fix(server): UDP readers exit when the server is closed

server.go

@@ -6,6 +6,7 @@ import (
 	"crypto/ed25519"
 	"encoding/binary"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"log/slog"
@@ -520,9 +521,19 @@ func (s *Server) readLoopBatches(conn *net.UDPConn) (err error, done bool) {
 	for {
 		n, rerr := pc.ReadBatch(msgs, 0)
 		if rerr != nil {
-			if opErr, ok := rerr.(*net.OpError); ok && opErr.Err.Error() == "use of closed network connection" {
+			// A closed socket, or a server being closed, ends the loop. On
+			// the Linux recvmmsg path the error is wrapped twice ("read udp
+			// ...: raw-read udp4 ...: use of closed network connection"),
+			// so comparing the inner error's text never matched there and
+			// every reader spun on its closed socket after Close.
+			if errors.Is(rerr, net.ErrClosed) {
 				return nil, true
 			}
+			select {
+			case <-s.done:
+				return nil, true
+			default:
+			}
 			slog.Debug("beacon read batch error", "err", rerr)
 			continue
 		}

zz_close_readers_test.go

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package beacon
+
+import (
+	"runtime"
+	"testing"
+	"time"
+)
+
+// TestCloseStopsReaders: Close ends every UDP reader. On Linux the
+// recvmmsg path wraps the closed-socket error twice, which the old check
+// (the inner error's exact text) never matched, so all 2×NumCPU readers
+// kept spinning on their closed sockets for the life of the process.
+func TestCloseStopsReaders(t *testing.T) {
+	runtime.GC()
+	base := runtime.NumGoroutine()
+
+	s := New()
+	go func() { _ = s.ListenAndServe("127.0.0.1:0") }()
+	select {
+	case <-s.Ready():
+	case <-time.After(5 * time.Second):
+		t.Fatal("beacon did not start")
+	}
+	time.Sleep(100 * time.Millisecond)
+	running := runtime.NumGoroutine()
+
+	if err := s.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+	deadline := time.Now().Add(5 * time.Second)
+	for {
+		n := runtime.NumGoroutine()
+		if n <= base+2 {
+			t.Logf("goroutines: base=%d running=%d after Close=%d", base, running, n)
+			return
+		}
+		if time.Now().After(deadline) {
+			buf := make([]byte, 1<<16)
+			t.Fatalf("goroutines still running after Close: base=%d running=%d now=%d\n%s",
+				base, running, n, buf[:runtime.Stack(buf, true)])
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+}