midx: fix bug that skips midx with alternates

The logic for constructing the linked list of multi-pack-indexes
in the object store is incorrect. If the local object store has
a multi-pack-index, but an alternate does not, then the list is
dropped.

Add tests that would have revealed this bug.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -331,7 +331,7 @@ int midx_contains_pack(struct multi_pack_index *m, const char *idx_name)
 
 int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, int local)
 {
-	struct multi_pack_index *m = r->objects->multi_pack_index;
+	struct multi_pack_index *m;
 	struct multi_pack_index *m_search;
 	int config_value;
 
@@ -339,14 +339,15 @@ int prepare_multi_pack_index_one(struct repository *r, const char *object_dir, i
 	    !config_value)
 		return 0;
 
-	for (m_search = m; m_search; m_search = m_search->next)
+	for (m_search = r->objects->multi_pack_index; m_search; m_search = m_search->next)
 		if (!strcmp(object_dir, m_search->object_dir))
 			return 1;
 
-	r->objects->multi_pack_index = load_multi_pack_index(object_dir, local);
+	m = load_multi_pack_index(object_dir, local);
 
-	if (r->objects->multi_pack_index) {
-		r->objects->multi_pack_index->next = m;
+	if (m) {
+		m->next = r->objects->multi_pack_index;
+		r->objects->multi_pack_index = m;
 		return 1;
 	}
 

t/t5319-multi-pack-index.sh

@@ -149,6 +149,23 @@ test_expect_success 'repack removes multi-pack-index' '
 
 compare_results_with_midx "after repack"
 
+test_expect_success 'multi-pack-index and alternates' '
+	git init --bare alt.git &&
+	echo $(pwd)/alt.git/objects >.git/objects/info/alternates &&
+	echo content1 >file1 &&
+	altblob=$(GIT_DIR=alt.git git hash-object -w file1) &&
+	git cat-file blob $altblob &&
+	git rev-list --all
+'
+
+compare_results_with_midx "with alternate (local midx)"
+
+test_expect_success 'multi-pack-index in an alternate' '
+	mv .git/objects/pack/* alt.git/objects/pack
+'
+
+compare_results_with_midx "with alternate (remote midx)"
+
 
 # usage: corrupt_data <file> <pos> [<data>]
 corrupt_data () {