update-index: integrate with sparse index

Enable use of the sparse index with `update-index`. Most variations of
`update-index` work without explicitly expanding the index or making any
other updates in or outside of `update-index.c`.

The one usage requiring additional changes is `--cacheinfo`; if a file
inside a sparse directory was specified, the index would not be expanded
until after the cache tree is invalidated, leading to a mismatch between the
index and cache tree. This scenario is handled by rearranging
`add_index_entry_with_check`, allowing `index_name_stage_pos` to expand the
index *before* attempting to invalidate the relevant cache tree path,
avoiding cache tree/index corruption.

Signed-off-by: Victoria Dye <vdye@github.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/update-index.c

@@ -1077,6 +1077,9 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 
 	git_config(git_default_config, NULL);
 
+	prepare_repo_settings(r);
+	the_repository->settings.command_requires_full_index = 0;
+
 	/* we will diagnose later if it turns out that we need to update it */
 	newfd = hold_locked_index(&lock_file, 0);
 	if (newfd < 0)

read-cache.c

@@ -1339,9 +1339,6 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 	int skip_df_check = option & ADD_CACHE_SKIP_DFCHECK;
 	int new_only = option & ADD_CACHE_NEW_ONLY;
 
-	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
-		cache_tree_invalidate_path(istate, ce->name);
-
 	/*
 	 * If this entry's path sorts after the last entry in the index,
 	 * we can avoid searching for it.
@@ -1352,6 +1349,13 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 	else
 		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce), EXPAND_SPARSE);
 
+	/*
+	 * Cache tree path should be invalidated only after index_name_stage_pos,
+	 * in case it expands a sparse index.
+	 */
+	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
+		cache_tree_invalidate_path(istate, ce->name);
+
 	/* existing match? Just replace it. */
 	if (pos >= 0) {
 		if (!new_only)

t/t1092-sparse-checkout-compatibility.sh

@@ -1253,6 +1253,21 @@ test_expect_success 'sparse index is not expanded: diff' '
 	ensure_not_expanded diff --cached
 '
 
+test_expect_success 'sparse index is not expanded: update-index' '
+	init_repos &&
+
+	deep_a_oid=$(git -C full-checkout rev-parse update-deep:deep/a) &&
+	ensure_not_expanded update-index --cacheinfo 100644 $deep_a_oid deep/a &&
+
+	echo "test" >sparse-index/README.md &&
+	echo "test2" >sparse-index/a &&
+	rm -f sparse-index/deep/a &&
+
+	ensure_not_expanded update-index --add README.md &&
+	ensure_not_expanded update-index a &&
+	ensure_not_expanded update-index --remove deep/a
+'
+
 test_expect_success 'sparse index is not expanded: blame' '
 	init_repos &&