fix: prevent service degradation on config reload failure

When applyConfig() fails during hot reload, the old proxy was being destroyed
before the new proxy was successfully initialized, causing partial service outage.

Changes:
- main.go: Use temporary variable to create and validate new proxy before
  replacing the global instance. On failure, cleanup new proxy and keep
  old proxy running.
- proxy.go: Fix bug where maxIdleConns was incorrectly assigned MaxIdleConnsPerHost

Author: dovtang

main.go

@@ -299,12 +299,26 @@ func proxyConfigChanged(cfgCp *config.ConnectionPool, rp *reverseProxy) bool {
 }
 
 func applyConfig(cfg *config.Config) error {
-	if proxy == nil || proxyConfigChanged(&cfg.ConnectionPool, proxy) {
-		proxy = newReverseProxy(&cfg.ConnectionPool)
-	}
-	if err := proxy.applyConfig(cfg); err != nil {
-		return err
+	needNewProxy := proxy == nil || proxyConfigChanged(&cfg.ConnectionPool, proxy)
+
+	if needNewProxy {
+		newProxy := newReverseProxy(&cfg.ConnectionPool)
+		if err := newProxy.applyConfig(cfg); err != nil {
+			close(newProxy.reloadSignal)
+			newProxy.reloadWG.Wait()
+			return err
+		}
+		if proxy != nil {
+			close(proxy.reloadSignal)
+			proxy.reloadWG.Wait()
+		}
+		proxy = newProxy
+	} else {
+		if err := proxy.applyConfig(cfg); err != nil {
+			return err
+		}
 	}
+
 	allowedNetworksHTTP.Store(&cfg.Server.HTTP.AllowedNetworks)
 	allowedNetworksHTTPS.Store(&cfg.Server.HTTPS.AllowedNetworks)
 	allowedNetworksMetrics.Store(&cfg.Server.Metrics.AllowedNetworks)

proxy.go

@@ -78,7 +78,7 @@ func newReverseProxy(cfgCp *config.ConnectionPool) *reverseProxy {
 		},
 		reloadSignal:        make(chan struct{}),
 		reloadWG:            sync.WaitGroup{},
-		maxIdleConns:        cfgCp.MaxIdleConnsPerHost,
+		maxIdleConns:        cfgCp.MaxIdleConns,
 		maxIdleConnsPerHost: cfgCp.MaxIdleConnsPerHost,
 	}
 }