Re: When are unused overlays garbage collected?

[Marcin Borkowski <mbork@mbork.pl>]

On 2021-05-26, at 14:30, Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Marcin Borkowski <mbork@mbork.pl>
>> Cc: help-gnu-emacs@gnu.org
>> Date: Wed, 26 May 2021 06:53:42 +0200
>> 
>> But what would happen if I (delete-overlay my-overlay), then it gets
>> garbage collected, and then I (move-overlay my-overlay ...)?
>> 
>> I just performed a simple experiment - I manually created an overlay in
>> a temporary buffer, bound a variable to it, deleted it, and called
>> (garbage-collect) - and C-h v'ing that variable still said:
>> 
>> aqq’s value is #<overlay in no buffer>
>> 
>> so I assume it was /not/ GCed.
>
> It wasn't GCed because it was referenced from a variable.

Stupid me, of course.

>> >> My guess would be that if the overlay is "deleted" (so it is not
>> >> attached to any buffer, either by means of `delete-overlay' or when its
>> >> buffer is killed) /and/ it can't be referenced from Elisp (e.g., there
>> >> is no variable bound to it).
>> >
>> > That's not entirely true.  An overlay (like any other Lisp object)
>> > that was deleted will not be collected as long as some variable
>> > _actually_references_ it.  That could be a Lisp variable or a C
>> > variable not exposed to Lisp.  The difference between what I wrote and
>> > what you wrote is that the reference must actually exist, not only be
>> > possible.
>> 
>> I am not sure if I grasp that difference.  Can you provide an example of
>> a situation when an object does not have an "actual reference" but still
>> "can be referenced"?  Do you mean e.g. it being an element of a list
>> bound to some variable?
>
> No, that's not the essence of the difference.  You said, above, "it
> can't be referenced from Lisp".  My point is that "can't" doesn't cut
> it, because an object can be GCed even if it _can_ be referenced.
> What matters is that it _is_not_ referenced, even though it _can_be_.
>
> IOW, any object can potentially be referenced by some variable, but GC
> only avoids recycling an object if some other object actually (not
> potentially) references it.  The GC's "mark" phase scans all the live
> Lisp objects and marks any other objects referenced by those live
> objects, recursively.  Any marked object will not be recycled by the
> "sweep" phase.

I think we misunderstood each other.  By "can't be referenced from Lisp"
I meant "there was no object - like a variable - referencing it".  IOW,
no possible Lisp code could get me a reference to that overlay.  So we
agree, we just used words in a different way.

>> Well, I meant something different - an overlay that is "live" in some
>> buffer, but no variable is bound to it.  Such an overlay /can/ be
>> referenced with `overlays-in' / `overlays-at', so obviously cannot be
>> GCed, right?
>
> No live overlay in a live buffer will ever be GCed, because when GC
> marks live buffers, it also marks the overlays in that buffer by
> walking the linked list of the buffer's overlays.

Yes, that was what I meant.  Thanks.

-- 
Marcin Borkowski
http://mbork.pl