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

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On Mon, Apr 30, 2018 at 4:42 PM, Duy Nguyen <pclouds@xxxxxxxxx> wrote:
> On Sun, Apr 29, 2018 at 10:53 PM, Johannes Schindelin
> <Johannes.Schindelin@xxxxxx> 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). This
file really messes my brain up.
-- 
Duy



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