[Tarantool-patches] [PATCH v4 09/12] raft: introduce raft_start/stop_candidate

[Serge Petrenko via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Extract raft_start_candidate and raft_stop_candidate functions from
raft_cfg_is_candidate.

These functions will be used in manual elections.

Prerequisite #3055
---
 src/lib/raft/raft.c   | 83 ++++++++++++++++++++++++++++---------------
 src/lib/raft/raft.h   | 13 +++++++
 test/unit/raft.c      | 33 +++++++++++++++--
 test/unit/raft.result | 10 +++++-
 4 files changed, 108 insertions(+), 31 deletions(-)

diff --git a/src/lib/raft/raft.c b/src/lib/raft/raft.c
index e9ce8cade..b21693642 100644
--- a/src/lib/raft/raft.c
+++ b/src/lib/raft/raft.c
@@ -848,38 +848,65 @@ raft_cfg_is_enabled(struct raft *raft, bool is_enabled)
 void
 raft_cfg_is_candidate(struct raft *raft, bool is_candidate)
 {
-	bool old_is_candidate = raft->is_candidate;
 	raft->is_cfg_candidate = is_candidate;
-	raft->is_candidate = is_candidate && raft->is_enabled;
-	if (raft->is_candidate == old_is_candidate)
-		return;
+	is_candidate = is_candidate && raft->is_enabled;
+	if (is_candidate)
+		raft_start_candidate(raft);
+	else
+		raft_stop_candidate(raft, true);
+}
 
-	if (raft->is_candidate) {
-		assert(raft->state == RAFT_STATE_FOLLOWER);
-		if (raft->is_write_in_progress) {
-			/*
-			 * If there is an on-going WAL write, it means there was
-			 * some node who sent newer data to this node. So it is
-			 * probably a better candidate. Anyway can't do anything
-			 * until the new state is fully persisted.
-			 */
-		} else if (raft->leader != 0) {
-			raft_sm_wait_leader_dead(raft);
-		} else {
-			raft_sm_wait_leader_found(raft);
-		}
+void
+raft_start_candidate(struct raft *raft)
+{
+	if (raft->is_candidate)
+		return;
+	raft->is_candidate = true;
+	assert(raft->state != RAFT_STATE_CANDIDATE);
+	/*
+	 * May still be the leader after raft_stop_candidate
+	 * with demote = false.
+	 */
+	if (raft->state == RAFT_STATE_LEADER)
+		return;
+	if (raft->is_write_in_progress) {
+		/*
+		 * If there is an on-going WAL write, it means there was
+		 * some node who sent newer data to this node. So it is
+		 * probably a better candidate. Anyway can't do anything
+		 * until the new state is fully persisted.
+		 */
+	} else if (raft->leader != 0) {
+		raft_sm_wait_leader_dead(raft);
 	} else {
-		if (raft->state != RAFT_STATE_LEADER) {
-			/* Do not wait for anything while being a voter. */
-			raft_ev_timer_stop(raft_loop(), &raft->timer);
-		}
-		if (raft->state != RAFT_STATE_FOLLOWER) {
-			if (raft->state == RAFT_STATE_LEADER)
-				raft->leader = 0;
-			raft->state = RAFT_STATE_FOLLOWER;
-			/* State is visible and changed - broadcast. */
-			raft_schedule_broadcast(raft);
+		raft_sm_wait_leader_found(raft);
+	}
+}
+
+void
+raft_stop_candidate(struct raft *raft, bool demote)
+{
+	if (!raft->is_candidate)
+		return;
+	raft->is_candidate = false;
+	if (raft->state != RAFT_STATE_LEADER) {
+		/* Do not wait for anything while being a voter. */
+		raft_ev_timer_stop(raft_loop(), &raft->timer);
+	}
+	if (raft->state != RAFT_STATE_FOLLOWER) {
+		if (raft->state == RAFT_STATE_LEADER) {
+			if (!demote) {
+				/*
+				 * Remain leader until someone
+				 * triggers new elections.
+				 */
+				return;
+			}
+			raft->leader = 0;
 		}
+		raft->state = RAFT_STATE_FOLLOWER;
+		/* State is visible and changed - broadcast. */
+		raft_schedule_broadcast(raft);
 	}
 }
 
