refs/reftable: batch refname availability checks

Refactor the "reftable" backend to batch the availability check for
refnames. This does not yet have an effect on performance as
`refs_verify_refnames_available()` effectively still performs the
availability check for each refname individually. But this will be
optimized in subsequent commits, where we learn to optimize some parts
of the logic when checking multiple refnames for availability.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs/reftable-backend.c

@@ -1069,6 +1069,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 		reftable_be_downcast(ref_store, REF_STORE_WRITE|REF_STORE_MAIN, "ref_transaction_prepare");
 	struct strbuf referent = STRBUF_INIT, head_referent = STRBUF_INIT;
 	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
+	struct string_list refnames_to_check = STRING_LIST_INIT_NODUP;
 	struct reftable_transaction_data *tx_data = NULL;
 	struct reftable_backend *be;
 	struct object_id head_oid;
@@ -1224,12 +1225,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 			 * can output a proper error message instead of failing
 			 * at a later point.
 			 */
-			ret = refs_verify_refname_available(ref_store, u->refname,
-							    &affected_refnames, NULL,
-							    transaction->flags & REF_TRANSACTION_FLAG_INITIAL,
-							    err);
-			if (ret < 0)
-				goto done;
+			string_list_append(&refnames_to_check, u->refname);
 
 			/*
 			 * There is no need to write the reference deletion
@@ -1379,6 +1375,12 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 		}
 	}
 
+	ret = refs_verify_refnames_available(ref_store, &refnames_to_check, &affected_refnames, NULL,
+					     transaction->flags & REF_TRANSACTION_FLAG_INITIAL,
+					     err);
+	if (ret < 0)
+		goto done;
+
 	transaction->backend_data = tx_data;
 	transaction->state = REF_TRANSACTION_PREPARED;
 
@@ -1394,6 +1396,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 	string_list_clear(&affected_refnames, 0);
 	strbuf_release(&referent);
 	strbuf_release(&head_referent);
+	string_list_clear(&refnames_to_check, 0);
 
 	return ret;
 }