raft: introduce on_update trigger

Raft state machine now has a trigger invoked each time when any of
the visible Raft attributes is changed: state, term, vote.

The trigger is needed to commit synchronous transactions of an old
leader, when a new leader is elected. This is done via a trigger
so as not to depend on box in raft code too much. That would make
it harder to extract it into a new module later.

The trigger is executed in the Raft worker fiber, so as not to
stop the state machine transitions anywhere, which currently don't
contain a single yield. And the synchronous transaction queue
clearance requires a yield, to write CONFIRM and ROLLBACK records
to WAL.

Part of #5339

src/box/raft.c

@@ -643,6 +643,7 @@ raft_worker_handle_broadcast(void)
 	}
 	replicaset_foreach(replica)
 		relay_push_raft(replica->relay, &req);
+	trigger_run(&raft.on_update, NULL);
 	raft.is_broadcast_scheduled = false;
 }
 
@@ -903,6 +904,12 @@ raft_serialize_for_disk(struct raft_request *req)
 	req->vote = raft.vote;
 }
 
+void
+raft_on_update(struct trigger *trigger)
+{
+	trigger_add(&raft.on_update, trigger);
+}
+
 void
 raft_cfg_is_enabled(bool is_enabled)
 {
@@ -1040,4 +1047,5 @@ void
 raft_init(void)
 {
 	ev_timer_init(&raft.timer, raft_sm_schedule_new_election_cb, 0, 0);
+	rlist_create(&raft.on_update);
 }

src/box/raft.h

@@ -32,6 +32,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include "tarantool_ev.h"
+#include "trigger.h"
 
 #if defined(__cplusplus)
 extern "C" {
@@ -150,6 +151,11 @@ struct raft {
 	struct fiber *worker;
 	/** Configured election timeout in seconds. */
 	double election_timeout;
+	/**
+	 * Trigger invoked each time any of the Raft node visible attributes are
+	 * changed.
+	 */
+	struct rlist on_update;
 };
 
 extern struct raft raft;
@@ -251,6 +257,13 @@ raft_serialize_for_network(struct raft_request *req, struct vclock *vclock);
 void
 raft_serialize_for_disk(struct raft_request *req);
 
+/**
+ * Add a trigger invoked each time any of the Raft node visible attributes are
+ * changed.
+ */
+void
+raft_on_update(struct trigger *trigger);
+
 /** Initialize Raft global data structures. */
 void
 raft_init(void);