app/vmauth: initialize reverse proxy only after flag.Parse() is called

This should properly take into accoun the `-maxIdleConnsPerBackend` command-line flag value.
Previously it was hardcoded to 100.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1300

app/vmauth/main.go

@@ -7,6 +7,7 @@ import (
 	"net/http/httputil"
 	"net/url"
 	"os"
+	"sync"
 	"time"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/buildinfo"
@@ -108,7 +109,7 @@ func proxyRequest(w http.ResponseWriter, r *http.Request) {
 		// Forward other panics to the caller.
 		panic(err)
 	}()
-	reverseProxy.ServeHTTP(w, r)
+	getReverseProxy().ServeHTTP(w, r)
 }
 
 var (
@@ -117,7 +118,19 @@ var (
 	missingRouteRequests     = metrics.NewCounter(`vmauth_http_request_errors_total{reason="missing_route"}`)
 )
 
-var reverseProxy = &httputil.ReverseProxy{
+var (
+	reverseProxy     *httputil.ReverseProxy
+	reverseProxyOnce sync.Once
+)
+
+func getReverseProxy() *httputil.ReverseProxy {
+	reverseProxyOnce.Do(initReverseProxy)
+	return reverseProxy
+}
+
+// initReverseProxy must be called after flag.Parse(), since it uses command-line flags.
+func initReverseProxy() {
+	reverseProxy = &httputil.ReverseProxy{
 		Director: func(r *http.Request) {
 			targetURL := r.Header.Get("vm-target-url")
 			target, err := url.Parse(targetURL)
@@ -140,6 +153,7 @@ var reverseProxy = &httputil.ReverseProxy{
 		}(),
 		FlushInterval: time.Second,
 		ErrorLog:      logger.StdErrorLogger(),
+	}
 }
 
 func usage() {

docs/CHANGELOG.md

@@ -6,6 +6,8 @@ sort: 15
 
 ## tip
 
+* BUGFIX: vmauth: properly take into account the value passed to `-maxIdleConnsPerBackend` command-line flag. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1300).
+
 
 ## [v1.69.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.69.0)