sparse-index: prevent repo root from becoming sparse

Prevent the repository root from being collapsed into a sparse directory by
treating an empty path as "inside the sparse-checkout". When collapsing a
sparse index (e.g. in 'git sparse-checkout reapply'), the root directory
typically could not become a sparse directory due to the presence of in-cone
root-level files and directories. However, if no such in-cone files or
directories were present, there was no explicit check signaling that the
"repository root path" (an empty string, in the case of
'convert_to_sparse(...)') was in-cone, and a sparse directory index entry
would be created from the repository root directory.

The documentation in Documentation/git-sparse-checkout.txt explicitly states
that the files in the root directory are expected to be in-cone for a
cone-mode sparse-checkout. Collapsing the root into a sparse directory entry
violates that assumption, as sparse directory entries are expected to be
outside the sparse cone and have SKIP_WORKTREE enabled. This invalid state
in turn causes issues with commands that interact with the index, e.g.
'git status'.

Treating an empty (root) path as in-cone prevents the creation of a root
sparse directory in 'convert_to_sparse(...)'. Because the repository root is
otherwise never compared with sparse patterns (in both cone-mode and
non-cone sparse-checkouts), the new check does not cause additional changes
to how sparse patterns are applied.

Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/dir.c b/dir.c
index d91295f2bc..a136377eb4 100644
--- a/dir.c
+++ b/dir.c
@@ -1463,10 +1463,11 @@ static int path_in_sparse_checkout_1(const char *path,
 	const char *end, *slash;
 
 	/*
-	 * We default to accepting a path if there are no patterns or
-	 * they are of the wrong type.
+	 * We default to accepting a path if the path is empty, there are no
+	 * patterns, or the patterns are of the wrong type.
 	 */
-	if (init_sparse_checkout_patterns(istate) ||
+	if (!*path ||
+	    init_sparse_checkout_patterns(istate) ||
 	    (require_cone_mode &&
 	     !istate->sparse_checkout_patterns->use_cone_patterns))
 		return 1;