Re: [PATCH v2 2/2] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

> "John Cai via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
>> diff --git a/reset.c b/reset.c
>> index e3383a93343..f8e32fcc240 100644
>> --- a/reset.c
>> +++ b/reset.c
>> @@ -101,6 +101,9 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
>>  	if (opts->branch_msg && !opts->branch)
>>  		BUG("branch reflog message given without a branch");
>>
>> +	if (switch_to_branch && opts->flags & RESET_HEAD_DETACH)
>
> It's just style thing but it probably is easier to read to have
> an extra () around the bitwise-&.
>
>> +		BUG("attempting to detach HEAD when branch is given");
>
> I wonder if there is a valid use case NOT to use RESET_HEAD_DETACH
> when switch_to_branch == NULL.  If there isn't, it could be that
> we can get rid of RESET_HEAD_DETACH bit and base this decision
> solely on switch_to_branch'es NULLness.
>
> But that is totally outside the scope of this fix.  I'd prefer to
> see a narrowly and sharply focused fix, and to be quite honest, I
> would be happier if this assertion weren't in this topic.

>
>>  	if (!refs_only && repo_hold_locked_index(r, &lock, LOCK_REPORT_ON_ERROR) < 0) {
>>  		ret = -1;
>>  		goto leave_reset_head;
>> diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
>> index 0643d015255..d5a8ee39fc4 100755
>> --- a/t/t3400-rebase.sh
>> +++ b/t/t3400-rebase.sh
>> @@ -394,6 +394,15 @@ test_expect_success 'switch to branch not checked out' '
>>  	git rebase main other
>>  '
>>
>> +test_expect_success 'switch to non-branch detaches HEAD' '
>> +	git checkout main &&
>> +	old_main=$(git rev-parse HEAD) &&
>> +	git rebase First Second^0 &&
>
> Good.  For reproduction, the fork-point "First" does not have to be
> a raw object name.  "Second^0" not being a branch name does matter.
>
>> +	test_cmp_rev HEAD Second &&
>> +	test_cmp_rev main $old_main &&
>> +	test_must_fail git symbolic-ref HEAD
>
> All correct and to the point.  Good.

>
>> +'
>> +
>>  test_expect_success 'refuse to switch to branch checked out elsewhere' '
>>  	git checkout main &&
>>  	git worktree add wt &&