control,health,ipn: move IP forwarding check to health tracker (#19007)

Currently IP forwarding health check is done on sending MapRequests.

Move ip forwarding to the health service to gain the benefits
of the health tracker and perodic monitoring out of band from
the MapRequest path. ipnlocal now provides a closure to
the health service to provide the check if forwarding is broken.

Removed `skipIPForwardingCheck` from controlclient/direct.go,
it wasn't being used as the comments describe it, that check
has moved to ipnlocal for the closure to the health tracker.

Updates #18976

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

health/health.go

@@ -132,6 +132,11 @@ type Tracker struct {
 	localLogConfigErr           error
 	tlsConnectionErrors         map[string]error // map[ServerName]error
 	metricHealthMessage         any              // nil or *metrics.MultiLabelMap[metricHealthMessageLabel]
+
+	// IP forwarding check
+	// If non-nil, called periodically to check if IP forwarding is broken.
+	// Should return true if broken, false if healthy.
+	isIPForwardingBroken func() bool
 }
 
 // NewTracker contructs a new [Tracker] and attaches the given eventbus.
@@ -1097,6 +1102,8 @@ func (t *Tracker) updateBuiltinWarnablesLocked() {
 		t.setHealthyLocked(NetworkStatusWarnable)
 	}
 
+	t.updateIPForwardingWarnableLocked()
+
 	if t.localLogConfigErr != nil {
 		t.setUnhealthyLocked(localLogWarnable, Args{
 			ArgError: t.localLogConfigErr.Error(),
@@ -1389,3 +1396,29 @@ func (t *Tracker) LastNoiseDialWasRecent() bool {
 	t.lastNoiseDial = now
 	return dur < 2*time.Minute
 }
+
+// SetIPForwardingCheck sets the function to check if IP forwarding is broken.
+// The function should return true if IP forwarding is broken, false if healthy.
+// Pass nil to disable IP forwarding checks.
+func (t *Tracker) SetIPForwardingCheck(checkFunc func() bool) {
+	if t.nil() {
+		return
+	}
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	t.isIPForwardingBroken = checkFunc
+
+	// Run an immediate check to set initial state
+	t.updateIPForwardingWarnableLocked()
+}
+
+// updateIPForwardingWarnableLocked checks the IP forwarding state and
+// sets or clears the ipForwardingWarnable accordingly.
+func (t *Tracker) updateIPForwardingWarnableLocked() {
+	if t.isIPForwardingBroken != nil && t.isIPForwardingBroken() {
+		t.setUnhealthyLocked(ipForwardingWarnable, Args{})
+	} else {
+		t.setHealthyLocked(ipForwardingWarnable)
+	}
+}