connected: refactor iterator to return next object ID directly

The object ID iterator used by the connectivity checks returns the next
object ID via an out-parameter and then uses a return code to indicate
whether an item was found. This is a bit roundabout: instead of a
separate error code, we can just return the next object ID directly and
use `NULL` pointers as indicator that the iterator got no items left.
Furthermore, this avoids a copy of the object ID.

Refactor the iterator and all its implementations to return object IDs
directly. This brings a tiny performance improvement when doing a mirror-fetch of a repository with about 2.3M refs:

    Benchmark #1: 328dc58b49919c43897240f2eabfa30be2ce32a4~: git-fetch
      Time (mean ± σ):     30.110 s ±  0.148 s    [User: 27.161 s, System: 5.075 s]
      Range (min … max):   29.934 s … 30.406 s    10 runs

    Benchmark #2: 328dc58b49919c43897240f2eabfa30be2ce32a4: git-fetch
      Time (mean ± σ):     29.899 s ±  0.109 s    [User: 26.916 s, System: 5.104 s]
      Range (min … max):   29.696 s … 29.996 s    10 runs

    Summary
      '328dc58b49919c43897240f2eabfa30be2ce32a4: git-fetch' ran
        1.01 ± 0.01 times faster than '328dc58b49919c43897240f2eabfa30be2ce32a4~: git-fetch'

While this 1% speedup could be labelled as statistically insignificant,
the speedup is consistent on my machine. Furthermore, this is an end to
end test, so it is expected that the improvement in the connectivity
check itself is more significant.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 13 insertions, 19 deletions

diff --git a/builtin/clone.c b/builtin/clone.c
index 66fe66679c..4a1056fcc2 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -657,7 +657,7 @@ static void write_followtags(const struct ref *refs, const char *msg)
 	}
 }
 
-static int iterate_ref_map(void *cb_data, struct object_id *oid)
+static const struct object_id *iterate_ref_map(void *cb_data)
 {
 	struct ref **rm = cb_data;
 	struct ref *ref = *rm;
@@ -668,13 +668,11 @@ static int iterate_ref_map(void *cb_data, struct object_id *oid)
 	 */
 	while (ref && !ref->peer_ref)
 		ref = ref->next;
-	/* Returning -1 notes "end of list" to the caller. */
 	if (!ref)
-		return -1;
+		return NULL;
 
-	oidcpy(oid, &ref->old_oid);
 	*rm = ref->next;
-	return 0;
+	return &ref->old_oid;
 }
 
 static void update_remote_refs(const struct ref *refs,
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 01513e6aea..cdf0d0d671 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -962,7 +962,7 @@ static int update_local_ref(struct ref *ref,
 	}
 }
 
-static int iterate_ref_map(void *cb_data, struct object_id *oid)
+static const struct object_id *iterate_ref_map(void *cb_data)
 {
 	struct ref **rm = cb_data;
 	struct ref *ref = *rm;
@@ -970,10 +970,9 @@ static int iterate_ref_map(void *cb_data, struct object_id *oid)
 	while (ref && ref->status == REF_STATUS_REJECT_SHALLOW)
 		ref = ref->next;
 	if (!ref)
-		return -1; /* end of the list */
+		return NULL;
 	*rm = ref->next;
-	oidcpy(oid, &ref->old_oid);
-	return 0;
+	return &ref->old_oid;
 }
 
 struct fetch_head {
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 2d1f97e1ca..041e915454 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1306,7 +1306,7 @@ static void refuse_unconfigured_deny_delete_current(void)
 	rp_error("%s", _(refuse_unconfigured_deny_delete_current_msg));
 }
 
-static int command_singleton_iterator(void *cb_data, struct object_id *oid);
+static const struct object_id *command_singleton_iterator(void *cb_data);
 static int update_shallow_ref(struct command *cmd, struct shallow_info *si)
 {
 	struct shallow_lock shallow_lock = SHALLOW_LOCK_INIT;
@@ -1731,16 +1731,15 @@ static void check_aliased_updates(struct command *commands)
 	string_list_clear(&ref_list, 0);
 }
 
-static int command_singleton_iterator(void *cb_data, struct object_id *oid)
+static const struct object_id *command_singleton_iterator(void *cb_data)
 {
 	struct command **cmd_list = cb_data;
 	struct command *cmd = *cmd_list;
 
 	if (!cmd || is_null_oid(&cmd->new_oid))
-		return -1; /* end of list */
+		return NULL;
 	*cmd_list = NULL; /* this returns only one */
-	oidcpy(oid, &cmd->new_oid);
-	return 0;
+	return &cmd->new_oid;
 }
 
 static void set_connectivity_errors(struct command *commands,
@@ -1770,7 +1769,7 @@ struct iterate_data {
 	struct shallow_info *si;
 };
 
-static int iterate_receive_command_list(void *cb_data, struct object_id *oid)
+static const struct object_id *iterate_receive_command_list(void *cb_data)
 {
 	struct iterate_data *data = cb_data;
 	struct command **cmd_list = &data->cmds;
@@ -1781,13 +1780,11 @@ static int iterate_receive_command_list(void *cb_data, struct object_id *oid)
 			/* to be checked in update_shallow_ref() */
 			continue;
 		if (!is_null_oid(&cmd->new_oid) && !cmd->skip_update) {
-			oidcpy(oid, &cmd->new_oid);
 			*cmd_list = cmd->next;
-			return 0;
+			return &cmd->new_oid;
 		}
 	}
-	*cmd_list = NULL;
-	return -1; /* end of list */
+	return NULL;
 }
 
 static void reject_updates_to_hidden(struct command *commands)