[PATCH v2 4/7] reset: integrate with sparse index

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

From: Victoria Dye <vdye@github.com>

`reset --soft` does not modify the index, so no compatibility changes are
needed for it to function without expanding the index. For all other reset
modes (`--mixed`, `--hard`, `--keep`, `--merge`), the full index is
explicitly expanded with `ensure_full_index` to maintain current behavior.

Additionally, the `read_cache()` check verifying an uncorrupted index is
moved after argument parsing and preparing the repo settings. The index is
not used by the preceding argument handling, but `read_cache()` does need to
be run after enabling sparse index for the command and before resetting.

Signed-off-by: Victoria Dye <vdye@github.com>
---
 builtin/reset.c | 10 +++++++---
 cache-tree.c    |  1 +
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/builtin/reset.c b/builtin/reset.c
index 3b75d3b2f20..e1f2a2bb2c4 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -184,6 +184,7 @@ static int read_from_tree(const struct pathspec *pathspec,
 	opt.flags.override_submodule_config = 1;
 	opt.repo = the_repository;
 
+	ensure_full_index(&the_index);
 	if (do_diff_cache(tree_oid, &opt))
 		return 1;
 	diffcore_std(&opt);
@@ -261,9 +262,6 @@ static void parse_args(struct pathspec *pathspec,
 	}
 	*rev_ret = rev;
 
-	if (read_cache() < 0)
-		die(_("index file corrupt"));
-
 	parse_pathspec(pathspec, 0,
 		       PATHSPEC_PREFER_FULL |
 		       (patch_mode ? PATHSPEC_PREFIX_ORIGIN : 0),
@@ -409,6 +407,12 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (intent_to_add && reset_type != MIXED)
 		die(_("-N can only be used with --mixed"));
 
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
+	if (read_cache() < 0)
+		die(_("index file corrupt"));
+
 	/* Soft reset does not touch the index file nor the working tree
 	 * at all, but requires them in a good order.  Other resets reset
 	 * the index file to the tree object we are switching to. */
diff --git a/cache-tree.c b/cache-tree.c
index 90919f9e345..9be19c85b66 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -776,6 +776,7 @@ void prime_cache_tree(struct repository *r,
 	cache_tree_free(&istate->cache_tree);
 	istate->cache_tree = cache_tree();
 
+	ensure_full_index(istate);
 	prime_cache_tree_rec(r, istate->cache_tree, tree);
 	istate->cache_changed |= CACHE_TREE_CHANGED;
 	trace2_region_leave("cache-tree", "prime_cache_tree", the_repository);
-- 
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