fix(msg): close control connection on write failure (#5550)

* fix(msg): close control connection on write failure

* fix(msg): handle shutdown enqueue race and preserve constructor API

pkg/msg/handler_shutdown_test.go

@@ -0,0 +1,87 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package msg
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"testing"
+)
+
+// Preserve the original exported constructor function type.
+var _ func(ReadWriter) *Dispatcher = NewDispatcher
+
+func TestDispatcherEnqueueRejectsStoppedDispatcher(t *testing.T) {
+	c := newRegressionControl(t)
+	d := NewDispatcher(c)
+	d.stop()
+
+	// Model shutdown after Send's initial check: the stop and buffered send
+	// cases are both ready. A send selected here must still return EOF.
+	enqueued := false
+	for range 100 {
+		if err := d.enqueue(&ReqWorkConn{}); !errors.Is(err, io.EOF) {
+			t.Fatalf("enqueue racing with shutdown: %v, want EOF", err)
+		}
+		if len(d.sendCh) > 0 {
+			enqueued = true
+			<-d.sendCh
+		}
+	}
+	if !enqueued {
+		t.Fatal("did not exercise the ready send case")
+	}
+}
+
+func TestDispatcherAcceptsReadWriterWithoutClose(t *testing.T) {
+	t.Run("codec factory", func(t *testing.T) {
+		d := NewDispatcher(NewV1ReadWriter(&bytes.Buffer{}))
+		d.Run()
+		regressionAwait(t, d.Done(), "non-closable codec EOF")
+	})
+
+	t.Run("write failure", func(t *testing.T) {
+		c := newRegressionControl(t)
+		regressionWarm(t, c)
+		// Hide Close to model an existing caller implementing only ReadWriter.
+		d := NewDispatcher(struct{ ReadWriter }{c})
+		d.Run()
+		t.Cleanup(func() {
+			c.transport.close()
+			regressionAwait(t, d.Done(), "externally interrupted reader")
+		})
+
+		c.transport.arm()
+		if err := d.Send(&ReqWorkConn{}); err != nil {
+			t.Fatal(err)
+		}
+		if err := regressionReceiveWrite(t, c); !errors.Is(err, errRegressionWriteTimeout) {
+			t.Fatalf("injected write failure: %v", err)
+		}
+		regressionAwait(t, d.stopCh, "write failure stops sends")
+		if err := d.Send(&ReqWorkConn{}); !errors.Is(err, io.EOF) {
+			t.Fatalf("Send after write failure: %v, want EOF", err)
+		}
+		if c.closeCalls.Load() != 0 {
+			t.Fatal("dispatcher closed a connection hidden by ReadWriter")
+		}
+		select {
+		case <-d.Done():
+			t.Fatal("Done closed before the non-closable reader returned")
+		default:
+		}
+	})
+}