sparse-checkout: toggle sparse index from builtin

The sparse index extension is used to signal that index writes should be
in sparse mode. This was only updated using GIT_TEST_SPARSE_INDEX=1.

Add a '--[no-]sparse-index' option to 'git sparse-checkout init' that
specifies if the sparse index should be used. It also updates the index
to use the correct format, either way. Add a warning in the
documentation that the use of a repository extension might reduce
compatibility with third-party tools. 'git sparse-checkout init' already
sets extension.worktreeConfig, which places most sparse-checkout users
outside of the scope of most third-party tools.

Update t1092-sparse-checkout-compatibility.sh to use this CLI instead of
GIT_TEST_SPARSE_INDEX=1.

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

1 files changed, 21 insertions, 12 deletions

diff --git a/sparse-index.c b/sparse-index.c
index 6f4d95d..4c73772 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -102,21 +102,32 @@ static int convert_to_sparse_rec(struct index_state *istate,
 	return num_converted - start_converted;
 }
 
-static int enable_sparse_index(struct repository *repo)
+static int set_index_sparse_config(struct repository *repo, int enable)
 {
-	const char *config_path = repo_git_path(repo, "config.worktree");
-
-	git_config_set_in_file_gently(config_path,
-				      "index.sparse",
-				      "true");
+	int res;
+	char *config_path = repo_git_path(repo, "config.worktree");
+	res = git_config_set_in_file_gently(config_path,
+					    "index.sparse",
+					    enable ? "true" : NULL);
+	free(config_path);
 
 	prepare_repo_settings(repo);
 	repo->settings.sparse_index = 1;
-	return 0;
+	return res;
+}
+
+int set_sparse_index_config(struct repository *repo, int enable)
+{
+	int res = set_index_sparse_config(repo, enable);
+
+	prepare_repo_settings(repo);
+	repo->settings.sparse_index = enable;
+	return res;
 }
 
 int convert_to_sparse(struct index_state *istate)
 {
+	int test_env;
 	if (istate->split_index || istate->sparse_index ||
 	    !core_apply_sparse_checkout || !core_sparse_checkout_cone)
 		return 0;
@@ -128,11 +139,9 @@ int convert_to_sparse(struct index_state *istate)
 	 * The GIT_TEST_SPARSE_INDEX environment variable triggers the
 	 * index.sparse config variable to be on.
 	 */
-	if (git_env_bool("GIT_TEST_SPARSE_INDEX", 0)) {
-		int err = enable_sparse_index(istate->repo);
-		if (err < 0)
-			return err;
-	}
+	test_env = git_env_bool("GIT_TEST_SPARSE_INDEX", -1);
+	if (test_env >= 0)
+		set_sparse_index_config(istate->repo, test_env);
 
 	/*
 	 * Only convert to sparse if index.sparse is set.