wgengine/magicsock, health, ipn/ipnstate: track DERP-advertised health

And add health check errors to ipnstate.Status (tailscale status --json).

Updates #2746
Updates #2775

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

ipn/ipnlocal/local.go

@@ -25,6 +25,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/go-multierror/multierror"
 	"inet.af/netaddr"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/control/controlclient"
@@ -331,6 +332,17 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 		s.Version = version.Long
 		s.BackendState = b.state.String()
 		s.AuthURL = b.authURLSticky
+
+		if err := health.OverallError(); err != nil {
+			switch e := err.(type) {
+			case multierror.MultipleErrors:
+				for _, err := range e {
+					s.Health = append(s.Health, err.Error())
+				}
+			default:
+				s.Health = append(s.Health, err.Error())
+			}
+		}
 		if b.netMap != nil {
 			s.MagicDNSSuffix = b.netMap.MagicDNSSuffix()
 			s.CertDomains = append([]string(nil), b.netMap.DNS.CertDomains...)

ipn/ipnstate/ipnstate.go

@@ -38,6 +38,11 @@ type Status struct {
 	TailscaleIPs []netaddr.IP // Tailscale IP(s) assigned to this node
 	Self         *PeerStatus
 
+	// Health contains health check problems.
+	// Empty means everything is good. (or at least that no known
+	// problems are detected)
+	Health []string
+
 	// MagicDNSSuffix is the network's MagicDNS suffix for nodes
 	// in the network such as "userfoo.tailscale.net".
 	// There are no surrounding dots.