Re: [PATCH v1 2/2] worktree: make add dwim

On Tue, Nov 14, 2017 at 3:45 AM, Thomas Gummerer <t.gummerer@gmail.com> wrote:

For my own edification...

git worktree add ../dir branch

* Checks out branch into ../dir if branch exists.

* Errors out if branch does not exist. However, if we consider the
history of the implementation of worktrees[*1*], then this really
ought to try the "origin/branch -> branch" DWIM before erroring-out.
Implementing this DWIM could be considered a regression fix according
to [*1*] and, as Junio points out, should not pose backward
compatibility worries.

git worktree add ../topic

* Correctly errors out, refusing to create a new branch named "topic",
if "topic" is already a branch.

* Creates a new branch named "topic" if no such local branch exists.

The desired new DWIMing would change the second bullet point to:

* If no local branch named "topic" exists, DWIM it from "origin/topic"
if possible, else create a new local branch named "topic".

But that's a behavior change since, with the existing implementation, a newly created local "topic" has no relation to, and does not track, any origin/topic branch. The proposed --guess option is to avoid users being hit by this backward incompatibility, however, one could also view the proposed DWIMing as some sort of bug fix since, at least some, users would expect the DWIMing since it already takes place elsewhere.

So, at least two options exist:

* Just make the new DWIMing the default behavior, accepting that it
might bite a few people. Fallout can be mitigated somewhat by
prominently announcing that the DWIMing took place, in which case the
user can take corrective action (say, by choosing a different worktree
name); nothing is lost and no damage done since this is happening only
at worktree creation time.

* Add a new option to enable DWIMing; perhaps name it -t/--track,
which is familiar terminology and still gives you a relatively short
and sweet "git worktree add -t ../topic".

Given the mentioned mitigation factor and that some/many users likely would expect it to DWIM "origin/topic -> topic" anyhow, I'd lean in favor of the first option (but perhaps I'm too daring with other people's workflows).

FOOTNOTES

[*1*]: When Duy first implemented worktree support, he incorporated it directly into the git-checkout command ("git checkout --to worktree ..."), which means that he got all the git-checkout features for free, including the "origin/branch -> branch" DWIM. When worktree support was later moved to git-worktree, it lost most of the features inherited implicitly from git-checkout, such as -b, -B, --detach, so those were added back to git-worktree explicitly. However, at that early stage, git-worktree was still piggy-backing atop git-checkout, thus likely was still getting the "origin/branch -> branch" DWIM for free. A final iteration converted git-worktree away from heavyweight git-checkout to lightweight git-reset, at which point he DWIMing was lost. If you take this history into account, then loss of "origin/branch -> branch" DWIMing is a regression, so restoring it could be considered a bug fix.