Hi Junio,

From: "Junio C Hamano" <gits...@pobox.com>
"Philip Oakley" <philipoak...@iee.org> writes:

> >"git checkout -b foo" (without -f -m or <start_point>) is defined in
> >the manual as being a shortcut for/equivalent to:
> >
> >        (1a) "git branch foo"
> >        (1b) "git checkout foo"
> >
> >However, it has been our experience in our observed use cases and all
> >the existing git tests, that it can be treated as equivalent to:
> >
> >        (2a) "git branch foo"
> >        (2b) "git symbolic-ref HEAD refs/heads/foo"
> >...
> >
> I am still not sure if I like the change of what "checkout -b" is this
> late in the game, though.

That said, you're much more on the frontline of receiving negative
feedback about doing that than I am. :)  How would you like to

I didn't see an initial confirmation as to what the issue really
was. You indicated the symptom ('a long checkout time'), but then we
missed out on hard facts and example repos, so that the issue was

I took it as a given, trivial and obvious optimization opportunity,
that it is wasteful having to traverse two trees to consolidate and
reflect their differences into the working tree when we know upfront
that these two trees are identical, no matter what the overhead for
doing so is.

I agree, and I believe Ben agrees.

At the moment there is the simple workaround of an alias that executes
that two step command dance to achieve what you needed, and Junio has
outlined the issues he needed to be covered from his maintainer
perspective (e.g. the detection of sparse checkouts). Confirming the
root causes would help in setting a baseline.

I hope that is of help - I'd seen that the discussion had gone quiet.

Some of the problems I have are:

(1) "git checkout -b NEW", "git checkout", "git checkout HEAD^0"
    and "git checkout HEAD" (no other parameters to any of them)
    ought to give identical index and working tree.  It is too
    confusing to leave subtly different results that will lead to
    hard to diagnose bugs for only one of them.

(2) The proposed log message talks only about "performance

while the purpose of the change is more about
    changing the definition

Here I think is the misunderstanding. His purpose is NOT to change the definition (IIUC). As I read the message you reference below (and Ben's other messages), I understood that he was trying to achieve what you said (i.e. optimise the trivial and obvious opportunity) of selecting for the common case (underlying conditions) where the two command sequences are identical. If the selected case / conditions is not identical then it is defined wrongly...

I suspect that it was Ben's 'soft' explanation that allowed the discussion to diverge.

of what "git checkout -b NEW" is from
    "git branch NEW && git checkout NEW" to "git branch NEW && git
    symbolic-ref HEAD refs/heads/NEW".  The explanation in a Ben's
    later message <007401d21278$445eba80$cd1c2f80$@gmail.com> does
    a much better job contrasting the two.

(3) I identified only one difference as an example sufficient to
    point out why the patch provided is not a pure optimization but
    behaviour change.  Fixing that example alone to avoid change in
    the behaviour is trivial (see if the "info/sparse-checkout"
    file is present and refrain from skipping the proper checkout),

This is probably the point Ben needs to take on board to narrow the conditions down. There may be others.

    but a much larger problem is that I do not know (and Ben does
    not, I suspect) know what other behaviour changes the patch is
    introducing, and worse, the checks are sufficiently dense too
    detailed and intimate to the implementation of unpack_trees()
    that it is impossible for anybody to make sure the exceptions
    defined in this patch and updates to other parts of the system
    will be kept in sync.

I did not believe he was proposing such a change to behaviour, hence his difficulty in responding (or at least that is my perception). I.e. he was digging a hole in the wrong place.

It is possible that he had accidentally introduced a behavious change, and having failed to explictly say "This patch (should) produces no behavious change", which then continued to re-inforce the misunderstanding.

So my inclination at this point, unless we see somebody invents a
clever way to solve (3), is that any change that violates (1),
i.e. as long as the patch does "Are we doing '-b NEW'?  Then we do
something subtly different", is not acceptable, and solving (3) in a
maintainable way smells like quite a hard thing to do.  But it would
be ideal if (3) is solved cleanly, as we will then not have to worry
about changing behaviour at all and can apply the optimization for
all of the four cases equally.  As a side effect, that approach
would solve problem (2) above.

If we were to punt on keeping the sanity (1) and introduce a subtly
different "create a new branch and point the HEAD at it", an easier
way out may be be one of

1. a totally new command, e.g. "git branch-switch NEW" that takes
   only a single argument and no other "checkout" options, or

2. a new option to "git checkout" that takes _ONLY_ a single
   argument and incompatible with any other option or command line
   argument, or

3. an alias that does "git branch" followed by "git symbolic-ref".

Neither of the first two sounds palatable, though.

It will need Ben to come back and clarify, if he did, or did not, want any behaviour change (beyond speed of action;-)



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