[Tarantool-patches] [PATCH v2 4/4] replication: do not promote local_vclock_at_subscribe unnecessarily

[sergepetrenko <sergepetrenko@tarantool.org>]

From: Serge Petrenko <sergepetrenko@tarantool.org>

When master processes a subscribe response, it responds with its vclock
at the moment of receiving the request. However, the fiber processing
the request may yield on coio_write_xrow, when sending the response to
the replica. In the meantime, master may apply additional rows coming
from the replica after it has issued SUBSCRIBE.
Then in relay_subscribe master sets its local vclock_at_subscribe to
a possibly updated value of replicaset.vclock
So, set local_vclock_at_subscribe to a remembered value, rather than an
updated one.

Follow-up #4739
---
 src/box/box.cc   |  2 +-
 src/box/relay.cc | 13 +++++++++++--
 src/box/relay.h  |  3 ++-
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/box/box.cc b/src/box/box.cc
index 952d60ad1..7dec1ae6b 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -1871,7 +1871,7 @@ box_process_subscribe(struct ev_io *io, struct xrow_header *header)
 	 * indefinitely).
 	 */
 	relay_subscribe(replica, io->fd, header->sync, &replica_clock,
-			replica_version_id);
+			replica_version_id, &vclock);
 }
 
 void
diff --git a/src/box/relay.cc b/src/box/relay.cc
index b89632273..b69646446 100644
--- a/src/box/relay.cc
+++ b/src/box/relay.cc
@@ -676,7 +676,8 @@ relay_subscribe_f(va_list ap)
 /** Replication acceptor fiber handler. */
 void
 relay_subscribe(struct replica *replica, int fd, uint64_t sync,
-		struct vclock *replica_clock, uint32_t replica_version_id)
+		struct vclock *replica_clock, uint32_t replica_version_id,
+		struct vclock *clock_at_subscribe)
 {
 	assert(replica->anon || replica->id != REPLICA_ID_NIL);
 	struct relay *relay = replica->relay;
@@ -699,7 +700,15 @@ relay_subscribe(struct replica *replica, int fd, uint64_t sync,
 		replica_on_relay_stop(replica);
 	});
 
-	vclock_copy(&relay->local_vclock_at_subscribe, &replicaset.vclock);
+	/*
+	 * It's too late to remember replicaset.vclock as local
+	 * vclock at subscribe. It might have incremented while we
+	 * were writing a subscribe response, and we don't want to
+	 * replicate back rows originating from the replica and
+	 * having arrived later than replica has issued
+	 * SUBSCRIBE.
+	 */
+	vclock_copy(&relay->local_vclock_at_subscribe, clock_at_subscribe);
 	relay->r = recovery_new(cfg_gets("wal_dir"), false,
 			        replica_clock);
 	vclock_copy(&relay->tx.vclock, replica_clock);
diff --git a/src/box/relay.h b/src/box/relay.h
index e1782d78f..54ebd6731 100644
--- a/src/box/relay.h
+++ b/src/box/relay.h
@@ -124,6 +124,7 @@ relay_final_join(int fd, uint64_t sync, struct vclock *start_vclock,
  */
 void
 relay_subscribe(struct replica *replica, int fd, uint64_t sync,
-		struct vclock *replica_vclock, uint32_t replica_version_id);
+		struct vclock *replica_vclock, uint32_t replica_version_id,
+		struct vclock *clock_at_subscribe);
 
 #endif /* TARANTOOL_REPLICATION_RELAY_H_INCLUDED */
-- 
2.20.1 (Apple Git-117)