Implement proxy back reference system for asyncio

Implemented a new registry architecture that allows the ``Async`` version
of an object, like ``AsyncSession``, ``AsyncConnection``, etc., to be
locatable given the proxied "sync" object, i.e. ``Session``,
``Connection``. Previously, to the degree such lookup functions were used,
an ``Async`` object would be re-created each time, which was less than
ideal as the identity and state of the "async" object would not be
preserved across calls.

From there, new helper functions :func:`_asyncio.async_object_session`,
:func:`_asyncio.async_session` as well as a new :class:`_orm.InstanceState`
attribute :attr:`_orm.InstanceState.asyncio_session` have been added, which
are used to retrieve the original :class:`_asyncio.AsyncSession` associated
with an ORM mapped object, a :class:`_orm.Session` associated with an
:class:`_asyncio.AsyncSession`, and an :class:`_asyncio.AsyncSession`
associated with an :class:`_orm.InstanceState`, respectively.

This patch also implements new methods
:meth:`_asyncio.AsyncSession.in_nested_transaction`,
:meth:`_asyncio.AsyncSession.get_transaction`,
:meth:`_asyncio.AsyncSession.get_nested_transaction`.

Fixes: #6319
Change-Id: Ia452a7e7ce9bad3ff8846c7dea8d45c839ac9fac

1 files changed, 43 insertions, 1 deletions

diff --git a/lib/sqlalchemy/ext/asyncio/base.py b/lib/sqlalchemy/ext/asyncio/base.py
index 76a2fbbde..3f2c084f4 100644
--- a/lib/sqlalchemy/ext/asyncio/base.py
+++ b/lib/sqlalchemy/ext/asyncio/base.py
@@ -1,8 +1,50 @@
 import abc
+import functools
+import weakref
 
 from . import exc as async_exc
 
 
+class ReversibleProxy:
+    # weakref.ref(async proxy object) -> weakref.ref(sync proxied object)
+    _proxy_objects = {}
+
+    def _assign_proxied(self, target):
+        if target is not None:
+            target_ref = weakref.ref(target, ReversibleProxy._target_gced)
+            proxy_ref = weakref.ref(
+                self,
+                functools.partial(ReversibleProxy._target_gced, target_ref),
+            )
+            ReversibleProxy._proxy_objects[target_ref] = proxy_ref
+
+        return target
+
+    @classmethod
+    def _target_gced(cls, ref, proxy_ref=None):
+        cls._proxy_objects.pop(ref, None)
+
+    @classmethod
+    def _regenerate_proxy_for_target(cls, target):
+        raise NotImplementedError()
+
+    @classmethod
+    def _retrieve_proxy_for_target(cls, target, regenerate=True):
+        try:
+            proxy_ref = cls._proxy_objects[weakref.ref(target)]
+        except KeyError:
+            pass
+        else:
+            proxy = proxy_ref()
+            if proxy is not None:
+                return proxy
+
+        if regenerate:
+            return cls._regenerate_proxy_for_target(target)
+        else:
+            return None
+
+
 class StartableContext(abc.ABC):
     @abc.abstractmethod
     async def start(self, is_ctxmanager=False):
@@ -25,7 +67,7 @@ class StartableContext(abc.ABC):
         )
 
 
-class ProxyComparable:
+class ProxyComparable(ReversibleProxy):
     def __hash__(self):
         return id(self)