Fixup line wrapper short write bug

[agcom]

TL;DR, without this patch, the following test would not pass:

func TestLineWrapperShortWrite(t *testing.T) {
    var sb strings.Builder
    w := &lineWrapper{w: &sb, maxLineLen: 76}

    b := []byte("The end.\r\n")
    n, err := w.Write(b)
    if err != nil {
        t.Fatalf("Write() = %v", err)
    }

    if n != len(b) {
        // FAILS HERE.
        t.Errorf("Written = %d, want %d", n, len(b))
    }
}

This is known as io.ErrShortWrite; I encountered it in bytes.Reader.WriteTo (via io.Copy):

package bytes

...

// WriteTo implements the [io.WriterTo] interface.
func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
	r.prevRune = -1
	if r.i >= int64(len(r.s)) {
		return 0, nil
	}
	b := r.s[r.i:]
	m, err := w.Write(b)
	if m > len(b) {
		panic("bytes.Reader.WriteTo: invalid Write count")
	}
	r.i += int64(m)
	n = int64(m)
	if m != len(b) && err == nil {
		err = io.ErrShortWrite
	}
	return
}

...

[emersion]

The io.Writer docs say:

It returns the number of bytes written from p (0 <= n <= len(p))

IOW: if we write extra bytes not contained in the input slice, I don't think we should count these?

[emersion]

Ah, but sometimes the \n that we write was in the input. How about this?

if lf {
    written++
}

[agcom]

Ah, but sometimes the \n that we write was in the input. How about this?

if lf {
    written++
}

Yes, your suggestion is correct. The count of written bytes should match the length of the given slice of bytes (unless returning an error); my initial solution would add two extra written bytes in case of reaching max line length and writing \r\n. I will update in a minute.

[agcom]

Also, just an observation: the underlying writer's written bytes count (n in n, err := w.w.Write(l)) should be accounted for, before checking the error; I guess this is a separate issue.

[emersion]

Thanks!