bug#24640: Crashes in 25.1

[Toby Cubitt]

On Wed, Oct 12, 2016 at 01:31:05PM +0300, Eli Zaretskii wrote:
> > Date: Tue, 11 Oct 2016 19:33:20 +0300
> > From: Eli Zaretskii <eliz@gnu.org>
> > Cc: 24640@debbugs.gnu.org
> > 
> > > ​Yes, the crashes appear to stop when I comment out 
> > > (global-undo-tree-mode) in vars.el.

You can also disable history loading without disabling
global-undo-tree-mode, by disabling undo-tree-auto-save-history. Might be
worth doing this more fine-grained check to confirm.

> > OK, so we have our prime suspect.  Can you tell where I can find the
> > exact version of undo-tree-mode you are using?
> > 
> > Phillip, could you please look into that package and see if you can
> > spot any potential problems with the Emacs 25 undo internals?  TIA.

>From the bug tracker discussion, it sounds like you're close to boiling
this down to some simple steps involving one undo-tree history file that
trigger the crash.

If you can send me the file and a list of steps to reproduce from
emacs -Q, I'd be happy to look into the undo-tree side of things.

> Some functions in undo-tree refer to or manipulate Emacs undo
> internals:
> 
>   undo-list-pop-changeset
>   undo-list-transfer-to-tree
>   undo-list-rebuild-from-tree
>   undo-tree-pull-undo-in-region-branch
>   undo-tree-pull-redo-in-region-branch
>   undo-tree-adjust-elements-to-elt
>   undo-tree-apply-deltas
>   undo-tree-undo-1
>   undo-tree-redo-1
> 
> Do they perhaps need some adjustments to Emacs 25's undo?

The only Emacs undo internal that undo-tree manipulates is the
buffer-undo-list variable. The only functions that do so are:

  undo-list-pop-changeset
  undo-list-transfer-to-tree
  undo-list-rebuild-from-tree

All the others either call one of these, or only touch buffer-undo-tree
which is a variable defined in the undo-tree package and which the Emacs
undo internals know nothing about.

Has the format of buffer-undo-list has changed at all? If so, then the
three above might need adjustment. If not, they should work as before.

The only other Emacs undo internals used by undo-tree are to call
primitive-undo and undo-boundary when undo/redoing.


> Another potential issue is the new undo timer we have in Emacs 25 (see
> undo-auto--boundary-ensure-timer in simple.el).  One way of checking
> whether this is related to the crashes is to modify that function to
> use a much larger value for the 1st argument of run-at-time, say
> 10000, so that the undo timer never fires during the startup.  Reuben,
> could you try that?

As far as I understand, the timer just adds undo boundaries to
buffer-undo-list in some of the open buffers. Undo-tree doesn't touch
buffer-undo-list (or do anything at all, in fact) until you call one of
its interactive commands.

Since the timer can't run whilst undo-tree lisp code is running, and
extra undo boundaries are no problem for undo-tree, I'd be surprised if
the new undo timer is the culprit.

Best,
Toby
-- 
Dr T. S. Cubitt
Royal Society University Research Fellow
Quantum Information Theory
Department of Computer Science
University College London

email: tsc25@cantab.net
web:   www.dr-qubit.org