[Tarantool-patches] [PATCH v2 4/5] election: during bootstrap prefer candidates

[Vladislav Shpilevoy via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

During cluster bootstrap the boot master election algorithm didn't
take into account election modes of the instances. It could be
that all nodes have box.cfg.read_only = false, none is booted,
all are read-only now. Then the node with the smallest UUID was
chosen even if it was a box.cfg.election_mode='voter' node.

It could neither boot nor register other nodes and the cluster
couldn't start.

The patch makes the boot master election prefer the instances
which can become a Raft leader. If all the other parameters didn't
help.

Part #6018
---
 src/box/replication.cc                        | 11 ++-
 .../gh-6018-election-boot-voter.result        | 70 +++++++++++++++++++
 .../gh-6018-election-boot-voter.test.lua      | 38 ++++++++++
 test/replication/gh-6018-master.lua           | 17 +++++
 test/replication/gh-6018-replica.lua          | 15 ++++
 test/replication/suite.cfg                    |  1 +
 6 files changed, 150 insertions(+), 2 deletions(-)
 create mode 100644 test/replication/gh-6018-election-boot-voter.result
 create mode 100644 test/replication/gh-6018-election-boot-voter.test.lua
 create mode 100644 test/replication/gh-6018-master.lua
 create mode 100644 test/replication/gh-6018-replica.lua

