fix: Better check of RPC type requests (#6927)

guessIsRPCReq() considers all POST requests as RPC but doesn't
check if this is an object operation API or not, which is actually
confusing bucket forwarder handler when it receives a new multipart
upload API which is a POST http request.

Due to this bug, users having a federated setup are not able to
upload a multipart object using an endpoint which doesn't actually
contain the specified bucket that will store the object.

Hence this commit will fix the described issue.

cmd/generic-handlers.go

@@ -228,7 +228,8 @@ func guessIsRPCReq(req *http.Request) bool {
 	if req == nil {
 		return false
 	}
-	return req.Method == http.MethodPost
+	return req.Method == http.MethodPost &&
+		strings.HasPrefix(req.URL.Path, minioReservedBucketPath+"/")
 }
 
 func (h redirectHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {

cmd/generic-handlers_test.go

@@ -78,9 +78,16 @@ func TestGuessIsRPC(t *testing.T) {
 	if guessIsRPCReq(nil) {
 		t.Fatal("Unexpected return for nil request")
 	}
+
+	u, err := url.Parse("http://localhost:9000/minio/lock")
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	r := &http.Request{
 		Proto:  "HTTP/1.0",
 		Method: http.MethodPost,
+		URL:    u,
 	}
 	if !guessIsRPCReq(r) {
 		t.Fatal("Test shouldn't fail for a possible net/rpc request.")