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

@@ -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