MESSAGE
| DATE | 2020-08-06 |
| FROM | From: "Jose E. Marchesi"
|
| SUBJECT | Re: [Hangout - NYLXS] Concerns about GNU Bison maintenance.
|
Hullo there.
As I see in what seems to be your last iteration in bug-bison, back in
January:
https://lists.gnu.org/archive/html/bug-bison/2020-01/msg00009.html
In that thread you raised some (all?) of these concerns in a single
email, and the Bison maintainer replied with some reasons, suggesting
some solutions, and making you some questions.
Seems like you didn't follow up.
Have you considered to do so?
> Without a doubt, GNU Bison is an a cornerstone piece of the GNU system,
> relied upon by many programs.
>
> Developers rely on Bison to be stable. What I mean by this is that a
> project which has a mature Bison grammar file that changes very little
> or not at all over a long period of time should not have to do anything
> to the code because of changes in the Bison upstream.
>
> For example, it should be possible to check out a ten-year-old version
> of the code (say during a "git bisect" operation, in uncovering the
> commit which caused a bug) and build it without problems with the
> whatever Bison is installed.
>
> Some developers write the grammar file such that it works with multiple
> implementations. That doesn't necessarily mean adhering to the POSIX
> Yacc specification. For instance, Berkeley Yacc has some GNU features
> like %pure-parser. This works fine with GNU Flex, just like the same
> feature in GNU Bison.
>
> However, over some years now there has been an unsettling trend in the
> development of Bison which can be summarized as the current maintainer
> treating it as a personal research project.
>
> Features are being introduced that are nice, but that nobody requires
> from GNU Bison. Tautologically, no existing code depends on a new
> feature. (So where are these requirements coming from? Who is
> gate-keeping them? What is the "product management" for Bison?) At the
> same time, stability and compatibility are showing the hairline cracks
> of fracture.
>
> Most recently, Bison 3.7 was just announced. I first saw the posting in
> the comp.compilers newsgroup, then in the Bison mailing list. Not soon
> afterward, the GNU Awk maintainer reported that it doesn't even
> build on Ubuntu 18.04, which is almost a poster child for "popular
> GNU/Linux distro". A storm of mailing list posts has ensued.
>
> Here is a problem I ran into fairly recently, after upgrading my
> environment to a newer GNU/Linux distribution with a newer Bison.
>
> Once upon a time, Bison introduced an extension to the language for
> making a re-entrant parser; it was keyed to the directive
> "%pure-parser". This went on to be adopted by other Yacc-like
> implementations such as Berkeley Yacc.
>
> The Bison maintainer believes that Bison "owns" this language feature
> and is free to deprecate it. Note that deprecating doesn't mean removing
> the *feature* of re-entrant parsing; just the *spelling* of the
> "%pure-parser" directive. As of some 3.x version, Bison now warns now
> that it's deprecated, and that one should use a different spelling
> for it.
>
> In a mailing list response, I was told that my "problem" is that I'm
> trying to write code that works with Byacc and Bison. (Writing code
> targeting multiple implementations is a problem? Now what are the odds
> that someone who thinks that way would end up breaking stuff?)
>
> The maintainer doesn't seem to understand that if I have to switch for
> some new spelling for an old feature to avoid the deprecation warning
> (and to anticipate the outright removal of the old spelling), the code
> then not only then doesn't work on Byacc, but it also doesn't work in
> older Bisons. The software no longer builds in operating system
> installations that have not updated to the latest Bison.
>
> Moreover, if Bison actually drops support for the spelling, then old
> baselines of my code will not build. Thus, for instance, I will not be
> easily able to do a "git bisect" to find where a bug was introduced. The
> old versions won't build unless I patch every commit I visit, or use a
> parallel installation of old Bison for the old baselines.
>
> Bison makes careless changes to the skeletons and other generated
> material. For instance, in Bison 3, a declaration of yyparse was
> introduced to "y.tab.h". I had to add a sed command into the makefile
> build recipe to filter it out textually.
>
> What is the problem with declaring yyparse in "y.tab.h"? The problem is
> that if you're using a re-entrant parser, the signature of yyparse
> contains custom types. For instance suppose we have this in the .y
> grammar file:
>
> %pure-parser
> %parse-param{scanner_t *scnr}
> %parse-param{parser_t *parser}
>
> The declaration of yyparse is this:
>
> int yyparse(scanner_t *scnr, parser_t *parser);
>
> It's not just something innocuous like:
>
> int yyparse(void);
>
> If the former is suddenly plonked into "y.tab.h" by the parser
> generator, it means that whoever is including that header now has to
> provide declarations of scanner_t and parser_t before the header.
>
> yyparse is not necessarily treated as a public function; programs can be
> written such that all the calls to yyparse occur in the same file.
>
> POSIX doesn't say anything abuot yyparse being declared in "y.tab.h".
> It says this:
>
> The header file shall contain #define statements that associate the
> token numbers with the token names. This allows source files other
> than the code file to access the token codes. If a %union declaration
> is used, the declaration for YYSTYPE and an extern YYSTYPE yylval
> declaration shall also be included in this file.
>
> The bottom line is that you can't just add material into a header file
> (whether it is static or generated). Due to the large number of
> programs which depend on it, you don't know what may break.
>
> The Bison project seems to lack proper focus. It now has parser
> generators for numerous languages, which distract from the main mission,
> which is to be a great replacement for Yacc, with some essential
> extensions.
>
> Bison could perhaps benefit from a split; do all the experimental new
> stuff and support for new languages and whatnot in a "Bison New
> Generation" project; and just keep "Regular Bison" working.
>
> All that said, I believe that the current maintainer is competent, and
> the situation can be turned around with a bit of an attitude
> readjustment.
>
> I think that understanding issues in software maintenance relating to
> backward compatibility is a separate skill apart from other software
> skills, and the Bison maintainer is lacking in this area; however, those
> things can be easily learned. (Often they can be deduced from first
> principles, if you think about the implications of every code change
> from that perspective.)
>
> I hasten to add the observation that no matter how much a maintainer
> cares about compatibility and stability and all that, one person will
> make mistakes anyway.
>
> Software shops nowadays deploy peer review systems, which require every
> change to be viewed by several other developers and approved. While
> I'm a good, reasonably conscientious coder with decades of experience,
> this has saved my proverbial butt. I can now palpably feel the
> disadvantage of not having a peer review crew in my side projects.
> The GNU project could benefit from a collaborative review system so that
> changes to an important, high-impact program that countless projects
> depend on, such as Bison, are not just down to a single fallible human
> being.
>
> There is a bit of collaboration in Bison in that some people, like
> Paul Eggert, regularly keep up with the baseline and post to the
> bug-bison mailing list. I feel that without their efforts, the situation
> would be worse.
>
> Lastly, it think it may be a good idea for at least every major release
> of Bison to be regression tested by building several GNU/Linux
> distributions from scratch with it. A distro build is a great test
> suite for a toolchain component. If that is available, why would you
> only rely on the tool's own limited suite when releasing?
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