clone, submodule: pass partial clone filters to submodules

When cloning a repo with a --filter and with --recurse-submodules
enabled, the partial clone filter only applies to the top-level repo.
This can lead to unexpected bandwidth and disk usage for projects which
include large submodules. For example, a user might wish to make a
partial clone of Gerrit and would run:
`git clone --recurse-submodules --filter=blob:5k https://gerrit.googlesource.com/gerrit`.
However, only the superproject would be a partial clone; all the
submodules would have all blobs downloaded regardless of their size.
With this change, the same filter can also be applied to submodules,
meaning the expected bandwidth and disk savings apply consistently.

To avoid changing default behavior, add a new clone flag,
`--also-filter-submodules`. When this is set along with `--filter` and
`--recurse-submodules`, the filter spec is passed along to git-submodule
and git-submodule--helper, such that submodule clones also have the
filter applied.

This applies the same filter to the superproject and all submodules.
Users who need to customize the filter per-submodule would need to clone
with `--no-recurse-submodules` and then manually initialize each
submodule with the proper filter.

Applying filters to submodules should be safe thanks to Jonathan Tan's
recent work [1, 2, 3] eliminating the use of alternates as a method of
accessing submodule objects, so any submodule object access now triggers
a lazy fetch from the submodule's promisor remote if the accessed object
is missing. This patch is a reworked version of [4], which was created
prior to Jonathan Tan's work.

[1]: 8721e2e (Merge branch 'jt/partial-clone-submodule-1', 2021-07-16)
[2]: 11e5d0a (Merge branch 'jt/grep-wo-submodule-odb-as-alternate',
	2021-09-20)
[3]: 162a13b (Merge branch 'jt/no-abuse-alternate-odb-for-submodules',
	2021-10-25)
[4]: https://lore.kernel.org/git/52bf9d45b8e2b72ff32aa773f2415bf7b2b86da2.1563322192.git.steadmon@google.com/

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 41 insertions, 0 deletions

diff --git a/t/t7814-grep-recurse-submodules.sh b/t/t7814-grep-recurse-submodules.sh
index 058e5d0c96..a4476dc492 100755
--- a/t/t7814-grep-recurse-submodules.sh
+++ b/t/t7814-grep-recurse-submodules.sh
@@ -544,4 +544,45 @@ test_expect_failure 'grep saves textconv cache in the appropriate repository' '
 	test_path_is_file "$sub_textconv_cache"
 '
 
+test_expect_success 'grep partially-cloned submodule' '
+	# Set up clean superproject and submodule for partial cloning.
+	git init super &&
+	git init super/sub &&
+	(
+		cd super &&
+		test_commit --no-tag "Add file in superproject" \
+			super-file "Some content for super-file" &&
+		test_commit -C sub --no-tag "Add file in submodule" \
+			sub-file "Some content for sub-file" &&
+		git submodule add ./sub &&
+		git commit -m "Add other as submodule sub" &&
+		test_tick &&
+		test_commit -C sub --no-tag --append "Update file in submodule" \
+			sub-file "Some more content for sub-file" &&
+		git add sub &&
+		git commit -m "Update submodule" &&
+		test_tick &&
+		git config --local uploadpack.allowfilter 1 &&
+		git config --local uploadpack.allowanysha1inwant 1 &&
+		git -C sub config --local uploadpack.allowfilter 1 &&
+		git -C sub config --local uploadpack.allowanysha1inwant 1
+	) &&
+	# Clone the superproject & submodule, then make sure we can lazy-fetch submodule objects.
+	git clone --filter=blob:none --also-filter-submodules \
+		--recurse-submodules "file://$(pwd)/super" partial &&
+	(
+		cd partial &&
+		cat >expect <<-\EOF &&
+		HEAD^:sub/sub-file:Some content for sub-file
+		HEAD^:super-file:Some content for super-file
+		EOF
+
+		GIT_TRACE2_EVENT="$(pwd)/trace2.log" git grep -e content \
+			--recurse-submodules HEAD^ >actual &&
+		test_cmp expect actual &&
+		# Verify that we actually fetched data from the promisor remote:
+		grep \"category\":\"promisor\",\"key\":\"fetch_count\",\"value\":\"1\" trace2.log
+	)
+'
+
 test_done