How Can I Create a \'tail -f\' Like Generator in Go?

"tail -f"-like Generator in Go

Problem

Tailing a file in Go requires a loop with a sleep on EOF, which can be error-prone and inefficient. Is there a cleaner way to do this?

Solution

Instead of using a goroutine, create a wrapper around a reader that sleeps on EOF:

<code class="go">type tailReader struct {
    io.ReadCloser
}

func (t tailReader) Read(b []byte) (int, error) {
    for {
        n, err := t.ReadCloser.Read(b)
        if n > 0 {
            return n, nil
        } else if err != io.EOF {
            return n, err
        }
        time.Sleep(10 * time.Millisecond)
    }
}

func newTailReader(fileName string) (tailReader, error) {
    f, err := os.Open(fileName)
    if err != nil {
        return tailReader{}, err
    }

    if _, err := f.Seek(0, 2); err != nil {
        return tailReader{}, err
    }
    return tailReader{f}, nil
}</code>

Usage

This reader can be used with any io.Reader, including bufio.Scanner and json.Decoder. For example:

<code class="go">t, err := newTailReader(&quot;somefile&quot;)
if err != nil {
    log.Fatal(err)
}
defer t.Close()

// Use with bufio.Scanner
scanner := bufio.NewScanner(t)
for scanner.Scan() {
    fmt.Println(scanner.Text())
}

// Use with json.Decoder
dec := json.NewDecoder(t)
for {
    var v SomeType
    if err := dec.Decode(&amp;v); err != nil {
       log.Fatal(err)
    }
    fmt.Println(&quot;the value is &quot;, v)
}</code>

Advantages

• Easy Shutdown: Just close the file.
• Works with Many Packages: Many packages (like bufio.Scanner and json.Decoder) work with io.Reader.
• Configurable Sleep: Adjust the sleep time to optimize latency or CPU usage.

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