ping
+        if reset_on_return in ('rollback', True, reset_rollback):
+            self._reset_on_return = reset_rollback
+        elif reset_on_return in ('none', None, False, reset_none):
+            self._reset_on_return = reset_none
+        elif reset_on_return in ('commit', reset_commit):
+            self._reset_on_return = reset_commit
+        else:
+            raise exc.ArgumentError(
+                "Invalid value for 'reset_on_return': %r"
+                % reset_on_return)
+
+        self.echo = echo
+
+        if _dispatch:
+            self.dispatch._update(_dispatch, only_propagate=False)
+        if dialect:
+            self._dialect = dialect
+        if events:
+            for fn, target in events:
+                event.listen(self, target, fn)
+        if listeners:
+            util.warn_deprecated(
+                "The 'listeners' argument to Pool (and "
+                "create_engine()) is deprecated.  Use event.listen().")
+            for l in listeners:
+                self.add_listener(l)
+
+    @property
+    def _creator(self):
+        return self.__dict__['_creator']
+
+    @_creator.setter
+    def _creator(self, creator):
+        self.__dict__['_creator'] = creator
+        self._invoke_creator = self._should_wrap_creator(creator)
+
+    def _should_wrap_creator(self, creator):
+        """Detect if creator accepts a single argument, or is sent
+        as a legacy style no-arg function.
+
+        """
+
+        try:
+            argspec = util.get_callable_argspec(self._creator, no_self=True)
+        except TypeError:
+            return lambda crec: creator()
+
+        defaulted = argspec[3] is not None and len(argspec[3]) or 0
+        positionals = len(argspec[0]) - defaulted
+
+        # look for the exact arg signature that DefaultStrategy
+        # sends us
+        if (argspec[0], argspec[3]) == (['connection_record'], (None,)):
+            return creator
+        # or just a single positional
+        elif positionals == 1:
+            return creator
+        # all other cases, just wrap and assume legacy "creator" callable
+        # thing
+        else:
+            return lambda crec: creator()
+
+    def _close_connection(self, connection):
+        self.logger.debug("Closing connection %r", connection)
+
+        try:
+            self._dialect.do_close(connection)
+        except Exception:
+            self.logger.error("Exception closing connection %r",
+                              connection, exc_info=True)
+
+    @util.deprecated(
+        2.7, "Pool.add_listener is deprecated.  Use event.listen()")
+    def add_listener(self, listener):
+        """Add a :class:`.PoolListener`-like object to this pool.
+
+        ``listener`` may be an object that implements some or all of
+        PoolListener, or a dictionary of callables containing implementations
+        of some or all of the named methods in PoolListener.
+
+        """
+        interfaces.PoolListener._adapt_listener(self, listener)
+
+    def unique_connection(self):
+        """Produce a DBAPI connection that is not referenced by any
+        thread-local context.
+
+        This method is equivalent to :meth:`.Pool.connect` when the
+        :paramref:`.Pool.use_threadlocal` flag is not set to True.
+        When :paramref:`.Pool.use_threadlocal` is True, the
+        :meth:`.Pool.unique_connection` method provides a means of bypassing
+        the threadlocal context.
+
+        """
+        return _ConnectionFairy._checkout(self)
+
+    def _create_connection(self):
+        """Called by subclasses to create a new ConnectionRecord."""
+
+        return _ConnectionRecord(self)
+
+    def _invalidate(self, connection, exception=None, _checkin=True):
+        """Mark all connections established within the generation
+        of the given connection as invalidated.
+
+        If this pool's last invalidate time is before when the given
+        connection was created, update the timestamp til now.  Otherwise,
+        no action is performed.
+
+        Connections with a start time prior to this pool's invalidation
+        time will be recycled upon next checkout.
+        """
+        rec = getattr(connection, "_connection_record", None)
+        if not rec or self._invalidate_time < rec.starttime:
+            self._invalidate_time = time.time()
+        if _checkin and getattr(connection, 'is_valid', False):
+            connection.invalidate(exception)
+
+    def recreate(self):
+        """Return a new :class:`.Pool`, of the same class as this one
+        and configured with identical creation arguments.
+
+        This method is used in conjunction with :meth:`dispose`
+        to close out an entire :class:`.Pool` and create a new one in
+        its place.
+
+        """
+
+        raise NotImplementedError()
+
+    def dispose(self):
+        """Dispose of this pool.
+
+        This method leaves the possibility of checked-out connections
+        remaining open, as it only affects connections that are
+        idle in the pool.
+
+        See also the :meth:`Pool.recreate` method.
+
+        """
+
+        raise NotImplementedError()
+
+    def connect(self):
+        """Return a DBAPI connection from the pool.
+
+        The connection is instrumented such that when its
+        ``close()`` method is called, the connection will be returned to
+        the pool.
+
+        """
+        if not self._use_threadlocal:
+            return _ConnectionFairy._checkout(self)
+
+        try:
+            rec = self._threadconns.current()
+        except AttributeError:
+            pass
+        else:
+            if rec is not None:
+                return rec._checkout_existing()
+
+        return _ConnectionFairy._checkout(self, self._threadconns)
+
+    def _return_conn(self, record):
+        """Given a _ConnectionRecord, return it to the :class:`.Pool`.
