[Libguestfs] Minimum OCaml compiler version (2021/2022 edition)

[Daniel P. Berrangé]

On Tue, Nov 09, 2021 at 09:04:40AM +0000, Richard W.M. Jones wrote:
> Previously:
> https://listman.redhat.com/archives/libguestfs/2020-March/msg00063.html
> https://listman.redhat.com/archives/libguestfs/2017-September/msg00203.html
> 
> Our current minimum version across projects is 4.03.
> 
> We still use "noalloc" in a few places which causes this warning:
> 
>   ocamlopt.opt -warn-error +A-3 -c NBDKit.ml -o NBDKit.cmx
>   File "NBDKit.ml", line 155, characters 0-70:
>   155 | external set_name : string -> unit = "ocaml_nbdkit_set_name" "noalloc"
>         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>   Alert deprecated: [@@noalloc] should be used instead of "noalloc"
> 
> I just noticed now that this change was made in 4.03 -- I will go
> ahead and fix this everywhere today.
> 
> Should we move to a newer minimum version?  If we moved to 4.07 then
> we could also get rid of the warnings about Pervasives (replaced by
> Stdlib), eg:
> 
>   File "std_utils.ml", line 329, characters 26-44:
>   329 |     let sort_uniq ?(cmp = Pervasives.compare) xs =
>                                   ^^^^^^^^^^^^^^^^^^
>   Alert deprecated: module Stdlib.Pervasives
>   Use Stdlib instead.
> 
> However 4.07 was only released in 2018, and this would mean removing
> RHEL 7 compatibility (officially -- it's sort of unofficially not
> supported already).  More irritatingly, FreeBSD is stuck on 4.05.

Note both libvirt and QEMU have explicitly dropped RHEL-7
as a supported build platform[1], under the rule that platforms
are dropped 2 years after the new major version is released.

It has been > 2 years since RHEL-8 GA, so they dropped RHEL-7.

IOW even if new libguestfs can build on RHEL-7, you'll be
stuck with old libvirt and QEMU.

> Here are the common distros and versions (< 4.07 marked with '*'):
> 
>   Arch (Extra)     OCaml 4.12
>   Debian stable    OCaml 4.11
>   Debian testing   OCaml 4.11
>   Fedora 31        OCaml 4.08
>   Fedora 35        OCaml 4.12
>   FreeBSD (ports)  OCaml 4.05  *
>   OpenSUSE         OCaml 4.13
>   RHEL 7           OCaml 4.05  *
>   RHEL 8           OCaml 4.07
>   RHEL 9           OCaml 4.11
>   Ubuntu 16.04     OCaml 4.02  *

Ubuntu 16.04 is explicitly dropped by both QEMU and libvirt too
a long time ago. It falls under both the 2 year cut off rule,
an the maximum of two major releases in concurrently rules.

NB, QEMU/libvirt only look at Ubuntu LTS releases for cutoffs

>   Ubuntu 18.04     OCaml 4.05  *
>   Ubuntu 20.04     OCaml 4.08
>   Ubuntu 21.04     OCaml 4.11
> 
> And here are the release dates of the OCaml compiler:
> 
>   OCaml version    Release date
>   4.02             2014-08
>   4.03             2016-04
>   4.04             2016-11
>   4.05             2017-07
>   4.06             2017-11
>   4.07             2018-07
>   4.08             2019-06
>   4.09             2019-09
>   4.10             2020-02
>   4.11             2020-08
>   4.12             2021-02
>   4.13             2021-09

Regards,
Daniel

[1] https://www.qemu.org/docs/master/about/build-platforms.html
    https://libvirt.org/platforms.html

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