Problem with undo and text properties.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

I noticed the following problem whilst solving bug #20266.  In that bug,
in a ~1500 line file.h, mark the entire buffer and yank it to the end of
itself, doubling its size, with C-x h, M-w, M->, C-y.  This went slowly,
and will soon go at a reasonable speed.

However, if you then undo this with C-x u, it takes almost forever to
complete.  The reason is that in the undo list there are lots (~6500) of
text property entries like

    (nil fontified nil 92222 . 92237)

which get individually undone.  Each such undoing is a buffer change, and
so triggers \(before\|after\)-change-functions.  This makes the operation
SLOW.

Perhaps it would be better to amend undo such that the removal (?and
re-application?) of text properties doesn't trigger the change hooks.  Or
perhaps a means could be devised of informing the change hooks that such
a change is what is being done.

What do you think?

-- 
Alan Mackenzie (Nuremberg, Germany).