+
+        This method is called when an instrumented DBAPI connection
+        has its ``close()`` method called.
+
+        """
+        if self._use_threadlocal:
+            try:
+                del self._threadconns.current
+            except AttributeError:
+                pass
+        self._do_return_conn(record)
+
+    def _do_get(self):
+        """Implementation for :meth:`get`, supplied by subclasses."""
+
+        raise NotImplementedError()
+
+    def _do_return_conn(self, conn):
+        """Implementation for :meth:`return_conn`, supplied by subclasses."""
+
+        raise NotImplementedError()
+
+    def status(self):
+        raise NotImplementedError()
+
+
+class _ConnectionRecord(object):
+
+    """Internal object which maintains an individual DBAPI connection
+    referenced by a :class:`.Pool`.
+
+    The :class:`._ConnectionRecord` object always exists for any particular
+    DBAPI connection whether or not that DBAPI connection has been
+    "checked out".  This is in contrast to the :class:`._ConnectionFairy`
+    which is only a public facade to the DBAPI connection while it is checked
+    out.
+
+    A :class:`._ConnectionRecord` may exist for a span longer than that
+    of a single DBAPI connection.  For example, if the
+    :meth:`._ConnectionRecord.invalidate`
+    method is called, the DBAPI connection associated with this
+    :class:`._ConnectionRecord`
+    will be discarded, but the :class:`._ConnectionRecord` may be used again,
+    in which case a new DBAPI connection is produced when the :class:`.Pool`
+    next uses this record.
+
+    The :class:`._ConnectionRecord` is delivered along with connection
+    pool events, including :meth:`.PoolEvents.connect` and
+    :meth:`.PoolEvents.checkout`, however :class:`._ConnectionRecord` still
+    remains an internal object whose API and internals may change.
+
+    .. seealso::
+
+        :class:`._ConnectionFairy`
+
+    """
+
+    def __init__(self, pool, connect=True):
+        self.__pool = pool
+        if connect:
+            self.__connect(first_connect_check=True)
+        self.finalize_callback = deque()
+
+    fairy_ref = None
+
+    starttime = None
+
+    connection = None
+    """A reference to the actual DBAPI connection being tracked.
+
+    May be ``None`` if this :class:`._ConnectionRecord` has been marked
+    as invalidated; a new DBAPI connection may replace it if the owning
+    pool calls upon this :class:`._ConnectionRecord` to reconnect.
+
+    """
+
+    _soft_invalidate_time = 0
+
+    @util.memoized_property
+    def info(self):
+        """The ``.info`` dictionary associated with the DBAPI connection.
+
+        This dictionary is shared among the :attr:`._ConnectionFairy.info`
+        and :attr:`.Connection.info` accessors.
+
+        .. note::
+
+            The lifespan of this dictionary is linked to the
+            DBAPI connection itself, meaning that it is **discarded** each time
+            the DBAPI connection is closed and/or invalidated.   The
+            :attr:`._ConnectionRecord.record_info` dictionary remains
+            persistent throughout the lifespan of the
+            :class:`._ConnectionRecord` container.
+
+        """
+        return {}
+
+    @util.memoized_property
+    def record_info(self):
+        """An "info' dictionary associated with the connection record
+        itself.
+
+        Unlike the :attr:`._ConnectionRecord.info` dictionary, which is linked
+        to the lifespan of the DBAPI connection, this dictionary is linked
+        to the lifespan of the :class:`._ConnectionRecord` container itself
+        and will remain persisent throughout the life of the
+        :class:`._ConnectionRecord`.
+
+        .. versionadded:: 1.1
+
+        """
+        return {}
+
+    @classmethod
+    def checkout(cls, pool):
+        rec = pool._do_get()
+        try:
+            dbapi_connection = rec.get_connection()
+        except Exception as err:
+            with util.safe_reraise():
+                rec._checkin_failed(err)
+        echo = pool._should_log_debug()
+        fairy = _ConnectionFairy(dbapi_connection, rec, echo)
+        rec.fairy_ref = weakref.ref(
+            fairy,
+            lambda ref: _finalize_fairy and
+            _finalize_fairy(
+                None,
+                rec, pool, ref, echo)
+        )
+        _refs.add(rec)
+        if echo:
+            pool.logger.debug("Connection %r checked out from pool",
+                              dbapi_connection)
+        return fairy
+
+    def _checkin_failed(self, err):
+        self.invalidate(e=err)
+        self.checkin(_no_fairy_ref=True)
+
+    def checkin(self, _no_fairy_ref=False):
+        if self.fairy_ref is None and not _no_fairy_ref:
+            util.warn("Double checkin attempted on %s" % self)
+            return
+        self.fairy_ref = None
+        connection = self.connection
+        pool = self.__pool
+        while self.finalize_callback:
+            finalizer = self.finalize_callback.pop()
+            finalizer(connection)
+        if pool.dispatch.checkin:
+            pool.dispatch.checkin(connection, self)
+        pool._return_conn(self)
+
+    @property
+    def in_use(self):
+        return self.fairy_ref is not None
+
+    @property
+    def last_connect_time(self):
+        return self.starttime
+
+    def close(self):
+        if self.connection is not None:
+            self.__close()
+
+    def invalidate(self, e=None, soft=False):
+        """Invalidate the DBAPI connection held by this :class:`._ConnectionRecord`.
