Re: Re: [PATCH v2 2/2] commit-graph.c: no lazy fetch in lookup_commit_in_graph()

[Han Xin <hanxin.hx@bytedance.com>]

On Sat, Jun 25, 2022 at 12:56 AM Junio C Hamano <gitster@pobox.com> wrote:
>
> Han Xin <hanxin.hx@bytedance.com> writes:
>
> > If a commit is in the commit graph, we would expect the commit to also
> > be present.
>
> > When we found the commit in the graph in lookup_commit_in_graph(),
> > but the commit is missing from the repository, we will try
> > promisor_remote_get_direct() and then enter another loop.  While
> > sometimes it will finally succeed because it cannot fork
> > subprocess,
>
> Is that a mode of "succeed"-ing?  Or merely a way to exit an endless
> loop that does not make any progress with a failure?

For the user, "fetch-pack" does succeed, because in
deref_without_lazy_fetch(), even if lookup_commit_in_graph() fails to
lazy fetch the lost commit, the following oid_object_info_extended()
will help us complete the previous work.

In a sense, this infinite loop is based on the fact that infinite processes
can be created.

However, your attempt to express the reasoning bellow is clearer.

>
> > it has exhausted the local process resources and can be harmful to the
> > remote service.
> >
> > Signed-off-by: Han Xin <hanxin.hx@bytedance.com>
> > ---
>
> I think the single-liner change in the patch is a good one, but I am
> having a hard time to agree with the reasoning above that explains
> why it is a good change.
>
> Here is an attempt to express a reasoning I can understand, can
> agree with, and (I think) better describes why the change is a good
> one.  Does my understanding of the problem and the solution totally
> misses the mark?
>
>         The commit-graph is used to opportunistically optimize
>         accesses to certain pieces of information on commit objects,
>         and lookup_commit_in_graph() tries to say "no" when the
>         requested commit does not locally exist by returning NULL,
>         in which case the caller can ask for (which may result in
>         on-demand fetching from a promisor remote) and parse the
>         commit object itself.
>
>         However, it uses a wrong helper, repo_has_object_file(), to
>         do so.  This helper not only checks if an object is
>         immediately available in the local object store, but also
>         tries to fetch from a promisor remote.  But the fetch
>         machinery calls lookup_commit_in_graph(), thus causing an
>         infinite loop.
>
>         We should make lookup_commit_in_graph() expect that a commit
>         given to it can be legitimately missing from the local
>         object store, by using the has_object_file() helper instead.
>
> > diff --git a/t/t5329-no-lazy-fetch-with-commit-graph.sh b/t/t5329-no-lazy-fetch-with-commit-graph.sh
> > new file mode 100755
> > index 0000000000..4d25d2c950
> > --- /dev/null
> > +++ b/t/t5329-no-lazy-fetch-with-commit-graph.sh
>
> Hmph, does this short-test need a completely new file?
>
> > @@ -0,0 +1,47 @@
> > +#!/bin/sh
> > +
> > +test_description='test for no lazy fetch with the commit-graph'
> > +
> > +. ./test-lib.sh
> > +
> > +test_expect_success 'setup: prepare a repository with a commit' '
> > +     git init with-commit &&
> > +     test_commit -C with-commit the-commit &&
> > +     oid=$(git -C with-commit rev-parse HEAD)
> > +'
> > +
> > +test_expect_success 'setup: prepare a repository with commit-graph contains the commit' '
> > +     git init with-commit-graph &&
> > +     echo "$(pwd)/with-commit/.git/objects" \
> > +             >with-commit-graph/.git/objects/info/alternates &&
> > +     # create a ref that points to the commit in alternates
> > +     git -C with-commit-graph update-ref refs/ref_to_the_commit "$oid" &&
> > +     # prepare some other objects to commit-graph
> > +     test_commit -C with-commit-graph somthing &&
>
> somthing? something?

Nod.

>
> > +     git -c gc.writeCommitGraph=true -C with-commit-graph gc &&
> > +     test_path_is_file with-commit-graph/.git/objects/info/commit-graph
> > +'
> > +
> > +test_expect_success 'setup: change the alternates to what without the commit' '
> > +     git init --bare without-commit &&
> > +     echo "$(pwd)/without-commit/objects" \
> > +             >with-commit-graph/.git/objects/info/alternates &&
>
> Doesn't this deliberately _corrupt_ the with-commit-graph repository
> that depended on the object whose name is $oid in with-commit
> repository?  Do we require a corrupt repository to trigger the "bug"?
>

The "bug" depends on the commit exist in the commit-graph but
missing in the repository.

I didn't find a better way to make this kind of scene.

This bug was first found when alternates and commit-graph were
both used. Since the promise did not maintain all the references,
I suspect that the "auto gc" during the update process of the promise
caused the loss of the unreachable commits in the promise.

> > +     test_must_fail git -C with-commit-graph cat-file -e $oid
> > +'
> > +
> > +test_expect_success 'setup: prepare another commit to fetch' '
> > +     test_commit -C with-commit another-commit &&
> > +     anycommit=$(git -C with-commit rev-parse HEAD)
>
> anycommit?  another_commit?  Be consistent in naming.
>

Nod.

> > +'
> > +
> > +test_expect_success ULIMIT_PROCESSES 'fetch any commit from promisor with the usage of the commit graph' '
>
> So we did all of the above set-up sequences only to skip the most
> interesting test, if we were testing with "dash"?  I suspect that it
> may be cleaner to put the prerequisite to the whole file with the
> "early test_done" trick like t0051 and t3008.
>

It make sense to me.

Thanks.
-Han Xin