net/netcheck: use DERP frames as a signal for home region liveness

This uses the fact that we've received a frame from a given DERP region
within a certain time as a signal that the region is stil present (and
thus can still be a node's PreferredDERP / home region) even if we don't
get a STUN response from that region during a netcheck.

This should help avoid DERP flaps that occur due to losing STUN probes
while still having a valid and active TCP connection to the DERP server.

RELNOTE=Reduce home DERP flapping when there's still an active connection

Updates #8603

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: If7da6312581e1d434d5c0811697319c621e187a0

health/health.go

@@ -360,6 +360,8 @@ func SetDERPRegionHealth(region int, problem string) {
 	selfCheckLocked()
 }
 
+// NoteDERPRegionReceivedFrame is called to note that a frame was received from
+// the given DERP region at the current time.
 func NoteDERPRegionReceivedFrame(region int) {
 	mu.Lock()
 	defer mu.Unlock()
@@ -367,6 +369,15 @@ func NoteDERPRegionReceivedFrame(region int) {
 	selfCheckLocked()
 }
 
+// GetDERPRegionReceivedTime returns the last time that a frame was received
+// from the given DERP region, or the zero time if no communication with that
+// region has occurred.
+func GetDERPRegionReceivedTime(region int) time.Time {
+	mu.Lock()
+	defer mu.Unlock()
+	return derpRegionLastFrame[region]
+}
+
 // state is an ipn.State.String() value: "Running", "Stopped", "NeedsLogin", etc.
 func SetIPNState(state string, wantRunning bool) {
 	mu.Lock()