r/vote_stm: assert we're not becoming a leader in the background

src/v/raft/vote_stm.cc

@@ -357,7 +357,7 @@ ss::future<> vote_stm::update_vote_state(ssx::semaphore_units u) {
                   term);
                 _ptr->_term = term;
                 _ptr->_voted_for = {};
-                _ptr->_vstate = consensus::vote_state::follower;
+                lose();
                 co_return;
             }
         }
@@ -379,7 +379,7 @@ ss::future<> vote_stm::update_vote_state(ssx::semaphore_units u) {
     }
     if (!_success) {
         vlog(_ctxlog.info, "[pre-vote: {}] vote failed", _prevote);
-        _ptr->_vstate = consensus::vote_state::follower;
+        lose();
         co_return;
     }
     /**
@@ -397,7 +397,7 @@ ss::future<> vote_stm::update_vote_state(ssx::semaphore_units u) {
           "[pre-vote: false] Ignoring successful vote. Node priority too low: "
           "{}",
           _ptr->_node_priority_override.value());
-        _ptr->_vstate = consensus::vote_state::follower;
+        lose();
         co_return;
     }
 
@@ -479,4 +479,13 @@ ss::future<> vote_stm::self_vote() {
     auto m = _replies.find(_ptr->self());
     m->second.set_value(reply);
 }
+
+void vote_stm::lose() {
+    vassert(
+      _ptr->_vstate != consensus::vote_state::leader
+        && _ptr->_hbeat != clock_type::time_point::max(),
+      "Became a leader outside current election");
+    _ptr->_vstate = consensus::vote_state::follower;
+}
+
 } // namespace raft

src/v/raft/vote_stm.h

@@ -120,6 +120,8 @@ class vote_stm {
 
     ss::future<> wait_for_next_reply();
 
+    void lose();
+
     friend std::ostream& operator<<(std::ostream&, const vmeta&);
 
     ss::future<election_success> do_vote();