Subject: bug#21824: 25.0.50; overlay modification-hooks called with wrong buffer

From: Noam Postavsky <[email-masked]>
Date: Sat, 7 Nov 2015 11:22:50 -0500

On Sat, Nov 7, 2015 at 3:28 AM, Eli Zaretskii <[email-masked]> wrote:
>> global-highlight-parentheses-mode puts overlays in every buffer.
>
> Does it really mean to do that in *Messages*?  If not, perhaps we
> should suggest to the developers of highlight-parentheses to exempt
> *Messages* from that, as that buffer is pretty specialized anyway.  In
> particular, due to the way the message logging in *Messages* is
> implemented, the overlay modification hooks will only ever be called
> in that buffer with AFTER non-nil, and even that will only happen when
> Emacs decides to delete some text from *Messages*; inserting a new
> message there will never call the overlay modification hooks.

Well, I suppose users may want parentheses highlighted in messages too.

But note that the overlays created by highlight-parentheses-mode do
*not* have any modification-hooks. It's the triggering
modification-hooks of (yasnippet's) overlays in another buffer in
addition to the existence of (highlight-parentheses') overlays in
*Messages* that triggers the bug.

> Yes, this looks good to me.  How about creating a new file
> buffer-tests.el (the offending code is in buffer.c)?

Okay, see attached.
From 565d8744271292bccb334460a62ff662802e6af3 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <[email-masked]>
Date: Sat, 7 Nov 2015 10:47:54 -0500
Subject: [PATCH] Add test for bug #21824

* test/automated/buffer-tests.el: New file.
(overlay-modification-hooks-message-other-buf): New test.
---
 test/automated/buffer-tests.el | 48 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)
 create mode 100644 test/automated/buffer-tests.el

diff --git a/test/automated/buffer-tests.el b/test/automated/buffer-tests.el
new file mode 100644
index 0000000..bb3c92d
--- /dev/null
+++ b/test/automated/buffer-tests.el
@@ -0,0 +1,48 @@
+;;; buffer-tests.el --- tests for buffer.c functions -*- lexical-binding: t -*-
+
+;; Copyright (C) 2015 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest overlay-modification-hooks-message-other-buf ()
+  "Test for bug#21824.
+After a modification-hook has been run and there is an overlay in
+the *Messages* buffer, the message coalescing [2 times] wrongly
+runs the modification-hook of the overlay in the 1st buffer, but
+with parameters from the *Messages* buffer modification."
+  (let ((buf nil)
+        (msg-ov nil))
+    (with-temp-buffer
+      (insert "123")
+      (overlay-put (make-overlay 1 3)
+                   'modification-hooks
+                   (list (lambda (&rest _)
+                           (setq buf (current-buffer)))))
+      (goto-char 2)
+      (insert "x")
+      (unwind-protect
+          (progn
+            (setq msg-ov (make-overlay 1 1 (get-buffer-create "*Messages*")))
+            (message "a message")
+            (message "a message")
+            (should (eq buf (current-buffer))))
+        (when msg-ov (delete-overlay msg-ov))))))
+
+;;; buffer-tests.el ends here
-- 
2.6.1