+
+        This method is called for all connection invalidations, including
+        when the :meth:`._ConnectionFairy.invalidate` or
+        :meth:`.Connection.invalidate` methods are called, as well as when any
+        so-called "automatic invalidation" condition occurs.
+
+        :param e: an exception object indicating a reason for the invalidation.
+
+        :param soft: if True, the connection isn't closed; instead, this
+         connection will be recycled on next checkout.
+
+         .. versionadded:: 1.0.3
+
+        .. seealso::
+
+            :ref:`pool_connection_invalidation`
+
+        """
+        # already invalidated
+        if self.connection is None:
+            return
+        if soft:
+            self.__pool.dispatch.soft_invalidate(self.connection, self, e)
+        else:
+            self.__pool.dispatch.invalidate(self.connection, self, e)
+        if e is not None:
+            self.__pool.logger.info(
+                "%sInvalidate connection %r (reason: %s:%s)",
+                "Soft " if soft else "",
+                self.connection, e.__class__.__name__, e)
+        else:
+            self.__pool.logger.info(
+                "%sInvalidate connection %r",
+                "Soft " if soft else "",
+                self.connection)
+        if soft:
+            self._soft_invalidate_time = time.time()
+        else:
+            self.__close()
+            self.connection = None
+
+    def get_connection(self):
+        recycle = False
+        if self.connection is None:
+            self.info.clear()
+            self.__connect()
+        elif self.__pool._recycle > -1 and \
+                time.time() - self.starttime > self.__pool._recycle:
+            self.__pool.logger.info(
+                "Connection %r exceeded timeout; recycling",
+                self.connection)
+            recycle = True
+        elif self.__pool._invalidate_time > self.starttime:
+            self.__pool.logger.info(
+                "Connection %r invalidated due to pool invalidation; " +
+                "recycling",
+                self.connection
+            )
+            recycle = True
+        elif self._soft_invalidate_time > self.starttime:
+            self.__pool.logger.info(
+                "Connection %r invalidated due to local soft invalidation; " +
+                "recycling",
+                self.connection
+            )
+            recycle = True
+
+        if recycle:
+            self.__close()
+            self.info.clear()
+
+            self.__connect()
+        return self.connection
+
+    def __close(self):
+        self.finalize_callback.clear()
+        if self.__pool.dispatch.close:
+            self.__pool.dispatch.close(self.connection, self)
+        self.__pool._close_connection(self.connection)
+        self.connection = None
+
+    def __connect(self, first_connect_check=False):
+        pool = self.__pool
+
+        # ensure any existing connection is removed, so that if
+        # creator fails, this attribute stays None
+        self.connection = None
+        try:
+            self.starttime = time.time()
+            connection = pool._invoke_creator(self)
+            pool.logger.debug("Created new connection %r", connection)
+            self.connection = connection
+        except Exception as e:
+            pool.logger.debug("Error on connect(): %s", e)
+            raise
+        else:
+            if first_connect_check:
+                pool.dispatch.first_connect.\
+                    for_modify(pool.dispatch).\
+                    exec_once(self.connection, self)
+            if pool.dispatch.connect:
+                pool.dispatch.connect(self.connection, self)
+
+
+def _finalize_fairy(connection, connection_record,
+                    pool, ref, echo, fairy=None):
+    """Cleanup for a :class:`._ConnectionFairy` whether or not it's already
+    been garbage collected.
+
+    """
+    _refs.discard(connection_record)
+
+    if ref is not None:
+        if connection_record.fairy_ref is not ref:
+            return
+        assert connection is None
+        connection = connection_record.connection
+
+    if connection is not None:
+        if connection_record and echo:
+            pool.logger.debug("Connection %r being returned to pool",
+                              connection)
+
+        try:
+            fairy = fairy or _ConnectionFairy(
+                connection, connection_record, echo)
+            assert fairy.connection is connection
+            fairy._reset(pool)
+
+            # Immediately close detached instances
+            if not connection_record:
+                if pool.dispatch.close_detached:
+                    pool.dispatch.close_detached(connection)
+                pool._close_connection(connection)
+        except BaseException as e:
+            pool.logger.error(
+                "Exception during reset or similar", exc_info=True)
+            if connection_record:
+                connection_record.invalidate(e=e)
+            if not isinstance(e, Exception):
+                raise
+
+    if connection_record and connection_record.fairy_ref is not None:
+        connection_record.checkin()
+
+
+_refs = set()
+
+
+class _ConnectionFairy(object):
+
+    """Proxies a DBAPI connection and provides return-on-dereference
+    support.
+
+    This is an internal object used by the :class:`.Pool` implementation
+    to provide context management to a DBAPI connection delivered by
+    that :class:`.Pool`.
+
+    The name "fairy" is inspired by the fact that the
+    :class:`._ConnectionFairy` object's lifespan is transitory, as it lasts
+    only for the length of a specific DBAPI connection being checked out from
+    the pool, and additionally that as a transparent proxy, it is mostly
+    invisible.
