diff --git a/apply.c b/apply.c
index 8582228047..821fe411ec 100644
--- a/apply.c
+++ b/apply.c
@@ -4105,7 +4105,7 @@ static int preimage_oid_in_gitlink_patch(struct patch *p, struct object_id *oid)
 static int build_fake_ancestor(struct apply_state *state, struct patch *list)
 {
 	struct patch *patch;
-	struct index_state result = { NULL };
+	struct index_state result = INDEX_STATE_INIT;
 	struct lock_file lock = LOCK_INIT;
 	int res;
 
diff --git a/builtin/difftool.c b/builtin/difftool.c
index d9b76226f6..758e7bd3b6 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -361,7 +361,7 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	struct hashmap symlinks2 = HASHMAP_INIT(pair_cmp, NULL);
 	struct hashmap_iter iter;
 	struct pair_entry *entry;
-	struct index_state wtindex;
+	struct index_state wtindex = INDEX_STATE_INIT;
 	struct checkout lstate, rstate;
 	int err = 0;
 	struct child_process cmd = CHILD_PROCESS_INIT;
@@ -387,8 +387,6 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	mkdir(ldir.buf, 0700);
 	mkdir(rdir.buf, 0700);
 
-	memset(&wtindex, 0, sizeof(wtindex));
-
 	memset(&lstate, 0, sizeof(lstate));
 	lstate.base_dir = lbase_dir = xstrdup(ldir.buf);
 	lstate.base_dir_len = ldir.len;
diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
index 58a22503f0..dc332c6d05 100644
--- a/builtin/sparse-checkout.c
+++ b/builtin/sparse-checkout.c
@@ -217,6 +217,7 @@ static int update_working_directory(struct pattern_list *pl)
 	o.head_idx = -1;
 	o.src_index = r->index;
 	o.dst_index = r->index;
+	index_state_init(&o.result);
 	o.skip_sparse_checkout = 0;
 	o.pl = pl;
 
diff --git a/builtin/stash.c b/builtin/stash.c
index bb0fd86143..a5f13fb1e9 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -1137,7 +1137,7 @@ static int save_untracked_files(struct stash_info *info, struct strbuf *msg,
 	int ret = 0;
 	struct strbuf untracked_msg = STRBUF_INIT;
 	struct child_process cp_upd_index = CHILD_PROCESS_INIT;
-	struct index_state istate = { NULL };
+	struct index_state istate = INDEX_STATE_INIT;
 
 	cp_upd_index.git_cmd = 1;
 	strvec_pushl(&cp_upd_index.args, "update-index", "-z", "--add",
@@ -1165,7 +1165,7 @@ static int save_untracked_files(struct stash_info *info, struct strbuf *msg,
 	}
 
 done:
-	discard_index(&istate);
+	release_index(&istate);
 	strbuf_release(&untracked_msg);
 	remove_path(stash_index_path.buf);
 	return ret;
@@ -1176,7 +1176,7 @@ static int stash_staged(struct stash_info *info, struct strbuf *out_patch,
 {
 	int ret = 0;
 	struct child_process cp_diff_tree = CHILD_PROCESS_INIT;
-	struct index_state istate = { NULL };
+	struct index_state istate = INDEX_STATE_INIT;
 
 	if (write_index_as_tree(&info->w_tree, &istate, the_repository->index_file,
 				0, NULL)) {
@@ -1199,7 +1199,7 @@ static int stash_staged(struct stash_info *info, struct strbuf *out_patch,
 	}
 
 done:
-	discard_index(&istate);
+	release_index(&istate);
 	return ret;
 }
 
@@ -1209,7 +1209,7 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 	int ret = 0;
 	struct child_process cp_read_tree = CHILD_PROCESS_INIT;
 	struct child_process cp_diff_tree = CHILD_PROCESS_INIT;
-	struct index_state istate = { NULL };
+	struct index_state istate = INDEX_STATE_INIT;
 	char *old_index_env = NULL, *old_repo_index_file;
 
 	remove_path(stash_index_path.buf);
@@ -1260,7 +1260,7 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 	}
 
 done:
-	discard_index(&istate);
+	release_index(&istate);
 	remove_path(stash_index_path.buf);
 	return ret;
 }
@@ -1271,7 +1271,7 @@ static int stash_working_tree(struct stash_info *info, const struct pathspec *ps
 	struct rev_info rev;
 	struct child_process cp_upd_index = CHILD_PROCESS_INIT;
 	struct strbuf diff_output = STRBUF_INIT;
-	struct index_state istate = { NULL };
+	struct index_state istate = INDEX_STATE_INIT;
 
