Why did not you use 
asserts:assert_server_follow_upstream(server, id)
from asserts.lua here?
 
+local function check_follow_master(server)
+ return t.assert_equals(
+ server:eval('return box.info.replication[1].upstream.status'), 'follow')
+end
 
 
Also, do we want to check this patch only for memtx? I asked because you haven’t provided any engine for the test.
 
--
Vitaliia Ioffe
 
 
>Среда, 27 октября 2021, 10:20 +03:00 от Serge Petrenko < sergepetrenko@tarantool.org >:
> 
>Replication reconfiguration used to work as follows: upon receiving a
>new config disconnect from all the existing masters and try to connect
>to all the masters from the new config.
>
>This lead to instances doing extra work when old config and new config
>had the same nodes in them: instead of doing nothing, tarantool
>reinitialized the connection.
>
>There was another problem: when an existing connection is broken, master
>takes some time to notice that. So, when replica resets the connection,
>it may try to reconnect faster than the master is able to notice its
>absence. In this case replica wouldn't be able to reconnect due to
>`duplicate connection with the same replica UUID`. So replication would
>stop for a replication_timeout, which may be quite large (seconds or
>tens of seconds).
>
>Let's prevent tarantool from reconnecting to the same instance, if there
>already is a working connection.
>
>Closes #4669
>---
> .../gh-4669-applier-reconfig-reconnect.md | 5 ++
> src/box/box.cc | 31 ++++++++---
> src/box/replication.cc | 41 +++++++++-----
> src/box/replication.h | 4 +-
> .../gh_4669_applier_reconnect_test.lua | 53 +++++++++++++++++++
> 5 files changed, 113 insertions(+), 21 deletions(-)
> create mode 100644 changelogs/unreleased/gh-4669-applier-reconfig-reconnect.md
> create mode 100644 test/replication-luatest/gh_4669_applier_reconnect_test.lua
>
>diff --git a/changelogs/unreleased/gh-4669-applier-reconfig-reconnect.md b/changelogs/unreleased/gh-4669-applier-reconfig-reconnect.md
>new file mode 100644
>index 000000000..2a776872b
>--- /dev/null
>+++ b/changelogs/unreleased/gh-4669-applier-reconfig-reconnect.md
>@@ -0,0 +1,5 @@
>+## bugfix/replication
>+
>+* Fixed replica reconnecting to a living master on any `box.cfg{replication=...}`
>+ change. Such reconnects could lead to replica failing to restore connection
>+ for `replication_timeout` seconds (gh-4669).
>diff --git a/src/box/box.cc b/src/box/box.cc
>index b1154affa..c2e4a55a7 100644
>--- a/src/box/box.cc
>+++ b/src/box/box.cc
>@@ -1261,7 +1261,7 @@ cfg_get_replication(int *p_count)
>  * don't start appliers.
>  */
> static void
>-box_sync_replication(bool connect_quorum)
>+box_sync_replication(bool do_quorum, bool do_reuse)
> {
>  int count = 0;
>  struct applier **appliers = cfg_get_replication(&count);
>@@ -1272,12 +1272,27 @@ box_sync_replication(bool connect_quorum)
>  for (int i = 0; i < count; i++)
>  applier_delete(appliers[i]); /* doesn't affect diag */
>  });
>-
>- replicaset_connect(appliers, count, connect_quorum);
>+ replicaset_connect(appliers, count, do_quorum, do_reuse);
> 
>  guard.is_active = false;
> }
> 
>+static inline void
>+box_restart_replication(void)
>+{
>+ const bool do_quorum = true;
>+ const bool do_reuse = false;
>+ box_sync_replication(do_quorum, do_reuse);
>+}
>+
>+static inline void
>+box_update_replication(void)
>+{
>+ const bool do_quorum = false;
>+ const bool do_reuse = true;
>+ box_sync_replication(do_quorum, do_reuse);
>+}
>+
> void
> box_set_replication(void)
> {
>@@ -1296,7 +1311,7 @@ box_set_replication(void)
>  * Stay in orphan mode in case we fail to connect to at least
>  * 'replication_connect_quorum' remote instances.