+
+    .. seealso::
+
+        :class:`._ConnectionRecord`
+
+    """
+
+    def __init__(self, dbapi_connection, connection_record, echo):
+        self.connection = dbapi_connection
+        self._connection_record = connection_record
+        self._echo = echo
+
+    connection = None
+    """A reference to the actual DBAPI connection being tracked."""
+
+    _connection_record = None
+    """A reference to the :class:`._ConnectionRecord` object associated
+    with the DBAPI connection.
+
+    This is currently an internal accessor which is subject to change.
+
+    """
+
+    _reset_agent = None
+    """Refer to an object with a ``.commit()`` and ``.rollback()`` method;
+    if non-None, the "reset-on-return" feature will call upon this object
+    rather than directly against the dialect-level do_rollback() and
+    do_commit() methods.
+
+    In practice, a :class:`.Connection` assigns a :class:`.Transaction` object
+    to this variable when one is in scope so that the :class:`.Transaction`
+    takes the job of committing or rolling back on return if
+    :meth:`.Connection.close` is called while the :class:`.Transaction`
+    still exists.
+
+    This is essentially an "event handler" of sorts but is simplified as an
+    instance variable both for performance/simplicity as well as that there
+    can only be one "reset agent" at a time.
+    """
+
+    @classmethod
+    def _checkout(cls, pool, threadconns=None, fairy=None):
+        if not fairy:
+            fairy = _ConnectionRecord.checkout(pool)
+
+            fairy._pool = pool
+            fairy._counter = 0
+
+            if threadconns is not None:
+                threadconns.current = weakref.ref(fairy)
+
+        if fairy.connection is None:
+            raise exc.InvalidRequestError("This connection is closed")
+        fairy._counter += 1
+
+        if (not pool.dispatch.checkout and not pool._pre_ping) or \
+                fairy._counter != 1:
+            return fairy
+
+        # Pool listeners can trigger a reconnection on checkout, as well
+        # as the pre-pinger.
+        # there are three attempts made here, but note that if the database
+        # is not accessible from a connection standpoint, those won't proceed
+        # here.
+        attempts = 2
+        while attempts > 0:
+            try:
+                if pool._pre_ping:
+                    if fairy._echo:
+                        pool.logger.debug(
+                            "Pool pre-ping on connection %s",
+                            fairy.connection)
+
+                    result = pool._dialect.do_ping(fairy.connection)
+                    if not result:
+                        if fairy._echo:
+                            pool.logger.debug(
+                                "Pool pre-ping on connection %s failed, "
+                                "will invalidate pool", fairy.connection)
+                        raise exc.InvalidatePoolError()
+
+                pool.dispatch.checkout(fairy.connection,
+                                       fairy._connection_record,
+                                       fairy)
+                return fairy
+            except exc.DisconnectionError as e:
+                if e.invalidate_pool:
+                    pool.logger.info(
+                        "Disconnection detected on checkout, "
+                        "invalidating all pooled connections prior to "
+                        "current timestamp (reason: %r)", e)
+                    fairy._connection_record.invalidate(e)
+                    pool._invalidate(fairy, e, _checkin=False)
+                else:
+                    pool.logger.info(
+                        "Disconnection detected on checkout, "
+                        "invalidating individual connection %s (reason: %r)",
+                        fairy.connection, e)
+                    fairy._connection_record.invalidate(e)
+                try:
+                    fairy.connection = \
+                        fairy._connection_record.get_connection()
+                except Exception as err:
+                    with util.safe_reraise():
+                        fairy._connection_record._checkin_failed(err)
+
+                attempts -= 1
+
+        pool.logger.info("Reconnection attempts exhausted on checkout")
+        fairy.invalidate()
+        raise exc.InvalidRequestError("This connection is closed")
+
+    def _checkout_existing(self):
+        return _ConnectionFairy._checkout(self._pool, fairy=self)
+
+    def _checkin(self):
+        _finalize_fairy(self.connection, self._connection_record,
+                        self._pool, None, self._echo, fairy=self)
+        self.connection = None
+        self._connection_record = None
+
+    _close = _checkin
+
+    def _reset(self, pool):
+        if pool.dispatch.reset:
+            pool.dispatch.reset(self, self._connection_record)
+        if pool._reset_on_return is reset_rollback:
+            if self._echo:
+                pool.logger.debug("Connection %s rollback-on-return%s",
+                                  self.connection,
+                                  ", via agent"
+                                  if self._reset_agent else "")
+            if self._reset_agent:
+                self._reset_agent.rollback()
+            else:
+                pool._dialect.do_rollback(self)
+        elif pool._reset_on_return is reset_commit:
+            if self._echo:
+                pool.logger.debug("Connection %s commit-on-return%s",
+                                  self.connection,
+                                  ", via agent"
+                                  if self._reset_agent else "")
+            if self._reset_agent:
+                self._reset_agent.commit()
+            else:
+                pool._dialect.do_commit(self)
+
+    @property
+    def _logger(self):
+        return self._pool.logger
+
+    @property
+    def is_valid(self):
+        """Return True if this :class:`._ConnectionFairy` still refers
+        to an active DBAPI connection."""
