[backport 2.11] replication: update raft instance id on anon replica register

changelogs/unreleased/gh-11938-stale-election-state-after-register.md

@@ -0,0 +1,4 @@
+## bugfix/replication
+
+* Fixed the election state corruption after an anonymous replica
+  becomes non-anonymous (gh-11938).

src/box/applier.cc

@@ -2561,6 +2561,19 @@ applier_f(va_list ap)
 				 * is retrying final join.
 				 */
 				applier_register(applier, was_anon);
+				/*
+				 * It's hard to catch the exact moment when
+				 * instance id settles, for example, it could
+				 * change multiple times during register, but we
+				 * have to configure raft with it before
+				 * `applier_subscribe` starts, because we'll
+				 * start receiving raft messages there.
+				 */
+				if (was_anon) {
+					assert(instance_id != 0);
+					raft_cfg_instance_id(box_raft(),
+							     instance_id);
+				}
 			}
 			applier_subscribe(applier);
 			/*

test/replication-luatest/gh_11938_anon_replica_register_election_crash_test.lua

@@ -0,0 +1,40 @@
+local t = require('luatest')
+local server = require('luatest.server')
+local replica_set = require('luatest.replica_set')
+
+local g = t.group()
+
+g.before_each(function(cg)
+    t.tarantool.skip_if_not_debug()
+    cg.replica_set = replica_set:new{}
+    cg.master = cg.replica_set:build_and_add_server({
+        box_cfg = {
+            election_mode = 'candidate',
+        },
+        alias = 'master',
+    })
+    cg.replica = cg.replica_set:build_and_add_server({
+        box_cfg = {
+            replication = {
+                server.build_listen_uri('master', cg.replica_set.id),
+            },
+            replication_anon = true,
+            read_only = true,
+        },
+        alias = 'replica',
+    })
+end)
+
+g.after_each(function(cg)
+    cg.replica_set:drop()
+end)
+
+g.test_anon_replica_election_state_is_consistent_after_register = function(cg)
+    cg.replica_set:start()
+    cg.replica:update_box_cfg({
+        replication_anon = false,
+    })
+    t.helpers.retrying({}, function()
+        cg.replica:assert_follows_upstream(cg.master:get_instance_id())
+    end)
+end