How to test EOF on io.Reader in Go?

Question

The Go's io.Reader documentation states that a Read() may return a non zero n value and an io.EOF at the same time. Unfortunately, the Read() method of a File doesn't do that.

When the EOF is reached and some bytes could still be read, the Read method of file returns non zero n and nil error. It is only when we try to read when already at the end of the file that we get back zero n and io.EOF as error.

I couldn't find a simple method to test if the EOF is reached without trying to read data from the file. If we perform a Read() with a buffer of 0 byte, we get back zero n and nil error although we are at the end of file.

To avoid this last read, the only solution I have found is to keep track myself of the number of bytes remaining to read in the file. Is there a simpler solution ?

Answer 1

You could create a new type, that keeps track of the number of bytes read so far. Then, at EOF check time, you could compare the expected number of bytes read with the actual number of bytes read. Here is a sample implementation. The eofReader keeps track of the number of bytes read and compares it to the file size, in case the underlying type is a file:

package main

// ... imports 

// eofReader can be checked for EOF, without a Read. 
type eofReader struct {
    r     io.Reader
    count uint64
}

// AtEOF returns true, if the number of bytes read equals the file size.
func (r *eofReader) AtEOF() (bool, error) {
    f, ok := r.r.(*os.File)
    if !ok {
        return false, nil
    }
    fi, err := f.Stat()
    if err != nil {
        return false, err
    }
    return r.Count() == uint64(fi.Size()), nil
}

// Read reads and counts.
func (r *eofReader) Read(buf []byte) (int, error) {
    n, err := r.r.Read(buf)
    atomic.AddUint64(&r.count, uint64(n))
    return n, err
}

// Count returns the count.
func (r *eofReader) Count() uint64 {
    return atomic.LoadUint64(&r.count)
}

You could use this type by wrapping any reader in an eofReader:

func main() {
    f, err := os.Open("main.go")
    if err != nil {
        log.Fatal(err)
    }

    r := &eofReader{r: f}
    log.Println(r.AtEOF())

    if _, err = ioutil.ReadAll(r); err != nil {
        log.Fatal(err)
    }

    log.Println(r.AtEOF())
}

// 2016/12/19 03:49:35 false <nil>
// 2016/12/19 03:49:35 true <nil>

Code as gist.