Re: [PATCH v3] unpack_trees: fix breakage when o->src_index != o->dst_index

> On Mon, Apr 30, 2018 at 4:42 PM, Duy Nguyen <pclouds@gmail.com> wrote:
>> On Sun, Apr 29, 2018 at 10:53 PM, Johannes Schindelin
>> <Johannes.Schindelin@gmx.de> wrote:
>>>> > @@ -1412,12 +1422,13 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
>>>> >                                                   WRITE_TREE_SILENT |
>>>> >                                                   WRITE_TREE_REPAIR);
>>>> >                 }
>>>> > -               move_index_extensions(&o->result, o->dst_index);
>>>> > +               move_index_extensions(&o->result, o->src_index);
>>>>
>>>> While this looks like the right thing to do on paper, I believe it's
>>>> actually broken for a specific case of untracked cache. In short,
>>>> please do not touch this line. I will send a patch to revert
>>>> edf3b90553 (unpack-trees: preserve index extensions - 2017-05-08),
>>>> which essentially deletes this line, with proper explanation and
>>>> perhaps a test if I could come up with one.
>>>>
>>>> When we update the index, we depend on the fact that all updates must
>>>> invalidate the right untracked cache correctly. In this unpack
>>>> operations, we start copying entries over from src to result. Since
>>>> 'result' (at least from the beginning) does not have an untracked
>>>> cache, it has nothing to invalidate when we copy entries over. By the
>>>> time we have done preparing 'result', what's recorded in src's (or
>>>> dst's for that matter) untracked cache may or may not apply to
>>>> 'result'  index anymore. This copying only leads to more problems when
>>>> untracked cache is used.
>>>
>>> Is there really no way to invalidate just individual entries?
>>
>> Grr.... the short answer is the current code (i.e. without Elijah's
>> changes) works but in a twisted way. So you get to keep untracked
>> cache in the end.
>
> GAAAHH.. it works _with_ Elijah's changes (since he made the change
> from dst to src) not without (and no performance regression).