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bug#31290: Fundamental bugs in syntax-propertize

From: Andreas Röhler
Subject: bug#31290: Fundamental bugs in syntax-propertize
Date: Sun, 13 May 2018 09:33:20 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 12.05.2018 13:26, Alan Mackenzie wrote:
Hello, Dmitry.

On Tue, May 08, 2018 at 15:35:14 +0300, Dmitry Gutov wrote:
On 4/28/18 12:08 AM, Alan Mackenzie wrote:

At least that would be true if syntax-propertize--done hadn't been
prematurely and spuriously increased, crudely to prevent an infinite
recursion, falsely indicating to the syntax-ppss infrastructure that the
syntax-table properties have already been applied to the region (BEGIN

      .... but it should not call `syntax-ppss-flush-cache', ....

Why not?  Because syntax-ppss-flush-cache sets syntax-propertize--done
back to its true value, allowing the wrongly allowed syntax-ppss calls at
a later position to cause a recursive loop.

Maybe we should "allow" it to loop, in certain cases? Leaving it to be
the responsibility of the programmer, to make sure the result doesn't
infloop, even if these rules are violated.

I'm not sure how this could work.  We would need to formalise the rules
very carefully, to avoid the need to read syntax.{c,el}'s source code.

      .... which means that it should not call `syntax-ppss' on some
      position and later modify the buffer on some earlier position.

This is a bad restriction, because sometimes syntax-table properties can
only be correctly determined by examining the syntax of later buffer
positions.  An example of this is giving the string-fence syntax-table
text property to an unbalanced opening string quote, but not to correctly
matched quotes.

I'm not exactly convinced by the given example (why would we use the
string-fence in that case?), but it might be better if something like
this was possible, indeed.

String fence can be used to signal to font lock that the delimiter
(together with the "mismatching" unescaped EOL) should be fontified in
warning face.

A better example might be C++ Mode's marking of a "< ... >" pair with
paren syntax.  This isn't done with syntax-propertize-function (as you
know), but it would be nice if this were possible.

2. syntax-propertize-function's are banned from using syntax-ppss, the
documentation instead directing them to use parse-partial-sexp directly.

The ones that currently call syntax-ppss, can't simply switch over to
parse-partial-sexp without becoming slower due to the lack of cache.

The cache at the pertinent buffer position doesn't exist at the time:
consistent syntax-table properties aren't on the preceding buffer

Before tackling this bug, I'd rather we see a real-world problem that it
caused, and pick a particular approach based on it.

My enhancements for bug#30393: "24.4; cperl-mode: indentation failure -
Documentation enhancements", where (almost) any change which affects the
syntactic state is programmed to call syntax-ppss-flush-cache from the C
level, clashes with the mechanism in this bug report.  Most of the time
it's fine, but when a change affecting the syntactic state is made from
inside a synax-propertize-function, Emacs goes into an infinite recursive

This isn't good.

But off the top of my head, we could introduce a "stricter but somewhat
slower" variation of syntax-ppss to be called inside
syntax-propertize-function's, which would treat the values in question
more carefully, somehow.

That's an idea worth exploring.

Hi folks,

from what I've seen month ago just may stress the term fundamental.
Gave up to follow details WRT to check-ins made.

That part needs some person treating the gordic knot according to its quality...

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