Added: reload server config by sending it a message (#4307)

 - [ ] Link to issue, e.g. `Resolves #NNN`
 - [ ] Documentation added (if applicable)
 - [x] Tests added
- [x] Branch rebased on top of current main (`git pull --rebase origin
main`)
- [x] Changes squashed to a single commit (described
[here](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html))
 - [x] Build is green in Travis CI
- [ ] You have certified that the contribution is your original work and
that you license the work to the project under the [Apache 2
license](https://github.com/nats-io/nats-server/blob/main/LICENSE)

Resolves #

### Changes proposed in this pull request:

- Reload server's configuration by sending a message to
`$SYS.REQ.SERVER.{server-id}.RELOAD`

server/events.go

@@ -60,6 +60,7 @@ const (
 	serverDirectReqSubj       = "$SYS.REQ.SERVER.%s.%s"
 	serverPingReqSubj         = "$SYS.REQ.SERVER.PING.%s"
 	serverStatsPingReqSubj    = "$SYS.REQ.SERVER.PING"             // use $SYS.REQ.SERVER.PING.STATSZ instead
+	serverReloadReqSubj       = "$SYS.REQ.SERVER.%s.RELOAD"        // with server ID
 	leafNodeConnectEventSubj  = "$SYS.ACCOUNT.%s.LEAFNODE.CONNECT" // for internal use only
 	remoteLatencyEventSubj    = "$SYS.LATENCY.M2.%s"
 	inboxRespSubj             = "$SYS._INBOX.%s.%s"
@@ -1187,6 +1188,12 @@ func (s *Server) initEventTracking() {
 	if _, err := s.sysSubscribeInternal(accSubsSubj, s.noInlineCallback(s.debugSubscribers)); err != nil {
 		s.Errorf("Error setting up internal debug service for subscribers: %v", err)
 	}
+
+	// Listen for requests to reload the server configuration.
+	subject = fmt.Sprintf(serverReloadReqSubj, s.info.ID)
+	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.reloadConfig)); err != nil {
+		s.Errorf("Error setting up server reload handler: %v", err)
+	}
 }
 
 // UserInfo returns basic information to a user about bound account and user permissions.
@@ -2659,6 +2666,18 @@ func (s *Server) nsubsRequest(sub *subscription, c *client, _ *Account, subject,
 	s.sendInternalMsgLocked(reply, _EMPTY_, nil, nsubs)
 }
 
+func (s *Server) reloadConfig(sub *subscription, c *client, _ *Account, subject, reply string, hdr, msg []byte) {
+	if !s.eventsRunning() {
+		return
+	}
+
+	optz := &EventFilterOptions{}
+	s.zReq(c, reply, hdr, msg, optz, optz, func() (interface{}, error) {
+		// Reload the server config, as requested.
+		return nil, s.Reload()
+	})
+}
+
 // Helper to grab account name for a client.
 func accForClient(c *client) string {
 	if c.acc != nil {

server/events_test.go

@@ -1666,7 +1666,7 @@ func TestSystemAccountWithGateways(t *testing.T) {
 
 	// If this tests fails with wrong number after 10 seconds we may have
 	// added a new inititial subscription for the eventing system.
-	checkExpectedSubs(t, 52, sa)
+	checkExpectedSubs(t, 53, sa)
 
 	// Create a client on B and see if we receive the event
 	urlb := fmt.Sprintf("nats://%s:%d", ob.Host, ob.Port)
@@ -2535,6 +2535,81 @@ func TestServerEventsStatszSingleServer(t *testing.T) {
 	checkSubsPending(t, sub, 1)
 }
 
+func TestServerEventsReload(t *testing.T) {
+	conf := createConfFile(t, []byte(`
+		listen: "127.0.0.1:-1"
+		accounts: {
+			$SYS { users [{user: "admin", password: "p1d"}]}
+			test { users [{user: "foo", password: "bar"}]}
+		}
+		ping_interval: "100ms"
+	`))
+	opts := LoadConfig(conf)
+	s := RunServer(opts)
+	defer s.Shutdown()
+	subject := fmt.Sprintf(serverReloadReqSubj, s.info.ID)
+
+	// Connect as a test user and make sure the reload endpoint is not
+	// accessible.
+	ncTest, _ := jsClientConnect(t, s, nats.UserInfo("foo", "bar"))
+	defer ncTest.Close()
+	testReply := ncTest.NewRespInbox()
+	sub, err := ncTest.SubscribeSync(testReply)
+	require_NoError(t, err)
+	err = ncTest.PublishRequest(subject, testReply, nil)
+	require_NoError(t, err)
+	_, err = sub.NextMsg(time.Second)
+	require_Error(t, err)
+
+	require_True(t, s.getOpts().PingInterval == 100*time.Millisecond)
+
+	// Connect as a system user.
+	nc, _ := jsClientConnect(t, s, nats.UserInfo("admin", "p1d"))
+	defer nc.Close()
+
+	// rewrite the config file with a different ping interval
+	err = os.WriteFile(conf, []byte(`
+		listen: "127.0.0.1:-1"
+		accounts: {
+			$SYS { users [{user: "admin", password: "p1d"}]}
+			test { users [{user: "foo", password: "bar"}]}
+		}
+		ping_interval: "200ms"
+	`), 0666)
+	require_NoError(t, err)
+
+	msg, err := nc.Request(subject, nil, time.Second)
+	require_NoError(t, err)
+
+	var apiResp = ServerAPIResponse{}
+	err = json.Unmarshal(msg.Data, &apiResp)
+	require_NoError(t, err)
+
+	require_True(t, apiResp.Data == nil)
+	require_True(t, apiResp.Error == nil)
+
+	// See that the ping interval has changed.
+	require_True(t, s.getOpts().PingInterval == 200*time.Millisecond)
+
+	// rewrite the config file with a different ping interval
+	err = os.WriteFile(conf, []byte(`garbage and nonsense`), 0666)
+	require_NoError(t, err)
+
+	// Request the server to reload and wait for the response.
+	msg, err = nc.Request(subject, nil, time.Second)
+	require_NoError(t, err)
+
+	apiResp = ServerAPIResponse{}
+	err = json.Unmarshal(msg.Data, &apiResp)
+	require_NoError(t, err)
+
+	require_True(t, apiResp.Data == nil)
+	require_Error(t, apiResp.Error, fmt.Errorf("Parse error on line 1: 'Expected a top-level value to end with a new line, comment or EOF, but got 'n' instead.'"))
+
+	// See that the ping interval has not changed.
+	require_True(t, s.getOpts().PingInterval == 200*time.Millisecond)
+}
+
 func Benchmark_GetHash(b *testing.B) {
 	b.StopTimer()
 	// Get 100 random names

server/monitor_test.go

@@ -3945,7 +3945,7 @@ func TestMonitorAccountz(t *testing.T) {
 	body = string(readBody(t, fmt.Sprintf("http://127.0.0.1:%d%s?acc=$SYS", s.MonitorAddr().Port, AccountzPath)))
 	require_Contains(t, body, `"account_detail": {`)
 	require_Contains(t, body, `"account_name": "$SYS",`)
-	require_Contains(t, body, `"subscriptions": 46,`)
+	require_Contains(t, body, `"subscriptions": 47,`)
 	require_Contains(t, body, `"is_system": true,`)
 	require_Contains(t, body, `"system_account": "$SYS"`)