Re: [Caml-list] Ephemerons: is this behavior correct ?

["François Bobot" <francois.bobot@cea.fr>]

Hi Bertrand,

On 10/10/2016 15:14, Bertrand Jeannet wrote:
> The following piece of code raises an assert false exception (in rare
> cases) with official version 4.03.0:
>
> match Ephemeron.K1.get_data c with
> | Some _ ->
>    match Ephemeron.K1.get_key c with
>    | Some _ -> (* ... *)
>    | None -> assert false (* reachable *)
 >
> The documentation of the Ephemeron module says:
> "When one of the keys is not considered alive anymore by the GC, the
> data is emptied from the ephemeron"
>
> I was expecting this to happen atomically from the programmer point of
> view,
Yes, atomicity is the intent.

> but here apparently the key was emptied but the data kept (at
> least temporarily).

I don't deduce that from the example. The cleaning can be done atomically between the return of 
get_data and the return of get_key (indeed get_key does an allocation). An example that would show 
non-atomicity would be:

 > match Ephemeron.K1.get_data c with
 > | None ->
 >    match Ephemeron.K1.get_key c with
 >    | Some _ -> assert false (* non atomic! *)
 >    | None -> ...
 >

or

 > match Ephemeron.K1.get_key c with
 > | None ->
 >    match Ephemeron.K1.get_data c with
 >    | Some _ -> assert false (* non atomic! *)
 >    | None -> ...
 >


>
> Was this behavior anticipated ?

Yes.

>
> IMHO, synchronized deletion is simpler and safer (in previous versions,
> I encountered the now-solved-bug that the deletion of several weak
> pointers to the same object was not synchronized).

Indeed I think it is a better behavior. I'm happy it solved your previous problems.

>
> Btw, the Ephemeron module does not appear on this url:
> http://caml.inria.fr/pub/docs/manual-ocaml/stdlib.html
> one has to go to
> http://caml.inria.fr/pub/docs/manual-ocaml/libref/index.html

I though that GPR#564[1] merged in 4.03 took care of this bug, so I don't understand.


Best,


[1]: https://github.com/ocaml/ocaml/pull/564

-- 
François