Re: [PATCH v3] checkout: eliminate unnecessary merge for trivial checkout

[Oleg Taranenko <olegtaranenko@gmail.com>]

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@pobox.com> wrote:
> Ben Peart <peartben@gmail.com> 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.