[tarantool-patches] [PATCH v2 3/5] Enforce applier out of order protection

[Georgy Kirichenko <georgy@tarantool.org>]

Do not skip row until the row is not processed by other appliers.

Prerequisite #980
---
 src/box/applier.cc | 35 ++++++++++++++++++-----------------
 1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/src/box/applier.cc b/src/box/applier.cc
index 87873e970..148c8ce5a 100644
--- a/src/box/applier.cc
+++ b/src/box/applier.cc
@@ -504,6 +504,22 @@ applier_subscribe(struct applier *applier)
 
 		applier->lag = ev_now(loop()) - row.tm;
 		applier->last_row_time = ev_monotonic_now(loop());
+		struct replica *replica = replica_by_id(row.replica_id);
+		struct latch *latch = (replica ? &replica->order_latch :
+				       &replicaset.applier.order_latch);
+		/*
+		 * In a full mesh topology, the same set
+		 * of changes may arrive via two
+		 * concurrently running appliers. Thanks
+		 * to vclock_follow() above, the first row
+		 * in the set will be skipped - but the
+		 * remaining may execute out of order,
+		 * when the following xstream_write()
+		 * yields on WAL. Hence we need a latch to
+		 * strictly order all changes which belong
+		 * to the same server id.
+		 */
+		latch_lock(latch);
 		if (vclock_get(&replicaset.applier.vclock,
 			       row.replica_id) < row.lsn) {
 			if (row.replica_id == instance_id &&
@@ -516,24 +532,7 @@ applier_subscribe(struct applier *applier)
 			int64_t old_lsn = vclock_get(&replicaset.applier.vclock,
 						     row.replica_id);
 			vclock_follow_xrow(&replicaset.applier.vclock, &row);
-			struct replica *replica = replica_by_id(row.replica_id);
-			struct latch *latch = (replica ? &replica->order_latch :
-					       &replicaset.applier.order_latch);
-			/*
-			 * In a full mesh topology, the same set
-			 * of changes may arrive via two
-			 * concurrently running appliers. Thanks
-			 * to vclock_follow() above, the first row
-			 * in the set will be skipped - but the
-			 * remaining may execute out of order,
-			 * when the following xstream_write()
-			 * yields on WAL. Hence we need a latch to
-			 * strictly order all changes which belong
-			 * to the same server id.
-			 */
-			latch_lock(latch);
 			int res = xstream_write(applier->subscribe_stream, &row);
-			latch_unlock(latch);
 			if (res != 0) {
 				struct error *e = diag_last_error(diag_get());
 				/**
@@ -548,11 +547,13 @@ applier_subscribe(struct applier *applier)
 					/* Rollback lsn to have a chance for a retry. */
 					vclock_set(&replicaset.applier.vclock,
 						   row.replica_id, old_lsn);
+					latch_unlock(latch);
 					diag_raise();
 				}
 			}
 		}
 done:
+		latch_unlock(latch);
 		/*
 		 * Stay 'orphan' until appliers catch up with
 		 * the remote vclock at the time of SUBSCRIBE
-- 
2.20.1