bug#29767: 26.0.90; Failing auto-fill in message-mode because of bad state (comment-skip-end)

[Stefan Monnier]

> Unfortunately, i cannot reproduce the bug reliably, but these are the
> symptoms:

I think I found the culprit: syntax-propertize relies incorrectly on
syntax-ppss to add syntax-ppss-flush-cache to before-change-functions.
This works fine as long as syntax-ppss is called, but in message-mode
buffers, syntax-propertize is always called but not syntax-ppss (it may
get called depending on the operations you perform, but it's not called
initially, e.g. because font-lock is told not to use the syntax-table to
highlight strings and comments).

The patch below seems to fix it for me.  Is it OK to push it to emacs-26?


        Stefan


diff --git a/lisp/emacs-lisp/syntax.el b/lisp/emacs-lisp/syntax.el
index a1b70b1869..a0493cc1ba 100644
--- a/lisp/emacs-lisp/syntax.el
+++ b/lisp/emacs-lisp/syntax.el
@@ -291,6 +291,9 @@ syntax-propertize
       ;; (message "Needs to syntax-propertize from %s to %s"
       ;;          syntax-propertize--done pos)
       (set (make-local-variable 'parse-sexp-lookup-properties) t)
+      (when (< syntax-propertize--done (point-min))
+        (add-hook 'before-change-functions
+                 #'syntax-ppss-flush-cache t t))
       (save-excursion
         (with-silent-modifications
           (make-local-variable 'syntax-propertize--done) ;Just in case!