[mypyc] Fix unsafe borrowing of instance attributes with free-threading

mypyc/irbuild/builder.py

@@ -1565,6 +1565,22 @@ def is_native_attr_ref(self, expr: MemberExpr) -> bool:
             and any(expr.name in ir.attributes for ir in obj_rtype.class_ir.mro)
         )
 
+    def is_final_native_attr_ref(self, expr: MemberExpr) -> bool:
+        """Is expr a direct reference to a Final native (struct) attribute of an instance?
+
+        A Final attribute is read-only at runtime (it has no setter), so it can never be
+        reassigned after construction. This makes it safe to borrow even on free-threaded
+        builds, since no concurrent store can invalidate the borrowed reference.
+        """
+        obj_rtype = self.node_type(expr.expr)
+        if not (isinstance(obj_rtype, RInstance) and obj_rtype.class_ir.is_ext_class):
+            return False
+        # Find the class that defines the attribute and check whether it's Final there.
+        for ir in obj_rtype.class_ir.mro:
+            if expr.name in ir.attributes:
+                return expr.name in ir.final_attributes
+        return False
+
     def mark_block_unreachable(self) -> None:
         """Mark statements in the innermost block being processed as unreachable.
 

mypyc/irbuild/expression.py

@@ -272,9 +272,16 @@ def transform_member_expr(builder: IRBuilder, expr: MemberExpr) -> Value:
     if isinstance(expr.node, MypyFile) and expr.node.fullname in builder.imports:
         return builder.load_module(expr.node.fullname)
 
-    can_borrow = builder.is_native_attr_ref(expr)
-    obj = builder.accept(expr.expr, can_borrow=can_borrow)
     rtype = builder.node_type(expr)
+    # Borrowing a native attribute read is unsafe on free-threaded builds, since another
+    # thread could concurrently reassign the attribute and free the old value. We still borrow
+    # in two cases:
+    #  - Native Final attributes are read-only at runtime, so they can never be reassigned.
+    #  - Vec-typed attributes require manual synchronization, so we borrow them liberally.
+    can_borrow = builder.is_native_attr_ref(expr) and (
+        not IS_FREE_THREADED or isinstance(rtype, RVec) or builder.is_final_native_attr_ref(expr)
+    )
+    obj = builder.accept(expr.expr, can_borrow=can_borrow)
 
     if (
         is_object_rprimitive(obj.type)

mypyc/test-data/exceptions.test

@@ -553,7 +553,7 @@ L3:
     r3 = <error> :: i64
     return r3
 
-[case testExceptionWithNativeAttributeGetAndSet]
+[case testExceptionWithNativeAttributeGetAndSet_withgil]
 class C:
     def __init__(self, x: int) -> None:
         self.x = x
@@ -578,6 +578,32 @@ L0:
     c.x = r1
     return 1
 
+[case testExceptionWithNativeAttributeGetAndSet_nogil]
+class C:
+    def __init__(self, x: int) -> None:
+        self.x = x
+
+def foo(c: C, x: int) -> None:
+    c.x = x - c.x
+[out]
+def C.__init__(self, x):
+    self :: __main__.C
+    x :: int
+L0:
+    inc_ref x :: int
+    self.x = x
+    return 1
+def foo(c, x):
+    c :: __main__.C
+    x, r0, r1 :: int
+    r2 :: bool
+L0:
+    r0 = c.x
+    r1 = CPyTagged_Subtract(x, r0)
+    dec_ref r0 :: int
+    c.x = r1
+    return 1
+
 [case testExceptionWithOverlappingFloatErrorValue]
 def f() -> float:
     return 0.0

mypyc/test-data/irbuild-basic.test

@@ -2063,7 +2063,7 @@ L7:
 L8:
     return r10
 
-[case testProperty]
+[case testProperty_withgil]
 class PropertyHolder:
     @property
     def value(self) -> int:

mypyc/test-data/irbuild-classes.test

@@ -26,7 +26,7 @@ L0:
     a.x = 2; r0 = is_error
     return 1
 
-[case testUserClassInList]
+[case testUserClassInList_withgil]
 class C:
     x: int
 
