[PATCH 05/12] alter: allocate triggers before the point of no return

[Vladimir Davydov <vdavydov.dev@gmail.com>]

Trigger allocation, as any other memory allocation, is allowed to fail.
If this happens in alter_space_do, the space will be left in an
inconsistent state. Let's move trigger allocation to the beginning of
alter_space_do and add a comment denoting the point of no return.
---
 src/box/alter.cc | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/box/alter.cc b/src/box/alter.cc
index 36310f1c..9d0c4c23 100644
--- a/src/box/alter.cc
+++ b/src/box/alter.cc
@@ -803,6 +803,11 @@ alter_space_rollback(struct trigger *trigger, void * /* event */)
 static void
 alter_space_do(struct txn *txn, struct alter_space *alter)
 {
+	/* Prepare triggers while we may fail. */
+	struct trigger *on_commit, *on_rollback;
+	on_commit = txn_alter_trigger_new(alter_space_commit, alter);
+	on_rollback = txn_alter_trigger_new(alter_space_rollback, alter);
+
 	/* Create a definition of the new space. */
 	space_dump_def(alter->old_space, &alter->key_list);
 	class AlterSpaceOp *op;
@@ -853,6 +858,11 @@ alter_space_do(struct txn *txn, struct alter_space *alter)
 		throw;
 	}
 
+	/*
+	 * This function must not throw exceptions or yield after
+	 * this point.
+	 */
+
 	/* Rebuild index maps once for all indexes. */
 	space_fill_index_map(alter->old_space);
 	space_fill_index_map(alter->new_space);
@@ -873,11 +883,7 @@ alter_space_do(struct txn *txn, struct alter_space *alter)
 	 * finish or rollback the DDL depending on the results of
 	 * writing to WAL.
 	 */
-	struct trigger *on_commit =
-		txn_alter_trigger_new(alter_space_commit, alter);
 	txn_on_commit(txn, on_commit);
-	struct trigger *on_rollback =
-		txn_alter_trigger_new(alter_space_rollback, alter);
 	txn_on_rollback(txn, on_rollback);
 }
 
-- 
2.11.0