+
+        return self.connection is not None
+
+    @util.memoized_property
+    def info(self):
+        """Info dictionary associated with the underlying DBAPI connection
+        referred to by this :class:`.ConnectionFairy`, allowing user-defined
+        data to be associated with the connection.
+
+        The data here will follow along with the DBAPI connection including
+        after it is returned to the connection pool and used again
+        in subsequent instances of :class:`._ConnectionFairy`.  It is shared
+        with the :attr:`._ConnectionRecord.info` and :attr:`.Connection.info`
+        accessors.
+
+        The dictionary associated with a particular DBAPI connection is
+        discarded when the connection itself is discarded.
+
+        """
+        return self._connection_record.info
+
+    @property
+    def record_info(self):
+        """Info dictionary associated with the :class:`._ConnectionRecord
+        container referred to by this :class:`.ConnectionFairy`.
+
+        Unlike the :attr:`._ConnectionFairy.info` dictionary, the lifespan
+        of this dictionary is persistent across connections that are
+        disconnected and/or invalidated within the lifespan of a
+        :class:`._ConnectionRecord`.
+
+        .. versionadded:: 1.1
+
+        """
+        if self._connection_record:
+            return self._connection_record.record_info
+        else:
+            return None
+
+    def invalidate(self, e=None, soft=False):
+        """Mark this connection as invalidated.
+
+        This method can be called directly, and is also called as a result
+        of the :meth:`.Connection.invalidate` method.   When invoked,
+        the DBAPI connection is immediately closed and discarded from
+        further use by the pool.  The invalidation mechanism proceeds
+        via the :meth:`._ConnectionRecord.invalidate` internal method.
+
+        :param e: an exception object indicating a reason for the invalidation.
+
+        :param soft: if True, the connection isn't closed; instead, this
+         connection will be recycled on next checkout.
+
+         .. versionadded:: 1.0.3
+
+        .. seealso::
+
+            :ref:`pool_connection_invalidation`
+
+        """
+
+        if self.connection is None:
+            util.warn("Can't invalidate an already-closed connection.")
+            return
+        if self._connection_record:
+            self._connection_record.invalidate(e=e, soft=soft)
+        if not soft:
+            self.connection = None
+            self._checkin()
+
+    def cursor(self, *args, **kwargs):
+        """Return a new DBAPI cursor for the underlying connection.
+
+        This method is a proxy for the ``connection.cursor()`` DBAPI
+        method.
+
+        """
+        return self.connection.cursor(*args, **kwargs)
+
+    def __getattr__(self, key):
+        return getattr(self.connection, key)
+
+    def detach(self):
+        """Separate this connection from its Pool.
+
+        This means that the connection will no longer be returned to the
+        pool when closed, and will instead be literally closed.  The
+        containing ConnectionRecord is separated from the DB-API connection,
+        and will create a new connection when next used.
+
+        Note that any overall connection limiting constraints imposed by a
+        Pool implementation may be violated after a detach, as the detached
+        connection is removed from the pool's knowledge and control.
+        """
+
+        if self._connection_record is not None:
+            rec = self._connection_record
+            _refs.remove(rec)
+            rec.fairy_ref = None
+            rec.connection = None
+            # TODO: should this be _return_conn?
+            self._pool._do_return_conn(self._connection_record)
+            self.info = self.info.copy()
+            self._connection_record = None
+
+            if self._pool.dispatch.detach:
+                self._pool.dispatch.detach(self.connection, rec)
+
+    def close(self):
+        self._counter -= 1
+        if self._counter == 0:
+            self._checkin()
+
+
diff --git a/lib/sqlalchemy/pool/dbapi_proxy.py b/lib/sqlalchemy/pool/dbapi_proxy.py
new file mode 100644
index 000000000..aa439bd23
--- /dev/null
+++ b/lib/sqlalchemy/pool/dbapi_proxy.py
@@ -0,0 +1,146 @@
+# sqlalchemy/pool/dbapi_proxy.py
+# Copyright (C) 2005-2018 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: http://www.opensource.org/licenses/mit-license.php
+
+
+"""DBAPI proxy utility.
+
+Provides transparent connection pooling on top of a Python DBAPI.
+
+This is legacy SQLAlchemy functionality that is not typically used
+today.
+
+"""
+
+from .impl import QueuePool
+from ..util import threading
+
+proxies = {}
+
+
+def manage(module, **params):
+    r"""Return a proxy for a DB-API module that automatically
+    pools connections.
+
+    Given a DB-API 2.0 module and pool management parameters, returns
+    a proxy for the module that will automatically pool connections,
+    creating new connection pools for each distinct set of connection
+    arguments sent to the decorated module's connect() function.
+
+    :param module: a DB-API 2.0 database module
+
+    :param poolclass: the class used by the pool module to provide
+      pooling.  Defaults to :class:`.QueuePool`.
+
+    :param \**params: will be passed through to *poolclass*
+
+    """
+    try:
+        return proxies[module]
+    except KeyError:
+        return proxies.setdefault(module, _DBProxy(module, **params))
+
+
+def clear_managers():
+    """Remove all current DB-API 2.0 managers.
