bug#26892: 25.2; whitespace-mode does not turn off correctly

[Nick Helm]

Emacs -Q
C-u 20 SPC RET RET RET RET   ; type some random whitespace
M-x global-whitespace-mode
M-x whitespace-mode
M-x whitespace-mode

The last command should toggle local whitespace-mode off, but note that the
whitespace display characters are not properly removed from the current
buffer.

I had a look at whitespace.el and I think the problem lies in
`whitespace-display-char-on'. Among other things, this function stores the
buffer display table and replaces it with a temporary display table to show
whitespace chars. The original is restored when the mode is turned off.

However, with global-whitespace-mode active, there is already a temporary
display table in place, which whitespace-mode incorrectly picks up and stores,
clobbering the original display table in the process. When whitespace-mode is
turned off, it restores the temporary display table set by
global-whitespace-mode, which leads to the problem.

Here's a little patch to check if global-whitespace-mode is active before
storing the display table. I've tried this for a few days with no obvious
problems.

--- whitespace-a.el     2017-05-12 00:59:34.000000000 +1200
+++ whitespace-b.el     2017-05-12 01:02:26.000000000 +1200
@@ -2496,9 +2496,10 @@
     (let (vecs vec)
       ;; Remember whether a buffer has a local display table.
       (unless whitespace-display-table-was-local
-       (setq whitespace-display-table-was-local t
-             whitespace-display-table
-             (copy-sequence buffer-display-table))
+       (setq whitespace-display-table-was-local t)
+        (unless global-whitespace-mode
+             (setq whitespace-display-table
+             (copy-sequence buffer-display-table)))
        ;; Assure `buffer-display-table' is unique
        ;; when two or more windows are visible.
        (setq buffer-display-table





In GNU Emacs 25.2.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 
10.9.5 (Build 13F1911))
 of 2017-04-22 built on builder10-9.porkrind.org
Windowing system distributor 'Apple', version 10.3.1504
Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES

Important settings:
  value of $LANG: en_NZ.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Messages

Minor modes in effect:
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  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  transient-mark-mode: t

Recent messages:
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Emacs start-up time: 1.0 seconds
For information about GNU Emacs and the GNU system, type C-h C-a.
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Cleaning up the recentf list...done (0 removed)
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s-b is undefined [2 times]

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