On Linux (and probably some other OS), you can only set the size of the buffers before the socket is connected/accepted. In the Python code, the buffer size is correctly set between the creation of the socket and the connect operation. In the Go code, Dial creates the socket and establishes the connection, so the SetReadBuffer call has no effect.

IMO, this is one flaw of the net API in Go: the API does not allow to properly configure some socket options before the socket is actually used.

A workaround is to directly execute the corresponding syscalls, but this is cumbersome.
More explanations were given by Mikio Hara in this thread.

Note that CL 72810 will probably offer a solution to this problem.

Thanks, setsockopt before connect solve the problem. The Control(Dialer.Control & ListenConfig.Control) callback seems nice, but I kind of dislike the nested callback thing.. also we can't actually read/write something from/to syscall.RawConn before dialing/binding, right?

Issue #9661 has been fixed for 1.11, so there will be a way to do this going forward.

It is correct that reading or writing to a syscall.RawConn before dialing or binding will not work. The calls will simply return an error.

We could see a good wayt to implement this correctly and safely without a nested callback. The problem is that the file must be locked during the operation, but permitting user code to lock files itself seems like a recipe for bugs.

Since it seems that people have identified the problem, and there doesn't seem to be anything to change in Go, I'm going to close this issue. Please comment if you disagree.

I think this would be nice:

type Dialer struct {
    Control func(network, address string, sfd int) error
}

func (d *Dialer) Dial(network, address string) (Conn, error) {
    ...
    if d.Control != nil {
        rawConn.Control(func(s uintptr) { d.Control(network, address, int(s)) })
    }
    ...
}

@damnever Windows does not use int for the file descriptor. In general the platform independent part of the os package avoids using any specific type for the system's representation of a file.

@ianlancetaylor I am not familiar with Windows. The API style is what I like better.

@damnever OK, understood, but we can't use that approach.