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

[Richard W.M. Jones]

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

Rich.

-- 
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