Re: adding new flua libraries

On 9/6/24 04:29, Baptiste Daroussin wrote:
> On Thu 05 Sep 18:51, Mark Johnston wrote:
>> FreeBSD ships a Lua 5.4 implementation, flua, in the base system.  It's
>> intended for use by the base system, and so far has a few consumers, but
>> not many so far.  (As an aside, flua's intended scope is not totally
>> clear to me.  Is it only for use by the base system?  What compatibility
>> guarantees does it provide, if any?)
>>
>> A few flua modules wrapping FreeBSD libraries (e.g., libjail) are
>> available, but they don't provide enough to make flua useful as a
>> general purpose programming tool.  It lacks interfaces for interacting
>> with the system (e.g., libc/libsys/libutil/etc wrappers) as well as
>> standard programming facilities (e.g., classes, higher-order functions,
>> etc.).  Here I'm mostly interested in discussing the former.
>>
>> I think flua could be a very useful alternative to shell scripts and C
>> code where performance is not critical.  It's very good at wrapping C
>> interfaces and thus could be used to make FreeBSD features (jails,
>> bhyve, ctl, pf, zfs, dtrace, ...) more accessible to developers who
>> don't want to interact with C.
>>
>> It's a lot of work to build up a set of flua modules that provide enough
>> functionality to be generally useful.  My feeling is that the easiest
>> way to get there is to wrap C interfaces directly (to the extent that
>> that's possible, but it's usually easy) and expose them in flua as a
>> stable interface.  Libraries written in pure Lua (or other languages
>> that interoperate well with Lua) could be built on top of that.
>>
>> I'm inclined to start by wrapping libc and libsys interfaces in a manner
>> similar to luaposix.  There, the namespace is partitioned by C headers,
>> so posix.unistd contains identifiers from unistd.h, posix.sys.stat
>> contains identifiers from sys/stat.h, and so on.  In fact, flua already
>> has a small subset of luaposix's functionality.  Wrapping C interfaces
>> isn't much fun, but it's easy, and it saves us having to come up with
>> names and interfaces (naming things is hard), and helps ensure that the
>> glue code is relatively small and doesn't impose a large testing burden.
>> Moreover, C interfaces don't change much and are subject to
>> well-understood compatibility constraints, which should mean that Lua
>> interfaces built on top of them are subject to the same constraints.
>>
>> Assuming there's general agreement on that approach, the question I'd
>> really like to answer is, what should the namespace look like?  It would
>> be useful to provide a posix module compatible with luaposix, but that
>> obviously won't contain FreeBSD-specific functionality.
>>
>> I propose having a top-level "freebsd" namespace for all modules
>> implemented in the base system, excluding posix and contrib libraries
>> which already define a Lua interface (libucl is the one example I see so
>> far; we could import sqlite bindings as well).  Then, if we follow
>> luaposix's convention, we could have freebsd.sys.capsicum.* for
>> Capsicum-related syscalls and constants, freebsd.sys.event.* for kevent
>> wrappers, and so on.  The posix module could simply provide a subset of
>> freebsd.*.
>>
>> Maybe it's better to separate C wrappers from native Lua modules though,
>> so it could be better to have freebsd.c.sys.capsicum.*, etc., and
>> provide higher-level interfaces for FreeBSD features under freebsd.pf,
>> freebsd.zfs, freebsd.jail, etc..  I'm not really sure.
>>
>> In the interest of prompting discussion a bit, I posted some patches to
>> add some example wrappers to flua:
>> https://reviews.freebsd.org/D46553
>> https://reviews.freebsd.org/D46554
>> https://reviews.freebsd.org/D46556
>>
>> Does anyone have opinions on anything I've brought up above?  I'm pretty
>> happy to write a lot of this glue code or find ways to automate it, as
>> I've already done a fair bit of it, but it's hard to make progress
>> without having some rigourous conventions for the flua namespace (again,
>> naming things is hard).
> 
> I like all of what I read and I am fully aligned.
> 
> What I don't see here, is I think we should have dynamic modules libucl, fbsd &
> friends are not dynamic and the reviews I see for the freebsd module reviews,
> this is still bundled.
> 

Right, this is an artifact of how flua's evolved that we need to move 
away from.  We didn't have module support at the beginning- that came 
later, but we didn't really move things out into modules.  We still 
can't move some things because of the bootstrap flua problem I mentioned 
elsewhere (doing so would block work to do `make sysent` type work as 
part of the build as it would break cross-builds), but at least 
libucl/fbsd would be fine.

> my proposal for unbundling, I need kldload for a project of mine, so I did:
> https://reviews.freebsd.org/D46558
> 
> Best regards,
> Bapt