bug#22295: viper-mode undo bug introduced between Nov 10 and Nov 14

[Jim Meyering <jim@meyering.net>]

On Sat, May 14, 2016 at 6:57 AM, Phillip Lord
<phillip.lord@russet.org.uk> wrote:
> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> Date: Sat, 14 May 2016 12:25:13 +0300
>>> From: Eli Zaretskii <eliz@gnu.org>
>>> Cc: 22295@debbugs.gnu.org, Jim Meyering <jim@meyering.net>
>>>
>>> > From: Jim Meyering <jim@meyering.net>
>>> > Date: Sat, 2 Jan 2016 20:01:36 -0800
>>> >
>>> > Hello,
>>> > I noticed that viper-mode's "undo" ('u') command began to undo too much
>>> > and was able quickly to determine that it worked fine with my snapshot
>>> > built from git master some time on Nov 10, yet that it began to undo
>>> > too much four days later.
>>> >
>>> > To demonstrate the problem (without risking changing anything in your
>>> > home directory), run this:
>>> >
>>> >   mkdir /tmp/x && HOME=/tmp/x emacs -Q -f viper-mode -nw
>>> > ~/previously-nonexistent-file
>>> >
>>> > then respond "y", "y", "5" to get past the "viperize" setup questions.
>>> > To reproduce the error, insert two lines, terminating each "insertion" with ESC,
>>> > so that each is recorded as a separate undo'able operation. I.e., type this
>>> >
>>> >    a 1 ESC
>>> >
>>> > to create the first line, then
>>> >
>>> >   o 2 ESC
>>> >
>>> > to create the second.
>>> > Finally, hit "u" to undo creation of the second and you'll see that it undoes
>>> > both operations, erasing both lines.  This is rather disruptive when that first
>>> > bit of text was a long paragraph or two -- the novice may think that it's lost,
>>> > because redo does not restore it -- however, it is available in emacs's
>>> > yank buffer.
>>>
>>> Phillip, could you please look into this?  This sounds like a annoying
>>> problem for users of viper-mode, and AFAIU it happens on the release
>>> branch as well.
>>
>> (Adding Michael to the addressees.)
>>
>> I took a short look, and it sounds like we need more experts here.
>> Undo in viper has its own implementation, which tries to do something
>> that is not immediately clear to me, and is not really documented
>> anywhere.  I guess vi users will know that, but I'm not one of them.
>>
>> The viper-undo command and related functions manipulate the Emacs undo
>> data structures directly, see viper-adjust-undo.  I guess the recent
>> changes in low-level undo implementation run afoul of what viper-mode
>> tries to do.
>>
>> I hope the above provides enough hints to find the reason for this
>> problem and solve it.
>
> Sorry for slow response -- was travelling.
>
> Yep, viper is doing strange things to undo -- it adds a symbol ('viper)
> to the undo list, then removes it later, amalgamating everything upto
> 'viper.
>
> I've got a complete test case (below in case anyone is interested --
> I'll make a proper unit test of it on master eventually).

Thank you for working on this. My fingers seem to have developed a
serious dependence on viper-mode over the last two decades.
I don't want to retrain them :-)