magicsock: test error case in sendDiscoMessage

In sendDiscoMessage there is a check of whether the connection is
closed, which is not being reliably exercised by other tests.
This shows up in code coverage reports, the lines of code in
sendDiscoMessage are alternately added and subtracted from
code coverage.

Add a test to specifically exercise and verify this code path.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -625,6 +625,58 @@ func TestConnClosing(t *testing.T) {
 	// (verified by deliberately breaking derpWriteChanOfAddr)
 }
 
+// Exercise a code path in sendDiscoMessage if the connection has been closed.
+func TestConnClosed(t *testing.T) {
+	mstun := &natlab.Machine{Name: "stun"}
+	m1 := &natlab.Machine{Name: "m1"}
+	m2 := &natlab.Machine{Name: "m2"}
+	inet := natlab.NewInternet()
+	sif := mstun.Attach("eth0", inet)
+	m1if := m1.Attach("eth0", inet)
+	m2if := m2.Attach("eth0", inet)
+
+	d := &devices{
+		m1:     m1,
+		m1IP:   m1if.V4(),
+		m2:     m2,
+		m2IP:   m2if.V4(),
+		stun:   mstun,
+		stunIP: sif.V4(),
+	}
+
+	derpMap, cleanup := runDERPAndStun(t, t.Logf, d.stun, d.stunIP)
+	defer cleanup()
+
+	ms1 := newMagicStack(t, logger.WithPrefix(t.Logf, "conn1: "), d.m1, derpMap)
+	defer ms1.Close()
+	ms2 := newMagicStack(t, logger.WithPrefix(t.Logf, "conn2: "), d.m2, derpMap)
+	defer ms2.Close()
+
+	cleanup = meshStacks(t.Logf, []*magicStack{ms1, ms2})
+	defer cleanup()
+
+	pkt := tuntest.Ping(ms2.IP(t).IPAddr().IP, ms1.IP(t).IPAddr().IP)
+
+	if len(ms1.conn.activeDerp) == 0 {
+		t.Errorf("unexpected DERP empty got: %v want: >0", len(ms1.conn.activeDerp))
+	}
+
+	ms1.conn.Close()
+	ms2.conn.Close()
+
+	// This should hit a c.closed conditional in sendDiscoMessage() and return immediately.
+	ms1.tun.Outbound <- pkt
+	select {
+	case <-ms2.tun.Inbound:
+		t.Error("unexpected response with connection closed")
+	case <-time.After(100 * time.Millisecond):
+	}
+
+	if len(ms1.conn.activeDerp) > 0 {
+		t.Errorf("unexpected DERP active got: %v want:0", len(ms1.conn.activeDerp))
+	}
+}
+
 func makeNestable(t *testing.T) (logf logger.Logf, setT func(t *testing.T)) {
 	var mu sync.RWMutex
 	cur := t