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

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/74fe398bbe2e53db21a998356b042c232d42a... > > 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/74fe398bbe2e53db21a998356b042c232d42a... > > 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/ffb2022797986324213891a59c02af46269b5... > > But the big difference is Gc.full_major: > > https://github.com/ocaml/ocaml/blob/ffb2022797986324213891a59c02af46269b5... > > 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.

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