Actively unset reset agent in discard transaction

The assumptions in _discard_transaction from
916e1fea25afcd07fa1d1d2f72043b372cd02223 were too narrow,
assuming that if the given transaction were not our
"current" one, that this would not be the reset agent.  however
as the legacy behvaior is that even a "nested" transaction gets
set as "self._transaction", this did not accommodate for the nested
transaction being thrown away.   We will attempt to refine all of this
logic in #5327 for 1.4 /master assuming this is feasible for the
full suite of current use cases.

Fixes: #5326
Change-Id: I6787e82c9e50c23317f87d0d094122c6a6f066da

3 files changed, 27 insertions, 16 deletions

diff --git a/doc/build/changelog/unreleased_13/5326.rst b/doc/build/changelog/unreleased_13/5326.rst
new file mode 100644
index 000000000..801ff4a42
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/5326.rst
@@ -0,0 +1,9 @@
+.. change::
+    :tags: bug, engine
+    :tickets: 5326
+
+    Further refinements to the fixes to the "reset" agent fixed in
+    :ticket:`5326`, which now emits a warning when it is not being correctly
+    invoked and corrects for the behavior.  Additional scenarios have been
+    identified and fixed where this warning was being emitted.
+
diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index ed0586cc7..e617f0fad 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -821,25 +821,13 @@ class Connection(Connectable):
                 assert trans._parent is trans
                 self._transaction = None
 
-                # test suite w/ SingletonThreadPool will have cases
-                # where _reset_agent is on a different Connection
-                # entirely so we can't assert this here.
-                # if (
-                #    not self._is_future
-                #    and self._still_open_and_connection_is_valid
-                # ):
-                #    assert self.__connection._reset_agent is None
             else:
                 assert trans._parent is not trans
                 self._transaction = trans._parent
 
-                # not doing this assertion for now, however this is how
-                # it would look:
-                # if self._still_open_and_connection_is_valid:
-                #    trans = self._transaction
-                #    while not trans._is_root:
-                #        trans = trans._parent
-                #    assert self.__connection._reset_agent is trans
+        if not self._is_future and self._still_open_and_connection_is_valid:
+            if self.__connection._reset_agent is trans:
+                self.__connection._reset_agent = None
 
     def _rollback_to_savepoint_impl(
         self, name, context, deactivate_only=False
diff --git a/test/engine/test_transaction.py b/test/engine/test_transaction.py
index 85f124d12..fbc1ffd83 100644
--- a/test/engine/test_transaction.py
+++ b/test/engine/test_transaction.py
@@ -663,7 +663,7 @@ class ResetAgentTest(fixtures.TestBase):
             conn.close()
 
     @testing.requires.savepoints
-    def test_begin_nested_trans_close(self):
+    def test_begin_nested_trans_close_one(self):
         with testing.db.connect() as connection:
             t1 = connection.begin()
             assert connection.connection._reset_agent is t1
@@ -678,6 +678,20 @@ class ResetAgentTest(fixtures.TestBase):
         assert not t1.is_active
 
     @testing.requires.savepoints
+    def test_begin_nested_trans_close_two(self):
+        with testing.db.connect() as connection:
+            t1 = connection.begin()
+            assert connection.connection._reset_agent is t1
+            t2 = connection.begin_nested()
+            assert connection.connection._reset_agent is t1
+            assert connection._transaction is t2
+
+            assert connection.connection._reset_agent is t1
+            t1.close()
+            assert connection.connection._reset_agent is None
+        assert not t1.is_active
+
+    @testing.requires.savepoints
     def test_begin_nested_trans_rollback(self):
         with testing.db.connect() as connection:
             t1 = connection.begin()