+
+    All pools and connections are disposed.
+    """
+
+    for manager in proxies.values():
+        manager.close()
+    proxies.clear()
+
+
+class _DBProxy(object):
+
+    """Layers connection pooling behavior on top of a standard DB-API module.
+
+    Proxies a DB-API 2.0 connect() call to a connection pool keyed to the
+    specific connect parameters. Other functions and attributes are delegated
+    to the underlying DB-API module.
+    """
+
+    def __init__(self, module, poolclass=QueuePool, **kw):
+        """Initializes a new proxy.
+
+        module
+          a DB-API 2.0 module
+
+        poolclass
+          a Pool class, defaulting to QueuePool
+
+        Other parameters are sent to the Pool object's constructor.
+
+        """
+
+        self.module = module
+        self.kw = kw
+        self.poolclass = poolclass
+        self.pools = {}
+        self._create_pool_mutex = threading.Lock()
+
+    def close(self):
+        for key in list(self.pools):
+            del self.pools[key]
+
+    def __del__(self):
+        self.close()
+
+    def __getattr__(self, key):
+        return getattr(self.module, key)
+
+    def get_pool(self, *args, **kw):
+        key = self._serialize(*args, **kw)
+        try:
+            return self.pools[key]
+        except KeyError:
+            self._create_pool_mutex.acquire()
+            try:
+                if key not in self.pools:
+                    kw.pop('sa_pool_key', None)
+                    pool = self.poolclass(
+                        lambda: self.module.connect(*args, **kw), **self.kw)
+                    self.pools[key] = pool
+                    return pool
+                else:
+                    return self.pools[key]
+            finally:
+                self._create_pool_mutex.release()
+
+    def connect(self, *args, **kw):
+        """Activate a connection to the database.
+
+        Connect to the database using this DBProxy's module and the given
+        connect arguments.  If the arguments match an existing pool, the
+        connection will be returned from the pool's current thread-local
+        connection instance, or if there is no thread-local connection
+        instance it will be checked out from the set of pooled connections.
+
+        If the pool has no available connections and allows new connections
+        to be created, a new database connection will be made.
+
+        """
+
+        return self.get_pool(*args, **kw).connect()
+
+    def dispose(self, *args, **kw):
+        """Dispose the pool referenced by the given connect arguments."""
+
+        key = self._serialize(*args, **kw)
+        try:
+            del self.pools[key]
+        except KeyError:
+            pass
+
+    def _serialize(self, *args, **kw):
+        if "sa_pool_key" in kw:
+            return kw['sa_pool_key']
+
+        return tuple(
+            list(args) +
+            [(k, kw[k]) for k in sorted(kw)]
+        )
diff --git a/lib/sqlalchemy/pool/impl.py b/lib/sqlalchemy/pool/impl.py
new file mode 100644
index 000000000..c45661895
--- /dev/null
+++ b/lib/sqlalchemy/pool/impl.py
@@ -0,0 +1,419 @@
+# sqlalchemy/pool.py
+# Copyright (C) 2005-2018 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: http://www.opensource.org/licenses/mit-license.php
+
+
+"""Pool implementation classes.
+
+"""
+
+import traceback
+import weakref
+
+from .base import Pool, _ConnectionRecord
+from .. import exc
+from .. import util
+from ..util import queue as sqla_queue
+from ..util import chop_traceback
+from ..util import threading
+
+
+class QueuePool(Pool):
+
+    """A :class:`.Pool` that imposes a limit on the number of open connections.
+
+    :class:`.QueuePool` is the default pooling implementation used for
+    all :class:`.Engine` objects, unless the SQLite dialect is in use.
+
+    """
+
+    def __init__(self, creator, pool_size=5, max_overflow=10, timeout=30,
+                 **kw):
+        r"""
+        Construct a QueuePool.
+
+        :param creator: a callable function that returns a DB-API
+          connection object, same as that of :paramref:`.Pool.creator`.
+
+        :param pool_size: The size of the pool to be maintained,
+          defaults to 5. This is the largest number of connections that
+          will be kept persistently in the pool. Note that the pool
+          begins with no connections; once this number of connections
+          is requested, that number of connections will remain.
+          ``pool_size`` can be set to 0 to indicate no size limit; to
+          disable pooling, use a :class:`~sqlalchemy.pool.NullPool`
+          instead.
+
+        :param max_overflow: The maximum overflow size of the
+          pool. When the number of checked-out connections reaches the
+          size set in pool_size, additional connections will be
+          returned up to this limit. When those additional connections
+          are returned to the pool, they are disconnected and
+          discarded. It follows then that the total number of
+          simultaneous connections the pool will allow is pool_size +
+          `max_overflow`, and the total number of "sleeping"
+          connections the pool will allow is pool_size. `max_overflow`
+          can be set to -1 to indicate no overflow limit; no limit
+          will be placed on the total number of concurrent
+          connections. Defaults to 10.
+
+        :param timeout: The number of seconds to wait before giving up
+          on returning a connection. Defaults to 30.
+
+        :param \**kw: Other keyword arguments including
+          :paramref:`.Pool.recycle`, :paramref:`.Pool.echo`,
+          :paramref:`.Pool.reset_on_return` and others are passed to the
+          :class:`.Pool` constructor.
