[Libguestfs] [PATCH libguestfs 1/2] ocaml/implicit_close test: collect all currently unreachable blocks

[Richard W.M. Jones]

On Wed, May 31, 2023 at 04:44:49PM +0200, Laszlo Ersek wrote:
> On 5/31/23 13:13, Richard W.M. Jones wrote:
> > On Wed, May 31, 2023 at 12:32:49PM +0200, Laszlo Ersek wrote:
> >> On 5/31/23 11:12, Richard W.M. Jones wrote:
> >>> On Sat, May 27, 2023 at 03:32:36PM +0200, Jürgen Hötzel wrote:
> >>>> Fixes failing implice_close test on OCaml 5.
> >>>> ---
> >>>>  ocaml/t/guestfs_065_implicit_close.ml | 4 ++--
> >>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/ocaml/t/guestfs_065_implicit_close.ml b/ocaml/t/guestfs_065_implicit_close.ml
> >>>> index 567e550b4..5e00c21ac 100644
> >>>> --- a/ocaml/t/guestfs_065_implicit_close.ml
> >>>> +++ b/ocaml/t/guestfs_065_implicit_close.ml
> >>>> @@ -30,8 +30,8 @@ let () =
> >>>>   *)
> >>>>  
> >>>>  (* This should cause the GC to close the handle. *)
> >>>> -let () = Gc.compact ()
> >>>> +let () = Gc.full_major ()
> >>>>  
> >>>>  let () = assert  (!close_invoked = 1)
> >>>>  
> >>>> -let () = Gc.compact ()
> >>>> +let () = Gc.full_major ()
> >>>
> >>> I don't understand this patch at all.  If there a test failing we need
> >>> to diagnose why it is failing, not paper over the symptoms.
> >>>
> >>> What is the exact failure?
> >>
> >> Well my assumption is that (a) we need to force a garbage collection for
> >> the (unreachable) handle to be closed actually (from earlier, nothing
> >> new regarding that), but (b) with OCaml 5, "compact" is not strong
> >> enough for that, while "full_major" is. Whether that means OCaml 5
> >> changed the semantics of these functions, or that even with OCaml 4
> >> we've only (consistently) lucky with "compact", I can't tell.
> > 
> > So it turns out there is a difference between OCaml 4.14 and 5 here.
> > 
> > In 4.14:
> > 
> > Gc.compact finishes the current major cycle and then compacts the
> > heap:
> > 
> > https://github.com/ocaml/ocaml/blob/74fe398bbe2e53db21a998356b042c232d42a4d8/runtime/gc_ctrl.c#L625
> > 
> > Gc.full_major finishes the major cycle and then sometimes compacts the
> > heap based on a threshold of how much heap is being used:
> > 
> > https://github.com/ocaml/ocaml/blob/74fe398bbe2e53db21a998356b042c232d42a4d8/runtime/gc_ctrl.c#L579
> > 
> > So Gc.full_major is (kind of) a subset of Gc.compact, which (mostly)
> > matches the documentation.
> > 
> > In 5.0:
> > 
> > Gc.compact finishes the major cycle, and as far as I can tell doesn't
> > compact the heap (bug?!):
> > 
> > https://github.com/ocaml/ocaml/blob/ffb2022797986324213891a59c02af46269b5c17/runtime/gc_ctrl.c#L297
> > 
> > But the big difference is Gc.full_major:
> > 
> > https://github.com/ocaml/ocaml/blob/ffb2022797986324213891a59c02af46269b5c17/runtime/gc_ctrl.c#L261
> > 
> > As you can see from a comment in the code, "[the new garbage
> > collector] can require up to 3 GC cycles for a currently-unreachable
> > object to be collected", and Gc.full_major does this.  (I don't
> > believe this is true for the old GC in OCaml <= 4.)
> > 
> > So in 5.0 full_major is quite a different operation from compact.  It
> > runs multiple major cycles to make sure everything is collected, and
> > doesn't compact the heap, but then neither does Gc.compact which is no
> > longer a superset of Gc.full_major.
> > 
> > The patch there is correct, for OCaml 5, but breaks OCaml 4,
> 
> Right, from your explanation above, this is what I've been expecting --
> what works for OCaml 5 may not work for OCaml 4.
> 
> In fact I don't understand the OCaml runtime's developers -- this is a
> compatibility breaking change! Every project that uses the Gc module
> will have to add compat code (version checking) now.

Yes, it's not a good change.

About the patch, we could try to conditionalize it on
Sys.ocaml_release.Sys.major >= 5 (see /usr/lib64/ocaml/sys.mli)

Rich.

> > so it
> > should probably have some kind of conditional on the OCaml version.
> > It also needs a much clearer explanation.
> 
> Thanks
> Laszlo

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