[mypyc] Fix memory leak when borrowing property getter return values

VaggelisD · VaggelisD · commit 32e0ca886d68 · 2026-04-14T20:29:06.000+03:00
`is_native_attr_ref()` uses `has_attr(name) and not get_method(name)` to decide if an attribute access can borrow. `has_attr()` is populated during preparation (always complete), but `get_method()` checks `ir.methods` which is populated during compilation. When cross-module classes haven't been compiled yet, `get_method()` returns None for property getters, incorrectly allowing borrowing. Property getters return new owned references, so skipping the DECREF leaks one reference per call. The fix checks `ir.attributes` directly (struct fields only, always populated during preparation). Properties are never in `ir.attributes`, so they always return False. This is the getter-side counterpart to python#21095.
diff --git a/mypyc/irbuild/builder.py b/mypyc/irbuild/builder.py
@@ -1553,8 +1553,7 @@ def is_native_attr_ref(self, expr: MemberExpr) -> bool:
         return (
             isinstance(obj_rtype, RInstance)
             and obj_rtype.class_ir.is_ext_class
-            and obj_rtype.class_ir.has_attr(expr.name)
-            and not obj_rtype.class_ir.get_method(expr.name)
+            and any(expr.name in ir.attributes for ir in obj_rtype.class_ir.mro)
         )
 
     def mark_block_unreachable(self) -> None:
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
@@ -5925,3 +5925,47 @@ assert NonExtDict.BASE == {"x": 1}
 assert NonExtDict.EXTENDED == {"x": 1, "y": 2}
 
 assert NonExtChained.Z == {10, 20, 30}
+
+[case testPropertyGetterLeak]
+class Bar:
+    pass
+
+class Foo:
+    def __init__(self) -> None:
+        self.obj: object = Bar()
+
+    @property
+    def val(self) -> object:
+        return self.obj
+
+[file other.py]
+import gc
+from native import Foo, Bar
+
+def check(foo: Foo) -> bool:
+    return isinstance(foo.val, Bar)
+
+def test_property_getter_no_leak() -> None:
+    foo = Foo()
+    gc.collect()
+    before = gc.get_objects()
+    for _ in range(100):
+        check(foo)
+    gc.collect()
+    after = gc.get_objects()
+    diff = len(after) - len(before)
+    assert diff <= 2, diff
+
+test_property_getter_no_leak()
+
+[file driver.py]
+import sys
+from other import check
+from native import Foo
+
+foo = Foo()
+init = sys.getrefcount(foo.obj)
+for _ in range(100):
+    check(foo)
+after = sys.getrefcount(foo.obj)
+assert after - init == 0, f"Leaked {after - init} refs"

Original file line number	Diff line number	Diff line change
`@@ -1553,8 +1553,7 @@ def is_native_attr_ref(self, expr: MemberExpr) -> bool:`
`1553`	`1553`	`return (`
`1554`	`1554`	`isinstance(obj_rtype, RInstance)`
`1555`	`1555`	`and obj_rtype.class_ir.is_ext_class`
`1556`		`- and obj_rtype.class_ir.has_attr(expr.name)`
`1557`		`- and not obj_rtype.class_ir.get_method(expr.name)`
	`1556`	`+ and any(expr.name in ir.attributes for ir in obj_rtype.class_ir.mro)`
`1558`	`1557`	`)`
`1559`	`1558`
`1560`	`1559`	`def mark_block_unreachable(self) -> None:`