all: leaking queries to OS resolver instead of SRVFAIL

So it would work in more general case than just captive portal network,
which ctrld have supported recently.

Uses who may want no leaking behavior can use a config to turn off this
feature.

cmd/cli/upstream_monitor.go

@@ -71,19 +71,19 @@ func (um *upstreamMonitor) reset(upstream string) {
 
 // checkUpstream checks the given upstream status, periodically sending query to upstream
 // until successfully. An upstream status/counter will be reset once it becomes reachable.
-func (um *upstreamMonitor) checkUpstream(upstream string, uc *ctrld.UpstreamConfig) {
-	um.mu.Lock()
-	isChecking := um.checking[upstream]
+func (p *prog) checkUpstream(upstream string, uc *ctrld.UpstreamConfig) {
+	p.um.mu.Lock()
+	isChecking := p.um.checking[upstream]
 	if isChecking {
-		um.mu.Unlock()
+		p.um.mu.Unlock()
 		return
 	}
-	um.checking[upstream] = true
-	um.mu.Unlock()
+	p.um.checking[upstream] = true
+	p.um.mu.Unlock()
 	defer func() {
-		um.mu.Lock()
-		um.checking[upstream] = false
-		um.mu.Unlock()
+		p.um.mu.Lock()
+		p.um.checking[upstream] = false
+		p.um.mu.Unlock()
 	}()
 
 	resolver, err := ctrld.NewResolver(uc)
@@ -104,7 +104,13 @@ func (um *upstreamMonitor) checkUpstream(upstream string, uc *ctrld.UpstreamConf
 	for {
 		if err := check(); err == nil {
 			mainLog.Load().Debug().Msgf("upstream %q is online", uc.Endpoint)
-			um.reset(upstream)
+			p.um.reset(upstream)
+			if p.leakingQuery.CompareAndSwap(true, false) {
+				p.leakingQueryMu.Lock()
+				p.leakingQueryWasRun = false
+				p.leakingQueryMu.Unlock()
+				mainLog.Load().Warn().Msg("stop leaking query")
+			}
 			return
 		}
 		time.Sleep(checkUpstreamBackoffSleep)