Re: An endless loop fetching issue with partial clone, alternates and commit graph

> We found an issue that could create an endless loop where alternates
> objects are used improperly.
>
> While do fetching in a partial cloned repository with a commit graph,
> deref_without_lazy_fetch_extended() will call lookup_commit_in_graph()
> to find the commit object. We can found the code in commit-graph.c:
>
>      struct commit *lookup_commit_in_graph(struct repository *repo, const struct object_id *id)
>      {
>           …
>           if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
>                return NULL;
>           if (!repo_has_object_file(repo, id))
>                return NULL;

> If we found the object in the commit graph, but missing it in the repository,
> we will go into an endless loop:
>      git fetch -> deref_without_lazy_fetch_extended() ->
>           lookup_commit_in_graph() -> repo_has_object_file() ->
>                promisor_remote_get_direct() -> fetch_objects() ->
>                     git fetch
>
> I know that the reason for this issue is due to improper use of
> alternates, we can ensure that objects will not be lost by maintaining
> all the references. But shouldn't we do something about this unusual
> usage, it will cause a fetch bombardment of the remote git service.
>
> We can reproduce this issue with the following test case, it will
> generate a lot of git processes, please be careful to stop it.
> ———————————————————————————
> #!/bin/sh
>
> test_description='test for an endless loop fetching’
>
> GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
> export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
>
> . ./test-lib.sh
>
> test_expect_success 'setup’ ‘
>     git init --bare dest.git &&
>     test_commit one &&
>    git checkout -b testbranch &&
>    test_commit two &&
>    git push dest.git --all
> '
>
> test_expect_success 'prepare a alternates repository without testbranch' '
>    git clone -b $GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME dest.git alternates &&
>    oid=$(git -C alternates rev-parse refs/remotes/origin/testbranch) &&
>    git -C alternates update-ref -d refs/remotes/origin/testbranch &&
>    git -C alternates gc --prune=now
> '
>
> test_expect_success 'prepare a repository with commit-graph' '
>    git init source &&
>    echo "$(pwd)/dest.git/objects" >source/.git/objects/info/alternates &&
>    git -C source remote add origin "$(pwd)/dest.git" &&
>    git -C source config remote.origin.promisor true &&
>    git -C source config remote.origin.partialclonefilter blob:none &&
>    git -C source fetch origin &&
>    (
>        cd source &&
>        test_commit three &&
>        git -c gc.writeCommitGraph=true gc
>    )
> '
>
> test_expect_success 'change alternates' '
>    echo "$(pwd)/alternates/.git/objects" >source/.git/objects/info/alternates &&
>    # this will bring an endless loop fetching
>    git -C source fetch origin $oid
> '
>
> test_done
>
> ------------------------------------------------------
>
> Thanks
> -Han Xin