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Golang / GoLang Concurrency Mastery Interview Questions

What is a livelock and how does it differ from a deadlock in Go programs?

A deadlock: all goroutines are blocked — no progress is possible. A livelock: all goroutines are actively running but constantly reacting to each other in a loop that prevents any meaningful progress — like two people in a corridor who keep stepping in the same direction to let the other pass.

// DEADLOCK: goroutines blocked — Go runtime detects it
ch := make(chan int)
ch <- 1  // send blocks, no receiver → runtime: 'all goroutines asleep'

// LIVELOCK: goroutines running but making no net progress
type lock struct{ taken bool }

func acquirePolite(own, other *lock) {
    for {
        own.taken = true
        time.Sleep(time.Millisecond)
        if other.taken {     // the other goroutine also has its lock
            own.taken = false // politely give up and retry → infinite loop!
            time.Sleep(time.Millisecond)
            continue
        }
        other.taken = true
        break // rarely reached
    }
}

a, b := &lock{}, &lock{}
go acquirePolite(a, b) // both goroutines busy but stuck
go acquirePolite(b, a)

// Go runtime does NOT detect livelocks — goroutines are 'running'

// Detection:
// - CPU at 100% with no observable progress
// - pprof CPU profile shows same functions in an infinite spin
// - go tool trace shows goroutines executing but state never advancing

// Prevention:
// - Randomised exponential backoff: rand.Sleep between retries
// - Consistent lock-acquisition ordering
// - Prefer context-based timeouts over spin-wait patterns

What is the key difference between a deadlock and a livelock?
Why does the Go runtime detect deadlocks but not livelocks?

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Explain Go's GMP scheduler model. What are M, P, and G and how do they interact? What is work stealing in Go's scheduler and why does it matter for performance? Why can Go run millions of goroutines while equivalent OS-thread workloads fail? What is the difference between unbuffered and buffered channels in Go? What happens when you send to, receive from, or close a nil or closed channel? How does the select statement work in Go and what are its key properties? What is a data race in Go, how do you detect it, and what are the three main fixes? When do you use sync.Mutex versus sync.RWMutex, and what are the critical usage rules? How does sync.WaitGroup work and what are the most common mistakes? What is sync.Once and what guarantees does it provide? What causes deadlocks in Go and how do you detect and prevent them? What are directional channels in Go and why use them in function signatures? What causes goroutine leaks and how do you prevent and detect them? Implement fan-out and fan-in concurrency patterns in Go. How do you use a nil channel in select to dynamically enable or disable cases? When should you use the sync/atomic package instead of sync.Mutex? Implement a bounded worker pool pattern in Go. What are the differences between time.After, time.NewTimer, and time.NewTicker, and which leaks resources? How does context.Context enable clean goroutine cancellation and why is 'defer cancel()' critical? How do you use a buffered channel as a semaphore to limit goroutine concurrency? What is the 'done channel' pattern and why has context.Context largely superseded it? What is the goroutine loop-variable capture bug and how do you fix it? How does golang.org/x/sync/errgroup simplify concurrent error handling? When should a channel carry 'chan struct{}' versus a typed value, and why is close() used for broadcast? What is GOMAXPROCS, how does it affect parallelism, and what is the container pitfall? How does Go's select handle multiple ready cases, and how do you implement true priority? What is sync.Cond and when do you use it instead of channels? Implement Go's canonical pipeline pattern with cancellation from the Go blog. What is Go's memory model and why does it matter for concurrent code? What is the check-then-act race condition (TOCTOU) and how do you fix it? What is asynchronous preemption in Go (1.14+) and why was it introduced? How do you safely use a map from multiple goroutines in Go? How do you use a buffered channel as a task queue with natural backpressure? How does Go handle goroutines that make blocking syscalls — what happens to M and P? Implement a simple publish-subscribe broker using Go channels. What is the lock-held-during-I/O anti-pattern and how do you fix it? Write a complete example of implementing operation timeouts in Go using select. How do you write tests that detect goroutine leaks automatically? Implement a concurrent word count across multiple files — a classic Go interview puzzle. How does GOMAXPROCS=1 change behaviour and when is it actually useful? How do you implement a high-performance sharded concurrent map in Go? What are the specific happens-before guarantees for channel operations in Go's memory model? How do you implement a hedged request pattern using select and goroutines? How does sync.Pool reduce GC pressure in high-throughput Go services? What is a livelock and how does it differ from a deadlock in Go programs? How do you implement backpressure in Go to prevent overloading downstream systems? Implement a lock-free stack using atomic CAS operations and explain the ABA problem. Summarise: channel vs mutex decision guide, and the top concurrency pitfalls.
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