 	init_revisions(&rev, NULL);
 	copy_pathspec(&rev.prune_data, ps);
@@ -1319,7 +1319,7 @@ static int stash_working_tree(struct stash_info *info, const struct pathspec *ps
 	}
 
 done:
-	discard_index(&istate);
+	release_index(&istate);
 	release_revisions(&rev);
 	strbuf_release(&diff_output);
 	remove_path(stash_index_path.buf);
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 591d659fae..311d6e9075 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -923,7 +923,7 @@ static int unlock_worktree(int ac, const char **av, const char *prefix)
 
 static void validate_no_submodules(const struct worktree *wt)
 {
-	struct index_state istate = { NULL };
+	struct index_state istate = INDEX_STATE_INIT;
 	struct strbuf path = STRBUF_INIT;
 	int i, found_submodules = 0;
 
diff --git a/cache.h b/cache.h
index 21ed9633b2..be28fce12a 100644
--- a/cache.h
+++ b/cache.h
@@ -360,6 +360,19 @@ struct index_state {
 	struct pattern_list *sparse_checkout_patterns;
 };
 
+/**
+ * A "struct index_state istate" must be initialized with
+ * INDEX_STATE_INIT or the corresponding index_state_init().
+ *
+ * If the variable won't be used again, use release_index() to free()
+ * its resources. If it needs to be used again use discard_index(),
+ * which does the same thing, but will use use index_state_init() at
+ * the end.
+ */
+#define INDEX_STATE_INIT { 0 }
+void index_state_init(struct index_state *istate);
+void release_index(struct index_state *istate);
+
 /* Name hashing */
 int test_lazy_init_name_hash(struct index_state *istate, int try_threaded);
 void add_name_hash(struct index_state *istate, struct cache_entry *ce);
diff --git a/merge-recursive.c b/merge-recursive.c
index 2fd0aa9687..0948b3ea96 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -412,7 +412,7 @@ static int unpack_trees_start(struct merge_options *opt,
 {
 	int rc;
 	struct tree_desc t[3];
-	struct index_state tmp_index = { NULL };
+	struct index_state tmp_index = INDEX_STATE_INIT;
 
 	memset(&opt->priv->unpack_opts, 0, sizeof(opt->priv->unpack_opts));
 	if (opt->priv->call_depth)
diff --git a/read-cache.c b/read-cache.c
index cf87ad7097..d191741f60 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2300,12 +2300,12 @@ static void set_new_index_sparsity(struct index_state *istate)
 	 * If the index's repo exists, mark it sparse according to
 	 * repo settings.
 	 */
-	if (istate->repo) {
-		prepare_repo_settings(istate->repo);
-		if (!istate->repo->settings.command_requires_full_index &&
-		    is_sparse_index_allowed(istate, 0))
-			istate->sparse_index = 1;
-	}
+	if (!istate->repo)
+		return;
+	prepare_repo_settings(istate->repo);
+	if (!istate->repo->settings.command_requires_full_index &&
+	    is_sparse_index_allowed(istate, 0))
+		istate->sparse_index = 1;
 }
 
 /* remember to discard_cache() before reading a different cache! */
@@ -2498,9 +2498,10 @@ int read_index_from(struct index_state *istate, const char *path,
 
 	trace_performance_enter();
 	if (split_index->base)
-		discard_index(split_index->base);
+		release_index(split_index->base);
 	else
-		CALLOC_ARRAY(split_index->base, 1);
+		ALLOC_ARRAY(split_index->base, 1);
+	index_state_init(split_index->base);
 
 	base_oid_hex = oid_to_hex(&split_index->base_oid);
 	base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
@@ -2539,7 +2540,13 @@ int is_index_unborn(struct index_state *istate)
 	return (!istate->cache_nr && !istate->timestamp.sec);
 }
 
-void discard_index(struct index_state *istate)
+void index_state_init(struct index_state *istate)
+{
+	struct index_state blank = INDEX_STATE_INIT;
+	memcpy(istate, &blank, sizeof(*istate));
+}
+
+void release_index(struct index_state *istate)
 {
 	/*
 	 * Cache entries in istate->cache[] should have been allocated
@@ -2551,20 +2558,12 @@ void discard_index(struct index_state *istate)
 	validate_cache_entries(istate);
 