+
+        """
+        Pool.__init__(self, creator, **kw)
+        self._pool = sqla_queue.Queue(pool_size)
+        self._overflow = 0 - pool_size
+        self._max_overflow = max_overflow
+        self._timeout = timeout
+        self._overflow_lock = threading.Lock()
+
+    def _do_return_conn(self, conn):
+        try:
+            self._pool.put(conn, False)
+        except sqla_queue.Full:
+            try:
+                conn.close()
+            finally:
+                self._dec_overflow()
+
+    def _do_get(self):
+        use_overflow = self._max_overflow > -1
+
+        try:
+            wait = use_overflow and self._overflow >= self._max_overflow
+            return self._pool.get(wait, self._timeout)
+        except sqla_queue.Empty:
+            # don't do things inside of "except Empty", because when we say
+            # we timed out or can't connect and raise, Python 3 tells
+            # people the real error is queue.Empty which it isn't.
+            pass
+        if use_overflow and self._overflow >= self._max_overflow:
+            if not wait:
+                return self._do_get()
+            else:
+                raise exc.TimeoutError(
+                    "QueuePool limit of size %d overflow %d reached, "
+                    "connection timed out, timeout %d" %
+                    (self.size(), self.overflow(), self._timeout), code="3o7r")
+
+        if self._inc_overflow():
+            try:
+                return self._create_connection()
+            except:
+                with util.safe_reraise():
+                    self._dec_overflow()
+        else:
+            return self._do_get()
+
+    def _inc_overflow(self):
+        if self._max_overflow == -1:
+            self._overflow += 1
+            return True
+        with self._overflow_lock:
+            if self._overflow < self._max_overflow:
+                self._overflow += 1
+                return True
+            else:
+                return False
+
+    def _dec_overflow(self):
+        if self._max_overflow == -1:
+            self._overflow -= 1
+            return True
+        with self._overflow_lock:
+            self._overflow -= 1
+            return True
+
+    def recreate(self):
+        self.logger.info("Pool recreating")
+        return self.__class__(self._creator, pool_size=self._pool.maxsize,
+                              max_overflow=self._max_overflow,
+                              timeout=self._timeout,
+                              recycle=self._recycle, echo=self.echo,
+                              logging_name=self._orig_logging_name,
+                              use_threadlocal=self._use_threadlocal,
+                              reset_on_return=self._reset_on_return,
+                              _dispatch=self.dispatch,
+                              dialect=self._dialect)
+
+    def dispose(self):
+        while True:
+            try:
+                conn = self._pool.get(False)
+                conn.close()
+            except sqla_queue.Empty:
+                break
+
+        self._overflow = 0 - self.size()
+        self.logger.info("Pool disposed. %s", self.status())
+
+    def status(self):
+        return "Pool size: %d  Connections in pool: %d "\
+            "Current Overflow: %d Current Checked out "\
+            "connections: %d" % (self.size(),
+                                 self.checkedin(),
+                                 self.overflow(),
+                                 self.checkedout())
+
+    def size(self):
+        return self._pool.maxsize
+
+    def checkedin(self):
+        return self._pool.qsize()
+
+    def overflow(self):
+        return self._overflow
+
+    def checkedout(self):
+        return self._pool.maxsize - self._pool.qsize() + self._overflow
+
+
+class NullPool(Pool):
+
+    """A Pool which does not pool connections.
+
+    Instead it literally opens and closes the underlying DB-API connection
+    per each connection open/close.
+
+    Reconnect-related functions such as ``recycle`` and connection
+    invalidation are not supported by this Pool implementation, since
+    no connections are held persistently.
+
+    .. versionchanged:: 0.7
+        :class:`.NullPool` is used by the SQlite dialect automatically
+        when a file-based database is used. See :ref:`sqlite_toplevel`.
+
+    """
+
+    def status(self):
+        return "NullPool"
+
+    def _do_return_conn(self, conn):
+        conn.close()
+
+    def _do_get(self):
+        return self._create_connection()
+
+    def recreate(self):
+        self.logger.info("Pool recreating")
+
+        return self.__class__(self._creator,
+                              recycle=self._recycle,
+                              echo=self.echo,
+                              logging_name=self._orig_logging_name,
+                              use_threadlocal=self._use_threadlocal,
+                              reset_on_return=self._reset_on_return,
+                              _dispatch=self.dispatch,
+                              dialect=self._dialect)
+
+    def dispose(self):
+        pass
+
+
+class SingletonThreadPool(Pool):
+
+    """A Pool that maintains one connection per thread.
+
+    Maintains one connection per each thread, never moving a connection to a
+    thread other than the one which it was created in.
+
+    .. warning::  the :class:`.SingletonThreadPool` will call ``.close()``
+       on arbitrary connections that exist beyond the size setting of
+       ``pool_size``, e.g. if more unique **thread identities**
+       than what ``pool_size`` states are used.   This cleanup is
+       non-deterministic and not sensitive to whether or not the connections
+       linked to those thread identities are currently in use.
