Re: [PATCH] cache-tree: do not lazy-fetch merge tree

[Derrick Stolee <stolee@gmail.com>]

On 9/3/2019 3:42 PM, Jonathan Tan wrote:
> When cherry-picking (for example), new trees may be constructed. During
> this process, Git checks whether these trees exist. However, in a
> partial clone, this causes a lazy fetch to occur, which is both
> unnecessary (because Git has already constructed this tree as part of
> the cherry-picking process) and likely to fail (because the remote
> probably doesn't have this tree).

If we have constructed the object already, then why do we not see it
and avoid fetching it? This must be a slightly strange timing issue
with objects being flushed to disk or added to the object cache.

One approach is to find all of these has_object_file() calls that should
really be one with OBJECT_INFO_SKIP_FETCH_OBJECT. Another would be to
find out why has_object_file() isn't seeing the object we constructed.

> Do not lazy fetch in this situation.
I agree that the patch has this effect.
 
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
> Another partial clone bug.
> 
> This raises the issue that failed fetches are currently fatal - if they
> weren't fatal, this cherry-pick would have worked (except with some
> delay as the fetch is attempted, and with a warning message about the
> fetch failing). My personal inclination right now is to leave things as
> it is (fatal failed fetches), but I'm open to other opinions.
> ---
>  cache-tree.c             |  2 +-
>  t/t0410-partial-clone.sh | 14 ++++++++++++++
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/cache-tree.c b/cache-tree.c
> index c22161f987..9e596893bc 100644
> --- a/cache-tree.c
> +++ b/cache-tree.c
> @@ -407,7 +407,7 @@ static int update_one(struct cache_tree *it,
>  	if (repair) {
>  		struct object_id oid;
>  		hash_object_file(buffer.buf, buffer.len, tree_type, &oid);
> -		if (has_object_file(&oid))
> +		if (has_object_file_with_flags(&oid, OBJECT_INFO_SKIP_FETCH_OBJECT))
>  			oidcpy(&it->oid, &oid);
>  		else
>  			to_invalidate = 1;
> diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
> index 6415063980..3e434b6a81 100755
> --- a/t/t0410-partial-clone.sh
> +++ b/t/t0410-partial-clone.sh
> @@ -492,6 +492,20 @@ test_expect_success 'gc stops traversal when a missing but promised object is re
>  	! grep "$TREE_HASH" out
>  '
>  
> +test_expect_success 'do not fetch when checking existence of tree we construct ourselves' '
> +	rm -rf repo &&
> +	test_create_repo repo &&
> +	test_commit -C repo base &&
> +	test_commit -C repo side1 &&
> +	git -C repo checkout base &&
> +	test_commit -C repo side2 &&
> +
> +	git -C repo config core.repositoryformatversion 1 &&
> +	git -C repo config extensions.partialclone "arbitrary string" &&
> +
> +	git -C repo cherry-pick side1
> +'
> +

I appreciate this test!

Thanks,
-Stolee