Wireshark-dev: Re: [Wireshark-dev] Lua 5.3
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From: João Valverde <joao.valverde@xxxxxxxxxxxxxxxxxx>
Date: Fri, 19 Aug 2016 17:43:53 +0100


On 08/19/2016 04:18 PM, Pascal Quantin wrote:



2016-08-19 17:05 GMT+02:00 João Valverde
<joao.valverde@xxxxxxxxxxxxxxxxxx
<mailto:joao.valverde@xxxxxxxxxxxxxxxxxx>>:



    On 08/19/2016 03:56 PM, João Valverde wrote:



        On 08/19/2016 02:54 PM, Peter Wu wrote:

            On Mon, Aug 08, 2016 at 09:17:35PM +0100, João Valverde wrote:



                On 08/08/2016 05:58 PM, Pascal Quantin wrote:

                    Hi João,

                    2016-08-08 18:52 GMT+02:00 João Valverde
                    <joao.valverde@xxxxxxxxxxxxxxxxxx
                    <mailto:joao.valverde@xxxxxxxxxxxxxxxxxx>
                    <mailto:joao.valverde@xxxxxxxxxxxxxxxxxx
                    <mailto:joao.valverde@xxxxxxxxxxxxxxxxxx>>>:

                        Is moving to Lua 5.3 something we should look into?

                        The 64 bit integer support seems really promising.


                    Hariel explained us that Lua 5.3 was a completely
                    different language (a
                    bit like what you have between Python 2.X and 3.X).
                    You can find much
                    more info (from people knowing what they are taling
                    about - so not me
                    ;)) in bug 10881.

                    Pascal.



                Thanks for that Pascal. The only sane way to approach
                the issue IMHO
                is to
                accept that this may and probably will break backward
                compatibility (not
                even think about supporting 5.1 or 5.2) and just
                consider whether
                that break
                is justified (hint: it is).


            Why is it justified to break backwards compatibility and
            move from 5.2
            to 5.3 without the ability to chose for 5.2? What is the
            killer feature
            of 5.3 that makes it totally worth to possibly break older
            dissectors?

            The disadvantage of C plugins is that it had to be
            recompiled for newer
            versions. With a move from 5.2 to 5.3 and also removing
            GRegex and bitop
            you make it quite likely to break Lua dissectors in some way.

            I have once written a Lua library in C, interfacing with
            Libgcrypt for
            which I studied the Lua manual. The API changes with 5.3
            were not that
            significant if I remember correctly (though you have to be
            careful with
            providing a compatibility layer), but the ABI is certainly not
            compatible.

            In the recent proposed patches, you seem to have no issues
            with breaking
            backwards compatibility. Have you developed Lua dissectors
            before?
            Breaking things for the sake of "shiny, new, future" is not an
            acceptable motivation, there must be something more
            appealing to justify
            such breakage. Having 64-bit integer support, but taking
            away the bitop
            library is a net loss without even considering the other
            factors.


        Doesn't Lua 5.3 provide native bit operators? If so there is not net
        loss of functionality. That was my reasoning at least.

        The language incompatibilities between 5.2 and 5.3 are minor. The
        wireshark API is exactly the same.

        LPeg is more powerful and Lua-thonic than lrexlib, but there is a
        learning curve for that, no doubt. For anyone relying on
        lrexlib, it's a
        significant break. We can keep lrexlib, that's not a problem and
        it is
        orthogonal to the other changes.

        As far as killer features go, besides the obvious, how about better
        UTF-8 support?

        I don't have time for a more detailed answer right now but I'd
        like to
        say I think this change is entirely justified but I also completely
        understand disagreeing with that opinion.


    I'm referring to the upgrade to Lua 5.3 here, i.e, breaking backward
    compatibility, same as any other Lua script moving from 5.1/5.2 to 5.3.


For what it is worth, I do not remember any user asking / pushing to
upgrade to Lua 5.3 yet. Breaking their script should be justified by a
huge win (I will not judge myself whether this is the case or not with
this upgrade as I'm not a Lua user, so I'm not qualified here; but we
must think about our existing users). C plugins are a bit of main to
recompile for each version, but are fast. ON the other side Lua scripts
were not as impacted as C ones with API changes, but are slower (from
what I understood). Any big change here should be done with caution I think.
I think the impact is very small compared to the benefits, it's an easy port to Lua 5.3 if it requires porting at all, but maybe someone else who uses this code heavily can comment on that. 

I really like the Wireshark Lua API, by the way. It's really cool.