[Tarantool-patches] [PATCH v2 2/4] replication: check for rows to skip in applier correctly

[sergepetrenko]

From: Serge Petrenko <sergepetrenko at tarantool.org>

Remove applier vclock initialization from replication_init(), where it
is zeroed-out, and place it in the end of box_cfg_xc(), where replicaset
vclock already has a meaningful value.
Do not apply rows originating form the current instance if replication
sync has ended.

Closes #4739
---
 src/box/applier.cc     | 17 +++++++++++++++--
 src/box/box.cc         |  6 ++++++
 src/box/replication.cc |  1 -
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/box/applier.cc b/src/box/applier.cc
index ae3d281a5..e931e1595 100644
--- a/src/box/applier.cc
+++ b/src/box/applier.cc
@@ -731,8 +731,21 @@ applier_apply_tx(struct stailq *rows)
 	struct latch *latch = (replica ? &replica->order_latch :
 			       &replicaset.applier.order_latch);
 	latch_lock(latch);
-	if (vclock_get(&replicaset.applier.vclock,
-		       first_row->replica_id) >= first_row->lsn) {
+	/*
+	 * Skip remote rows either if one of the appliers has
+	 * sent them to write or if the rows originate from the
+	 * local instance and we've already synced with the
+	 * replica. The latter is important because relay gets
+	 * notified about WAL write before tx does, so it is
+	 * possible that a remote instance receives our rows
+	 * via replication before we update replicaset vclock and
+	 * even sends these rows back to us. An attemt to apply
+	 * such rows will lead to having entries with duplicate
+	 * LSNs in WAL.
+	 */
+	if (vclock_get(&replicaset.applier.vclock, first_row->replica_id) >=
+	    first_row->lsn || (first_row->replica_id == instance_id &&
+	    !box_is_orphan())) {
 		latch_unlock(latch);
 		return 0;
 	}
diff --git a/src/box/box.cc b/src/box/box.cc
index 0290578b2..952d60ad1 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -2425,6 +2425,12 @@ box_cfg_xc(void)
 
 	rmean_cleanup(rmean_box);
 
+	/*
+	 * Local recovery is over so it's fine to update applier
+	 * vclock now.
+	 */
+	vclock_copy(&replicaset.applier.vclock, &replicaset.vclock);
+
 	/* Follow replica */
 	replicaset_follow();
 
diff --git a/src/box/replication.cc b/src/box/replication.cc
index e7bfa22ab..7b04573a4 100644
--- a/src/box/replication.cc
+++ b/src/box/replication.cc
@@ -93,7 +93,6 @@ replication_init(void)
 	latch_create(&replicaset.applier.order_latch);
 
 	vclock_create(&replicaset.applier.vclock);
-	vclock_copy(&replicaset.applier.vclock, &replicaset.vclock);
 	rlist_create(&replicaset.applier.on_rollback);
 	rlist_create(&replicaset.applier.on_commit);
 
-- 
2.20.1 (Apple Git-117)