Make git_odb_foreach_cb take const param

This makes the first OID param of the ODB callback a const pointer
and also propogates that change all the way to the backends.

include/git2/odb.h

@@ -23,11 +23,6 @@
 GIT_BEGIN_DECL
 
 /**
- * Function type for callbacks from git_odb_foreach.
- */
-typedef int (*git_odb_foreach_cb)(git_oid *id, void *payload);
-
-/**
  * Create a new object database with no backends.
  *
  * Before the ODB can be used for read/writing, a custom database

include/git2/odb_backend.h

@@ -24,7 +24,14 @@ GIT_BEGIN_DECL
 struct git_odb_stream;
 struct git_odb_writepack;
 
-/** An instance for a custom backend */
+/**
+ * Function type for callbacks from git_odb_foreach.
+ */
+typedef int (*git_odb_foreach_cb)(const git_oid *id, void *payload);
+
+/**
+ * An instance for a custom backend
+ */
 struct git_odb_backend {
 	git_odb *odb;
 
@@ -79,7 +86,7 @@ struct git_odb_backend {
 
 	int (* foreach)(
 			struct git_odb_backend *,
-			int (*cb)(git_oid *oid, void *payload),
+			git_odb_foreach_cb cb,
 			void *payload);
 
 	int (* writepack)(

src/odb_loose.c

@@ -678,7 +678,7 @@ static int loose_backend__exists(git_odb_backend *backend, const git_oid *oid)
 
 struct foreach_state {
 	size_t dir_len;
-	int (*cb)(git_oid *oid, void *data);
+	git_odb_foreach_cb cb;
 	void *data;
 	int cb_error;
 };
@@ -734,7 +734,7 @@ static int foreach_cb(void *_state, git_buf *path)
 	return git_path_direach(path, foreach_object_dir_cb, state);
 }
 
-static int loose_backend__foreach(git_odb_backend *_backend, int (*cb)(git_oid *oid, void *data), void *data)
+static int loose_backend__foreach(git_odb_backend *_backend, git_odb_foreach_cb cb, void *data)
 {
 	char *objects_dir;
 	int error;

src/odb_pack.c

@@ -458,7 +458,7 @@ static int pack_backend__exists(git_odb_backend *backend, const git_oid *oid)
 	return pack_entry_find(&e, (struct pack_backend *)backend, oid) == 0;
 }
 
-static int pack_backend__foreach(git_odb_backend *_backend, int (*cb)(git_oid *oid, void *data), void *data)
+static int pack_backend__foreach(git_odb_backend *_backend, git_odb_foreach_cb cb, void *data)
 {
 	int error;
 	struct git_pack_file *p;

src/pack.c

@@ -698,7 +698,7 @@ static int git__memcmp4(const void *a, const void *b) {
 
 int git_pack_foreach_entry(
 	struct git_pack_file *p,
-	int (*cb)(git_oid *oid, void *data),
+	git_odb_foreach_cb cb,
 	void *data)
 {
 	const unsigned char *index = p->index_map.data, *current;

src/pack.h

@@ -105,7 +105,7 @@ int git_pack_entry_find(
 		size_t len);
 int git_pack_foreach_entry(
 		struct git_pack_file *p,
-		int (*cb)(git_oid *oid, void *data),
+		git_odb_foreach_cb cb,
 		void *data);
 
 #endif

tests-clar/odb/foreach.c

@@ -16,7 +16,7 @@ void test_odb_foreach__cleanup(void)
 	_repo = NULL;
 }
 
-static int foreach_cb(git_oid *oid, void *data)
+static int foreach_cb(const git_oid *oid, void *data)
 {
 	GIT_UNUSED(data);
 	GIT_UNUSED(oid);
@@ -59,7 +59,7 @@ void test_odb_foreach__one_pack(void)
 	cl_assert(nobj == 1628);
 }
 
-static int foreach_stop_cb(git_oid *oid, void *data)
+static int foreach_stop_cb(const git_oid *oid, void *data)
 {
 	GIT_UNUSED(data);
 	GIT_UNUSED(oid);