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bug#16377: Undo Tree regression: (error "Unrecognized entry in undo list
Keith David Bershatsky
bug#16377: Undo Tree regression: (error "Unrecognized entry in undo list undo-tree-canary")
Wed, 05 Jul 2017 23:25:10 -0700
Thank you for the helpful critique to substitute `window-buffer` for
I like to keep `debug-on-error` always set to `t`, and known situations I
generally limit to showing me just a message in the echo area with the name of
the function that generated it. That's why I've been using the `quit` signal
instead of popping open a `*Backrace*` buffer for familiar situations.
The code that the Emacs team implemented in response to bug#25599 is probably
fine. I just hadn't seen that before today, and my code predates its
Substituting the `warn` pop-up buffer for a plain old `message` was just a
matter of personal preference. Nothing substantive there was intended.
The existence and discussion surrounding `undo-tree.el` predates my usage of
that library, but from what I can tell, at some point in the past
`primitive-undo' was "improved" to start checking for errors and this resulted
in bug #16377. From an untrained layman's way of thinking, I am "guessing"
that if undo-tree worked fine before `primitive-undo` was modified to throw an
error, then perhaps it is not "a problem" with undo-tree except to the extent
that undo-tree may need to "evolve" to play nice with the current version of
`primitive-undo`. Based on that "optional evolution theory", I created the
workaround to just throw an informative message instead of an error. However,
I do not know if that approach could lead to problems.
I've spent quite a bit of time studying certain sections of the undo-tree.el
library, but there are sections of the code that are still "Greek to me". My
understanding of the `undo-tree-canary` symbol inside the `buffer-undo-list` is
that it is a way for undo-tree to check if it has interacted with the
`buffer-undo-list`. If the canary is not there, then handle the situation
differently. If the canary is there, then process the `buffer-undo-list` until
reaching the canary. It could be any arbitrary symbol so long as it remains at
the tail end of the `buffer-undo-list` with a `nil` before it while undo-tree
is being used in the buffer. There is only one situation I am aware of where
the `undo-tree-canary` disappears, and it happens sometimes with garbage
collection (bug #27214). Whatever symbol is used, it needs to remain in the
`buffer-undo-list` until `undo-tree-mode` is deactivated. I suppose the design
could have been different, but Dr. Cubitt probably had several additiona
l reasons for using a constant symbol such as the `undo-tree-canary`.
Bug #16377 might very well be resolvable by tweaking/fixing undo-tree.el;
however, the undo/redo in region code is still a few light years beyond my
> > + (_
> > + (if (eq next 'undo-tree-canary)
> > + (message "undo-tree--primitive-undo: catch-all found `%s'."
> > next)
> > + (error "Unrecognized entry in undo list %S" next)))))
> This might make sense to work around the problem, but is clearly not an
> actual fix. IIUC Tony said it looked like a bug in undo-tree.
> Has there been any progress on finding/fixing the bug there?
> What is this "canary" meant to do? If it shouldn't signal an error
> here, maybe rather than the constant `undo-tree-canary`, undo-tree
> should use another constant value, i.e. one that is a valid (and
> harmless) undo entry.