health, control/controlclient, wgengine: report when router unhealthy

Updates tailscale/corp#1338

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/auto.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"golang.org/x/oauth2"
+	"tailscale.com/health"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/empty"
@@ -116,6 +117,8 @@ type Client struct {
 	closed   bool
 	newMapCh chan struct{} // readable when we must restart a map request
 
+	unregisterHealthWatch func()
+
 	mu         sync.Mutex   // mutex guards the following fields
 	statusFunc func(Status) // called to update Client status
 
@@ -171,7 +174,14 @@ func NewNoStart(opts Options) (*Client, error) {
 	}
 	c.authCtx, c.authCancel = context.WithCancel(context.Background())
 	c.mapCtx, c.mapCancel = context.WithCancel(context.Background())
+	c.unregisterHealthWatch = health.RegisterWatcher(c.onHealthChange)
 	return c, nil
+
+}
+
+func (c *Client) onHealthChange(key string, err error) {
+	c.logf("controlclient: restarting map request for %q health change to new state: %v", key, err)
+	c.cancelMapSafely()
 }
 
 // SetPaused controls whether HTTP activity should be paused.
@@ -700,6 +710,7 @@ func (c *Client) Shutdown() {
 
 	c.logf("client.Shutdown: inSendStatus=%v", inSendStatus)
 	if !closed {
+		c.unregisterHealthWatch()
 		close(c.quit)
 		c.cancelAuth()
 		<-c.authDone

control/controlclient/direct.go

@@ -33,6 +33,7 @@ import (
 	"golang.org/x/crypto/nacl/box"
 	"golang.org/x/oauth2"
 	"inet.af/netaddr"
+	"tailscale.com/health"
 	"tailscale.com/log/logheap"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/netns"
@@ -537,9 +538,16 @@ func (c *Direct) sendMapRequest(ctx context.Context, maxPolls int, cb func(*netm
 		DebugFlags: c.debugFlags,
 		OmitPeers:  cb == nil,
 	}
+	var extraDebugFlags []string
 	if hostinfo != nil && ipForwardingBroken(hostinfo.RoutableIPs) {
+		extraDebugFlags = append(extraDebugFlags, "warn-ip-forwarding-off")
+	}
+	if health.RouterHealth() != nil {
+		extraDebugFlags = append(extraDebugFlags, "warn-router-unhealthy")
+	}
+	if len(extraDebugFlags) > 0 {
 		old := request.DebugFlags
-		request.DebugFlags = append(old[:len(old):len(old)], "warn-ip-forwarding-off")
+		request.DebugFlags = append(old[:len(old):len(old)], extraDebugFlags...)
 	}
 	if c.newDecompressor != nil {
 		request.Compress = "zstd"