consider aliased mappers in cycles also

Fixed endless loop which could occur when using "relationship to aliased
class" feature and also indicating a recursive eager loader such as
``lazy="selectinload"`` in the loader, in combination with another eager
loader on the opposite side. The check for cycles has been fixed to include
aliased class relationships.

Fixes: #9590
Change-Id: I8d340882f040ff9289c209bedd8fbdfd7186f944
(cherry picked from commit e79ab08165e01dc7af50fcffadb31468ace51b6c)

3 files changed, 74 insertions, 1 deletions

diff --git a/doc/build/changelog/unreleased_14/9590.rst b/doc/build/changelog/unreleased_14/9590.rst
new file mode 100644
index 000000000..472cfc70e
--- /dev/null
+++ b/doc/build/changelog/unreleased_14/9590.rst
@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, orm
+    :tickets: 9590
+    :versions: 2.0.9
+
+    Fixed endless loop which could occur when using "relationship to aliased
+    class" feature and also indicating a recursive eager loader such as
+    ``lazy="selectinload"`` in the loader, in combination with another eager
+    loader on the opposite side. The check for cycles has been fixed to include
+    aliased class relationships.
diff --git a/lib/sqlalchemy/orm/path_registry.py b/lib/sqlalchemy/orm/path_registry.py
index 4deb96b1f..dad1fd46c 100644
--- a/lib/sqlalchemy/orm/path_registry.py
+++ b/lib/sqlalchemy/orm/path_registry.py
@@ -120,7 +120,7 @@ class PathRegistry(HasCacheKey):
 
     def contains_mapper(self, mapper):
         for path_mapper in [self.path[i] for i in range(0, len(self.path), 2)]:
-            if path_mapper.is_mapper and path_mapper.isa(mapper):
+            if path_mapper.mapper.isa(mapper):
                 return True
         else:
             return False
diff --git a/test/orm/test_ac_relationships.py b/test/orm/test_ac_relationships.py
index f59d704f3..57e2b25e9 100644
--- a/test/orm/test_ac_relationships.py
+++ b/test/orm/test_ac_relationships.py
@@ -14,6 +14,7 @@ from sqlalchemy.orm import relationship
 from sqlalchemy.orm import selectinload
 from sqlalchemy.orm import Session
 from sqlalchemy.testing import eq_
+from sqlalchemy.testing import expect_warnings
 from sqlalchemy.testing import fixtures
 from sqlalchemy.testing.assertions import expect_raises_message
 from sqlalchemy.testing.assertsql import CompiledSQL
@@ -331,3 +332,65 @@ class AltSelectableTest(
             "FROM a JOIN (b JOIN d ON d.b_id = b.id "
             "JOIN c ON c.id = d.c_id) ON a.b_id = b.id",
         )
+
+
+class StructuralEagerLoadCycleTest(fixtures.DeclarativeMappedTest):
+    @classmethod
+    def setup_classes(cls):
+        Base = cls.DeclarativeBasic
+
+        class A(Base):
+            __tablename__ = "a"
+            id = Column(Integer, primary_key=True)
+
+            bs = relationship(lambda: B, back_populates="a")
+
+        class B(Base):
+            __tablename__ = "b"
+            id = Column(Integer, primary_key=True)
+            a_id = Column(ForeignKey("a.id"))
+
+            a = relationship(A, lazy="joined", back_populates="bs")
+
+        partitioned_b = aliased(B)
+
+        A.partitioned_bs = relationship(
+            partitioned_b, lazy="selectin", viewonly=True
+        )
+
+    @classmethod
+    def insert_data(cls, connection):
+        A, B = cls.classes("A", "B")
+
+        s = Session(connection)
+        a = A()
+        a.bs = [B() for _ in range(5)]
+        s.add(a)
+
+        s.commit()
+
+    @testing.variation("ensure_no_warning", [True, False])
+    def test_no_endless_loop(self, ensure_no_warning):
+        """test #9590"""
+
+        A = self.classes.A
+
+        sess = fixture_session()
+
+        results = sess.scalars(select(A))
+
+        # the correct behavior is 1. no warnings and 2. no endless loop.
+        # however when the failure mode is occurring, it correctly warns,
+        # but then we don't get to see the endless loop happen.
+        # so test it both ways even though when things are "working", there's
+        # no problem
+        if ensure_no_warning:
+
+            a = results.first()
+        else:
+            with expect_warnings(
+                "Loader depth for query is excessively deep", assert_=False
+            ):
+                a = results.first()
+
+        a.bs