bug#30186: 27.0.50; Password is not hidden in read-passwd

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 27 Jan 2018 11:37:13 +0000
> Cc: juri@linkov.net, 30186@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > I'm not sure it would be possible to provide such a flag.  Did you
> > look at the internals involved, and if so, can you tell where do we
> > know which kind of change caused the hooks to run?
> 
> I envisage adding an extra boolean argument to prepare_to_modify_buffer,
> and to signal_after_change.  When called from the text property
> routines, that argument would be true, otherwise it would be false.

What happens when both the text and the properties are changed?

> So, I'm changing my mind, after looking into it a bit more.  Removing
> the with-silent-modifications from remove-yank-excluded-properties would
> not slow down undo in CC Mode buffers noticeably.

So let's do that now.  I think the problem with read-passwd is a
security issue, so it should go to emacs-26, do you agree?

> It might slow down other modes which make extensive use of
> before/after-change-functions.

Let's see if any such modes show up.

> The extra flag for the change hooks might still be a good idea.  It no
> longer seems pertinent for solving the current bug, though.

If it can be definitive, I might agree with you.

Alternatively, we could introduce a mechanism for interested modes to
prevent such changes from getting into buffer-undo-list.