ensure pool.reset event always called for reset

Added new parameter :paramref:`.PoolEvents.reset.reset_state` parameter to
the :meth:`.PoolEvents.reset` event, with deprecation logic in place that
will continue to accept event hooks using the previous set of arguments.
This indicates various state information about how the reset is taking
place and is used to allow custom reset schemes to take place with full
context given.

Within this change a fix that's also backported to 1.4 is included which
re-enables the :meth:`.PoolEvents.reset` event to continue to take place
under all circumstances, including when :class:`.Connection` has already
"reset" the connection.

The two changes together allow custom reset schemes to be implemented using
the :meth:`.PoolEvents.reset` event, instead of the
:meth:`.PoolEvents.checkin` event (which continues to function as it always
has).

Change-Id: Ie17c4f55d02beb6f570b9de6b3044baffa7d6df6
Fixes: #8717

1 files changed, 64 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 5eab8f47c..482e36594 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -233,6 +233,70 @@ SERIALIZABLE isolation.
 .. versionadded:: 1.4 added support for the ``postgresql_readonly``
    and ``postgresql_deferrable`` execution options.
 
+.. _postgresql_reset_on_return:
+
+Temporary Table / Resource Reset for Connection Pooling
+-------------------------------------------------------
+
+The :class:`.QueuePool` connection pool implementation used
+by the SQLAlchemy :class:`.Engine` object includes
+:ref:`reset on return <pool_reset_on_return>` behavior that will invoke
+the DBAPI ``.rollback()`` method when connections are returned to the pool.
+While this rollback will clear out the immediate state used by the previous
+transaction, it does not cover a wider range of session-level state, including
+temporary tables as well as other server state such as prepared statement
+handles and statement caches.   The PostgreSQL database includes a variety
+of commands which may be used to reset this state, including
+``DISCARD``, ``RESET``, ``DEALLOCATE``, and ``UNLISTEN``.
+
+
+To install
+one or more of these commands as the means of performing reset-on-return,
+the :meth:`.PoolEvents.reset` event hook may be used, as demonstrated
+in the example below. The implementation
+will end transactions in progress as well as discard temporary tables
+using the ``CLOSE``, ``RESET`` and ``DISCARD`` commands; see the PostgreSQL
+documentation for background on what each of these statements do.
+
+The :paramref:`_sa.create_engine.pool_reset_on_return` parameter
+is set to ``None`` so that the custom scheme can replace the default behavior
+completely.   The custom hook implementation calls ``.rollback()`` in any case,
+as it's usually important that the DBAPI's own tracking of commit/rollback
+will remain consistent with the state of the transaction::
+
+
+    from sqlalchemy import create_engine
+    from sqlalchemy import event
+
+    postgresql_engine = create_engine(
+        "postgresql+pyscopg2://scott:tiger@hostname/dbname",
+
+        # disable default reset-on-return scheme
+        pool_reset_on_return=None,
+    )
+
+
+    @event.listens_for(postgresql_engine, "reset")
+    def _reset_mssql(dbapi_connection, connection_record, reset_state):
+        if not reset_state.terminate_only:
+            dbapi_connection.execute("CLOSE ALL")
+            dbapi_connection.execute("RESET ALL")
+            dbapi_connection.execute("DISCARD TEMP")
+
+        # so that the DBAPI itself knows that the connection has been
+        # reset
+        dbapi_connection.rollback()
+
+.. versionchanged:: 2.0.0b3  Added additional state arguments to
+   the :meth:`.PoolEvents.reset` event and additionally ensured the event
+   is invoked for all "reset" occurrences, so that it's appropriate
+   as a place for custom "reset" handlers.   Previous schemes which
+   use the :meth:`.PoolEvents.checkin` handler remain usable as well.
+
+.. seealso::
+
+    :ref:`pool_reset_on_return` - in the :ref:`pooling_toplevel` documentation
+
 .. _postgresql_alternate_search_path:
 
 Setting Alternate Search Paths on Connect