teach add_submodule_odb() to look for alternates

Since we allow to link other object databases when loading a submodules
database we should also load possible alternates.
Signed-off-by: NHeiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -960,6 +960,7 @@ extern struct alternate_object_database {
 	char base[FLEX_ARRAY]; /* more */
 } *alt_odb_list;
 extern void prepare_alt_odb(void);
+extern void read_info_alternates(const char * relative_base, int depth);
 extern void add_to_alternates_file(const char *reference);
 typedef int alt_odb_fn(struct alternate_object_database *, void *);
 extern void foreach_alt_odb(alt_odb_fn, void*);

sha1_file.c

@@ -228,7 +228,6 @@ char *sha1_pack_index_name(const unsigned char *sha1)
 struct alternate_object_database *alt_odb_list;
 static struct alternate_object_database **alt_odb_tail;
 
-static void read_info_alternates(const char * alternates, int depth);
 static int git_open_noatime(const char *name);
 
 /*
@@ -353,7 +352,7 @@ static void link_alt_odb_entries(const char *alt, const char *ep, int sep,
 	}
 }
 
-static void read_info_alternates(const char * relative_base, int depth)
+void read_info_alternates(const char * relative_base, int depth)
 {
 	char *map;
 	size_t mapsz;

submodule.c

@@ -63,6 +63,9 @@ static int add_submodule_odb(const char *path)
 	alt_odb->name[40] = '\0';
 	alt_odb->name[41] = '\0';
 	alt_odb_list = alt_odb;
+
+	/* add possible alternates from the submodule */
+	read_info_alternates(objects_directory.buf, 0);
 	prepare_alt_odb();
 done:
 	strbuf_release(&objects_directory);

t/t4041-diff-submodule-option.sh

@@ -458,4 +458,38 @@ EOF
 	test_cmp expected actual
 '
 
+test_expect_success 'diff --submodule with objects referenced by alternates' '
+	mkdir sub_alt &&
+	(cd sub_alt &&
+		git init &&
+		echo a >a &&
+		git add a &&
+		git commit -m a
+	) &&
+	mkdir super &&
+	(cd super &&
+		git clone -s ../sub_alt sub &&
+		git init &&
+		git add sub &&
+		git commit -m "sub a"
+	) &&
+	(cd sub_alt &&
+		sha1_before=$(git rev-parse --short HEAD)
+		echo b >b &&
+		git add b &&
+		git commit -m b
+		sha1_after=$(git rev-parse --short HEAD)
+		echo "Submodule sub $sha1_before..$sha1_after:
+  > b" >../expected
+	) &&
+	(cd super &&
+		(cd sub &&
+			git fetch &&
+			git checkout origin/master
+		) &&
+		git diff --submodule > ../actual
+	)
+	test_cmp expected actual
+'
+
 test_done