app/vmauth: properly handle http.ErrAbortHandler panic

This panic can be raised by the reverseProxy on aborted request to the backend.
So handle it (e.g. suppress) at reverseProxy.ServeHTTP call.

Do not suppress the panic at lib/httpserver generic HTTP handler,
since it may result in an inconsistent state left after the panicking handler.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1353

app/vmauth/main.go

@@ -81,10 +81,24 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 		return true
 	}
 	r.Header.Set("vm-target-url", targetURL.String())
-	reverseProxy.ServeHTTP(w, r)
+	proxyRequest(w, r)
 	return true
 }
 
+func proxyRequest(w http.ResponseWriter, r *http.Request) {
+	defer func() {
+		err := recover()
+		if err == nil || err == http.ErrAbortHandler {
+			// Suppress http.ErrAbortHandler panic.
+			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1353
+			return
+		}
+		// Forward other panics to the caller.
+		panic(err)
+	}()
+	reverseProxy.ServeHTTP(w, r)
+}
+
 var configReloadRequests = metrics.NewCounter(`vmagent_http_requests_total{path="/-/reload"}`)
 
 var reverseProxy = &httputil.ReverseProxy{

lib/httpserver/httpserver.go

@@ -212,9 +212,7 @@ func handlerWrapper(s *server, w http.ResponseWriter, r *http.Request, rh Reques
 	// The following recover() code works around this by explicitly stopping the process after logging the panic.
 	// See https://github.com/golang/go/issues/16542#issuecomment-246549902 for details.
 	defer func() {
-		// need to check for abortHandler
-		// https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1353
-		if err := recover(); err != nil && err != http.ErrAbortHandler {
+		if err := recover(); err != nil {
 			buf := make([]byte, 1<<20)
 			n := runtime.Stack(buf, false)
 			fmt.Fprintf(os.Stderr, "panic: %v\n\n%s", err, buf[:n])