bug#10047: Looking for help debugging an undo issue

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Thank you Stefan, that's very much appreciated. (Will I find that
> code in the new 24.0.92 pretest, or do I need to build from version
> control?)

Yes, it should be in 24.0.92.

>> - swap 4 and 5 (actually it was swapped in Emacs-22, so that would
>> revert to the behavior of Emacs-21).
> I see what you mean, and yet one presumes that change was made for
> a reason. Is there any indication of why that change was made?

Yes, it's because the truncation now does a few more things, one of
which can resulting in prompting the user, and that can't be done where
truncation used to take place (because it's a spot where the heap is in
a special inconsistent state).
So to swap things back, we'll need to split the "old plain truncation"
which can be done late from the "new prompting truncation" which will
have to stay where it is.

> On a related note to all of this, I found what seems like a good
> question, enquiring about how to determine safe/sensible values
> for increasing the undo limits, but it never got an answer.  It
> would be nice to see that answered:
> http://lists.gnu.org/archive/html/help-gnu-emacs/2011-02/msg00112.html

There is no good answer: increasing those values will increase your
memory use, which may be perfectly OK or not depending on
your circumstance.  The current default values have nothing magical to
them, they're just choices which seemed reasonable at some point.
We could probably increase them nowadays since memory resources have
increased.


        Stefan