bug#30823: 25.3; modification-hooks of overlays are not run in some cases

[Ren Victor <victorhge@gmail.com>]

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On Thu, Mar 15, 2018 at 2:00 PM, Eli Zaretskii <eliz@gnu.org> wrote:
>> From: Ren Victor <victorhge@gmail.com>
>> Date: Thu, 15 Mar 2018 12:15:57 +0800
>>
>> Two factors：
>>
>>    1.  A commit of Emacs， modification-hooks might not be run in some cases:
>>     http://git.savannah.gnu.org/cgit/emacs.git/commit/src?id=564d811725596f15ecf543777e11504b47d2af86
>>
>>    2.  In ggtags, an overlay is deleted in the overlay's modification-hooks:
>>     https://github.com/leoliu/ggtags/blob/eec392d2d639030c5a51bce8431f2815ad8e7bc5/ggtags.el#L2306
>>
>> Deleted overlay ceases to be attached to the buffer. If the buffer of
>> the first overlay in the saved array doesn't match the current buffer,
>> then all the modification hooks will not be run in this buffer.
>>
>> Thus modes that depends on modification-hooks won't work together with
>> ggtags-highlight-tag mode.
>>
>> I think Emacs should support `delete-overlay' in modificaiton-hooks of
>> overlays, like ggtags.  So I report this bug.
>
> Thanks.  Can you provide a recipe starting from "emacs -Q" to
> reproduce the problem?

I wrote a ert case which is encolsed.

emacs -Q -batch -l ert -l bug30823.el -f ert-run-tests-batch-and-exit

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(require 'ert)

(ert-deftest test-modification-hooks ()
  "Test for bug#30823."
  (let ((check-point nil)
	(ov-delete nil)
	(ov-set nil))
    (with-temp-buffer
      (insert "abc")
      (setq ov-set (make-overlay 1 3))
      (overlay-put ov-set 'modification-hooks
		   (list (lambda (o after &rest _args)
			   (and after (setq check-point t)))))
      (setq ov-delete (make-overlay 1 3))
      (overlay-put ov-delete 'modification-hooks
		   (list (lambda (o after &rest _args)
			   (and (not after) (delete-overlay o)))))
      (goto-char 2)
      (insert "1")
      (should (eq check-point t)))))