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

Deferring GZIP Writer Closure Leads to Data Loss

In Go, using defer to close a gzip.Writer can result in unexpected EOF errors when reading from the zipped data. To resolve this issue, let's delve into the specifics of the problem and provide an alternative solution.

Understanding the Issue:

The gzip.Writer's Close method performs two tasks: it flushes any unwritten data to the underlying writer and writes the GZIP footer. However, in the code provided:

<code class="go">func zipData(originData []byte) ([]byte, error) {
    // ...

    defer gw.Close()

    // ...
}</code>

The defer statement delays the execution of gw.Close() until the surrounding function zipData returns. Therefore, when zipData finishes and returns, the footer is written to an unsaved buffer and not included in the returned byte array. This causes unexpected EOF errors when attempting to read from the zipped data.

Alternative Solution:

To resolve the issue, it is recommended to close the writer before returning the zipped data:

<code class="go">func zipData(originData []byte) ([]byte, error) {
    // ...

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

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

    // ...
}</code>

By explicitly closing the writer before returning, you ensure that the GZIP footer is written to the saved buffer and thus included in the returned byte array. This prevents unexpected EOF errors and guarantees the integrity of the zipped data.

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