Ignore reservedBucket checks for net/rpc requests (#4884)

All `net/rpc` requests go to `/minio`, so the existing
generic handler for reserved bucket check would essentially
erroneously send errors leading to distributed setups to
wait infinitely.

For `net/rpc` requests alone we should skip this check and
allow resource bucket names to be from `/minio` .

cmd/generic-handlers.go

@@ -156,6 +156,14 @@ func guessIsBrowserReq(req *http.Request) bool {
 	return strings.Contains(req.Header.Get("User-Agent"), "Mozilla")
 }
 
+// guessIsRPCReq - returns true if the request is for an RPC endpoint.
+func guessIsRPCReq(req *http.Request) bool {
+	if req == nil {
+		return false
+	}
+	return req.Method == http.MethodConnect && req.Proto == "HTTP/1.0"
+}
+
 func (h redirectHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	aType := getRequestAuthType(r)
 	// Re-direct only for JWT and anonymous requests from browser.
@@ -202,20 +210,22 @@ func (h cacheControlHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 }
 
 // Adds verification for incoming paths.
-type minioPrivateBucketHandler struct {
+type minioReservedBucketHandler struct {
 	handler http.Handler
 }
 
-func setPrivateBucketHandler(h http.Handler) http.Handler {
-	return minioPrivateBucketHandler{h}
+func setReservedBucketHandler(h http.Handler) http.Handler {
+	return minioReservedBucketHandler{h}
 }
 
-func (h minioPrivateBucketHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	// For all non browser requests, reject access to 'minioReservedBucketPath'.
-	bucketName, _ := urlPath2BucketObjectName(r.URL)
-	if !guessIsBrowserReq(r) && (isMinioReservedBucket(bucketName) || isMinioMetaBucket(bucketName)) {
-		writeErrorResponse(w, ErrAllAccessDisabled, r.URL)
-		return
+func (h minioReservedBucketHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if !guessIsRPCReq(r) && !guessIsBrowserReq(r) {
+		// For all non browser, non RPC requests, reject access to 'minioReservedBucketPath'.
+		bucketName, _ := urlPath2BucketObjectName(r.URL)
+		if isMinioReservedBucket(bucketName) || isMinioMetaBucket(bucketName) {
+			writeErrorResponse(w, ErrAllAccessDisabled, r.URL)
+			return
+		}
 	}
 	h.handler.ServeHTTP(w, r)
 }