refactor refs_from_alternate_cb to allow passing extra data

The foreach_alt_odb function triggers a callback for each
alternate object db we have, with room for a single void
pointer as data. Currently, we always call refs_from_alternate_cb
as the callback function, and then pass another callback (to
receive each ref individually) as the void pointer.

This has two problems:

  1. C technically forbids stuffing a function pointer into
     a "void *". In practice, this probably doesn't matter
     on any architectures git runs on, but it never hurts to
     follow the letter of the law.

  2. There is no room for an extra data pointer. Indeed, the
     alternate_ref_fn that refs_from_alternate_cb calls
     takes a void* for data, but we always pass it NULL.

Instead, let's properly stuff our function pointer into a
data struct, which also leaves room for an extra
caller-supplied data pointer. And to keep things simple for
existing callers, let's make a for_each_alternate_ref
function that takes care of creating the extra struct.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch-pack.c

@@ -226,7 +226,7 @@ static void insert_one_alternate_ref(const struct ref *ref, void *unused)
 
 static void insert_alternate_refs(void)
 {
-	foreach_alt_odb(refs_from_alternate_cb, insert_one_alternate_ref);
+	for_each_alternate_ref(insert_one_alternate_ref, NULL);
 }
 
 #define INITIAL_FLUSH 16

builtin/receive-pack.c

@@ -738,7 +738,7 @@ static void add_one_alternate_ref(const struct ref *ref, void *unused)
 
 static void add_alternate_refs(void)
 {
-	foreach_alt_odb(refs_from_alternate_cb, add_one_alternate_ref);
+	for_each_alternate_ref(add_one_alternate_ref, NULL);
 }
 
 int cmd_receive_pack(int argc, const char **argv, const char *prefix)

transport.c

@@ -1190,14 +1190,20 @@ literal_copy:
 	return xstrdup(url);
 }
 
-int refs_from_alternate_cb(struct alternate_object_database *e, void *cb)
+struct alternate_refs_data {
+	alternate_ref_fn *fn;
+	void *data;
+};
+
+static int refs_from_alternate_cb(struct alternate_object_database *e,
+				  void *data)
 {
 	char *other;
 	size_t len;
 	struct remote *remote;
 	struct transport *transport;
 	const struct ref *extra;
-	alternate_ref_fn *ref_fn = cb;
+	struct alternate_refs_data *cb = data;
 
 	e->name[-1] = '\0';
 	other = xstrdup(real_path(e->base));
@@ -1218,8 +1224,16 @@ int refs_from_alternate_cb(struct alternate_object_database *e, void *cb)
 	for (extra = transport_get_remote_refs(transport);
 	     extra;
 	     extra = extra->next)
-		ref_fn(extra, NULL);
+		cb->fn(extra, cb->data);
 	transport_disconnect(transport);
 	free(other);
 	return 0;
 }
+
+void for_each_alternate_ref(alternate_ref_fn fn, void *data)
+{
+	struct alternate_refs_data cb;
+	cb.fn = fn;
+	cb.data = data;
+	foreach_alt_odb(refs_from_alternate_cb, &cb);
+}

transport.h

@@ -167,6 +167,6 @@ void transport_print_push_status(const char *dest, struct ref *refs,
 		  int verbose, int porcelain, int *nonfastforward);
 
 typedef void alternate_ref_fn(const struct ref *, void *);
-extern int refs_from_alternate_cb(struct alternate_object_database *e, void *cb);
+extern void for_each_alternate_ref(alternate_ref_fn, void *);
 
 #endif