Golang Bufio writer .Flush() does not write small data when big buffer size

Question

Golang Bufio writer .Flush() does not write small data when big buffer size (example 4096(standard size)*2)

package main
import (
    "log"
    "os"
    "bufio"
)
func main() {
    file, err := os.Create("test")
    defer file.Close()
    w := bufio.NewWriter(file)

    w = bufio.NewWriterSize(
        w,
        4096*2,
    )
    bytesAvailable := w.Available()
    log.Printf("Available %v\n", bytesAvailable)
    bw, _ := w.Write(
      []byte("A"),
    )
    log.Printf("written bytes: %v\n", bw)
    bytesAvailable = w.Available()
    log.Printf("Available: %v\n", bytesAvailable)
    buf := w.Buffered()
    log.Printf("buffered: %d\n", buf)
    err = w.Flush()
    if err != nil {
        log.Fatal(err)
    }
}

When I use the standard size or I write more data it works as expected.

Answer 1

The problem is that the application has two layers of bufio writers:

w := bufio.NewWriter(file)
w = bufio.NewWriterSize(
    w,
    4096*2,
)

One bufio.Writer wraps the other. The application flushes the outer bufio.Writer, but there's no code that flushes the inner bufio.Writer. Change the code to use a single bufio.Writer and the program will work as expected.

w := bufio.NewWriterSize(
    file,
    4096*2,
)