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

From: Duy Nguyen <pclouds@gmail.com>
Date: Apr 30, 2018, 14:42 UTC
Message-ID: <CACsJy8Cqq5c1QC=2EjfajS3=+ofAF8eoFG2V4QeY+LzSykJOFw@mail.gmail.com>
In-Reply-To: <nycvar.QRO.7.76.6.1804292251000.79@tvgsbejvaqbjf.bet>

On Sun, Apr 29, 2018 at 10:53 PM, Johannes Schindelin <Johannes.Schindelin@gmx.de> wrote:

Show 25 quoted lines

>> > @@ -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.

I was right about the invalidation stuff. I knew about invalidate_ce_path() in this file. What I didn't remember was this function actually invalidates entries from the _source_ index, not the result one. What kind of logic is that? You copy/move entries from source to result than you go invalidate the source. Since the original move_index_extensions() call moves extensions from the source, these are already properly invalidated (both untracked cache and cache tree), it it looks like it does the right thing. Two wrongs make a right, I guess.

Sorry for venting. I was not happy with what I found. And sorry for wasting your time making this move_index.. change then remove it.

Show 10 quoted lines

> I have a couple of worktrees which are *huge*. And edf3b90553 really
> helped relieve the pain a bit when running `git status`. Now you say that
> even a `git checkout -b new-branch` would blow the untracked cache away
> again?
>
> It would be *really* nice if we could prevent that performance regression
> somehow.
>
> Ciao,
> Dscho

-- 
Duy