Re: tools for easily "uncommitting" parts of a patch I just commited?

[Jeff King <peff@peff.net>]

On Sun, Oct 23, 2016 at 08:23:01AM +0700, Duy Nguyen wrote:

> I hit the same problem sometimes, but in my case sometimes I
> accidentally do "git add" after "git add -p" and a configuration in
> "git commit -a" won't help me. I'd prefer we could undo changes in
> index instead. Something like reflog but for index.

An index write always writes the whole file from scratch, so you really
just need to save a copy of the old file. Perhaps something like:

  rm -f $GIT_DIR/index.old
  ln $GIT_DIR/index.old $GIT_DIR/index
  ... and then open $GIT_DIR/index.tmp ...
  ... and then rename(index.tmp, index) ...

could do it cheaply. It's a little more complicated if you want to save
a sequence of versions, and eventually would take a lot of space, but
presumably a handful of saved indexes would be sufficient.

Another option would be an index format that journals, and you could
potentially walk back the journal to a point. That seems like a much
bigger change (and has weird layering, because deciding when to fold in
the journal is usually a performance thing, but obviously this would
have user-visible impact about how far back you could undo).

-Peff