Subject: Re: [PATCH] index-pack: remove fetch_if_missing=0

From: Kousik Sanagavarapu <[email-masked]>
Date: Sat, 11 Mar 2023 11:30:12 +0530

On Sat, 11 Mar 2023 at 02:01, Junio C Hamano <[email-masked]> wrote:
>
> [...]
>
> Two requests.
>
>  * Could you substantiate this claim for future readers of "git
>    log"?  A reference to an old mailing list discussion or a log
>    message of the commit that added the temporary measure that says
>    the above plan would be perfect.
>
>  * What exactly does "once the commands have been taught"?  Which
>    commands?  Could you clarify?
>

Will do the change.

> > Hence, use has_object() to check for the existence of an object, which
> > has the default behavior of not lazy-fetching in a partial clone. It is
> > worth mentioning that this is the only place where there is potential for
> > lazy-fetching and all other cases are properly handled, making it safe to
> > remove this global here.
>
> This paragraph is very well explained.
>

Thanks.

> > @@ -1728,14 +1727,6 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
> >       int report_end_of_input = 0;
> >       int hash_algo = 0;
> > 
> > -     /*
> > -      * index-pack never needs to fetch missing objects except when
> > -      * REF_DELTA bases are missing (which are explicitly handled). It only
> > -      * accesses the repo to do hash collision checks and to check which
> > -      * REF_DELTA bases need to be fetched.
> > -      */
>
> OK.  The comment describes the design choice we made to flip the
> fetch_if_missing flag off.  The old world-view was that we would
> notice a breakage by non-functioning index-pack when a lazy clone is
> missing objects that we need by disabling auto-fetching, and we
> instead explicitly handle any missing and necessary objects by lazy
> fetching (like "when we lack REF_DELTA bases").  It does sound like
> a conservative thing to do, compared to the opposite approach we are
> taking with this patch, i.e. we would not fail if we tried to access
> objects we do not need to, because we have lazy fetching enabled,
> and we just ended up with bloated object store nobody may notice.
>
> To protect us from future breakage that can come from the new
> approach, it is a very good thing that you added new tests to ensure
> no unnecessary lazy fetching is done (I am not offhand sure if that
> test is sufficient, though).
>
> > -     fetch_if_missing = 0;
>
> Looking good to me.  Jonathan, who reviewed the previous round, do
> you have any comments?
>
> Thanks, all.  Will queue.
>

It does seem that the tests need to be changed significantly. Will do.

> > diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
> > index f519d2a87a..46af8698ce 100755
> > --- a/t/t5616-partial-clone.sh
> > +++ b/t/t5616-partial-clone.sh
> > @@ -644,6 +644,41 @@ test_expect_success 'repack does not loosen promisor objects' '
> >       grep "loosen_unused_packed_objects/loosened:0" trace
> >  '
> > 
> > +test_expect_success 'index-pack does not lazy-fetch when checking for sha1 collsions' '
> > +     rm -rf server promisor-remote client repo trace &&
> > +
> > +     # setup
> > +     git init server &&
> > +     for i in 1 2 3 4
> > +     do
> > +             echo $i >server/file$i &&
> > +             git -C server add file$i &&
> > +             git -C server commit -am "Commit $i" || return 1
> > +     done &&
> > +     git -C server config --local uploadpack.allowFilter 1 &&
> > +     git -C server config --local uploadpack.allowAnySha1InWant 1 &&
> > +     HASH=$(git -C server hash-object file3) &&
> > +
> > +     git init promisor-remote &&
> > +     git -C promisor-remote fetch --keep "file://$(pwd)/server" &&
> > +
> > +     git clone --no-checkout --filter=blob:none "file://$(pwd)/server" client &&
> > +     git -C client remote set-url origin "file://$(pwd)/promisor-remote" &&
> > +     git -C client config extensions.partialClone 1 &&
> > +     git -C client config remote.origin.promisor 1 &&
> > +
> > +     git init repo &&
> > +     echo "5" >repo/file5 &&
> > +     git -C repo config --local uploadpack.allowFilter 1 &&
> > +     git -C repo config --local uploadpack.allowAnySha1InWant 1 &&
> > +
> > +     # verify that no lazy-fetching is done when fetching from another repo
> > +     GIT_TRACE_PACKET="$(pwd)/trace" git -C client \
> > +                                     fetch --keep "file://$(pwd)/repo" main &&
> > +
> > +     ! grep "want $HASH" trace
> > +'
> > +
> >  test_expect_success 'lazy-fetch in submodule succeeds' '
> >       # setup
> >       test_config_global protocol.file.allow always &&

Thanks