golang File WriteString cover - go

Question

I want get increment id with file, code as below:
// get increment id
func GetID() uint64 {
appIdLock.Lock()
defer appIdLock.Unlock()
f, err := os.OpenFile(idPath, os.O_RDWR, 0666)
if err != nil {
return 0
}
defer f.Close()
// Read
bufferTemp := make([]byte, 16)
bufferResult := make([]byte, 0)
for {
n, _ := f.Read(bufferTemp)
if n > 0 {
bufferResult = append(bufferResult, bufferTemp[:n]...)
} else {
break
}
}
if len(bufferResult) == 0 {
return 0
}
s := common.StringToUint64(string(bufferResult))
s += 1
// Write (how to cover?)
f.WriteString(strconv.FormatUint(s, 10))
return s
}
f.WriteString function was append, example, my file content: 123, run GetID() I hope my file content is: 124, But result was: 123124

Answer 1

Without changing much of your code, here is the solution that will work and do what you want to do. No loops or more than a single byte slice.
func GetID() uint64 {
appIdLock.Lock()
defer appIdLock.Unlock()
// Added + os.O_CREATE to create the file if it doesn't exist.
f, err := os.OpenFile(idPath, os.O_RDWR + os.O_CREATE, 0666)
if err != nil {
fmt.Println(err)
return 0
}
defer f.Close()
// Know file content beforehand so I allocate a suitable bytes slice.
fileStat, err := f.Stat()
if err != nil {
fmt.Println(err)
return 0
}
buffer := make([]byte, fileStat.Size())
_, err = f.Read(buffer)
if err != nil {
fmt.Println(err)
return 0
}
s, _ := strconv.ParseUint(string(buffer), 10, 64)
s += 1
// The Magic is here ~ Writes bytes at 0 index of the file.
_, err = f.WriteAt([]byte(strconv.FormatUint(s, 10)), 0)
if err != nil {
fmt.Println(err)
return 0
}
return s
}
Hope it helps!