bug#22983: syntax-ppss returns wrong result.

[Alan Mackenzie <acm@muc.de>]

Hello, John.

On Tue, Sep 05, 2017 at 13:28:52 +0100, John Wiegley wrote:
> >>>>> Dmitry Gutov <dgutov@yandex.ru> writes:

> > I'm sure we want to fix design flaws. As long as there is a solid plan that
> > does not swap one flaw for another.

> Can we have a summary of the current proposal(s) on the table? It would help
> to clarify, rather than navigating past discussions. Alan has told me that
> this issue is affecting people and has been outstanding for some time; I'd
> like to get a better idea of its seriousness/scope, and what effect the
> available solutions would have (as Dmitry says, we don't want to replace one
> flaw with another).

First, I think it's worthwhile emphasising what the function purports to
do:

    syntax-ppss is a compiled Lisp function in `syntax.el'.

    (syntax-ppss &optional POS)

    Parse-Partial-Sexp State at POS, defaulting to point.
    The returned value is the same as that of `parse-partial-sexp'
    run from `point-min' to POS except that values at positions 2 and 6
    in the returned list (counting from 0) cannot be relied upon.
    Point is at POS when this function returns.

The solution I propose is to introduce a second cache into syntax-ppss,
and this cache would be used whenever (not (eq (point-min) 1)).
Whenever point-min changes, and isn't 1, this second cached would be
calculated again from scratch.

This proposal has these advantages:

(i) It would make the function deliver what its unchanged doc string
says.  This is important, given that syntax-ppss has been very widely
used within Emacs, and likely by external packages too; these will
typically have assumed the advertised behaviour of the function, without
having tested it in narrowed buffers.

(i) In the case which currently works, namely a non-narrowed buffer,
there would be only a minute slow-down (basically, there would be extra
code to check point-min and select the cache to use).

(ii) The cache for use in a narrowed buffer might well be sufficiently
fast in normal use.  If it is not, it could be enhanced readily.

I think Dmitry also proposed a method of solution some months ago,
though I don't remember in detail what it was.  Dmitry, do you still
think your solution would work?  If so, please elaborate on it.

> -- 
> John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
> http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2

-- 
Alan Mackenzie (Nuremberg, Germany).