>  */
>- box_sync_replication(false);
>+ box_update_replication();
>  /* Follow replica */
>  replicaset_follow();
>  /* Wait until appliers are in sync */
>@@ -1416,7 +1431,7 @@ box_set_replication_anon(void)
>  * them can register and others resend a
>  * non-anonymous subscribe.
>  */
>- box_sync_replication(true);
>+ box_restart_replication();
>  /*
>  * Wait until the master has registered this
>  * instance.
>@@ -3279,7 +3294,7 @@ bootstrap(const struct tt_uuid *instance_uuid,
>  * with connecting to 'replication_connect_quorum' masters.
>  * If this also fails, throw an error.
>  */
>- box_sync_replication(true);
>+ box_restart_replication();
> 
>  struct replica *master = replicaset_find_join_master();
>  assert(master == NULL || master->applier != NULL);
>@@ -3356,7 +3371,7 @@ local_recovery(const struct tt_uuid *instance_uuid,
>  if (wal_dir_lock >= 0) {
>  if (box_listen() != 0)
>  diag_raise();
>- box_sync_replication(false);
>+ box_update_replication();
> 
>  struct replica *master;
>  if (replicaset_needs_rejoin(&master)) {
>@@ -3435,7 +3450,7 @@ local_recovery(const struct tt_uuid *instance_uuid,
>  vclock_copy(&replicaset.vclock, &recovery->vclock);
>  if (box_listen() != 0)
>  diag_raise();
>- box_sync_replication(false);
>+ box_update_replication();
>  }
>  stream_guard.is_active = false;
>  recovery_finalize(recovery);
>diff --git a/src/box/replication.cc b/src/box/replication.cc
>index 1288bc9b1..10b4ac915 100644
>--- a/src/box/replication.cc
>+++ b/src/box/replication.cc
>@@ -507,7 +507,7 @@ replica_on_applier_state_f(struct trigger *trigger, void *event)
>  * upon reconfiguration of box.cfg.replication.
>  */
> static void
>-replicaset_update(struct applier **appliers, int count)
>+replicaset_update(struct applier **appliers, int count, bool keep_connect)
> {
>  replica_hash_t uniq;
>  memset(&uniq, 0, sizeof(uniq));
>@@ -572,22 +572,39 @@ replicaset_update(struct applier **appliers, int count)
>  applier_stop(applier);
>  applier_delete(applier);
>  }
>+
>+ replicaset.applier.total = count;
>+ replicaset.applier.connected = 0;
>+ replicaset.applier.loading = 0;
>+ replicaset.applier.synced = 0;
>  replicaset_foreach(replica) {
>  if (replica->applier == NULL)
>  continue;
>- applier = replica->applier;
>- replica_clear_applier(replica);
>- replica->applier_sync_state = APPLIER_DISCONNECTED;
>+ struct replica *other = replica_hash_search(&uniq, replica);
>+ if (keep_connect && other != NULL &&
>+ (replica->applier->state == APPLIER_FOLLOW ||
>+ replica->applier->state == APPLIER_SYNC)) {
>+ /*
>+ * Try not to interrupt working appliers upon
>+ * reconfiguration.
