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

> Hi Duy,
>
> On Mon, Apr 30, 2018 at 7:45 AM, Duy Nguyen <pclouds@gmail.com> wrote:
>> 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).
>
> So...is that an Acked-by for the patch

> or does the "two wrong make a
> right, I guess" comment suggest that we should still drop the
> move_index_extensions change (essentially reverting to v1 of the PATCH
> as found at 20180421193736.12722-1-newren@gmail.com), and you'll fix
> things up further in a separate series?