Why Does Deferring gzip.NewWriter Closure Cause Data Loss in Go?

Deferring Closure Causes Data Loss in gzip Writer

When utilizing the gzip.NewWriter in Go to compress a byte slice, it's commonly observed that deferring the closure of the writer results in data loss. This phenomenon becomes evident when reading the compressed data, as it prematurely terminates with an unexpected end-of-file (EOF) error.

To understand this, let's examine the provided code snippet:

<code class="go">func zipData(originData []byte) ([]byte, error) {
    // Create a buffer to store the compressed data
    var bf bytes.Buffer
    
    // Initialize the gzip writer with the buffer
    gw := gzip.NewWriter(&bf)
    
    // Defer closure of the writer
    defer gw.Close()
    
    // Write the original data to the writer
    _, err := gw.Write(originData)
    if err != nil {
        return nil, err
    }
    
    // Flush the writer to write any buffered data
    if err = gw.Flush(); err != nil {
        return nil, err
    }
    
    // Return the compressed data
    return bf.Bytes(), nil
}</code>

The issue stems from the use of defer to close the gzip writer (gw). According to the documentation for Close():

"Close closes the Writer by flushing any unwritten data to the underlying io.Writer and writing the GZIP footer."

In this case, the deferred closure executes after the function returns the compressed data stored in the buffer (bf). However, the footer is not written until the Close() method is invoked, leaving the compressed data incomplete when it's returned.

This issue can be resolved by manually closing the writer before returning the compressed data:

<code class="go">func zipData(originData []byte) ([]byte, error) {
    // ... (same code as before) ...
    
    // Close the writer to write the footer
    if err := gw.Close(); err != nil {
        return nil, err
    }
    
    // Return the compressed data
    return bf.Bytes(), nil
}</code>

By closing the writer before returning, the GZIP footer is successfully written, and the compressed data can be read without encountering the EOF error.

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