bug#29381: show-paren-mode should not be globalized

[Dmitry Gutov <dgutov@yandex.ru>]

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On 03.09.2021 10:50, Lars Ingebrigtsen wrote:
> "Paul W. Rankin"<pwr@bydasein.com>  writes:
> 
>> There are plenty of minor modes that use FOO-mode (global) and
>> FOO-local-mode
> That's a good point; didn't know that.  Perhaps we should flip this
> around as suggested, then, and I'm reopening this bug report.

Here's a patch, modeled after electric-indent-local-mode.

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diff --git a/lisp/paren.el b/lisp/paren.el
index a45a08abd3..5c3c85828b 100644
--- a/lisp/paren.el
+++ b/lisp/paren.el
@@ -114,8 +114,28 @@ show-paren-mode
                                 show-paren-delay t
                                 #'show-paren-function))
   (unless show-paren-mode
-    (delete-overlay show-paren--overlay)
-    (delete-overlay show-paren--overlay-1)))
+    (show-paren--delete-overlays)))
+
+(defun show-paren--delete-overlays ()
+  (delete-overlay show-paren--overlay)
+  (delete-overlay show-paren--overlay-1))
+
+;;;###autoload
+(define-minor-mode show-paren-local-mode
+  "Toggle `show-paren-mode' only in this buffer."
+  :variable (buffer-local-value 'show-paren-mode (current-buffer))
+  (cond
+   ((eq show-paren-mode (default-value 'show-paren-mode))
+    (unless show-paren-mode
+      (show-paren--delete-overlays))
+    (kill-local-variable 'show-paren-mode))
+   ((not (default-value 'show-paren-mode))
+    ;; Locally enabled, but globally disabled.
+    (show-paren-mode 1)                ; Setup the timer.
+    (setq-default show-paren-mode nil) ; But keep it globally disabled.
+    )
+   (t ;; Locally disabled only.
+    (show-paren--delete-overlays))))
 
 (defun show-paren--unescaped-p (pos)
   "Determine whether the paren after POS is unescaped."