Subject: Re: [PATCH v1] commit-graph.c: no lazy fetch in lookup_commit_in_graph()

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 20 Jun 2022 09:07:24 +0200

On Sat, Jun 18, 2022 at 11:01:30AM +0800, Han Xin wrote:
> If a commit is in the commit graph, we would expect the commit to also
> be present. So we should use has_object() instead of
> repo_has_object_file(), which will help us avoid getting into an endless
> loop of lazy fetch.
> 
> We can see the endless loop issue via this[1].
> 
> 1. https://lore.kernel.org/git/[email-masked]/
> 
> Signed-off-by: Han Xin <[email-masked]>

Thanks a lot for working on this issue!

> ---
>  commit-graph.c                             |  2 +-
>  t/t5329-no-lazy-fetch-with-commit-graph.sh | 50 ++++++++++++++++++++++
>  2 files changed, 51 insertions(+), 1 deletion(-)
>  create mode 100755 t/t5329-no-lazy-fetch-with-commit-graph.sh
> 
> diff --git a/commit-graph.c b/commit-graph.c
> index 2b52818731..2dd9bcc7ea 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -907,7 +907,7 @@ struct commit *lookup_commit_in_graph(struct repository *repo, const struct obje
>  		return NULL;
>  	if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
>  		return NULL;
> -	if (!repo_has_object_file(repo, id))
> +	if (!has_object(repo, id, 0))
>  		return NULL;

Agreed, this change makes sense to me.

>  	commit = lookup_commit(repo, id);
> 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..ea5940b9f1
> --- /dev/null
> +++ b/t/t5329-no-lazy-fetch-with-commit-graph.sh
> @@ -0,0 +1,50 @@
> +#!/bin/sh
> +
> +test_description='test for no lazy fetch with the commit-graph'
> +
> +. ./test-lib.sh
> +
> +test_expect_success 'setup' '

Nit: I find it a bit confusing that you only call the first test case
`setup`, while all the other tests except for the last one are also only
setting up the necessary state.

> +	git init --bare dest.git &&
> +	test_commit one &&
> +	git checkout -b tmp &&
> +	test_commit two &&
> +	git push dest.git --all
> +'
> +
> +test_expect_success 'prepare a alternates repository without commit two' '
> 
> +	git clone --bare dest.git alternates &&
> 
> +	oid=$(git -C alternates rev-parse refs/heads/tmp) &&
> +	git -C alternates update-ref -d refs/heads/tmp &&
> +	git -C alternates gc --prune=now &&
> +	pack=$(echo alternates/objects/pack/*.pack) &&
> +	git verify-pack -v "$pack" >have &&
> +	! grep "$oid" have
> +'

Instead of going into the low-level details you could just verify that
`git cat-file -e $oid` returns `1`.

> +
> +test_expect_success 'prepare a repository with a commit-graph contains commit two' '
> +	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 &&
> +	# the source repository has the whole refs contains refs/heads/tmp
> +	git -C source fetch origin &&
> +	(
> +		cd source &&
> +		test_commit three &&
> +		git -c gc.writeCommitGraph=true gc
> +	)
> +'
> +
> +test_expect_success 'change the alternates of source to that without commit two' '
> +	# now we have a commit-graph in the source repository but without the commit two
> +	echo "$(pwd)/alternates/objects" >source/.git/objects/info/alternates
> +'
> +
> +test_expect_success 'fetch the missing commit' '
> +	git -C source fetch origin $oid 2>fetch.out &&
> +	grep "$oid" fetch.out
> +'

This test passes even without your fix, albeit a lot slower compared
to with it. Can we somehow cause it to fail reliably so that the test
becomes effective in catching a regression here?

> +test_done
> -- 
> 2.36.1

Patrick