[ruby-core:108678] [Ruby master Bug#18791] Unexpected behavior of Socket#connect_nonblock

Issue #18791 has been updated by jeremyevans0 (Jeremy Evans).

Status changed from Open to Rejected

This doesn't look like a bug.  If the `connect` system call returns `0`, then you were able to connect, and `connect_nonblock` in Ruby returns `0` as well.  Yes, you should treat `0` as indicating the connection was successful.  The documentation states `Returns 0 if successful, otherwise an exception is raised.`  I think that explains the behavior correctly, so this does not appear to be a documentation bug either.

I think maybe you are expecting `Errno::EISCONN` must be raised to indicate a successful connection.  That would also indicate a successful connection, but having the method return `0` also indicates the connection was successful.  The example code is written so either returning `0` or raising `Errno::EISCONN` is treated as a successful connection, so the example code does not appear to be incorrect.



----------------------------------------
Bug #18791: Unexpected behavior of Socket#connect_nonblock
https://bugs.ruby-lang.org/issues/18791#change-97719

* Author: midnight (Sarun R)
* Status: Rejected
* Priority: Normal
* ruby -v: ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
I followed an example of [Socket#connect_nonblock](https://ruby-doc.org/stdlib-3.1.2/libdoc/socket/rdoc/Socket.html#method-i-connect_nonblock) on the document.
Waiting for multiple Socket connections at once.

~~~Ruby
# Pull down Google's web page
require 'socket'
include Socket::Constants
socket = Socket.new(AF_INET, SOCK_STREAM, 0)
sockaddr = Socket.sockaddr_in(80, 'www.google.com')
begin # emulate blocking connect
  socket.connect_nonblock(sockaddr)
rescue IO::WaitWritable
  IO.select(nil, [socket]) # wait 3-way handshake completion
  begin
    # ****************** The problem is here ********************
    socket.connect_nonblock(sockaddr) # check connection failure
  rescue Errno::EISCONN
  end
end
socket.write("GET / HTTP/1.0\r\n\r\n")
results = socket.read
~~~

The first call to `connect_nonblock` raises `IO::WaitWritable` as expected.
But the confirmation call did not raise `Errno::EISCONN`; instead, it returned `0`.

Upon source code inspection, 0 was returned from `connect` and is supposed to mean success.

~~~C
static VALUE
sock_connect_nonblock(VALUE sock, VALUE addr, VALUE ex)
{
    VALUE rai;
    rb_io_t *fptr;
    int n;

    SockAddrStringValueWithAddrinfo(addr, rai);
    addr = rb_str_new4(addr);
    GetOpenFile(sock, fptr);
    rb_io_set_nonblock(fptr);
    n = connect(fptr->fd, (struct sockaddr*)RSTRING_PTR(addr), RSTRING_SOCKLEN(addr));
    if (n < 0) {
	int e = errno;
	if (e == EINPROGRESS) {
            if (ex == Qfalse) {
                return sym_wait_writable;
            }
            rb_readwrite_syserr_fail(RB_IO_WAIT_WRITABLE, e, "connect(2) would block");
	}
	if (e == EISCONN) {
            if (ex == Qfalse) {
                return INT2FIX(0);
            }
	}
	rsock_syserr_fail_raddrinfo_or_sockaddr(e, "connect(2)", addr, rai);
    }

    return INT2FIX(n);
}
~~~

I made sure `ex` is `true`, so it is not `return INT2FIX(0);` that get returned but the last statement `return INT2FIX(n);`.
Is this the intended behavior? It does surprise me and clearly doesn't explain very well in the document.
The example shown implied that the only way to confirm the connection is `Errno::EISCONN`; never mention the return code.



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