+
+       :class:`.SingletonThreadPool` may be improved in a future release,
+       however in its current status it is generally used only for test
+       scenarios using a SQLite ``:memory:`` database and is not recommended
+       for production use.
+
+
+    Options are the same as those of :class:`.Pool`, as well as:
+
+    :param pool_size: The number of threads in which to maintain connections
+        at once.  Defaults to five.
+
+    :class:`.SingletonThreadPool` is used by the SQLite dialect
+    automatically when a memory-based database is used.
+    See :ref:`sqlite_toplevel`.
+
+    """
+
+    def __init__(self, creator, pool_size=5, **kw):
+        kw['use_threadlocal'] = True
+        Pool.__init__(self, creator, **kw)
+        self._conn = threading.local()
+        self._all_conns = set()
+        self.size = pool_size
+
+    def recreate(self):
+        self.logger.info("Pool recreating")
+        return self.__class__(self._creator,
+                              pool_size=self.size,
+                              recycle=self._recycle,
+                              echo=self.echo,
+                              logging_name=self._orig_logging_name,
+                              use_threadlocal=self._use_threadlocal,
+                              reset_on_return=self._reset_on_return,
+                              _dispatch=self.dispatch,
+                              dialect=self._dialect)
+
+    def dispose(self):
+        """Dispose of this pool."""
+
+        for conn in self._all_conns:
+            try:
+                conn.close()
+            except Exception:
+                # pysqlite won't even let you close a conn from a thread
+                # that didn't create it
+                pass
+
+        self._all_conns.clear()
+
+    def _cleanup(self):
+        while len(self._all_conns) >= self.size:
+            c = self._all_conns.pop()
+            c.close()
+
+    def status(self):
+        return "SingletonThreadPool id:%d size: %d" % \
+            (id(self), len(self._all_conns))
+
+    def _do_return_conn(self, conn):
+        pass
+
+    def _do_get(self):
+        try:
+            c = self._conn.current()
+            if c:
+                return c
+        except AttributeError:
+            pass
+        c = self._create_connection()
+        self._conn.current = weakref.ref(c)
+        if len(self._all_conns) >= self.size:
+            self._cleanup()
+        self._all_conns.add(c)
+        return c
+
+
+class StaticPool(Pool):
+
+    """A Pool of exactly one connection, used for all requests.
+
+    Reconnect-related functions such as ``recycle`` and connection
+    invalidation (which is also used to support auto-reconnect) are not
+    currently supported by this Pool implementation but may be implemented
+    in a future release.
+
+    """
+
+    @util.memoized_property
+    def _conn(self):
+        return self._creator()
+
+    @util.memoized_property
+    def connection(self):
+        return _ConnectionRecord(self)
+
+    def status(self):
+        return "StaticPool"
+
+    def dispose(self):
+        if '_conn' in self.__dict__:
+            self._conn.close()
+            self._conn = None
+
+    def recreate(self):
+        self.logger.info("Pool recreating")
+        return self.__class__(creator=self._creator,
+                              recycle=self._recycle,
+                              use_threadlocal=self._use_threadlocal,
+                              reset_on_return=self._reset_on_return,
+                              echo=self.echo,
+                              logging_name=self._orig_logging_name,
+                              _dispatch=self.dispatch,
+                              dialect=self._dialect)
+
+    def _create_connection(self):
+        return self._conn
+
+    def _do_return_conn(self, conn):
+        pass
+
+    def _do_get(self):
+        return self.connection
+
+
+class AssertionPool(Pool):
+
+    """A :class:`.Pool` that allows at most one checked out connection at
+    any given time.
+
+    This will raise an exception if more than one connection is checked out
+    at a time.  Useful for debugging code that is using more connections
+    than desired.
+
+    .. versionchanged:: 0.7
+        :class:`.AssertionPool` also logs a traceback of where
+        the original connection was checked out, and reports
+        this in the assertion error raised.
+
+    """
+
+    def __init__(self, *args, **kw):
+        self._conn = None
+        self._checked_out = False
+        self._store_traceback = kw.pop('store_traceback', True)
+        self._checkout_traceback = None
+        Pool.__init__(self, *args, **kw)
+
+    def status(self):
+        return "AssertionPool"
+
+    def _do_return_conn(self, conn):
+        if not self._checked_out:
+            raise AssertionError("connection is not checked out")
+        self._checked_out = False
+        assert conn is self._conn
+
+    def dispose(self):
+        self._checked_out = False
+        if self._conn:
+            self._conn.close()
+
+    def recreate(self):
+        self.logger.info("Pool recreating")
+        return self.__class__(self._creator, echo=self.echo,
+                              logging_name=self._orig_logging_name,
+                              _dispatch=self.dispatch,
+                              dialect=self._dialect)
+
+    def _do_get(self):
+        if self._checked_out:
+            if self._checkout_traceback:
+                suffix = ' at:\n%s' % ''.join(
+                    chop_traceback(self._checkout_traceback))
+            else:
+                suffix = ''
+            raise AssertionError("connection is already checked out" + suffix)
+
+        if not self._conn:
+            self._conn = self._create_connection()
+
+        self._checked_out = True
+        if self._store_traceback:
+            self._checkout_traceback = traceback.format_stack()
+        return self._conn