Sorry for bothering, why not introduce a brand new option like git
checkout -b foo --skip-worktree-merge for such rare optimization use
case?
On Wed, Sep 14, 2016 at 12:34 AM, Junio C Hamano <gitster@xxxxxxxxx> wrote:
> Ben Peart <peartben@xxxxxxxxx> writes:
>
>> +static int needs_working_tree_merge(const struct checkout_opts *opts,
>> + const struct branch_info *old,
>> + const struct branch_info *new)
>> +{
>> +...
>> +}
>
> I do not think I need to repeat the same remarks on the conditions
> in this helper, which hasn't changed since v2. Many "comments" in
> the code do not explain why skipping is justified, or what they
> claim to check looks to me just plain wrong.
>
> For example, there is
>
> /*
> * If we're not creating a new branch, by definition we're changing
> * the existing one so need to do the merge
> */
> if (!opts->new_branch)
> return 1;
>
> but "git checkout" (no other argument) hits this condition. It
> disables the most trivial optimization opportunity, because we are
> not "creating".
>
> "By definition, we're changing"? Really? Not quite.
>
> If you disable this bogus check, "git checkout" (no other argument)
> would be allowed to skip the merge_working_tree(), and that in turn
> reveals another case that the helper is not checking when
> unpack_trees() MUST be called.
>
> Note: namely, when sparse checkout is in effect, switching from
> HEAD to HEAD can nuke existing working tree files outside the
> sparse pattern -- YUCK! See penultimate test in t1011 for
> an example.
>
> This yuckiness is not your fault, but needs_working_tree_merge()
> logic you added needs to refrain from skipping unpack_trees() call
> when sparse thing is in effect. I'd expect "git checkout -b foo"
> instead of "git checkout" (no other argument) would fail to honor
> the sparse thing and reveal this bug, because the above bogus
> "!opts->new_branch" check will not protect you for that case.
>
> In other words, these random series of "if (...) return 1" are bugs
> hiding other real bugs and we need to reason about which ones are
> bugs that are hiding what other bugs that are not covered by this
> function. As Peff said earlier for v1, this is still an unreadable
> mess. We need to figure out a way to make sure we are skipping on
> the right condition and not accidentally hiding a bug of failing to
> check the right condition. I offhand do not have a good suggestion
> on this; sorry.
>
>> static int merge_working_tree(const struct checkout_opts *opts,
>> struct branch_info *old,
>> struct branch_info *new,
>> int *writeout_error)
>> {
>> + /*
>> + * Optimize the performance of "git checkout -b foo" by avoiding
>> + * the expensive merge, index and working directory updates if they
>> + * are not needed.
>> + */
>> + if (!needs_working_tree_merge(opts, old, new))
>> + return 0;
>> +
>> int ret;
>> struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
>
> With the change you made at the beginning of this function, it no
> longer compiles with -Wdecl-after-stmt, but that is the smallest of
> the problems.
>
> It is a small step in the right direction to move the call to the
> helper from the caller to this function, but it is a bit too small.
>
> Notice that the lines after the above context look like this:
>
> hold_locked_index(lock_file, 1);
> if (read_cache_preload(NULL) < 0)
> return error(_("index file corrupt"));
>
> resolve_undo_clear();
> if (opts->force) {
> ret = reset_tree(new->commit->tree, opts, 1, writeout_error);
> if (ret)
> return ret;
> } else {
> struct tree_desc trees[2];
> ...
>
> I would have expected that the check goes inside the "else" thing
> that actually does a two-tree merge, and the helper loses the check
> with opts->force, at least. That would still be a change smaller
> than desired, but at least a meaningful improvement compared to the
> previous one. As I have already pointed out, in the "else" clause
> there is a check "is the index free of conflicted entries? if so
> error out", and that must be honored in !opt->force case, no matter
> what your needs_working_tree_merge() says. I also was hoping that
> you would notice, when you were told about the unmerged check, by
> reading the remainder of the merge_working_tree(), that we need to
> call show_local_changes() when we are not doing force and when we
> are not quiet---returning early like the above patch will never be
> able to call that one downstream in the function.
>
> Regardless of what the actual checks end up to be, the right place
> to do this "optimization" would look more like:
>
> builtin/checkout.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/builtin/checkout.c b/builtin/checkout.c
> index 2b50a49..a6b9e17 100644
> --- a/builtin/checkout.c
> +++ b/builtin/checkout.c
> @@ -508,14 +508,19 @@ static int merge_working_tree(const struct checkout_opts *opts,
> topts.dir->flags |= DIR_SHOW_IGNORED;
> setup_standard_excludes(topts.dir);
> }
> +
> + if ( we know we can skip the unpack ) {
> + ret = 0;
> + } else {
> tree = parse_tree_indirect(old->commit ?
> old->commit->object.oid.hash :
> EMPTY_TREE_SHA1_BIN);
> init_tree_desc(&trees[0], tree->buffer, tree->size);
> tree = parse_tree_indirect(new->commit->object.oid.hash);
> init_tree_desc(&trees[1], tree->buffer, tree->size);
> -
> ret = unpack_trees(2, trees, &topts);
> + }
> +
> if (ret == -1) {
> /*
> * Unpack couldn't do a trivial merge; either
>
> I'd think. Note that the determination of "we can skip" would
> involve knowing the object names of the two trees involved, so for
> performance reasons, some of the parse-tree calls may have to come
> before the call to "do we know we can skip?", but that does not
> fundamentally change the basic code structure.
>
> Thanks.