Lazy config load behind a factory closure

byob-config.3 config

Problem: if main() always reads, parses, and validates the config file, mytool --version and mytool --help pay a filesystem cost and can fail on a broken config file they don't even need.

Idea: put config loading inside a lazy factory closure. Only commands that actually dereference f.Config() trigger the load. sync.Once guarantees one load per process.

Tradeoffs: you must remember f.Config() (invoke) instead of a bare field. Alternative: eager load. Simpler to reason about, slower to start, and turns "broken config file" into "can't see --help".

Design

func (f *Factory) configProvider() func() (*Config, error) {
    var (
        once sync.Once
        cfg  *Config
        err  error
    )
    return func() (*Config, error) {
        once.Do(func() {
            path, e := FindConfigUp("mytool.toml")
            if e != nil { cfg, err = defaultConfig(), nil; return }
            cfg, err = loadAndValidate(path)
        })
        return cfg, err
    }
}

// main() never reads config; 'mytool --help' is filesystem-free.