[RFC PATCH 0/2] Re: An endless loop fetching issue with partial clone, alternates and commit graph

From: Han Xin <hanxin.hx@bytedance.com>
Date: Jun 16, 2022, 03:38 UTC
Message-ID: <cover.1655350442.git.hanxin.hx@bytedance.com>
In-Reply-To: <YqlBjET0tf7V9/sg@nand.local>

On Wed, Jun 15, 2022 at 10:18 AM Taylor Blau <me@ttaylorr.com> wrote:

Show 16 quoted lines

>
> [+cc Stolee]
>
> On Tue, Jun 14, 2022 at 03:25:13PM +0800, Haiyng Tan wrote:
> > I think it's caused by using lazy-fetch in
> > deref_without_lazy_fetch_extended().  In lookup_commit_in_graph(),
> > lazy-fetch is initiated by repo_has_object_file() used.  has_object()
> > should be used, it's no-lazy-fetch.
>
> Hmm. Are there cases where lookup_commit_in_graph() is expected to
> lazily fetch missing objects from promisor remotes? If so, then this
> wouldn't quite work. If not, then this seems like an appropriate fix to
> me.
>
> Thanks,
> Taylor

We can see the use of has_object() in RelNotes/2.29.0.txt[1]：
   * A new helper function has_object() has been introduced to make it
     easier to mark object existence checks that do and don't want to
     trigger lazy fetches, and a few such checks are converted using it.

Let's see the difference between has_object() and repo_has_object_file():
    int has_object(struct repository *r, const struct object_id *oid,
            unsigned flags)
    {
        int quick = !(flags & HAS_OBJECT_RECHECK_PACKED);
        unsigned object_info_flags = OBJECT_INFO_SKIP_FETCH_OBJECT |
            (quick ? OBJECT_INFO_QUICK : 0);

        if (!startup_info->have_repository)
            return 0;
        return oid_object_info_extended(r, oid, NULL, object_info_flags) >= 0;
    }

    int repo_has_object_file_with_flags(struct repository *r,
                        const struct object_id *oid, int flags)
    {
        if (!startup_info->have_repository)
            return 0;
        return oid_object_info_extended(r, oid, NULL, flags) >= 0;
    }

    int repo_has_object_file(struct repository *r,
                const struct object_id *oid)
    {
        return repo_has_object_file_with_flags(r, oid, 0);
    }

Now we kown that has_object() add OBJECT_INFO_SKIP_FETCH_OBJECT to skip fetch object.

I found that Ævar Arnfjörð Bjarmason added deref_without_lazy_fetch()
4 weeks ago[2]:
    static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
                            int mark_tags_complete)
    {
        enum object_type type;
        unsigned flags = OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK;
        return deref_without_lazy_fetch_extended(oid, mark_tags_complete,
                            &type, flags);
    }

But oi_flags is only used by oid_object_info_extended() and is missed by
lookup_commit_in_graph():
    static struct commit *deref_without_lazy_fetch_extended(const struct object_id *oid,
                                int mark_tags_complete,
                                enum object_type *type,
                                unsigned int oi_flags)
    {
        struct object_info info = { .typep = type };
        struct commit *commit;

        commit = lookup_commit_in_graph(the_repository, oid);
        if (commit)
            return commit;

        while (1) {
            if (oid_object_info_extended(the_repository, oid, &info,
                            oi_flags))

So, an appropriate fix can be that let lookup_commit_in_graph() pickup oi_flags and pass it to oid_object_info_extended(), then the fetching loop will be prevent by the given flag OBJECT_INFO_SKIP_FETCH_OBJECT.

1. https://github.com/git/git/blob/master/Documentation/RelNotes/2.29.0.txt
2. https://lore.kernel.org/git/2a563b5f18cc9c42cb71a9547344a5435f6bc058.1652731865.git.gitgitgadget@gmail.com/

Thanks -Han Xin

Han Xin (2):
  commit-graph.c: add "flags" to lookup_commit_in_graph()
  fetch-pack.c: pass "oi_flags" to lookup_commit_in_graph()

 builtin/fetch.c                    |  4 ++-
 commit-graph.c                     |  5 ++--
 commit-graph.h                     |  3 +-
 fetch-pack.c                       | 10 +++----
 revision.c                         |  2 +-
 t/t5583-fetch-with-commit-graph.sh | 47 ++++++++++++++++++++++++++++++
 upload-pack.c                      |  5 ++--
 7 files changed, 64 insertions(+), 12 deletions(-)
 create mode 100644 t/t5583-fetch-with-commit-graph.sh

-- 
2.36.1