从阅读器读取并以Golang写入Writer时是否需要检查非零长度？

When I try to copy from a Reader to a Writer manually, I notice that this works:

func fromAToB(a, b net.Conn) {
    buf := make([]byte, 1024*32)
    for {
        n, err := a.Read(buf)
        if n > 0 {
            if err != nil {
                log.Fatal(err)
            }
            b.Write(buf[0:n])
        }
    }
} 

But this doesn't

func fromAToB(a, b net.Conn) {
    buf := make([]byte, 1024*32)
    for {
        _, err := a.Read(buf)
        if err != nil {
            log.Fatal(err)
        }
        b.Write(buf)
    }
}

So the questions are:

• Why is the check if n>0 necessary?
• Is this only necessary for net.Conn or for any type that implements the Reader and Writer interfaces?

EDIT: The second snippet runs fine without any runtime error, just that the behavior is not correct. I want to know what is the effect of that n>0 check and what will happen under the surface when I remove it.

There's already a function io.Copy to do exactly this. You can see how it's implemented for a good example. It works with all io.Reader/io.Writer types.

I figured it out: without n, it will write the whole buffer (32*1024 bytes) to the Writer instead of just n bytes, and that's the source of the weird behavior.