Re: [PATCH 13/26] rust: qom: automatically use Drop trait to implement instance_finalize

[Paolo Bonzini]

On Tue, Dec 10, 2024 at 4:58 PM Zhao Liu <zhao1.liu@intel.com> wrote:
> Great idea. It nicely balances the differences between Rust and C QOM
> conventions.

Except it does not work. :(  Suppose you have

       pub struct MySuperclass {
           parent: DeviceState,
           field: Box<MyData>,
           ...
       }

       impl Drop for MySuperclass {
           ...
       }

       pub struct MySubclass {
           parent: MySuperclass,
           ...
       }

When instance_finalize is called for MySubclass, it will walk the
struct's list of fields and call the drop method for MySuperclass.
Then, object_deinit recurses to the superclass and calls the same drop
method again.  This will cause double-freeing of the Box<MyData>, or
more in general double-dropping.

What's happening here is that QOM wants to control the drop order of
MySuperclass and MySubclass's fields.  To do so, the parent field must
be marked ManuallyDrop<>, which is quite ugly.  Perhaps we can add a
wrapper type ParentField<> that is specific to QOM.  This hides the
implementation detail of *what* is special about the ParentField, and
it will also be easy to check for in the #[derive(Object)] macro.
Maybe in the future it will even make sense to have special functions
implemented on ParentField, I don't know...

Paolo