Subject: Re: [PATCH 4/4] promisor-remote: teach lazy-fetch in any repo

From: Taylor Blau <[email-masked]>
Date: Fri, 4 Jun 2021 17:25:46 -0400

On Tue, Jun 01, 2021 at 02:34:19PM -0700, Jonathan Tan wrote:
> This is one step towards supporting partial clone submodules.
>
> Even after this patch, we will still lack partial clone submodules
> support, primarily because a lot of Git code that accesses submodule
> objects does so by adding their object stores as alternates, meaning
> that any lazy fetches that would occur in the submodule would be done
> based on the config of the superproject, not of the submodule. This also
> prevents testing of the functionality in this patch by user-facing
> commands. So for now, test this mechanism using a test helper.

OK. Everything you wrote seemed reasonable to me, but I did have a
couple of questions on the test you added:

> diff --git a/t/helper/test-partial-clone.c b/t/helper/test-partial-clone.c
> new file mode 100644
> index 0000000000..e7bc7eb21f
> --- /dev/null
> +++ b/t/helper/test-partial-clone.c
> @@ -0,0 +1,34 @@
> +#include "cache.h"
> +#include "test-tool.h"
> +#include "repository.h"
> +#include "object-store.h"
> +
> +static void object_info(const char *gitdir, const char *oid_hex)
> +{
> +	struct repository r;
> +	struct object_id oid;
> +	unsigned long size;
> +	struct object_info oi = {.sizep = &size};
> +	const char *p;
> +
> +	if (repo_init(&r, gitdir, NULL))
> +		die("could not init repo");
> +	if (parse_oid_hex(oid_hex, &oid, &p))
> +		die("could not parse oid");
> +	if (oid_object_info_extended(&r, &oid, &oi, 0))
> +		die("could not obtain object info");
> +	printf("%d\n", (int) size);
> +}

Hmm. Is there a reason that the same couldn't be implemented by calling "git
cat-file -s" from the partial clone?

> diff --git a/t/t0410-partial-clone.sh b/t/t0410-partial-clone.sh
> index 584a039b85..e804d267e6 100755
> --- a/t/t0410-partial-clone.sh
> +++ b/t/t0410-partial-clone.sh
> @@ -604,6 +604,30 @@ test_expect_success 'do not fetch when checking existence of tree we construct o
>  	git -C repo cherry-pick side1
>  '
>
> +test_expect_success 'lazy-fetch when accessing object not in the_repository' '
> +	rm -rf full partial.git &&
> +	test_create_repo full &&
> +	printf 12345 >full/file.txt &&
> +	git -C full add file.txt &&
> +	git -C full commit -m "first commit" &&

This is a stylistic nit, but I think using test_commit is better here
for a non-superficial reason. My guess is that you wanted to avoid
specifying a message and file (which are required positional arguments
to test_commit before you can specify the contents). But I think there
are two good reasons to use test_commit here:

  - It saves three lines of test script here.
  - You don't have to make the expected size a magic number (i.e.,
    because you knew ahead of time that the contents was "12345").

I probably would have expected this test to end with:

  git -C full cat-file -s $FILE_HASH >expect &&
  git -C partial.git cat-file -s $FILE_HASH >actual &&
  test_cmp expect actual

which reads more clearly to me (although I think the much more important
test is that $FILE_HASH doesn't show up in the output of the rev-list
--missing=print that is run in the partial clone).

> +
> +	test_config -C full uploadpack.allowfilter 1 &&
> +	test_config -C full uploadpack.allowanysha1inwant 1 &&
> +	git clone --filter=blob:none --bare "file://$(pwd)/full" partial.git &&
> +	FILE_HASH=$(git hash-object --stdin <full/file.txt) &&

This works for me, although I wouldn't have been sad to see the
sub-shell contain "git -C full rev-parse HEAD:file.txt" instead.

> +	# Sanity check that the file is missing
> +	git -C partial.git rev-list --objects --missing=print HEAD >out &&
> +	grep "[?]$FILE_HASH" out &&
> +
> +	OUT=$(test-tool partial-clone object-info partial.git "$FILE_HASH") &&

Coming back to my point about the utility of the partial-clone helper,
could this be replaced by saying just OUT="$(git -C partial.git cat-file
-s "$FILE_HASH")" instead?

Thanks,
Taylor