File scanner loop does not execute - file

Question

I'm getting the last line of a text file, and try to read it.
get last line:
func getLastLine(file *os.File) (result int) {
s := bufio.NewScanner(file)
result = 0
for s.Scan() {
result++
}
err := s.Err()
if err != nil {
log.Fatal(err)
}
return
}
read file:
func readFileFrom(file *os.File) {
s := bufio.NewScanner(file)
for s.Scan() {
fmt.Println(s.Text())
}
err := s.Err()
if err != nil {
log.Fatal(err)
}
}
If i write this in main.go:
getLastLine(file)
readFileFrom(file)
It will not execute the block:
for s.Scan() {
fmt.Println(s.Text())
}
If I remove the line getLastLine(file), the reading works as expected.
I think it's because 2 Scanners are accessing the same file.

Answer 1

os.File maintains the position where the next read or write operation will work. Reading from / writing to the file updates this position.
If you use a single file, passing it to getLastLine() will read it till its end, so its pointer will point to the end of the file. Now passing it to readFileFrom() will not read and print anything because there is no more data after the end of the file (that's the definition of the "end").
You need to either rewind the pointer using File.Seek(), or you need to close and reopen it. Obviously just rewinding is more efficient. To set the pointer to the file start:
if _, err := file.Seek(0, io.SeekStart); err != nil {
panic(err)
}
So do this between the 2 function calls:
getLastLine(file)
if _, err := file.Seek(0, io.SeekStart); err != nil {
panic(err)
}
readFileFrom(file)
Also note that if you would open the file twice, you would not need to rewind it, and you could also run the 2 functions concurrently without interfering with each other, because they only read the file and each os.File has its own pointer.
file1, err := os.Open("a.txt")
// handle err
defer file1.Close()
file2, err := os.Open("a.txt")
// handle err
defer file2.Close()
wg := sync.WaitGroup()
wg.Add(1)
go func() {
defer wg.Done()
getLastLine(file1)
}()
readFileFrom(file2)
wg.Wait() // Wait for getLastLine() to complete