httpFancyWriter.ReadFrom double-counts BytesWritten when Tee is set

[queelius]

Description

httpFancyWriter.ReadFrom in middleware/wrap_writer.go double-counts the bytes written when a Tee writer is set.

When tee != nil, ReadFrom uses io.Copy(&f.basicWriter, r) which routes writes through basicWriter.Write(). The Write method already increments basicWriter.bytes on line 112. However, after io.Copy returns, ReadFrom also adds n to basicWriter.bytes again on line 212:

func (f *httpFancyWriter) ReadFrom(r io.Reader) (int64, error) {
	if f.basicWriter.tee != nil {
		n, err := io.Copy(&f.basicWriter, r)
		f.basicWriter.bytes += int(n)  // BUG: double-counts, basicWriter.Write already incremented bytes
		return n, err
	}
	// ...non-tee path correctly counts since rf.ReadFrom bypasses basicWriter.Write
}

This causes BytesWritten() to return double the actual number of bytes when using ReadFrom with a Tee writer.

Reproduction

func TestHttpFancyWriterReadFromWithTee(t *testing.T) {
	original := &httptest.ResponseRecorder{
		HeaderMap: make(http.Header),
		Body:      new(bytes.Buffer),
	}
	f := &httpFancyWriter{basicWriter: basicWriter{ResponseWriter: original}}

	var teeBuf bytes.Buffer
	f.Tee(&teeBuf)

	input := "hello world"
	n, err := f.ReadFrom(strings.NewReader(input))
	// n == 11, err == nil
	// f.BytesWritten() == 22  -- BUG: should be 11
}

Fix

Remove the redundant f.basicWriter.bytes += int(n) line in the tee branch, since io.Copy through basicWriter.Write already handles byte counting.

[alliasgher]

Opened #1085 for this.