net/netns, net/dns/resolver, etc: make netmon required in most places

The goal is to move more network state accessors to netmon.Monitor
where they can be cheaper/cached. But first (this change and others)
we need to make sure the one netmon.Monitor is plumbed everywhere.

Some notable bits:

* tsdial.NewDialer is added, taking a now-required netmon

* because a tsdial.Dialer always has a netmon, anything taking both
  a Dialer and a NetMon is now redundant; take only the Dialer and
  get the NetMon from that if/when needed.

* netmon.NewStatic is added, primarily for tests

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: I877f9cb87618c4eb037cee098241d18da9c01691
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -56,6 +56,7 @@ import (
 	"tailscale.com/util/singleflight"
 	"tailscale.com/util/syspolicy"
 	"tailscale.com/util/systemd"
+	"tailscale.com/util/testenv"
 	"tailscale.com/util/zstdframe"
 )
 
@@ -68,7 +69,7 @@ type Direct struct {
 	serverURL                  string              // URL of the tailcontrol server
 	clock                      tstime.Clock
 	logf                       logger.Logf
-	netMon                     *netmon.Monitor // or nil
+	netMon                     *netmon.Monitor // non-nil
 	health                     *health.Tracker
 	discoPubKey                key.DiscoPublic
 	getMachinePrivKey          func() (key.MachinePrivate, error)
@@ -120,10 +121,9 @@ type Options struct {
 	Hostinfo                   *tailcfg.Hostinfo // non-nil passes ownership, nil means to use default using os.Hostname, etc
 	DiscoPublicKey             key.DiscoPublic
 	Logf                       logger.Logf
-	HTTPTestClient             *http.Client    // optional HTTP client to use (for tests only)
-	NoiseTestClient            *http.Client    // optional HTTP client to use for noise RPCs (tests only)
-	DebugFlags                 []string        // debug settings to send to control
-	NetMon                     *netmon.Monitor // optional network monitor
+	HTTPTestClient             *http.Client // optional HTTP client to use (for tests only)
+	NoiseTestClient            *http.Client // optional HTTP client to use for noise RPCs (tests only)
+	DebugFlags                 []string     // debug settings to send to control
 	HealthTracker              *health.Tracker
 	PopBrowserURL              func(url string)             // optional func to open browser
 	OnClientVersion            func(*tailcfg.ClientVersion) // optional func to inform GUI of client version status
@@ -213,6 +213,19 @@ func NewDirect(opts Options) (*Direct, error) {
 	if opts.GetMachinePrivateKey == nil {
 		return nil, errors.New("controlclient.New: no GetMachinePrivateKey specified")
 	}
+	if opts.Dialer == nil {
+		if testenv.InTest() {
+			panic("no Dialer")
+		}
+		return nil, errors.New("controlclient.New: no Dialer specified")
+	}
+	netMon := opts.Dialer.NetMon()
+	if netMon == nil {
+		if testenv.InTest() {
+			panic("no NetMon in Dialer")
+		}
+		return nil, errors.New("controlclient.New: Dialer has nil NetMon")
+	}
 	if opts.ControlKnobs == nil {
 		opts.ControlKnobs = &controlknobs.Knobs{}
 	}
@@ -233,9 +246,8 @@ func NewDirect(opts Options) (*Direct, error) {
 	dnsCache := &dnscache.Resolver{
 		Forward:          dnscache.Get().Forward, // use default cache's forwarder
 		UseLastGood:      true,
-		LookupIPFallback: dnsfallback.MakeLookupFunc(opts.Logf, opts.NetMon),
+		LookupIPFallback: dnsfallback.MakeLookupFunc(opts.Logf, netMon),
 		Logf:             opts.Logf,
-		NetMon:           opts.NetMon,
 	}
 
 	httpc := opts.HTTPTestClient
@@ -272,7 +284,7 @@ func NewDirect(opts Options) (*Direct, error) {
 		authKey:                    opts.AuthKey,
 		discoPubKey:                opts.DiscoPublicKey,
 		debugFlags:                 opts.DebugFlags,
-		netMon:                     opts.NetMon,
+		netMon:                     netMon,
 		health:                     opts.HealthTracker,
 		skipIPForwardingCheck:      opts.SkipIPForwardingCheck,
 		pinger:                     opts.Pinger,

control/controlclient/direct_test.go

@@ -14,6 +14,7 @@ import (
 
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -31,7 +32,7 @@ func TestNewDirect(t *testing.T) {
 		GetMachinePrivateKey: func() (key.MachinePrivate, error) {
 			return k, nil
 		},
-		Dialer: new(tsdial.Dialer),
+		Dialer: tsdial.NewDialer(netmon.NewStatic()),
 	}
 	c, err := NewDirect(opts)
 	if err != nil {
@@ -107,7 +108,7 @@ func TestTsmpPing(t *testing.T) {
 		GetMachinePrivateKey: func() (key.MachinePrivate, error) {
 			return k, nil
 		},
-		Dialer: new(tsdial.Dialer),
+		Dialer: tsdial.NewDialer(netmon.NewStatic()),
 	}
 
 	c, err := NewDirect(opts)

control/controlclient/noise_test.go

@@ -16,6 +16,7 @@ import (
 
 	"golang.org/x/net/http2"
 	"tailscale.com/control/controlhttp"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
@@ -73,7 +74,7 @@ func (tt noiseClientTest) run(t *testing.T) {
 	})
 	defer hs.Close()
 
-	dialer := new(tsdial.Dialer)
+	dialer := tsdial.NewDialer(netmon.NewStatic())
 	nc, err := NewNoiseClient(NoiseOpts{
 		PrivKey:      clientPrivate,
 		ServerPubKey: serverPrivate.Public(),