Deleting bytes from a io.ReadWriteSeeker(via file) - go

Question

Let's say I have an io.ReadWriteSeeker that is reading an writing from a certain file.
At some point I decide I want to remove some bytes from the file (specifically the end) and reduce its length.
Whats the right way to go about this?
I can't just overwrite it by seeking because I want to end up with a shorter file.
io.CopyN will similarly not work (as src is smaller than dest)

Answer 1

If the io.ReadWriteSeeker value also supports a Truncate method, like an *os.File value.
For example,
package main
import (
"fmt"
"io"
"io/ioutil"
"os"
)
func truncate(rws io.ReadWriteSeeker, size int64) error {
type Truncater interface {
Truncate(size int64) error
}
t, ok := rws.(Truncater)
if !ok {
return fmt.Errorf("truncate: unable to truncate")
}
return t.Truncate(size)
}
func main() {
filename := `/tmp/truncate.test.file`
f, err := os.Create(filename)
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
defer f.Close()
n, err := f.Write([]byte("test data"))
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
data, err := ioutil.ReadFile(filename)
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
fmt.Println(string(data))
// truncate io.ReadWriteSeeker
err = truncate(f, int64(n-1))
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
data, err = ioutil.ReadFile(filename)
if err != nil {
fmt.Fprintln(os.Stderr, err)
return
}
fmt.Println(string(data))
os.Remove(filename)
}
Playground: https://play.golang.org/p/pp4IUSoKo4M
Output:
test data
test dat