net/sockstats: remove explicit dependency on wgengine/monitor

Followup to #7177 to avoid adding extra dependencies to the CLI. We
instead declare an interface for the link monitor.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/interfaces/interfaces.go

@@ -353,6 +353,12 @@ func (s *State) String() string {
 	return sb.String()
 }
 
+// ChangeFunc is a callback function (usually registered with
+// wgengine/monitor's Mon) that's called when the network
+// changed. The changed parameter is whether the network changed
+// enough for State to have changed since the last callback.
+type ChangeFunc func(changed bool, state *State)
+
 // An InterfaceFilter indicates whether EqualFiltered should use i when deciding whether two States are equal.
 // ips are all the IPPrefixes associated with i.
 type InterfaceFilter func(i Interface, ips []netip.Prefix) bool

net/sockstats/sockstats.go

@@ -11,7 +11,7 @@ package sockstats
 import (
 	"context"
 
-	"tailscale.com/wgengine/monitor"
+	"tailscale.com/net/interfaces"
 )
 
 type SockStats struct {
@@ -34,6 +34,13 @@ func Get() *SockStats {
 	return get()
 }
 
-func SetLinkMonitor(lm *monitor.Mon) {
+// LinkMonitor is the interface for the parts of wgengine/mointor's Mon that we
+// need, to avoid the dependency.
+type LinkMonitor interface {
+	InterfaceState() *interfaces.State
+	RegisterChangeCallback(interfaces.ChangeFunc) (unregister func())
+}
+
+func SetLinkMonitor(lm LinkMonitor) {
 	setLinkMonitor(lm)
 }

net/sockstats/sockstats_noop.go

@@ -7,8 +7,6 @@ package sockstats
 
 import (
 	"context"
-
-	"tailscale.com/wgengine/monitor"
 )
 
 func withSockStats(ctx context.Context, label string) context.Context {
@@ -19,5 +17,5 @@ func get() *SockStats {
 	return nil
 }
 
-func setLinkMonitor(lm *monitor.Mon) {
+func setLinkMonitor(lm LinkMonitor) {
 }

net/sockstats/sockstats_tsgo.go

@@ -13,7 +13,6 @@ import (
 	"sync/atomic"
 
 	"tailscale.com/net/interfaces"
-	"tailscale.com/wgengine/monitor"
 )
 
 type sockStatCounters struct {
@@ -113,7 +112,7 @@ func get() *SockStats {
 	return r
 }
 
-func setLinkMonitor(lm *monitor.Mon) {
+func setLinkMonitor(lm LinkMonitor) {
 	sockStats.mu.Lock()
 	defer sockStats.mu.Unlock()