sparse-index API: BUG() out on NULL ensure_full_index()

Make the ensure_full_index() function stricter, and have it only
accept a non-NULL "struct index_state". This function (and this
behavior) was added in [1].

The only reason it needed to be this lax was due to interaction with
repo_index_has_changes(). See the addition of that code in [2].

The other reason for why this was needed dates back to interaction
with code added in [3]. In [4] we started calling ensure_full_index()
in unpack_trees(), but the caller added in 34110cd wants to pass
us a NULL "dst_index". Let's instead do the NULL check in
unpack_trees() itself.

1. 4300f84 (sparse-index: implement ensure_full_index(), 2021-03-30)
2. 0c18c05 (read-cache: ensure full index, 2021-04-01)
3. 34110cd (Make 'unpack_trees()' have a separate source and
   destination index, 2008-03-06)
4. 6863df3 (unpack-trees: ensure full index, 2021-03-30)

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Acked-by: Derrick Stolee <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

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);
 }
 

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 &&