Re: [PATCH v3 18/23] checkout: retire --to option

On Mon, Jul 6, 2015 at 3:41 PM, Junio C Hamano [off-list ref] wrote:

Eric Sunshine [off-list ref] writes:

quoted

Now that "git worktree add" has achieved user-facing feature-parity with
"git checkout --to", retire the latter.
[...]
This effectively reverts changes to checkout.c by 529fef2 (checkout:
support checking out into a new working directory, 2014-11-30) with the
exception of merge_working_tree() and switch_branches() which still
require specialized knowledge that a the checkout is occurring in a
newly-created linked worktree (signaled to them by the private
GIT_CHECKOUT_NEW_WORKTREE environment variable).

I do not quite understand why we still need the hidden environment
variable.  Is this a sign that the implementation is shared too much
between unrelated codepaths (or to put it another way, perhaps API
functions that are not best fit are being used)?

Duy had responded[1][2] to my inquiry about these two remaining bits
of checkout.c code with intimate knowledge of the initial
linked-worktree checkout (merge_working_tree & switch_branches), but I
don't know the checkout code well enough (yet) to fully understand his
responses or to answer your question satisfactorily. This is also why
I was afraid to rip out those final two bits of code, even though in
my own testing, having ripped them out locally, I was unable to
trigger any sort of "bad" behavior, as far as I could tell. Thus, I
wasn't sure if those two bits of code were needed, and was hoping
someone more qualified (Duy, for instance) would be able to provide a
more authoritative answer.

Having now re-read and finally digested Duy's response about
switch_branches(), when I rip out the `new_worktree_mode` check
locally, I find that I can trigger the misleading warning about an
orphaned commit, so that bit of code still serves a practical purpose.
This doesn't justify that such ugly intimacy between git-worktree and
git-checkout is desirable; only that it still seems to be needed until
"git reset --hard" can be swapped in to replace "git checkout".

[1]: http://article.gmane.org/gmane.comp.version-control.git/273225
[2]: http://article.gmane.org/gmane.comp.version-control.git/273226

Stepping back a bit, with or without the new "linked worktree"
feature, when you came across a repository whose working tree does
not have any file (i.e. somebody ran "git ls-files | xargs rm"), you
do not know and care what is in .git/index right now, you do not
know and care what branch its .git/HEAD points at, but you *do* know
what branch you want to be on (or where you want its HEAD detached
at), what would be the command you would use?

The state immediately after a new worktree is constructed by
populating /path/main/.git/worktrees/test-next/ and pointing it from
/path/other/test-next/.git but before the index or the files under
/path/other/test-next/ are populated is exactly that situation, no?
Wouldn't "symbolic-ref HEAD the-branch-i-want" (or "update-ref HEAD
the-commit-i-want" in the detached case) followed by "reset --hard"
the more natural thing to use, instead of merge-working-tree and
switch-branches that are implementation details of "checkout"?

It seems so to me. I didn't repeat it in the v3 cover letter, but the
v2 cover letter said this (which still holds true for v3):

    v2 does not attempt either of the suggestions by Junio[*3*] or
    Duy[*4*] for eliminating git-checkout from the equation, which
    would allow us to remove the final couple bits of code in
    git-checkout which require intimate knowledge that the checkout
    is occurring in a newly created linked worktree. This series is
    already too long, and I didn't want it to grow further by
    implementing either of those ideas. Instead, this series leaves
    git-worktree at a state where one or the other of those
    suggestions can be done as follow-on patches touching only the
    underlying machinery, without affecting the user-facing
    interface.

    [*3*]: via private email which suggested using "git-reset --hard"
           rather than "git checkout" to populate the new linked
           worktree.
    [*4*]: http://thread.gmane.org/gmane.comp.version-control.git/273032/focus=273226

In order to use "git reset --hard" in place of "git checkout",
git-worktree will need to handle -b/-B, --detach, --force (and
possibly --track and --orphan) itself. I'm still in the process of
familiarizing myself with the code needed to perform those actions, as
well as whatever else is needed to make "git reset --hard" a reality,
so I am not yet in a position to say now much time or work will be
required to swap out "git checkout" for "git reset --hard". As such, I
didn't want to hold up the series for an unknown amount of time while
studying the issue; and, as the series stands, the remaining ugly
intimate knowledge between git-worktree and git-checkout is
behind-the-scenes and localized (not affecting the user experience).