Re: [PATCH 2/2] qom: Assert that objects being destroyed have no parent

[Paolo Bonzini]

On 16/12/20 08:53, Marc-André Lureau wrote:
> On the principle, I fully agree. But the risk is high to introduce 
> regression if objects are manipulated in strange ways.
>
> I remember I wanted object_unref() to automatically remove itself from 
> the parent when the last ref is dropped. I think there were similar 
> concerns.

unref and unparent are two very different operations; the former means 
*I* am done with this object, the latter means *QEMU* is done with this 
object (even though there may be a few reference held, e.g. on the call 
stack or by RCU).  Since object_unparent operates on global state, you 
can even call object_unparent if you don't own yourself a reference to 
the object and just got the pointer from the caller.

While unref is a "mechanical" operation of dropping a reference and 
possibly freeing the object, unparent is an active operation that 
includes for example dropping reference cycles or in general detaching 
from other places that are known to hold references to this object.

This is not a concept that is specific to QEMU, I think I read somewhere 
that LibreOffice's UI library does something similar, calling it "dispose".

Paolo