sparse-checkout: add 'cone' mode

The sparse-checkout feature can have quadratic performance as
the number of patterns and number of entries in the index grow.
If there are 1,000 patterns and 1,000,000 entries, this time can
be very significant.

Create a new Boolean config option, core.sparseCheckoutCone, to
indicate that we expect the sparse-checkout file to contain a
more limited set of patterns. This is a separate config setting
from core.sparseCheckout to avoid breaking older clients by
introducing a tri-state option.

The config option does nothing right now, but will be expanded
upon in a later commit.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
index c385c62c92..0b2715db52 100755
--- a/t/t1091-sparse-checkout-builtin.sh
+++ b/t/t1091-sparse-checkout-builtin.sh
@@ -148,6 +148,20 @@ test_expect_success 'set sparse-checkout using --stdin' '
 	test_cmp expect dir
 '
 
+test_expect_success 'cone mode: match patterns' '
+	git -C repo config --worktree core.sparseCheckoutCone true &&
+	rm -rf repo/a repo/folder1 repo/folder2 &&
+	git -C repo read-tree -mu HEAD &&
+	git -C repo reset --hard &&
+	ls repo >dir  &&
+	cat >expect <<-EOF &&
+		a
+		folder1
+		folder2
+	EOF
+	test_cmp expect dir
+'
+
 test_expect_success 'sparse-checkout disable' '
 	git -C repo sparse-checkout disable &&
 	test_path_is_missing repo/.git/info/sparse-checkout &&