When using UDPConn.SetReadBuffer, I expect to be able to set the buffer up to the kernel limit kern.ipc.maxsockbuf, but I am getting failures on Darwin and FreeBSD when trying to do that.

From what I can tell, this should be supported, assuming the code is doing a setsockopt on the SO_RCVBUF constant, from the bsd setsockopt man page:

SO_SNDBUF and SO_RCVBUF are options to adjust the normal buffer sizes
     allocated for output and input buffers, respectively.  The buffer size
     may be increased for high-volume connections, or may be decreased to
     limit the possible backlog of incoming data.  The system places an abso-
     lute maximum on these values, which is accessible through the sysctl(3)
     MIB variable ``kern.ipc.maxsockbuf''.

Versions

$ go version
go version go1.5.1 darwin/amd64
$ uname -srm
Darwin 15.0.0 x86_64

and FreeBSD:

$ uname -srm
FreeBSD 10.1-RELEASE amd64

What did you do?

This script fails on any BSD/Darwin architecture, where ReadBuffer is set equal to kern.ipc.maxsockbuf, error output I get is set udp [::]:55647: setsockopt: no buffer space available

// setreadbuffer.go
package main

import (
    "fmt"
    "net"
)

const (
    // Darwin Limit
    // sysctl kern.ipc.maxsockbuf -> 8388608
    ReadBuffer = 8388608 // - (911 * 1024) // (this works if you subtract 911kb)

    // FreeBSD Limit
    // sysctl kern.ipc.maxsockbuf -> 2097152
    // ReadBuffer = 2097152

    // Linux Limit
    // sysctl net.core.rmem_max -> 212992
    // ReadBuffer = 212992 // (this works on Linux)
)

func main() {
    addr, _ := net.ResolveUDPAddr("udp", ":80891")

    conn, err := net.ListenUDP("udp", addr)
    if err != nil {
        fmt.Printf("Failed to set up UDP listener at address %s: %s", ":8089", err)
        panic(err)
    }
    defer conn.Close()

    err = conn.SetReadBuffer(ReadBuffer) // THIS FAILS
    if err != nil {
        fmt.Printf("Failed to set UDP read buffer to %d: %s",
            ReadBuffer, err)
    }
}

What did you expect to see?

I would expect conn.SetReadBuffer(ReadBuffer) to succeed when ReadBuffer is less than or equal to kern.ipc.maxsockbuf

What did you see instead?

On BSD/Darwin, conn.SetReadBuffer(ReadBuffer) fails when ReadBuffer is equal to kern.ipc.maxsockbuf. From my limited testing it doesn't start passing until you subtract ~12% from the max.