sha1-file: support OBJECT_INFO_FOR_PREFETCH

Teach oid_object_info_extended() to support a new flag that inhibits
fetching of missing objects. This is equivalent to setting
fetch_is_missing to 0, calling oid_object_info_extended(), then setting
fetch_if_missing to whatever it was before. Update unpack-trees.c to use
this new flag instead of repeatedly setting fetch_if_missing.

This new flag complicates things slightly in that there are now 2 ways
to do the same thing. But this eliminates the need to repeatedly set a
global variable, and more importantly, allows prefetching to be done in
parallel (in the future); hence, this patch.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object-store.h

@@ -280,6 +280,12 @@ struct object_info {
 #define OBJECT_INFO_QUICK 8
 /* Do not check loose object */
 #define OBJECT_INFO_IGNORE_LOOSE 16
+/*
+ * Do not attempt to fetch the object if missing (even if fetch_is_missing is
+ * nonzero). This is meant for bulk prefetching of missing blobs in a partial
+ * clone. Implies OBJECT_INFO_QUICK.
+ */
+#define OBJECT_INFO_FOR_PREFETCH (32 + OBJECT_INFO_QUICK)
 
 int oid_object_info_extended(struct repository *r,
 			     const struct object_id *,

sha1-file.c

@@ -1370,7 +1370,8 @@ int oid_object_info_extended(struct repository *r, const struct object_id *oid,
 
 		/* Check if it is a missing object */
 		if (fetch_if_missing && repository_format_partial_clone &&
-		    !already_retried && r == the_repository) {
+		    !already_retried && r == the_repository &&
+		    !(flags & OBJECT_INFO_FOR_PREFETCH)) {
 			/*
 			 * TODO Investigate having fetch_object() return
 			 * TODO error/success and stopping the music here.

unpack-trees.c

@@ -404,20 +404,21 @@ static int check_updates(struct unpack_trees_options *o)
 		 * below.
 		 */
 		struct oid_array to_fetch = OID_ARRAY_INIT;
-		int fetch_if_missing_store = fetch_if_missing;
-		fetch_if_missing = 0;
 		for (i = 0; i < index->cache_nr; i++) {
 			struct cache_entry *ce = index->cache[i];
-			if ((ce->ce_flags & CE_UPDATE) &&
-			    !S_ISGITLINK(ce->ce_mode)) {
-				if (!has_object_file(&ce->oid))
-					oid_array_append(&to_fetch, &ce->oid);
-			}
+
+			if (!(ce->ce_flags & CE_UPDATE) ||
+			    S_ISGITLINK(ce->ce_mode))
+				continue;
+			if (!oid_object_info_extended(the_repository, &ce->oid,
+						      NULL,
+						      OBJECT_INFO_FOR_PREFETCH))
+				continue;
+			oid_array_append(&to_fetch, &ce->oid);
 		}
 		if (to_fetch.nr)
 			fetch_objects(repository_format_partial_clone,
 				      to_fetch.oid, to_fetch.nr);
-		fetch_if_missing = fetch_if_missing_store;
 		oid_array_clear(&to_fetch);
 	}
 	for (i = 0; i < index->cache_nr; i++) {