Fix reOpenDevNull

[chenchun]

We should open /dev/null with os.O_RDWR, otherwise it won't be
possible written to it

Signed-off-by: Chun Chen ramichen@tencent.com

Found this bug when I tried to set docker containers' stdout/stderr to null if launching with --log-drive=none.
See the following simple program. It won't work without os.O_RDWR.

func main() {
    if err := reOpenDevNull(); err != nil {
        fmt.Println("Error reOpenDevNull ", err, "\n")
    }
    fmt.Println(syscall.Exec("/bin/echo", []string{"echo"}, os.Environ()))
}

func reOpenDevNull() error {
    file, err := os.Open("/dev/null")
//  file, err := os.OpenFile("/dev/null", os.O_RDWR, 0) // should open it with os.O_RDWR
    if err != nil {
        return fmt.Errorf("Failed to open /dev/null - %s", err)
    }
    defer file.Close()
    for fd := 1; fd < 3; fd++ {
        if err := syscall.Dup3(int(file.Fd()), fd, 0); err != nil {
            return err
        }
    }
    return nil
}

[crosbymichael]

LGTM

[LK4D4]

LGTM