ipn, wgengine, magicsock, tsdns: be quieter and less aggressive when offline

If no interfaces are up, calm down and stop spamming so much. It was
noticed as especially bad on Windows, but probably was bad
everywhere. I just have the best network conditions testing on a
Windows VM.

Updates #604

wgengine/magicsock/magicsock.go

@@ -208,6 +208,11 @@ type Conn struct {
 	// necessarily have a netcheck.Report and don't want to skip
 	// logging.
 	noV4, noV6 syncs.AtomicBool
+
+	// networkUp is whether the network is up (some interface is up
+	// with IPv4 or IPv6). It's used to suppress log spam and prevent
+	// new connection that'll fail.
+	networkUp syncs.AtomicBool
 }
 
 // derpRoute is a route entry for a public key, saying that a certain
@@ -345,6 +350,7 @@ func newConn() *Conn {
 		discoOfAddr:     make(map[netaddr.IPPort]tailcfg.DiscoKey),
 	}
 	c.muCond = sync.NewCond(&c.mu)
+	c.networkUp.Set(true) // assume up until told otherwise
 	return c
 }
 
@@ -969,8 +975,15 @@ func (as *AddrSet) appendDests(dsts []netaddr.IPPort, b []byte) (_ []netaddr.IPP
 }
 
 var errNoDestinations = errors.New("magicsock: no destinations")
+var errNetworkDown = errors.New("magicsock: network down")
+
+func (c *Conn) networkDown() bool { return !c.networkUp.Get() }
 
 func (c *Conn) Send(b []byte, ep conn.Endpoint) error {
+	if c.networkDown() {
+		return errNetworkDown
+	}
+
 	var as *AddrSet
 	switch v := ep.(type) {
 	default:
@@ -1111,6 +1124,10 @@ func (c *Conn) derpWriteChanOfAddr(addr netaddr.IPPort, peer key.Public) chan<-
 	}
 	regionID := int(addr.Port)
 
+	if c.networkDown() {
+		return nil
+	}
+
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if !c.wantDerpLocked() || c.closed {
@@ -1304,15 +1321,19 @@ func (c *Conn) runDerpReader(ctx context.Context, derpFakeAddr netaddr.IPPort, d
 
 	for {
 		msg, err := dc.Recv()
-		if err == derphttp.ErrClientClosed {
-			return
-		}
 		if err != nil {
 			// Forget that all these peers have routes.
 			for peer := range peerPresent {
 				delete(peerPresent, peer)
 				c.removeDerpPeerRoute(peer, regionID, dc)
 			}
+			if err == derphttp.ErrClientClosed {
+				return
+			}
+			if c.networkDown() {
+				c.logf("magicsock: derp.Recv(derp-%d): network down, closing", regionID)
+				return
+			}
 			select {
 			case <-ctx.Done():
 				return
@@ -1691,7 +1712,9 @@ func (c *Conn) sendDiscoMessage(dst netaddr.IPPort, dstKey tailcfg.NodeKey, dstD
 	} else if err == nil {
 		// Can't send. (e.g. no IPv6 locally)
 	} else {
-		c.logf("magicsock: disco: failed to send %T to %v: %v", m, dst, err)
+		if !c.networkDown() {
+			c.logf("magicsock: disco: failed to send %T to %v: %v", m, dst, err)
+		}
 	}
 	return sent, err
 }
@@ -1956,6 +1979,21 @@ func (c *Conn) sharedDiscoKeyLocked(k tailcfg.DiscoKey) *[32]byte {
 	return shared
 }
 
+func (c *Conn) SetNetworkUp(up bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.networkUp.Get() == up {
+		return
+	}
+
+	c.logf("magicsock: SetNetworkUp(%v)", up)
+	c.networkUp.Set(up)
+
+	if !up {
+		c.closeAllDerpLocked("network-down")
+	}
+}
+
 // SetPrivateKey sets the connection's private key.
 //
 // This is only used to be able prove our identity when connecting to
@@ -2282,6 +2320,10 @@ func maxIdleBeforeSTUNShutdown() time.Duration {
 }
 
 func (c *Conn) shouldDoPeriodicReSTUN() bool {
+	if c.networkDown() {
+		return false
+	}
+
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if len(c.peerSet) == 0 {