[Tarantool-patches] [PATCH v2 7/9] replication: introduce a new election mode: "manual"

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

When an instance is configured in "manual" election mode, it behaves as
a voter for most of the time, until `box.ctl.promote()` is called.

Once `box.ctl.promote()` is called the instance will behave as a
candidate for a full election round, e.g. until the leader is known. If
the instance wins the elections, it remains in `leader` state until the
next elections. Otherwise the instance returns to `voter` state.

Follow-up #5445
Part of #3055
---
 src/box/box.cc                         | 37 +++++++++++++++++---------
 src/box/raft.c                         |  2 ++
 src/box/raft.h                         | 17 ++++++++++++
 test/replication/election_basic.result |  4 +--
 4 files changed, 45 insertions(+), 15 deletions(-)

diff --git a/src/box/box.cc b/src/box/box.cc
index aae57ec29..b77b0a08d 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -680,17 +680,27 @@ box_check_uri(const char *source, const char *option_name)
 	}
 }
 
-static const char *
+static enum election_mode
 box_check_election_mode(void)
 {
 	const char *mode = cfg_gets("election_mode");
-	if (mode == NULL || (strcmp(mode, "off") != 0 &&
-	    strcmp(mode, "voter") != 0 && strcmp(mode, "candidate") != 0)) {
-		diag_set(ClientError, ER_CFG, "election_mode", "the value must "
-			 "be a string 'off' or 'voter' or 'candidate'");
-		return NULL;
-	}
-	return mode;
+	if (mode == NULL)
+		goto error;
+
+	if (strcmp(mode, "off") == 0)
+		return ELECTION_MODE_OFF;
+	else if (strcmp(mode, "voter") == 0)
+		return ELECTION_MODE_VOTER;
+	else if (strcmp(mode, "manual") == 0)
+		return ELECTION_MODE_MANUAL;
+	else if (strcmp(mode, "candidate") == 0)
+		return ELECTION_MODE_CANDIDATE;
+
+error:
+	diag_set(ClientError, ER_CFG, "election_mode",
+		"the value must be one of the following strings: "
+		"'off', 'voter', 'candidate', 'manual'");
+	return ELECTION_MODE_INVALID;
 }
 
 static double
@@ -1113,7 +1123,7 @@ box_check_config(void)
 	box_check_uri(cfg_gets("listen"), "listen");
 	box_check_instance_uuid(&uuid);
 	box_check_replicaset_uuid(&uuid);
-	if (box_check_election_mode() == NULL)
+	if (box_check_election_mode() == ELECTION_MODE_INVALID)
 		diag_raise();
 	if (box_check_election_timeout() < 0)
 		diag_raise();
@@ -1147,11 +1157,12 @@ box_check_config(void)
 int
 box_set_election_mode(void)
 {
-	const char *mode = box_check_election_mode();
-	if (mode == NULL)
+	enum election_mode mode = box_check_election_mode();
+	if (mode == ELECTION_MODE_INVALID)
 		return -1;
-	raft_cfg_is_candidate(box_raft(), strcmp(mode, "candidate") == 0);
-	raft_cfg_is_enabled(box_raft(), strcmp(mode, "off") != 0);
+	box_election_mode = mode;
+	raft_cfg_is_candidate(box_raft(), mode == ELECTION_MODE_CANDIDATE);
+	raft_cfg_is_enabled(box_raft(), mode != ELECTION_MODE_OFF);
 	return 0;
 }
 
diff --git a/src/box/raft.c b/src/box/raft.c
index cfd898db0..285dbe4fd 100644
--- a/src/box/raft.c
+++ b/src/box/raft.c
@@ -44,6 +44,8 @@ struct raft box_raft_global = {
 	.state = 0,
 };
 
+enum election_mode box_election_mode = ELECTION_MODE_INVALID;
+
 /**
  * A trigger executed each time the Raft state machine updates any
  * of its visible attributes.
diff --git a/src/box/raft.h b/src/box/raft.h
index 1c59f17e6..15f4e80d9 100644
--- a/src/box/raft.h
+++ b/src/box/raft.h
@@ -35,8 +35,25 @@
 extern "C" {
 #endif
 
+enum election_mode {
+	ELECTION_MODE_INVALID = -1,
+	ELECTION_MODE_OFF = 0,
+	ELECTION_MODE_VOTER = 1,
+	ELECTION_MODE_MANUAL = 2,
+	ELECTION_MODE_CANDIDATE = 3,
+};
+
 struct raft_request;
 
+/**
+ * box_election_mode - current mode of operation for raft. Some modes correspond
+ * to RAFT operation modes directly, like CANDIDATE, VOTER and OFF.
+ * There's a mode which does not map to raft operation mode directly:
+ * MANUAL. In this mode RAFT usually operates as a voter, but it may become a
+ * candidate for some period of time when user calls `box.ctl.promote()`
+ */
+extern enum election_mode box_election_mode;
+
 /** Raft state of this instance. */
 static inline struct raft *
 box_raft(void)
diff --git a/test/replication/election_basic.result b/test/replication/election_basic.result
index 4d7d33f2b..d5320b3ff 100644
--- a/test/replication/election_basic.result
+++ b/test/replication/election_basic.result
@@ -22,8 +22,8 @@ box.cfg{election_mode = 100}
  | ...
 box.cfg{election_mode = '100'}
  | ---
- | - error: 'Incorrect value for option ''election_mode'': the value must be a string
- |     ''off'' or ''voter'' or ''candidate'''
+ | - error: 'Incorrect value for option ''election_mode'': the value must be one of the
+ |     following strings: ''off'', ''voter'', ''candidate'', ''manual'''
  | ...
 box.cfg{election_timeout = -1}
  | ---
-- 
2.24.3 (Apple Git-128)