Fix live status on CS2: accept RCON output tagged with the end packet's id

CS2 tags a command's output with the id of the empty end packet when both
arrive together, and replies to the end packet with a bare \x00\x01. The
client dropped the output as stale, so the panel reported WebPanelBridge as
missing. Also log each new kind of poll failure once, and recovery.

internal/live/live.go

@@ -151,14 +151,23 @@ func (p *Poller) Refresh() Status {
 	st.Updated = time.Now()
 	if err != nil {
 		st = Status{Error: err.Error(), Updated: st.Updated}
+		// Log each new kind of failure once, so a wrong password or missing plugin shows up in the
+		// log without repeating every few seconds. ErrDown only repeats the previous failure.
 		p.mu.RLock()
-		was := p.current.Online || p.current.Updated.IsZero()
+		changed := p.current.Online || p.current.Updated.IsZero() || p.current.Error != st.Error
 		p.mu.RUnlock()
-		if was && !errors.Is(err, rcon.ErrDown) {
+		if changed && !errors.Is(err, rcon.ErrDown) {
 			slog.Warn("game server status unavailable", "err", err)
 		}
 	}
 	p.mu.Lock()
+	if st.Online && !p.current.Online && !p.current.Updated.IsZero() {
+		slog.Info("game server status available again")
+	}
+	// While backing off, keep the underlying error rather than "retrying shortly".
+	if errors.Is(err, rcon.ErrDown) && p.current.Error != "" {
+		st.Error = p.current.Error
+	}
 	p.current = st
 	p.mu.Unlock()
 	return st

internal/rcon/rcon.go

@@ -2,8 +2,12 @@
 //
 // One connection is kept open and shared; commands are serialised with a mutex. A response can
 // span several packets, so every command is followed by an empty SERVERDATA_RESPONSE_VALUE packet:
-// the server answers packets in order, so seeing that packet's echo means the command's output is
+// the server answers packets in order, so its reply to that packet means the command's output is
-// complete. If the server never echoes it, reading stops after a short idle timeout instead.
+// complete. If the server never replies to it, reading stops after a short idle timeout instead.
+//
+// CS2 differs from older Source servers in two ways this handles: when both packets arrive
+// together it tags the command's output with the end packet's id, and it answers the end packet
+// with a single "\x00\x01" body rather than an empty packet followed by that one.
 package rcon
 
 import (
@@ -25,6 +29,10 @@ const (
 	typeAuth          = 3
 
 	maxPacket = 4096 + 14
+
+	// The body a server sends in reply to an empty SERVERDATA_RESPONSE_VALUE packet (0x00 0x01
+	// 0x00 0x00, less the trailing nulls read() trims).
+	endMarker = "\x00\x01"
 )
 
 // ErrAuth means the server rejected the RCON password.
@@ -112,15 +120,20 @@ func (c *Client) exec(cmd string) (string, error) {
 			}
 			return "", err
 		}
-		switch pid {
+		if pid != id && pid != endID {
-		case id:
+			continue // left over from an earlier command
-			out.WriteString(body)
+		}
-			got = true
+		switch {
-		case endID:
+		case pid == endID && body == endMarker:
-			// Some servers answer the empty packet with a second, odd-looking packet. Drain it.
+			return out.String(), nil
+		case pid == endID && body == "":
+			// Older servers echo the empty packet, then send endMarker. Drain it.
 			_ = c.conn.SetReadDeadline(time.Now().Add(50 * time.Millisecond))
 			_, _, _, _ = c.read()
 			return out.String(), nil
+		default:
+			out.WriteString(body)
+			got = true
 		}
 	}
 }

internal/rcon/rcon_test.go

@@ -30,7 +30,7 @@ func TestArg(t *testing.T) {
 
 // fakeServer answers auth, then replies to each command with its text split over two packets,
 // and mirrors the empty end-marker packet the way srcds does.
-func fakeServer(t *testing.T, password string) string {
+func fakeServer(t *testing.T, password string, cs2 bool) string {
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
 	if err != nil {
 		t.Fatal(err)
@@ -42,7 +42,7 @@ func fakeServer(t *testing.T, password string) string {
 			if err != nil {
 				return
 			}
-			go serve(c, password)
+			go serve(c, password, cs2)
 		}
 	}()
 	return ln.Addr().String()
@@ -69,8 +69,9 @@ func writePkt(w io.Writer, id, typ int32, body string) {
 	w.Write(buf)
 }
 
-func serve(c net.Conn, password string) {
+func serve(c net.Conn, password string, cs2 bool) {
 	defer c.Close()
+	pending := ""
 	for {
 		id, typ, body, err := readPkt(c)
 		if err != nil {
@@ -85,10 +86,20 @@ func serve(c net.Conn, password string) {
 				writePkt(c, -1, typeAuthResponse, "")
 			}
 		case typeExecCommand:
+			if cs2 {
+				// CS2 answers after reading the end packet too, tagging the output with its id.
+				pending = "echo:" + body + "\n"
+				continue
+			}
 			out := "echo:" + body + "\n" + strings.Repeat("x", 5000)
 			writePkt(c, id, typeResponseValue, out[:4000])
 			writePkt(c, id, typeResponseValue, out[4000:])
 		case typeResponseValue:
+			if cs2 {
+				writePkt(c, id, typeResponseValue, pending)
+				writePkt(c, id, typeResponseValue, "\x00\x01")
+				continue
+			}
 			writePkt(c, id, typeResponseValue, "")
 			writePkt(c, id, typeResponseValue, "\x00\x01\x00\x00")
 		}
@@ -96,7 +107,7 @@ func serve(c net.Conn, password string) {
 }
 
 func TestExecMultiPacket(t *testing.T) {
-	addr := fakeServer(t, "pw")
+	addr := fakeServer(t, "pw", false)
 	c := New(addr, "pw")
 	defer c.Close()
 	for i := 0; i < 3; i++ {
@@ -111,7 +122,7 @@ func TestExecMultiPacket(t *testing.T) {
 }
 
 func TestWrongPassword(t *testing.T) {
-	addr := fakeServer(t, "pw")
+	addr := fakeServer(t, "pw", false)
 	c := New(addr, "nope")
 	if _, err := c.Exec("status"); err != ErrAuth {
 		t.Fatalf("got %v, want ErrAuth", err)
@@ -130,3 +141,18 @@ func TestBackoff(t *testing.T) {
 		t.Fatalf("second call should fail fast with ErrDown, got %v", err)
 	}
 }
+
+// CS2 tags output with the end packet's id and ends with a bare "\x00\x01" (seen live on fr04).
+func TestExecCS2Ids(t *testing.T) {
+	c := New(fakeServer(t, "pw", true), "pw")
+	defer c.Close()
+	for i := 0; i < 3; i++ {
+		out, err := c.Exec("css_webpanel_status")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if out != "echo:css_webpanel_status\n" {
+			t.Fatalf("got %q", out)
+		}
+	}
+}