What causes deadlocks in Go and how do you detect and prevent them?

A deadlock occurs when every goroutine is blocked waiting for a resource held by another — a circular dependency from which no goroutine can proceed. The Go runtime detects simple all-goroutine deadlocks and panics with 'all goroutines are asleep — deadlock!'.

// DEADLOCK 1: circular channel wait
ch1 := make(chan int)
ch2 := make(chan int)
go func() { v := <-ch1; ch2 <- v }() // waits for ch1, then sends ch2
go func() { v := <-ch2; ch1 <- v }() // waits for ch2, then sends ch1
// ALL goroutines blocked → runtime: 'all goroutines are asleep'

// DEADLOCK 2: non-reentrant mutex locked twice
var mu sync.Mutex
mu.Lock()
mu.Lock()   // DEADLOCK — tries to acquire a lock already held

// DEADLOCK 3: inconsistent lock-acquisition order
var muA, muB sync.Mutex
// goroutine 1: muA.Lock() then muB.Lock()  (A→B order)
// goroutine 2: muB.Lock() then muA.Lock()  (B→A order) → DEADLOCK

// FIX for #3: enforce a global consistent ordering — always A before B

// DEADLOCK 4: unbuffered send with no receiver
// ch := make(chan int)
// ch <- 1  // blocks forever

// Detection tools:
// 1. Go runtime: 'all goroutines are asleep'
// 2. CTRL+\  sends SIGQUIT → dumps all goroutine stacks
// 3. pprof goroutine endpoint: /debug/pprof/goroutine?debug=2
// 4. context.WithTimeout prevents indefinite blocking

Prevention rules: (1) acquire locks in a globally consistent order. (2) Prefer context.Context with deadlines over raw channel waits. (3) Use select with default or a timeout to avoid indefinite blocking. (4) Keep critical sections short and never hold a lock while performing I/O.

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