+ if (fprintf(out, "%s", PACKED_REFS_HEADER) < 0)
+ goto write_error;
+
+ /*
+ * We iterate in parallel through the current list of refs and
+ * the list of updates, processing an entry from at least one
+ * of the lists each time through the loop. When the current
+ * list of refs is exhausted, set iter to NULL. When the list
+ * of updates is exhausted, leave i set to updates->nr.
+ */
+ iter = packed_ref_iterator_begin(&refs->base, "",
+ DO_FOR_EACH_INCLUDE_BROKEN);
+ if ((ok = ref_iterator_advance(iter)) != ITER_OK)
+ iter = NULL;
+
+ i = 0;
+
+ while (iter || i < updates->nr) {
+ struct ref_update *update = NULL;
+ int cmp;
+
+ if (i >= updates->nr) {
+ cmp = -1;
+ } else {
+ update = updates->items[i].util;
+
+ if (!iter)
+ cmp = +1;
+ else
+ cmp = strcmp(iter->refname, update->refname);
+ }
+
+ if (!cmp) {
+ /*
+ * There is both an old value and an update
+ * for this reference. Check the old value if
+ * necessary:
+ */
+ if ((update->flags & REF_HAVE_OLD)) {
+ if (is_null_oid(&update->old_oid)) {
+ strbuf_addf(err, "cannot update ref '%s': "
+ "reference already exists",
+ update->refname);
+ goto error;
+ } else if (oidcmp(&update->old_oid, iter->oid)) {
+ strbuf_addf(err, "cannot update ref '%s': "
+ "is at %s but expected %s",
+ update->refname,
+ oid_to_hex(iter->oid),
+ oid_to_hex(&update->old_oid));
+ goto error;
+ }
+ }
+
+ /* Now figure out what to use for the new value: */
+ if ((update->flags & REF_HAVE_NEW)) {
+ /*
+ * The update takes precedence. Skip
+ * the iterator over the unneeded
+ * value.
+ */
+ if ((ok = ref_iterator_advance(iter)) != ITER_OK)
+ iter = NULL;
+ cmp = +1;
+ } else {
+ /*
+ * The update doesn't actually want to
+ * change anything. We're done with it.
+ */
+ i++;
+ cmp = -1;
+ }
+ } else if (cmp > 0) {
+ /*
+ * There is no old value but there is an
+ * update for this reference. Make sure that
+ * the update didn't expect an existing value:
+ */
+ if ((update->flags & REF_HAVE_OLD) &&
+ !is_null_oid(&update->old_oid)) {
+ strbuf_addf(err, "cannot update ref '%s': "
+ "reference is missing but expected %s",
+ update->refname,
+ oid_to_hex(&update->old_oid));
+ goto error;
+ }
+ }