>+ */
>+ replicaset.applier.connected++;
>+ replicaset.applier.synced++;
>+ replica_hash_remove(&uniq, other);
>+ applier = other->applier;
>+ replica_clear_applier(other);
>+ replica_delete(other);
>+ } else {
>+ applier = replica->applier;
>+ replica_clear_applier(replica);
>+ replica->applier_sync_state = APPLIER_DISCONNECTED;
>+ }
>  applier_stop(applier);
>  applier_delete(applier);
>  }
> 
>- /* Save new appliers */
>- replicaset.applier.total = count;
>- replicaset.applier.connected = 0;
>- replicaset.applier.loading = 0;
>- replicaset.applier.synced = 0;
> 
>+ /* Save new appliers */
>  replica_hash_foreach_safe(&uniq, replica, next) {
>  replica_hash_remove(&uniq, replica);
> 
>@@ -664,11 +681,11 @@ applier_on_connect_f(struct trigger *trigger, void *event)
> 
> void
> replicaset_connect(struct applier **appliers, int count,
>- bool connect_quorum)
>+ bool connect_quorum, bool keep_connect)
> {
>  if (count == 0) {
>  /* Cleanup the replica set. */
>- replicaset_update(appliers, count);
>+ replicaset_update(appliers, 0, false);
>  return;
>  }
> 
>@@ -772,7 +789,7 @@ replicaset_connect(struct applier **appliers, int count,
> 
>  /* Now all the appliers are connected, update the replica set. */
>  try {
>- replicaset_update(appliers, count);
>+ replicaset_update(appliers, count, keep_connect);
>  } catch (Exception *e) {
>  goto error;
>  }
>diff --git a/src/box/replication.h b/src/box/replication.h
>index 4c82cd839..a8fed45e8 100644
>--- a/src/box/replication.h
>+++ b/src/box/replication.h
>@@ -439,10 +439,12 @@ replicaset_add_anon(const struct tt_uuid *replica_uuid);
>  * \param connect_quorum if this flag is set, fail unless at
>  * least replication_connect_quorum
>  * appliers have successfully connected.
>+ * \param keep_connect if this flag is set do not force a reconnect if the
>+ * old connection to the replica is fine.
>  */
> void
> replicaset_connect(struct applier **appliers, int count,
>- bool connect_quorum);
>+ bool connect_quorum, bool keep_connect);
> 
> /**
>  * Check if the current instance fell too much behind its
>diff --git a/test/replication-luatest/gh_4669_applier_reconnect_test.lua b/test/replication-luatest/gh_4669_applier_reconnect_test.lua
>new file mode 100644
>index 000000000..9c6d26036
>--- /dev/null
>+++ b/test/replication-luatest/gh_4669_applier_reconnect_test.lua
>@@ -0,0 +1,53 @@
>+local t = require('luatest')
>+local cluster = require('test.luatest_helpers.cluster')
>+local helpers = require('test.luatest_helpers')
>+
>+local g = t.group('gh-4669-applier-reconnect')
>+
>+local function check_follow_master(server)
>+ return t.assert_equals(
>+ server:eval('return box.info.replication[1].upstream.status'), 'follow')
>+end
>+
>+g.before_each(function()
>+ g.cluster = cluster:new({})
>+ g.master = g.cluster:build_server({alias = 'master'})
>+ local box_cfg = {
>+ replication = ('%s/master.iproto'):format(helpers.SOCKET_DIR),
>+ }
>+ g.replica = g.cluster:build_server({alias = 'replica', box_cfg = box_cfg})
>+ g.replica2 = g.cluster:build_server({alias = 'replica2', box_cfg = box_cfg})
>+
>+ g.cluster:add_server(g.master)
>+ g.cluster:add_server(g.replica)
>+ g.cluster:add_server(g.replica2)
>+ g.cluster:start()
>+ check_follow_master(g.replica)
>+end)
>+
>+g.after_each(function()
>+ g.cluster:stop()
>+end)
>+
>+-- Test that appliers aren't recreated upon replication reconfiguration.
>+-- Add and then remove two extra replicas to the configuration. The master
>+-- connection should stay intact.
>+g.test_applier_connection_on_reconfig = function(g)
>+ g.replica:eval(([[
>+ box.cfg{
>+ replication = {
>+ box.cfg.listen,
>+ box.cfg.replication[1],
>+ "%s/replica2.iproto",
>+ }
>+ }]]):format(helpers.SOCKET_DIR))
>+ g.replica:eval([[
>+ box.cfg{
>+ replication = {
>+ box.cfg.replication[2]
>+ }
>+ }
>+ ]])
>+ check_follow_master(g.replica)
>+ t.assert_equals(g.master:grep_log("exiting the relay loop"), nil)
>+end
>--
>2.30.1 (Apple Git-130)