## Why Does Deferring GZIP Writer Closure Lead to Data Loss in Go?

Deferring GZIP Writer Closure Can Lead to Data Loss

The GZIP writer is a commonly used utility in Go for data compression. This writer has a Close method that not only flushes data but also writes the GZIP footer, which is required for successful decompression.

In certain scenarios, it's tempting to defer the Close call to ensure it's executed at the end of a function. However, this approach can result in data loss.

To understand why, consider the following code:

<code class="go">func main() {
    data := []byte("Hello World")
    compressedData, err := zipData(data)
    if err != nil {
        panic(err)
    }

    uncompressedData, err := unzipData(compressedData)
    if err != nil {
        panic(err)
    }

    fmt.Println(string(uncompressedData))
}</code>

The zipData function uses a GZIP writer, and the unzipData function reads the compressed data:

<code class="go">func zipData(data []byte) ([]byte, error) {
    buf := new(bytes.Buffer)
    gw := gzip.NewWriter(buf)

    // Deferring Close here causes data loss!
    defer gw.Close()

    _, err := gw.Write(data)
    if err != nil {
        return nil, err
    }

    err = gw.Flush()
    if err != nil {
        return nil, err
    }

    return buf.Bytes(), nil
}</code>

<code class="go">func unzipData(data []byte) ([]byte, error) {
    r := bytes.NewReader(data)
    gr, err := gzip.NewReader(r)
    if err != nil {
        return nil, err
    }
    defer gr.Close()

    uncompressed, err := ioutil.ReadAll(gr)
    if err != nil {
        return nil, err
    }
    return uncompressed, nil
}</code>

The issue arises when deferring the Close call in zipData. This means the function returns before the GZIP footer is written, resulting in the omission of essential data from the compressed byte array. This error manifests as an unexpected EOF when attempting to read the decompressed data.

To resolve this issue, the Close method of the GZIP writer should be called before returning the compressed data. This ensures that the footer is written, and the data is complete.

<code class="go">func zipData(data []byte) ([]byte, error) {
    buf := new(bytes.Buffer)
    gw := gzip.NewWriter(buf)

    // Close the writer before returning
    defer gw.Close()

    _, err := gw.Write(data)
    if err != nil {
        return nil, err
    }

    err = gw.Flush()
    if err != nil {
        return nil, err
    }

    return buf.Bytes(), nil
}</code>

By closing the writer before returning, we guarantee that the complete compressed data, including the GZIP footer, is available for decompression without any data loss.

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