 	resolve_undo_clear_index(istate);
-	istate->cache_nr = 0;
-	istate->cache_changed = 0;
-	istate->timestamp.sec = 0;
-	istate->timestamp.nsec = 0;
 	free_name_hash(istate);
 	cache_tree_free(&(istate->cache_tree));
-	istate->initialized = 0;
-	istate->fsmonitor_has_run_once = 0;
-	FREE_AND_NULL(istate->fsmonitor_last_update);
-	FREE_AND_NULL(istate->cache);
-	istate->cache_alloc = 0;
+	free(istate->fsmonitor_last_update);
+	free(istate->cache);
 	discard_split_index(istate);
 	free_untracked_cache(istate->untracked);
-	istate->untracked = NULL;
 
 	if (istate->sparse_checkout_patterns) {
 		clear_pattern_list(istate->sparse_checkout_patterns);
@@ -2577,6 +2576,12 @@ void discard_index(struct index_state *istate)
 	}
 }
 
+void discard_index(struct index_state *istate)
+{
+	release_index(istate);
+	index_state_init(istate);
+}
+
 /*
  * Validate the cache entries of this index.
  * All cache entries associated with this index
diff --git a/repository.c b/repository.c
index 3427085fd6..a5805fa33b 100644
--- a/repository.c
+++ b/repository.c
@@ -302,8 +302,10 @@ int repo_read_index(struct repository *repo)
 {
 	int res;
 
-	if (!repo->index)
-		CALLOC_ARRAY(repo->index, 1);
+	if (!repo->index) {
+		ALLOC_ARRAY(repo->index, 1);
+		index_state_init(repo->index);
+	}
 
 	/* Complete the double-reference */
 	if (!repo->index->repo)
diff --git a/revision.c b/revision.c
index 100e5ad511..fb090886f5 100644
--- a/revision.c
+++ b/revision.c
@@ -1813,7 +1813,7 @@ void add_index_objects_to_pending(struct rev_info *revs, unsigned int flags)
 	worktrees = get_worktrees();
 	for (p = worktrees; *p; p++) {
 		struct worktree *wt = *p;
-		struct index_state istate = { NULL };
+		struct index_state istate = INDEX_STATE_INIT;
 
 		if (wt->is_current)
 			continue; /* current index already taken care of */
diff --git a/sparse-index.c b/sparse-index.c
index 8c269dab80..86e3b99870 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -299,7 +299,7 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 	 * If the index is already full, then keep it full. We will convert
 	 * it to a sparse index on write, if possible.
 	 */
-	if (!istate || istate->sparse_index == INDEX_EXPANDED)
+	if (istate->sparse_index == INDEX_EXPANDED)
 		return;
 
 	/*
@@ -424,6 +424,8 @@ void expand_index(struct index_state *istate, struct pattern_list *pl)
 
 void ensure_full_index(struct index_state *istate)
 {
+	if (!istate)
+		BUG("ensure_full_index() must get an index!");
 	expand_index(istate, NULL);
 }
 
@@ -547,7 +549,7 @@ void expand_to_path(struct index_state *istate,
 	if (in_expand_to_path)
 		return;
 
-	if (!istate || !istate->sparse_index)
+	if (!istate->sparse_index)
 		return;
 
 	if (!istate->repo)
diff --git a/split-index.c b/split-index.c
index 9d0ccc30d0..a5b56c0c37 100644
--- a/split-index.c
+++ b/split-index.c
@@ -90,7 +90,8 @@ void move_cache_to_base_index(struct index_state *istate)
 		mem_pool_combine(istate->ce_mem_pool, istate->split_index->base->ce_mem_pool);
 	}
 
-	CALLOC_ARRAY(si->base, 1);
+	ALLOC_ARRAY(si->base, 1);
+	index_state_init(si->base);
 	si->base->version = istate->version;
 	/* zero timestamp disables racy test in ce_write_index() */
 	si->base->timestamp = istate->timestamp;
diff --git a/unpack-trees.c b/unpack-trees.c
index ea09023b01..d1d1b0f319 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1880,7 +1880,8 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 	prepare_repo_settings(repo);
 	if (repo->settings.command_requires_full_index) {
 		ensure_full_index(o->src_index);
-		ensure_full_index(o->dst_index);
+		if (o->dst_index)
+			ensure_full_index(o->dst_index);
 	}
 
 	if (o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED &&
@@ -1904,7 +1905,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		populate_from_existing_patterns(o, &pl);
 	}
 
-	memset(&o->result, 0, sizeof(o->result));
+	index_state_init(&o->result);
 	o->result.initialized = 1;
 	o->result.timestamp.sec = o->src_index->timestamp.sec;
 	o->result.timestamp.nsec = o->src_index->timestamp.nsec;