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

[Richard W.M. Jones]

On Wed, May 31, 2023 at 05:33:13PM +0200, Laszlo Ersek wrote:
> On 5/31/23 13:23, Richard W.M. Jones wrote:
> > 
> > ... And while I'm scrawling my throughts into this thread ...
> > 
> > What we intend here are two slightly different operations:
> > 
> > (A) Free every unreachable object.  That's what we want in this
> > specific place in the code.
> > 
> > (B) Provide a soft test that the OCaml heap hasn't been screwed
> > up because of bugs in our bindings.  That's what we use eg here:
> > https://gitlab.com/nbdkit/libnbd/-/blob/master/ocaml/tests/test_100_handle.ml#L24
> > 
> > In OCaml 4 Gc.compact provides (A) and (B), in particular compaction
> > is quite an aggressive & costly operation that requires examining and
> > moving every object on the heap.  In practice we found this quickly
> > reveals bugs in bindings.
> > 
> > OCaml 5 provides (A) through Gc.full_major--albeit not documented--and
> > doesn't seem to provide any way to do (B) as far as I can tell.
> 
> Re: (A) -- I didn't expect to see
> <https://github.com/rpm-software-management/dnf/commit/5ce5ed1ea08ad>
> again, just in different clothing. :/
> 
> At least we have a way to force (A), going forward.
> 
> Re: (B) -- can we keep testing our OCaml bindings on OCaml 4, kind of as
> a "requirement", until goal (B) becomes possible on OCaml 5 too? That
> wouldn't test (A)+(B) on OCaml 5, but we'd still cover (A)+(B) in total,
> across multiple OCaml versions.
> 
> Meanwhile we should probably chime in on one of these upstream
> discussions. For that, how would we we "formalize" goal (B)? Call it
> "heap fsck" (cf. "git-fsck", which is an existent utility)?
> 
> Ticket <https://github.com/ocaml/ocaml/issues/11812> mentions, "OCaml 5
> does not implement heap compaction. It is not yet clear whether we will
> need one." -- a heap consistency check (for the sake of C language
> extensions) would be one reason.

To start with I have opened a discussion topic.  We can think about
opening bugs later.

https://discuss.ocaml.org/t/ocaml-heap-fsck-and-forcing-collection-of-unreachable-objects/12281

Rich.

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