[PATCH 6/7] reset: make --mixed sparse-aware

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

From: Victoria Dye <vdye@github.com>

Sparse directory entries are "diffed" as trees in `diff_cache` (used
internally by `reset --mixed`), following a code path separate from
individual file handling. The use of `diff_tree_oid` there requires setting
explicit `change` and `add_remove` functions to process the internal
contents of a sparse directory.

Additionally, the `recursive` diff option handles cases in which `reset
--mixed` must diff/merge files that are nested multiple levels deep in a
sparse directory.

Signed-off-by: Victoria Dye <vdye@github.com>
---
 builtin/reset.c                          | 30 +++++++++++++++++++++++-
 t/t1092-sparse-checkout-compatibility.sh | 13 +++++++++-
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/builtin/reset.c b/builtin/reset.c
index 92b9a3815c7..2d95ce76f20 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -196,6 +196,8 @@ static int read_from_tree(const struct pathspec *pathspec,
 			  int intent_to_add)
 {
 	struct diff_options opt;
+	unsigned int i;
+	char *skip_worktree_seen = NULL;
 
 	memset(&opt, 0, sizeof(opt));
 	copy_pathspec(&opt.pathspec, pathspec);
@@ -203,9 +205,35 @@ static int read_from_tree(const struct pathspec *pathspec,
 	opt.format_callback = update_index_from_diff;
 	opt.format_callback_data = &intent_to_add;
 	opt.flags.override_submodule_config = 1;
+	opt.flags.recursive = 1;
 	opt.repo = the_repository;
+	opt.change = diff_change;
+	opt.add_remove = diff_addremove;
+
+	/*
+	 * When pathspec is given for resetting a cone-mode sparse checkout, it may
+	 * identify entries that are nested in sparse directories, in which case the
+	 * index should be expanded. For the sake of efficiency, this check is
+	 * overly-cautious: anything with a wildcard or a magic prefix requires
+	 * expansion, as well as literal paths that aren't in the sparse checkout
+	 * definition AND don't match any directory in the index.
+	 */
+	if (pathspec->nr && the_index.sparse_index) {
+		if (pathspec->magic || pathspec->has_wildcard) {
+			ensure_full_index(&the_index);
+		} else {
+			for (i = 0; i < pathspec->nr; i++) {
+				if (!path_in_cone_mode_sparse_checkout(pathspec->items[i].original, &the_index) &&
+				    !matches_skip_worktree(pathspec, i, &skip_worktree_seen)) {
+					ensure_full_index(&the_index);
+					break;
+				}
+			}
+		}
+	}
+
+	free(skip_worktree_seen);
 
-	ensure_full_index(&the_index);
 	if (do_diff_cache(tree_oid, &opt))
 		return 1;
 	diffcore_std(&opt);
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 0b6ff0de17d..c9b9ef4992c 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -801,14 +801,25 @@ test_expect_success 'sparse-index is not expanded' '
 	for ref in update-deep update-folder1 update-folder2 update-deep
 	do
 		echo >>sparse-index/README.md &&
+		ensure_not_expanded reset --mixed $ref
 		ensure_not_expanded reset --hard $ref || return 1
 	done &&
 
 	ensure_not_expanded reset --hard update-deep &&
 	ensure_not_expanded reset --keep base &&
 	ensure_not_expanded reset --merge update-deep &&
-	ensure_not_expanded reset --hard &&
 
+	ensure_not_expanded reset base -- deep/a &&
+	ensure_not_expanded reset base -- nonexistent-file &&
+	ensure_not_expanded reset deepest -- deep &&
+
+	# Although folder1 is outside the sparse definition, it exists as a
+	# directory entry in the index, so it will be reset without needing to
+	# expand the full index.
+	ensure_not_expanded reset --hard update-folder1 &&
+	ensure_not_expanded reset base -- folder1 &&
+
+	ensure_not_expanded reset --hard update-deep &&
 	ensure_not_expanded checkout -f update-deep &&
 	test_config -C sparse-index pull.twohead ort &&
 	(
-- 
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