https://github.com/prometheus/prometheus/issues/484 minimal test case

gistfile1.go

package main

import (
  "flag"
  "fmt"
  "os"
)

var path = flag.String("path", "./abc", "Path to create test file")

func main() {
  flag.Parse()
  f, err := os.Create(*path)
  if err != nil {
    panic(err)
  }
  f.Write([]byte("a\n"))
  f.Close()

  f, err = os.OpenFile(*path, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0640)
  if err != nil {
    panic(err)
  }
  f.Write([]byte("bc\n"))

  offset, err := f.Seek(0, os.SEEK_CUR)
  if err != nil {
    panic(err)
  }
  fmt.Printf("lseek returns: %d Expected: 5\n", offset)
}