defer and cleanup in Go

Guaranteed cleanup, and returning a deferred error.

func copyFile(src, dst string) (err error) {
    in, err := os.Open(src)
    if err != nil {
        return fmt.Errorf("open %s: %w", src, err)
    }
    defer in.Close()

    out, err := os.Create(dst)
    if err != nil {
        return err
    }
    defer func() {
        if cerr := out.Close(); err == nil {
            err = cerr
        }
    }()
    _, err = out.ReadFrom(in)
    return err
}

How it works

1. defer runs when the function returns, in LIFO order.
2. A named result lets a deferred closure set the error.
3. %w wraps the failure with context.

Keywords and builtins used here

• defer
• error
• func
• if
• return
• string

The run, in numbers

Lines

19

Characters to type

311

Tokens

102

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 58 seconds.

Type this snippet

Step 1 of 4 in defer, panic & recover, step 10 of 15 in Errors.

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