fix: size attribute of VLA structs is now computed properly

Author: MrIndeciso

libdestruct/common/struct/struct_impl.py

@@ -59,13 +59,22 @@ def __init__(self: struct_impl, resolver: Resolver | None = None, **kwargs: ...)
 
     def __getattribute__(self: struct_impl, name: str) -> object:
         """Return the attribute, checking struct members first to avoid collisions with obj properties."""
-        # Check _members dict directly to avoid infinite recursion
         try:
             members = object.__getattribute__(self, "_members")
-            if name in members:
-                return members[name]
         except AttributeError:
-            pass
+            return super().__getattribute__(name)
+        if name in members:
+            return members[name]
+        if name == "size":
+            # VLA structs store _vla_fixed_offset instead of an instance size attr;
+            # without this, `instance.size` would fall back to the static class size
+            # (set by compute_own_size), missing the dynamic VLA contribution.
+            try:
+                vla_offset = object.__getattribute__(self, "_vla_fixed_offset")
+            except AttributeError:
+                pass
+            else:
+                return vla_offset + next(reversed(members.values())).size
         return super().__getattribute__(name)
 
     def __setattr__(self: struct_impl, name: str, value: object) -> None:

test/scripts/vla_test.py

@@ -313,5 +313,33 @@ class packet_t(struct):
             lib.inflate(packet_t, 0)
 
 
+class VLAInstanceSizeTest(unittest.TestCase):
+    """`instance.size` on a VLA struct must report the dynamic size, not the class static size."""
+
+    def test_vla_instance_size_matches_size_of(self):
+        class packet_t(struct):
+            length: c_int
+            data: array[c_int, "length"]
+
+        data = pystruct.pack("<i", 3) + pystruct.pack("<iii", 10, 20, 30)
+        memory = bytearray(data)
+        pkt = packet_t.from_bytes(memory)
+        self.assertEqual(pkt.size, size_of(pkt))
+        self.assertEqual(pkt.size, 16)
+
+    def test_vla_instance_size_updates_when_count_changes(self):
+        class packet_t(struct):
+            length: c_int
+            data: array[c_int, "length"]
+
+        memory = bytearray(pystruct.pack("<i", 2) + pystruct.pack("<ii", 10, 20) + b"\x00" * 16)
+        lib = inflater(memory)
+        pkt = lib.inflate(packet_t, 0)
+        self.assertEqual(pkt.size, 12)
+
+        memory[0:4] = pystruct.pack("<i", 4)
+        self.assertEqual(pkt.size, 20)
+
+
 if __name__ == "__main__":
     unittest.main()