Subject: 26.3; Narrowing can prevent syntax propertization
From: Ikumi Keita <[email-masked]>
To: [email-masked]
Date: Mon, 25 May 2020 17:42:09 +0900
List-Id: <debbugs-submit.debbugs.gnu.org>

When syntax propertization occurs inside narrowed area of a buffer,
the outside area sometimes doesn't get syntax propertization even after
the narrowing is removed.

[How to reproduce]
1. Save the attached sample file as /tmp/sample.sh
2. emacs -Q -geometry 80x25 /tmp/sample.sh
3. Type C-u C-n C-e to go to the end of the (visible) fourth line.
4. Type C-x C-e as the comment there instructs.
5. Then the face of here-doc bleeds over the "EOF" token and continues
   to the end of the buffer.
   Expected result is that the here-doc face ends at "EOF" and the rest
   of the buffer is fontified with various suitable faces.

[Analysis]
In the above example, `narrow-to-region' restricts the buffer into the
last three lines of Local Variables section where syntax propertization
is not yet done. There, `syntax-ppss' triggers syntax propertization and
set `syntax-propertize--done' to (point-max). But actually that
`syntax-ppss' didn't syntax-properize outside the narrowing so it didn't
give the suitable `syntax-table' property to the newline following the
"EOF". Since `syntax-propertize--done' tells emacs that all portion of
the buffer has been propertized already, emacs does not give the
suitable `syntax-table' property to the newline following the "EOF" even
after `widen'. Thus the fontification of here-doc continues beyond "EOF".

In a similar way, if a lisp function goes to the point where syntax
propertization has not yet took place, makes narrowing and calls some
syntax-aware function which sets `syntax-propertize--done' to a value
inside the narrowing, a large portion of the buffer can be left without
suitable `syntax-table' property. This can lead to incorrect result
for not only font-lock but also syntax-aware functions such as
`forward-comment'.

Best regards,
Ikumi Keita

In GNU Emacs 26.3 (build 1, x86_64-unknown-freebsd12.0, GTK+ Version 3.24.9)
 of 2019-08-29 built on freebsd.vmware
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
Recent messages:
Indentation variables are now local.
Indentation setup for shell type sh
 [2 times]
Mark set
Saving file /home/keita/scr/sample.sh...
Wrote /home/keita/scr/sample.sh
Setting up indent for shell type sh
Indentation variables are now local.
Indentation setup for shell type sh

Configured using:
 'configure --with-canna --with-canna-includes=/usr/local/canna/include
--with-canna-libraries=/usr/local/canna/lib --without-xim
--disable-largefile --with-sound=yes --with-file-notification=yes
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Configured features:
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Load-path shadows:
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/home/keita/.emacs.d/elpa/soap-client-3.1.5/soap-client hides /usr/local/share/emacs/26.3/lisp/net/soap-client
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