net/memnet: allow listener address reuse (#17342)

Listen address reuse is allowed as soon as the previous listener is
closed. There is no attempt made to emulate more complex address reuse
logic.

Updates tailscale/corp#28078

Change-Id: I56be1c4848e7b3f9fc97fd4ef13a2de9dcfab0f2

Signed-off-by: Brian Palmer <brianp@tailscale.com>

net/memnet/listener.go

@@ -22,6 +22,7 @@ type Listener struct {
 	ch        chan Conn
 	closeOnce sync.Once
 	closed    chan struct{}
+	onClose   func() // or nil
 
 	// NewConn, if non-nil, is called to create a new pair of connections
 	// when dialing. If nil, NewConn is used.
@@ -44,9 +45,14 @@ func (l *Listener) Addr() net.Addr {
 
 // Close closes the pipe listener.
 func (l *Listener) Close() error {
+	var cleanup func()
 	l.closeOnce.Do(func() {
+		cleanup = l.onClose
 		close(l.closed)
 	})
+	if cleanup != nil {
+		cleanup()
+	}
 	return nil
 }
 

net/memnet/memnet.go

@@ -61,6 +61,11 @@ func (m *Network) Listen(network, address string) (net.Listener, error) {
 		}
 		ln := Listen(key)
 		m.lns[key] = ln
+		ln.onClose = func() {
+			m.mu.Lock()
+			delete(m.lns, key)
+			m.mu.Unlock()
+		}
 		return ln, nil
 	}
 }

net/memnet/memnet_test.go

@@ -0,0 +1,23 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package memnet
+
+import "testing"
+
+func TestListenAddressReuse(t *testing.T) {
+	var nw Network
+	ln1, err := nw.Listen("tcp", "127.0.0.1:80")
+	if err != nil {
+		t.Fatalf("listen failed: %v", err)
+	}
+	if _, err := nw.Listen("tcp", "127.0.0.1:80"); err == nil {
+		t.Errorf("listen on in-use address succeeded")
+	}
+	if err := ln1.Close(); err != nil {
+		t.Fatalf("close failed: %v", err)
+	}
+	if _, err := nw.Listen("tcp", "127.0.0.1:80"); err != nil {
+		t.Errorf("listen on same address after close failed: %v", err)
+	}
+}