diff --git a/src/lib/raft/raft.h b/src/lib/raft/raft.h
index a5f7e08d9..69dec63c6 100644
--- a/src/lib/raft/raft.h
+++ b/src/lib/raft/raft.h
@@ -327,6 +327,19 @@ raft_cfg_is_enabled(struct raft *raft, bool is_enabled);
 void
 raft_cfg_is_candidate(struct raft *raft, bool is_candidate);
 
+/**
+ * Make the instance a candidate.
+ */
+void
+raft_start_candidate(struct raft *raft);
+
+/**
+ * Make the instance stop taking part in new elections.
+ * @param demote whether to stop being a leader immediately or not.
+ */
+void
+raft_stop_candidate(struct  raft *raft, bool demote);
+
 /** Configure Raft leader election timeout. */
 void
 raft_cfg_election_timeout(struct raft *raft, double timeout);
diff --git a/test/unit/raft.c b/test/unit/raft.c
index 0306cefcd..575886932 100644
--- a/test/unit/raft.c
+++ b/test/unit/raft.c
@@ -1296,15 +1296,43 @@ raft_test_term_filter(void)
 	ok(!raft_is_node_outdated(&node.raft, 3), "node doesn't become "
 						  "outdated");
 
-
 	raft_node_destroy(&node);
 	raft_finish_test();
 }
 
+static void
+raft_test_start_stop_candidate(void)
+{
+	raft_start_test(4);
+	struct raft_node node;
+	raft_node_create(&node);
+
+	raft_node_cfg_is_candidate(&node, false);
+	raft_node_cfg_election_quorum(&node, 1);
+
+	raft_start_candidate(&node.raft);
+	raft_run_next_event();
+	is(node.raft.state, RAFT_STATE_LEADER, "became leader after "
+					       "start_candidate");
+	raft_stop_candidate(&node.raft, false);
+	raft_run_for(node.cfg_death_timeout);
+	is(node.raft.state, RAFT_STATE_LEADER, "remain leader after "
+					       "stop_candidate");
+
+	is(raft_node_send_vote_request(&node,
+		3 /* Term. */,
+		"{}" /* Vclock. */,
+		2 /* Source. */
+	), 0, "vote request from 2");
+	is(node.raft.state, RAFT_STATE_FOLLOWER, "demote once new election "
+						 "starts");
+	raft_finish_test();
+}
+
 static int
 main_f(va_list ap)
 {
-	raft_start_test(14);
+	raft_start_test(15);
 
 	(void) ap;
 	fakeev_init();
@@ -1323,6 +1351,7 @@ main_f(va_list ap)
 	raft_test_enable_disable();
 	raft_test_too_long_wal_write();
 	raft_test_term_filter();
+	raft_test_start_stop_candidate();
 
 	fakeev_free();
 
diff --git a/test/unit/raft.result b/test/unit/raft.result
index ecb962e42..bb799936b 100644
--- a/test/unit/raft.result
+++ b/test/unit/raft.result
@@ -1,5 +1,5 @@
 	*** main_f ***
-1..14
+1..15
 	*** raft_test_leader_election ***
     1..24
     ok 1 - 1 pending message at start
@@ -233,4 +233,12 @@ ok 13 - subtests
     ok 9 - node doesn't become outdated
 ok 14 - subtests
 	*** raft_test_term_filter: done ***
+	*** raft_test_start_stop_candidate ***
+    1..4
+    ok 1 - became leader after start_candidate
+    ok 2 - remain leader after stop_candidate
+    ok 3 - vote request from 2
+    ok 4 - demote once new election starts
+ok 15 - subtests
+	*** raft_test_start_stop_candidate: done ***
 	*** main_f: done ***
-- 
2.24.3 (Apple Git-128)