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#16377
Undo Tree regression: (error "Unrecognized entry in undo list undo-tree-canary")
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Your bug report
#16377: Undo Tree regression: (error "Unrecognized entry in undo list undo-tree-canary")
which was filed against the emacs package, has been closed.
The explanation is attached below, along with your original report.
If you require more details, please reply to 16377 <at> debbugs.gnu.org.
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16377: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16377
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On Wed, Jan 22, 2014 at 9:17 AM, Toby Cubitt <
toby-dated-1391609828.ede38b <at> dr-qubit.org> wrote:
> On Tue, Jan 21, 2014 at 07:05:38PM -0500, Barry OReilly wrote:
> > > The "Unrecognised entry" error suggests the undo-tree-canary symbol
> > > has somehow ended up in a `buffer-undo-tree' entry.
> >
> > You mean "buffer-undo-list" not "buffer-undo-tree" right?
>
> No, I mean `buffer-undo-tree'. The canary should never end up there. In
> undo-tree-mode, primitive-undo only ever gets called on an entry copied
> from buffer-undo-tree. Hence my statement, above.
>
> > I checked Emacs 24.3 and as I suspected it's quite easy to make
> > undo-tree-canary appear in the buffer-undo-list.
>
> It's *supposed* to be in buffer-undo-list. It's not supposed to ever be
> in buffer-undo-tree. (And maybe it isn't, I'm just guessing from the
> error message here. I haven't had time to investigate yet.)
>
> > What changed is the error checking in core Emacs. If you expected that
> > undo-tree-canary would never be there between commands, that has not
> > been so for some time.
>
> I didn't expect that.
>
> > Could you tell me more about the purpose of undo-tree-canary?
>
> It lets undo-tree-mode detect when Emacs has discarded undo history from
> buffer-undo-list "behind undo-tree-mode's back". If the canary has
> vanished when undo-tree-mode looks at buffer-undo-list, Emacs must have
> purged some undo history.
>
> The new error checking in primitive-undo shouldn't affect undo-tree-mode
> in any way. I still strongly suspect this is a bug in the very delicate
> and relatively untested undo-in-region code, and the change to
> primitive-undo is a red herring.
>
>
In undo-tree-redo-1:
(setf (undo-tree-node-undo current)
(undo-list-pop-changeset 'discard-pos))
In undo-list-pop-changeset:
(if (eq (car buffer-undo-list) 'undo-tree-canary)
(push nil buffer-undo-list)
[...])
The push call returns (nil 'undo-tree-canary). This is how it gets
into the buffer-undo-tree in my reproduction.
I'll close the Emacs bug since we're fairly sure at this point it's an
undo-tree bug.
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Don't know yet if the code change belongs in Undo Tree or Emacs but
the recipe is:
• Start Emacs in scratch buffer with global-undo-tree-mode enabled
• Go to BOB and insert "xxx"
• Select first line
• undo-tree-undo in region
• undo-tree-redo in region
• undo-tree-visualize
• Navigate down (redo)*
• Navigate up (undo)
Upon the navigate up, this error occurs:
Debugger entered--Lisp error: (error "Unrecognized entry in undo list
undo-tree-canary")
signal(error ("Unrecognized entry in undo list undo-tree-canary"))
error("Unrecognized entry in undo list %S" undo-tree-canary)
primitive-undo(1 (undo-tree-canary))
undo-tree-undo-1(1)
undo-tree-visualize-undo(1)
call-interactively(undo-tree-visualize-undo nil nil)
command-execute(undo-tree-visualize-undo)
* Sometimes the navigate down hangs with backtrace:
Debugger entered--Lisp error: (quit)
undo-tree-copy-list((nil))
undo-tree-redo-1(1)
undo-tree-visualize-redo(1)
call-interactively(undo-tree-visualize-redo nil nil)
command-execute(undo-tree-visualize-redo)
Don't know if it's related or not.
This recipe does not cause a problem with Emacs 24.3.
The following commit is suspicious:
commit 0c180ea96c022923ef18a255d0492bdb5ad04e02
Author: Aaron S. Hawley <aaron.s.hawley <at> gmail.com>
Date: Tue Jan 8 14:13:31 2013 -0500
* lisp/simple.el (primitive-undo): Move from undo.c.
* src/undo.c (Fprimitive_undo): Move to simple.el.
(syms_of_undo): Remove declaration for Sprimitive_undo.
* test/automated/undo-tests.el: New file.
The commit moved primitive-undo from C to Elisp. In the process, it
added an else condition not present in the C code:
+ (t (error "Unrecognized entry in undo list %S" next)))))
The consequence of this bug is that when using Undo Tree, recent undo
history can become completely inaccessible.
I should note that if I don't undo in region, Undo Tree has never run
into any problems with the Emacs on trunk.
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This bug report was last modified 8 years and 12 days ago.
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