Implement connection binding for AsyncSession

Implemented "connection-binding" for :class:`.AsyncSession`, the ability to
pass an :class:`.AsyncConnection` to create an :class:`.AsyncSession`.
Previously, this use case was not implemented and would use the associated
engine when the connection were passed.  This fixes the issue where the
"join a session to an external transaction" use case would not work
correctly for the :class:`.AsyncSession`.  Additionally, added methods
:meth:`.AsyncConnection.in_transaction`,
:meth:`.AsyncConnection.in_nested_transaction`,
:meth:`.AsyncConnection.get_transaction`.

The :class:`.AsyncEngine`, :class:`.AsyncConnection` and
:class:`.AsyncTransaction` objects may be compared using Python ``==`` or
``!=``, which will compare the two given objects based on the "sync" object
they are proxying towards. This is useful as there are cases particularly
for :class:`.AsyncTransaction` where multiple instances of
:class:`.AsyncTransaction` can be proxying towards the same sync
:class:`_engine.Transaction`, and are actually equivalent.   The
:meth:`.AsyncConnection.get_transaction` method will currently return a new
proxying :class:`.AsyncTransaction` each time as the
:class:`.AsyncTransaction` is not otherwise statefully associated with its
originating :class:`.AsyncConnection`.

Fixes: #5811
Change-Id: I5a3a6b2f088541eee7b0e0f393510e61bc9f986b

1 files changed, 17 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/asyncio/base.py b/lib/sqlalchemy/ext/asyncio/base.py
index 051f9e21a..fa8c5006e 100644
--- a/lib/sqlalchemy/ext/asyncio/base.py
+++ b/lib/sqlalchemy/ext/asyncio/base.py
@@ -23,3 +23,20 @@ class StartableContext(abc.ABC):
             "%s context has not been started and object has not been awaited."
             % (self.__class__.__name__)
         )
+
+
+class ProxyComparable:
+    def __hash__(self):
+        return id(self)
+
+    def __eq__(self, other):
+        return (
+            isinstance(other, self.__class__)
+            and self._proxied == other._proxied
+        )
+
+    def __ne__(self, other):
+        return (
+            not isinstance(other, self.__class__)
+            or self._proxied != other._proxied
+        )