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Golang / Golang Internals and Memory Management Interview Questions

How does sync.Once work and what are its use cases?

sync.Once guarantees that a function is executed exactly once, regardless of how many goroutines call it concurrently. It is the idiomatic Go way to implement lazy initialisation and singletons without explicit locking in user code.

var (
    instance *Database
    once     sync.Once
)

func GetDB() *Database {
    once.Do(func() {
        // This function runs exactly once across all goroutines
        instance = &Database{
            conn: openConnection(config),
        }
    })
    return instance // safe to return — once.Do blocks until init completes
}

// All concurrent callers either:
// - Execute the function themselves (first caller)
// - Block until the function completes and then return (all others)

// ERROR HANDLING in sync.Once — no built-in mechanism
// Pattern: store the error alongside the result
type singleton struct {
    db  *Database
    err error
}
var s singleton
var initOnce sync.Once

func getDB() (*Database, error) {
    initOnce.Do(func() {
        s.db, s.err = openDB()
    })
    return s.db, s.err
}

// sync.OnceFunc (Go 1.21) — wraps a function so it runs at most once
initDB := sync.OnceFunc(func() { /* initialise */ })
initDB() // safe to call from multiple goroutines
initDB() // no-op — function already ran

// sync.OnceValue / sync.OnceValues (Go 1.21) — return a value
getConfig := sync.OnceValue(func() *Config { return loadConfig() })
cfg := getConfig() // computed once, cached

sync.Once has no reset mechanism — once the function has run, there is no way to make it run again. This is by design. If you need resettable one-time execution, use a mutex-protected boolean flag instead. Go 1.21 added sync.OnceFunc, sync.OnceValue, and sync.OnceValues as ergonomic wrappers.

What does sync.Once.Do() guarantee about the wrapped function?
What happens if the function passed to sync.Once.Do() panics?

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