diff --git a/src/box/replication.cc b/src/box/replication.cc
index a0b3e0186..45ad03dfd 100644
--- a/src/box/replication.cc
+++ b/src/box/replication.cc
@@ -978,12 +978,19 @@ replicaset_find_join_master(void)
 		 * config is stronger because if it is configured as read-only,
 		 * it is in read-only state for sure, until the config is
 		 * changed.
+		 *
+		 * In a cluster with leader election enabled all instances might
+		 * look equal by the scores above. Then must prefer the ones
+		 * which can be elected as a leader, because only they would be
+		 * able to boot themselves and register the others.
 		 */
 		if (ballot->is_booted)
-			score += 10;
+			score += 1000;
 		if (!ballot->is_ro_cfg)
-			score += 5;
+			score += 100;
 		if (!ballot->is_ro)
+			score += 10;
+		if (ballot->can_lead)
 			score += 1;
 		if (leader_score < score)
 			goto elect;
diff --git a/test/replication/gh-6018-election-boot-voter.result b/test/replication/gh-6018-election-boot-voter.result
new file mode 100644
index 000000000..6b05f0825
--- /dev/null
+++ b/test/replication/gh-6018-election-boot-voter.result
@@ -0,0 +1,70 @@
+-- test-run result file version 2
+--
+-- gh-6018: in an auto-election cluster nodes with voter state could be selected
+-- as bootstrap leaders. They should not, because a voter can't be ever writable
+-- and it can neither boot itself nor register other nodes.
+--
+test_run = require('test_run').new()
+ | ---
+ | ...
+
+function boot_with_master_election_mode(mode)                                   \
+        test_run:cmd('create server master with '..                             \
+                     'script="replication/gh-6018-master.lua"')                 \
+        test_run:cmd('start server master with wait=False, args="'..mode..'"')  \
+        test_run:cmd('create server replica with '..                            \
+                     'script="replication/gh-6018-replica.lua"')                \
+        test_run:cmd('start server replica')                                    \
+end
+ | ---
+ | ...
+
+function stop_cluster()                                                         \
+    test_run:cmd('stop server replica')                                         \
+    test_run:cmd('stop server master')                                          \
+    test_run:cmd('delete server replica')                                       \
+    test_run:cmd('delete server master')                                        \
+end
+ | ---
+ | ...
+
+--
+-- Candidate leader.
+--
+boot_with_master_election_mode('candidate')
+ | ---
+ | ...
+
+test_run:switch('master')
+ | ---
+ | - true
+ | ...
+test_run:wait_cond(function() return not box.info.ro end)
+ | ---
+ | - true
+ | ...
+assert(box.info.election.state == 'leader')
+ | ---
+ | - true
+ | ...
+
+test_run:switch('replica')
+ | ---
+ | - true
+ | ...
+assert(box.info.ro)
+ | ---
+ | - true
+ | ...
+assert(box.info.election.state == 'follower')
+ | ---
+ | - true
+ | ...
+
+test_run:switch('default')
+ | ---
+ | - true
+ | ...
+stop_cluster()
+ | ---
+ | ...
diff --git a/test/replication/gh-6018-election-boot-voter.test.lua b/test/replication/gh-6018-election-boot-voter.test.lua
new file mode 100644
index 000000000..7222beb19
--- /dev/null
+++ b/test/replication/gh-6018-election-boot-voter.test.lua
@@ -0,0 +1,38 @@
+--
+-- gh-6018: in an auto-election cluster nodes with voter state could be selected
+-- as bootstrap leaders. They should not, because a voter can't be ever writable
+-- and it can neither boot itself nor register other nodes.
+--
+test_run = require('test_run').new()
+
+function boot_with_master_election_mode(mode)                                   \
+        test_run:cmd('create server master with '..                             \
+                     'script="replication/gh-6018-master.lua"')                 \
+        test_run:cmd('start server master with wait=False, args="'..mode..'"')  \
+        test_run:cmd('create server replica with '..                            \
+                     'script="replication/gh-6018-replica.lua"')                \
+        test_run:cmd('start server replica')                                    \
+end
+
+function stop_cluster()                                                         \
+    test_run:cmd('stop server replica')                                         \
+    test_run:cmd('stop server master')                                          \
+    test_run:cmd('delete server replica')                                       \
+    test_run:cmd('delete server master')                                        \
+end
+
+--
+-- Candidate leader.
+--
+boot_with_master_election_mode('candidate')
+
+test_run:switch('master')
+test_run:wait_cond(function() return not box.info.ro end)
+assert(box.info.election.state == 'leader')
+
+test_run:switch('replica')
+assert(box.info.ro)
+assert(box.info.election.state == 'follower')
+
+test_run:switch('default')
+stop_cluster()
diff --git a/test/replication/gh-6018-master.lua b/test/replication/gh-6018-master.lua
new file mode 100644
index 000000000..1192204ff
--- /dev/null
+++ b/test/replication/gh-6018-master.lua
@@ -0,0 +1,17 @@
+#!/usr/bin/env tarantool
+
+require('console').listen(os.getenv('ADMIN'))
+
+box.cfg({
+    listen = 'unix/:./gh-6018-master.sock',
+    replication = {
+	'unix/:./gh-6018-master.sock',
+	'unix/:./gh-6018-replica.sock',
+    },
+    election_mode = arg[1],
+    instance_uuid = 'cbf06940-0790-498b-948d-042b62cf3d29',
+    replication_timeout = 0.1,
+})
+
+box.ctl.wait_rw()
+box.schema.user.grant('guest', 'super')
diff --git a/test/replication/gh-6018-replica.lua b/test/replication/gh-6018-replica.lua
new file mode 100644
index 000000000..71e669141
--- /dev/null
+++ b/test/replication/gh-6018-replica.lua
@@ -0,0 +1,15 @@
+#!/usr/bin/env tarantool
+
+require('console').listen(os.getenv('ADMIN'))
+
+box.cfg({
+    listen = 'unix/:./gh-6018-replica.sock',
+    replication = {
+	'unix/:./gh-6018-master.sock',
+	'unix/:./gh-6018-replica.sock',
+    },
+    election_mode = 'voter',
+    -- Smaller than master UUID.
+    instance_uuid = 'cbf06940-0790-498b-948d-042b62cf3d28',
+    replication_timeout = 0.1,
+})
diff --git a/test/replication/suite.cfg b/test/replication/suite.cfg
index 69f2f3511..2bfc3b845 100644
--- a/test/replication/suite.cfg
+++ b/test/replication/suite.cfg
@@ -45,6 +45,7 @@
     "gh-5536-wal-limit.test.lua": {},
     "gh-5566-final-join-synchro.test.lua": {},
     "gh-5613-bootstrap-prefer-booted.test.lua": {},
+    "gh-6018-election-boot-voter.test.lua": {},
     "gh-6027-applier-error-show.test.lua": {},
     "gh-6032-promote-wal-write.test.lua": {},
     "gh-6057-qsync-confirm-async-no-wal.test.lua": {},
-- 
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