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>

1 files changed, 8 insertions, 7 deletions

diff --git a/connected.c b/connected.c
index b18299fdf0..35bd4a2638 100644
--- a/connected.c
+++ b/connected.c
@@ -24,7 +24,7 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 	struct child_process rev_list = CHILD_PROCESS_INIT;
 	FILE *rev_list_in;
 	struct check_connected_options defaults = CHECK_CONNECTED_INIT;
-	struct object_id oid;
+	const struct object_id *oid;
 	int err = 0;
 	struct packed_git *new_pack = NULL;
 	struct transport *transport;
@@ -34,7 +34,8 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 		opt = &defaults;
 	transport = opt->transport;
 
-	if (fn(cb_data, &oid)) {
+	oid = fn(cb_data);
+	if (!oid) {
 		if (opt->err_fd)
 			close(opt->err_fd);
 		return err;
@@ -73,7 +74,7 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 			for (p = get_all_packs(the_repository); p; p = p->next) {
 				if (!p->pack_promisor)
 					continue;
-				if (find_pack_entry_one(oid.hash, p))
+				if (find_pack_entry_one(oid->hash, p))
 					goto promisor_pack_found;
 			}
 			/*
@@ -83,7 +84,7 @@ int check_connected(oid_iterate_fn fn, void *cb_data,
 			goto no_promisor_pack_found;
 promisor_pack_found:
 			;
-		} while (!fn(cb_data, &oid));
+		} while ((oid = fn(cb_data)) != NULL);
 		return 0;
 	}
 
@@ -132,12 +133,12 @@ no_promisor_pack_found:
 		 * are sure the ref is good and not sending it to
 		 * rev-list for verification.
 		 */
-		if (new_pack && find_pack_entry_one(oid.hash, new_pack))
+		if (new_pack && find_pack_entry_one(oid->hash, new_pack))
 			continue;
 
-		if (fprintf(rev_list_in, "%s\n", oid_to_hex(&oid)) < 0)
+		if (fprintf(rev_list_in, "%s\n", oid_to_hex(oid)) < 0)
 			break;
-	} while (!fn(cb_data, &oid));
+	} while ((oid = fn(cb_data)) != NULL);
 
 	if (ferror(rev_list_in) || fflush(rev_list_in)) {
 		if (errno != EPIPE && errno != EINVAL)