possible libuv issue with macos 10.15: empty udp message not registering

[daviehh]

• Version:
  v12.12.0
• Platform:
  macos catalina/10.15 (19A602)
• Subsystem:
  n/a

When sending an empty udp message, the message will not be received until another non-empty udp message is received.

Example/Reproduce: (may only affect macos 10.15?)

file server.js

var PORT = 33333;
var HOST = '127.0.0.1';

var dgram = require('dgram');
var server = dgram.createSocket('udp4');

server.on('listening', function(){
 var address = server.address();
 console.log('UDP Server listening on ' + address.address + ':' + address.port);
});

server.on('message', function(message, remote){
 console.log(remote.address + ':' + remote.port +' - ' + message);
});

server.bind(PORT, HOST);

file client_test.js

var PORT = 33333;
var HOST = '127.0.0.1';

var dgram = require('dgram');


var client = dgram.createSocket('udp4');
var message = Buffer.from('abc');

client.send(message, 0, message.length, PORT, HOST, function(err, bytes){
 if (err) throw err;
 console.log('UDP message sent to ' + HOST +':'+ PORT);
 client.close();
});

file client_0.js

var PORT = 33333;
var HOST = '127.0.0.1';

var dgram = require('dgram');


var client = dgram.createSocket('udp4');
var message = Buffer.from('');

client.send(message, 0, message.length, PORT, HOST, function(err, bytes){
 if (err) throw err;
 console.log('UDP message sent to ' + HOST +':'+ PORT);
 client.close();
});

run the server code, then run the client_0.js: no response in server.
followed by running client_test.js: server got empty and the "abc" message.

[addaleax]

Does this also happen with Node 12.11.1?

[daviehh]

@addaleax Yes, just tried with the binary using https://nodejs.org/dist/v12.11.1/

[addaleax]

/cc @nodejs/platform-macos

@daviehh Have you tried recreating this with e.g. Python? I’m not on a mac but I have the suspicion that this may actually be just a platform quirk of macOS that we can’t do anything about.

[daviehh]

@addaleax I have actually tried Python, empty udp message works there:

receiving:

import socket

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(("127.0.0.1", 6000))

while True:
 data, addr = sock.recvfrom(1024)
 print("received message:", data)

sending:

import socket

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(b"", ("127.0.0.1", 6000))

[lpinca]

It seems our test suite also fails due to this on macOS Catalina.

=== release test-dgram-connect-send-empty-buffer === 
Path: parallel/test-dgram-connect-send-empty-buffer
Command: out/Release/node /Users/luigi/code/node/test/parallel/test-dgram-connect-send-empty-buffer.js
--- TIMEOUT ---
=== release test-dgram-connect-send-empty-array === 
Path: parallel/test-dgram-connect-send-empty-array
Command: out/Release/node /Users/luigi/code/node/test/parallel/test-dgram-connect-send-empty-array.js
--- TIMEOUT ---
=== release test-dgram-connect-send-empty-packet === 
Path: parallel/test-dgram-connect-send-empty-packet
Command: out/Release/node /Users/luigi/code/node/test/parallel/test-dgram-connect-send-empty-packet.js
--- TIMEOUT ---
=== release test-dgram-send-empty-array === 
Path: parallel/test-dgram-send-empty-array
Command: out/Release/node /Users/luigi/code/node/test/parallel/test-dgram-send-empty-array.js
--- TIMEOUT ---
=== release test-dgram-send-empty-buffer === 
Path: parallel/test-dgram-send-empty-buffer
Command: out/Release/node /Users/luigi/code/node/test/parallel/test-dgram-send-empty-buffer.js
--- TIMEOUT ---
=== release test-dgram-send-empty-packet ===
Path: parallel/test-dgram-send-empty-packet
Command: out/Release/node /Users/luigi/code/node/test/parallel/test-dgram-send-empty-packet.js
--- TIMEOUT ---
[03:46|% 100|+ 2791|- 6]: Done

[daviehh]

Thanks! I first noticed this issue with a beta version of another language that uses libuv (julia), so it's likely that it's a libuv bug; is it good for the bug report to be here or is it better to report to libuv? Not sure how to write a simple c code that directly uses libuv to reproduce the issue though.

[santigimeno]

Yeah, I've reproduced the issue in libuv. Apparently the message is actually sent (I've checked it with tcpdump) but not received.

[santigimeno]

It looks like a problem with kqueue not reporting the event. The python example does not seem to apply to our case as it's waiting on recvfrom syscall.