Wrapping existing net.Socket does not work

[marco-a]

• Version: 14.16.0
• Platform: MacOS
• Subsystem: macOS Catalina 10.15.7

What steps will reproduce the bug?

const net = require("net")

const srv = net.createServer()

srv.on("connection", c => {
    const sock = new net.Socket({
        fd: c._handle.fd
    })
})

srv.listen(4444)

How often does it reproduce? Is there a required condition?

No required condition (as far as I can tell).

What is the expected behavior?

That a new net.Socket instance is created which behaves exactly like the original instance (c in this case).

What do you see instead?

An error is thrown (EEXISTS):

net.js:337
        throw errnoException(err, 'open');
        ^

Error: open EEXIST
    at new Socket (net.js:337:15)
    at Server.<anonymous> (test.js:6:15)
    at Server.emit (events.js:315:20)
    at TCP.onconnection (net.js:1560:8) {
  errno: -17,
  code: 'EEXIST',
  syscall: 'open'
}

Additional information

[Ayase-252]

I don't think you need to wrap with net.Socket again, since c is already an opened net.Socket.

https://nodejs.org/api/net.html#net_event_connection

[marco-a]

I'm aware that c is already a net.Socket.

[Ayase-252]

@marco-a

Hi, Sorry. I'm just aware that the expected behavior section is missing. Could you provide what you are expecting from the snippet?

[marco-a]

Sure.

What I was expecting is that the net.Socket instance is created and that it behaves exactly like the original instance (c in this case).

[marco-a]

I will adapt my code to use c directly, but I still think this is a bug or the documentation should be updated.

fd If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.

In my view net.Socket (or c) is an existing socket, also I'm not aware of any other socket type than net.Socket in node (but I could be wrong).

[Ayase-252]

Sure, I agree with you that the part of document needs improve if it leads misuse.

Would you interest in opening a PR to correct that?

[Ayase-252]

I also need help to understand the exact meaning of words in document.

fd If specified, wrap around an existing socket with the given file descriptor, otherwise a new socket will be created.

Since wrapping an existing socket seems throwing error here, what type of existing socket can be passed here?

cc @nodejs/net

[theanarkh]

Error is thrown from Libuv, code is as follow.

int uv__fd_exists(uv_loop_t* loop, int fd) {
  return (unsigned) fd < loop->nwatchers && loop->watchers[fd] != NULL;
}

related:
#3604
libuv/libuv#1851