This is some go example code of two readers reading words/bytes from input data before passing a limited size buffer on to a server.

wordByteReader.go

func GetExampleData() []byte {
	return []byte(`"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."`)
}

var ErrWordTooLong = errors.New("word too long")
var ErrBufferEmpty = errors.New("buffer empty")
var ErrEndOfWord = errors.New("end of word")

func TestWordBuffer(t *testing.T) {
	b := bytes.NewBuffer(GetExampleData())
	r := bufio.NewReaderSize(b, b.Len())
	maxSize := 10

	for {
		w, err := readWord(maxSize, r)
		if err != nil {
			if err == io.EOF {
				t.Logf("%s", string(*w))
				break
			}

			if err == ErrEndOfWord {
				t.Logf("%s", string(*w))
				continue
			}

			if err == ErrWordTooLong {
				t.Error(err.Error())
				continue
			}

			t.Error(err)
			continue
		}

		if w != nil {
			t.Logf("%s", string(*w))
		}
	}
}

func readWord(maxSize int, r *bufio.Reader) (*[]byte, error) {
	size := 0
	word := []byte{}

	for {
		if size > maxSize {
			return nil, ErrWordTooLong
		}

		b, err := r.ReadByte()
		if err != nil {
			if err == io.EOF {
				return &word, io.EOF
			}

			return nil, err
		}

		if unicode.IsSpace(rune(b)) {
			// end of word
			if len(word) == 0 {
				// jump over whitespace
				continue
			}

			return &word, ErrEndOfWord
		}

		word = append(word, b)
		size++
	}
}

func TestPassOnBytes(t *testing.T) {
	// build a server
	mux := http.NewServeMux()
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusAccepted)
		defer r.Body.Close()

		b, err := io.ReadAll(r.Body)
		if err != nil {
			t.Errorf("error reading body: %s", err.Error())
			w.WriteHeader(http.StatusBadRequest)
			_, err := w.Write([]byte(err.Error()))
			if err != nil {
				t.Errorf("error writing response: %s", err.Error())
				return
			}
			return
		}

		t.Logf("server received: %s", string(b))

		_, err = w.Write([]byte(`ok`))
		if err != nil {
			t.Errorf("error writing response: %s", err.Error())
			return
		}
	})

	srv := httptest.NewServer(mux)
	defer srv.Close()

	b := bytes.NewBuffer(GetExampleData())

	client := http.DefaultClient
	client.Timeout = 5 * time.Second

	for w := range scanBytes(2, bufio.NewScanner(b)) {
		func() {
			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
			defer cancel()

			r := bytes.NewReader(w)
			req, err := http.NewRequestWithContext(ctx, http.MethodPost, srv.URL, r)
			if err != nil {
				t.Errorf("error creating request: %s", err.Error())
				return
			}

			res, err := client.Do(req)
			if err != nil {
				t.Errorf("error sending request: %s", err.Error())
				return
			}

			if res.StatusCode != http.StatusAccepted {
				t.Errorf("unexpected status code: %d", res.StatusCode)
				return
			}
		}()

		time.Sleep(250 * time.Millisecond)
	}
}

func scanBytes(amount int, scan *bufio.Scanner) <-chan []byte {
	out := make(chan []byte)

	go func() {
		defer close(out)

		scan.Split(bufio.ScanBytes)

		batch := make([]byte, 0, amount)
		for scan.Scan() {
			word := scan.Bytes()
			batch = append(batch, word...)

			if len(batch) == amount {
				out <- batch
				batch = make([]byte, 0, amount)
			}
		}
		out <- batch
	}()

	return out
}