GNU bug report logs - #36136
syntax-ppss fails to invalidate its cache on changes to syntax-table text properties

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: bug-gnu-emacs <at> gnu.org
Subject: syntax-ppss fails to invalidate its cache on changes to syntax-table
 text properties
Date: Sat, 8 Jun 2019 13:17:24 +0000

Hello, Emacs.

The syntax-ppss cache is not invalidated when syntax-table text
properties are set or cleared.  This is because the invalidation
function, syntax-ppss-flush-cache is invoked only as a before-change
function, but typical (?all) syntax-table property changes happen when
before-change-functions is inactive.

This is a bug.

In my debugging of a CC Mode scenario, a buffer change causes a
syntax-table text property change at an earlier part of the buffer.  This
is to do with the change making previously non-matching C++ raw string
identifiers match up.  Font lock follows the syntax-ppss state, which
spuriously records that the end part of the buffer is still in a string.
Hence the non-string part of the buffer is still fontified with
font-lock-string-face.

Suggested fix: the functions set_properties, add_properties,
remove_properties in textprop.c should check for changes to,
specifically, syntax-table properties.  When these changes are detected,
a hook called something like syntax-table-props-change-alert-hook should
be called (with some appropriate position parameters, tbd).
syntax-ppss-flush-cache will be added to this hook.

-- 
Alan Mackenzie (Nuremberg, Germany).

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