raft: make promote bump term and vote at once

box.ctl.promote() was impplemented as follows: an instance bumps the
term and marks itself a candidate, but doesn't vote for self
immediately. Instead it relies on the machinery which makes a candidate
vote for self as soon as it persists a new term.

This differs from a normal election start due to leader timeout: there
term and vote are bumped at once.

Besides, this increases probability of box.ctl.promote() resulting in
other node getting elected: if a node first broadcasts a term without a
vote, it is not considered a candidate, so other candidates might start
elections and vote for themselves.

Let's bring promote into line with automatic elections.

Closes tarantool#8497

NO_DOC=bugfix

changelogs/unreleased/gh-8497-promote-atomic.md

@@ -0,0 +1,4 @@
+## bugfix/replication
+
+* Fixed possible failure to promote the desired node by `box.ctl.promote()` on
+  a cluster with nodes configured with `election_mode = "candidate"` (gh-8497).

src/lib/raft/raft.c

@@ -1211,6 +1211,7 @@ raft_promote(struct raft *raft)
 		return;
 	raft_sm_schedule_new_term(raft, raft->volatile_term + 1);
 	raft_start_candidate(raft);
+	raft_sm_schedule_new_vote(raft, raft->self, raft->vclock);
 }
 
 void

test/replication-luatest/gh_6036_qsync_order_test.lua

@@ -187,7 +187,7 @@ g.test_promote_order = function(cg)
     cg.r1:exec(function() box.cfg{replication=""} end)
     cg.r2:exec(function()
         box.cfg{replication = {}}
-        box.error.injection.set('ERRINJ_WAL_DELAY_COUNTDOWN', 2)
+        box.error.injection.set('ERRINJ_WAL_DELAY_COUNTDOWN', 1)
         require('fiber').create(function() box.ctl.promote() end)
     end)
     t.helpers.retrying({}, function()

test/replication-luatest/gh_8497_atomic_promote_test.lua

@@ -0,0 +1,56 @@
+local t = require('luatest')
+local server = require('luatest.server')
+local replica_set = require('luatest.replica_set')
+
+local g = t.group('gh-8497-atomic-promote')
+
+g.before_each(function(cg)
+    t.tarantool.skip_if_not_debug()
+    cg.replica_set = replica_set:new({})
+    cg.box_cfg = {
+        replication_timeout = 0.1,
+        replication = {
+            server.build_listen_uri('server1', cg.replica_set.id),
+            server.build_listen_uri('server2', cg.replica_set.id),
+        },
+    }
+    cg.box_cfg.election_mode = 'candidate'
+    cg.server1 = cg.replica_set:build_and_add_server{
+        alias = 'server1',
+        box_cfg = cg.box_cfg,
+    }
+    cg.box_cfg.election_mode = 'voter'
+    cg.server2 =cg.replica_set:build_and_add_server{
+        alias = 'server2',
+        box_cfg = cg.box_cfg,
+    }
+    cg.replica_set:start()
+    cg.replica_set:wait_for_fullmesh()
+    cg.server1:wait_for_election_leader()
+end)
+
+g.after_each(function(cg)
+    cg.replica_set:drop()
+end)
+
+g.test_election_promote_finishes_in_one_term = function(cg)
+    cg.server2:update_box_cfg{election_mode = 'candidate'}
+    local term = cg.server1:get_election_term()
+    t.assert_equals(term, cg.server2:get_election_term(),
+                    'The cluster is stable')
+    local ok, err = cg.server2:exec(function()
+        local fiber = require('fiber')
+        box.error.injection.set('ERRINJ_WAL_DELAY_COUNTDOWN', 1)
+        local fib = fiber.new(box.ctl.promote)
+        fib:set_joinable(true)
+        fiber.sleep(2 * box.cfg.replication_timeout)
+        box.error.injection.set('ERRINJ_WAL_DELAY', false)
+        return fib:join()
+    end)
+    t.assert_equals({ok, err}, {true, nil}, 'No error in promote')
+    cg.server2:wait_for_election_leader()
+    t.assert_equals(term + 1, cg.server1:get_election_term(),
+                    'The term is bumped once')
+    t.assert_equals(term + 1, cg.server2:get_election_term(),
+                    'The term is bumped once')
+end

test/replication/election_basic.result

@@ -403,32 +403,23 @@ box.ctl.promote()
  | ---
  | ...
 
-test_run:wait_cond(function() return #election_tbl == 9 end)
+test_run:wait_cond(function() return #election_tbl == 8 end)
  | ---
  | - true
  | ...
-assert(election_tbl[7].state == 'follower')
+assert(election_tbl[7].state == 'candidate')
  | ---
  | - true
  | ...
 assert(election_tbl[7].term == election_tbl[6].term + 1)
  | ---
  | - true
  | ...
--- Vote is visible here already, but it is volatile.
 assert(election_tbl[7].vote == 1)
  | ---
  | - true
  | ...
-assert(election_tbl[8].state == 'candidate')
- | ---
- | - true
- | ...
-assert(election_tbl[8].vote == 1)
- | ---
- | - true
- | ...
-assert(election_tbl[9].state == 'leader')
+assert(election_tbl[8].state == 'leader')
  | ---
  | - true
  | ...

test/replication/election_basic.test.lua

@@ -165,14 +165,11 @@ assert(election_tbl[6].state == 'follower')
 
 box.ctl.promote()
 
-test_run:wait_cond(function() return #election_tbl == 9 end)
-assert(election_tbl[7].state == 'follower')
+test_run:wait_cond(function() return #election_tbl == 8 end)
+assert(election_tbl[7].state == 'candidate')
 assert(election_tbl[7].term == election_tbl[6].term + 1)
--- Vote is visible here already, but it is volatile.
 assert(election_tbl[7].vote == 1)
-assert(election_tbl[8].state == 'candidate')
-assert(election_tbl[8].vote == 1)
-assert(election_tbl[9].state == 'leader')
+assert(election_tbl[8].state == 'leader')
 
 test_run:cmd('stop server replica')
 test_run:cmd('delete server replica')

test/unit/raft.c

@@ -2348,7 +2348,7 @@ raft_test_resign(void)
 		1 /* Vote. */,
 		2 /* Volatile term. */,
 		1 /* Volatile vote. */,
-		"{0: 2}" /* Vclock. */
+		"{0: 1}" /* Vclock. */
 	), "became leader");
 
 	raft_node_resign(&node);
@@ -2361,7 +2361,7 @@ raft_test_resign(void)
 		1 /* Vote. */,
 		2 /* Volatile term. */,
 		1 /* Volatile vote. */,
-		"{0: 2}" /* Vclock. */
+		"{0: 1}" /* Vclock. */
 	), "resigned from leader state");
 
 	raft_node_destroy(&node);