disallow adding to identity map that's been discarded

Fixed bug where iterating a :class:`.Result` from a :class:`_orm.Session`
after that :class:`_orm.Session` were closed would partially attach objects
to that session in an essentially invalid state.   It now raises an exception
with a link to new documentation if an **un-buffered** result is iterated
from a :class:`_orm.Session` that was closed or otherwise had the
:meth:`_orm.Session.expunge_all` method called after that :class:`.Result`
was generated.  The "prebuffer_rows" execution option, as is used by the
asyncio extension, may be used to produce a :class:`.Result` where the ORM
objects are prebuffered, and in this case iterating the result will produce
a series of detached objects.

Fixes: #7128
Change-Id: I59f0ae32a83a64587937741b80f31ff825bbb574

1 files changed, 14 insertions, 0 deletions

diff --git a/lib/sqlalchemy/orm/identity.py b/lib/sqlalchemy/orm/identity.py
index 7eec4fd8d..6aea0d185 100644
--- a/lib/sqlalchemy/orm/identity.py
+++ b/lib/sqlalchemy/orm/identity.py
@@ -18,6 +18,9 @@ class IdentityMap(object):
         self._modified = set()
         self._wr = weakref.ref(self)
 
+    def _kill(self):
+        self._add_unpresent = _killed
+
     def keys(self):
         return self._dict.keys()
 
@@ -238,3 +241,14 @@ class WeakInstanceDict(IdentityMap):
                 if st is state:
                     self._dict.pop(state.key, None)
                     self._manage_removed_state(state)
+
+
+def _killed(state, key):
+    # external function to avoid creating cycles when assigned to
+    # the IdentityMap
+    raise sa_exc.InvalidRequestError(
+        "Object %s cannot be converted to 'persistent' state, as this "
+        "identity map is no longer valid.  Has the owning Session "
+        "been closed?" % orm_util.state_str(state),
+        code="lkrp",
+    )