Re: [PATCH 1/4] built-in rebase: no need to check out `onto` twice

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Phillip,

On Thu, 28 Feb 2019, Phillip Wood wrote:

> On 28/02/2019 15:27, Johannes Schindelin via GitGitGadget wrote:
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> > 
> > In the case that the rebase boils down to a fast-forward, the built-in
> > rebase reset the working tree twice: once to start the rebase at `onto`,
> > then realizing that the original HEAD was an ancestor, `reset_head()`
> > was called to update the original ref and to point HEAD back to it.
> > 
> > That second `reset_head()` call does not need to touch the working tree,
> > though, as it does not change the actual tip commit. So let's avoid that
> > unnecessary work.
> 
> I'm confused by this I think I must be missing something. If we've checked out
> onto then why does the working copy not need updating when we fast forward.
> (also why to we checkout onto before seeing if we can fast-forward but that's
> not related to this patch series)

Sorry, I really try to learn how to express things clearly. Still learning
a lot.

So what happens is this: we detect the situation where the pre-rebase
`HEAD` is an ancestor of `onto`. We do this *after* checking out `onto`.
So we now know that we essentially fast-forwarded to the post-rebase
state.

What we still need to do is to update the original ref (unless we were on
a detached HEAD when the rebase was started).

The original shell code updates the original ref to the current HEAD, and
the updates HEAD to point to that symbolic ref instead of being detached.

In the C code, we abused `reset_head()` to do the same. But `reset_head()`
does more: it does a two-way merge (in this instance, because
`RESET_HEAD_HARD` was not part of the flags). Which is unnecessary.

That's all this commit is about.

Ciao,
Dscho

> 
> Best Wishes
> 
> Phillip
> 
> > 
> > Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> > ---
> >   builtin/rebase.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/builtin/rebase.c b/builtin/rebase.c
> > index 08ec4d52c7..813ec284ca 100644
> > --- a/builtin/rebase.c
> > +++ b/builtin/rebase.c
> > @@ -1740,8 +1740,8 @@ int cmd_rebase(int argc, const char **argv, const char
> > *prefix)
> >     strbuf_addf(&msg, "rebase finished: %s onto %s",
> >      options.head_name ? options.head_name : "detached HEAD",
> >      oid_to_hex(&options.onto->object.oid));
> > -		reset_head(NULL, "Fast-forwarded", options.head_name, 0,
> > -			   "HEAD", msg.buf);
> > +		reset_head(NULL, "Fast-forwarded", options.head_name,
> > +			   RESET_HEAD_REFS_ONLY, "HEAD", msg.buf);
> >     strbuf_release(&msg);
> >     ret = !!finish_rebase(&options);
> >     goto cleanup;
> > 
>