@@ -103,7 +103,7 @@ L0:
     r0 = a.n
     return r0
 
-[case testOptionalMember]
+[case testOptionalMember_withgil]
 from typing import Optional
 class Node:
     next: Optional[Node]
@@ -1239,7 +1239,7 @@ L0:
     __mypyc_self__.s = r0
     return 1
 
-[case testBorrowAttribute]
+[case testBorrowAttribute_withgil]
 def f(d: D) -> int:
     return d.c.x
 
@@ -1258,6 +1258,24 @@ L0:
     keep_alive d
     return r1
 
+[case testCannotBorrowAttribute_nogil]
+def f(d: D) -> int:
+    return d.c.x
+
+class C:
+    x: int
+class D:
+    c: C
+[out]
+def f(d):
+    d :: __main__.D
+    r0 :: __main__.C
+    r1 :: int
+L0:
+    r0 = d.c
+    r1 = r0.x
+    return r1
+
 [case testNoBorrowOverPropertyAccess]
 class C:
     d: D

mypyc/test-data/irbuild-generics.test

@@ -40,7 +40,7 @@ L0:
     y = r3
     return 1
 
-[case testGenericAttrAndTypeApplication]
+[case testGenericAttrAndTypeApplication_withgil]
 from typing import TypeVar, Generic
 T = TypeVar('T')
 class C(Generic[T]):

mypyc/test-data/irbuild-glue-methods.test

@@ -91,7 +91,7 @@ L0:
     r0 = x.foo(y)
     return r0
 
-[case testPropertyDerivedGen]
+[case testPropertyDerivedGen_withgil]
 from typing import Callable
 class BaseProperty:
     @property

mypyc/test-data/irbuild-i64.test

@@ -1032,7 +1032,7 @@ L4:
     y = r3
     return y
 
-[case testBorrowOverI64Arithmetic]
+[case testBorrowOverI64Arithmetic_withgil]
 from mypy_extensions import i64
 
 def add_simple(c: C) -> i64:
@@ -1084,7 +1084,7 @@ L0:
     keep_alive d, d
     return r4
 
-[case testBorrowOverI64Bitwise]
+[case testBorrowOverI64Bitwise_withgil]
 from mypy_extensions import i64
 
 def bitwise_simple(c: C) -> i64:
@@ -2070,7 +2070,7 @@ L4:
     r6 = CPyFloat_FromTagged(r3)
     return r6
 
-[case testI64IsinstanceNarrowing]
+[case testI64IsinstanceNarrowing_withgil]
 from typing import Union
 from mypy_extensions import i64
 

mypyc/test-data/irbuild-isinstance.test

@@ -48,7 +48,7 @@ L2:
 L3:
     return r1
 
-[case testBorrowSpecialCaseWithIsinstance]
+[case testBorrowSpecialCaseWithIsinstance_withgil]
 class C:
     s: str
 

mypyc/test-data/irbuild-optional.test

@@ -237,7 +237,7 @@ L3:
 L4:
     return 1
 
-[case testUnionType]
+[case testUnionType_withgil]
 from typing import Union
 
 class A:

mypyc/test-data/irbuild-vec-t.test

@@ -512,15 +512,25 @@ L0:
     return r2
 
 [case testVecTBorrowGetItem_64bit]
+from typing import Final
+
 from librt.vecs import vec
 from mypy_extensions import i64
 
 class A:
-    x: str
+    def __init__(self, x: str) -> None:
+        # Final to allow borrowing on free-threaded builds
+        self.x: Final = x
 
 def f(v: vec[A], n: i64) -> int:
     return len(v[n].x)
 [out]
+def A.__init__(self, x):
+    self :: __main__.A
+    x :: str
+L0:
+    self.x = x
+    return 1
 def f(v, n):
     v :: vec[__main__.A]
     n :: i64

mypyc/test-data/opt-copy-propagation.test

@@ -185,7 +185,7 @@ L1:
 L2:
     return 2
 
-[case testIRTransformRegisterOps1]
+[case testIRTransformRegisterOps1_withgil]
 from __future__ import annotations
 from typing import cast