cmd/cli: new flow for leaking queries to OS resolver

The current flow involves marking OS resolver as down, which is not
right at all, since ctrld depends on it for leaking queries.

This commits implements new flow, which ctrld will restore DNS settings
once leaking marked, allowing queries go to OS resolver until the
internet connection is established.

cmd/cli/dns_proxy.go

@@ -419,12 +419,7 @@ func (p *prog) proxy(ctx context.Context, req *proxyRequest) *proxyResponse {
 	upstreamConfigs := p.upstreamConfigsFromUpstreamNumbers(upstreams)
 
 	leaked := false
-	// If ctrld is going to leak query to OS resolver, check remote upstream in background,
-	// so ctrld could be back to normal operation as long as the network is back online.
 	if len(upstreamConfigs) > 0 && p.leakingQuery.Load() {
-		for n, uc := range upstreamConfigs {
-			go p.checkUpstream(upstreams[n], uc)
-		}
 		upstreamConfigs = nil
 		leaked = true
 		ctrld.Log(ctx, mainLog.Load().Debug(), "%v is down, leaking query to OS resolver", upstreams)
@@ -936,11 +931,25 @@ func (p *prog) performLeakingQuery() {
 	mainLog.Load().Warn().Msg("leaking query to OS resolver")
 	// Signal dns watchers to stop, so changes made below won't be reverted.
 	p.leakingQuery.Store(true)
+	defer func() {
+		p.leakingQuery.Store(false)
+		p.leakingQueryMu.Lock()
+		p.leakingQueryWasRun = false
+		p.leakingQueryMu.Unlock()
+	}()
+	// Reset DNS, so queries are forwarded to OS resolver normally.
 	p.resetDNS()
+	// Check remote upstream in background, so ctrld could be back to normal
+	// operation as long as the network is back online.
+	for name, uc := range p.cfg.Upstream {
+		p.checkUpstream(name, uc)
+	}
+	// After all upstream back, re-initializing OS resolver.
 	ns := ctrld.InitializeOsResolver()
 	mainLog.Load().Debug().Msgf("re-initialized OS resolver with nameservers: %v", ns)
 	p.dnsWg.Wait()
 	p.setDNS()
+	mainLog.Load().Warn().Msg("stop leaking query")
 }
 
 // forceFetchingAPI sends signal to force syncing API config if run in cd mode,