What is a data race in Go and how do you detect one?

A data race occurs when two or more goroutines access the same memory location concurrently, at least one access is a write, and there is no synchronisation between them. Data races produce undefined, non-deterministic behaviour — results vary between runs and can silently corrupt data.

// DATA RACE — unsafe counter increment from multiple goroutines
var counter int
var wg sync.WaitGroup

for i := 0; i < 1000; i++ {
    wg.Add(1)
    go func() {
        defer wg.Done()
        counter++  // READ + INCREMENT + WRITE — not atomic!
    }()
}
wg.Wait()
fmt.Println(counter)  // less than 1000 — data was lost!

// DETECT: go run -race main.go  or  go test -race ./...
// Race detector output:
// ==================
// WARNING: DATA RACE
// Write at 0x... by goroutine 8: main.main.func1() :12
// Previous write at 0x... by goroutine 7: main.main.func1() :12
// ==================

// FIX 1: sync.Mutex
var mu sync.Mutex
go func() { mu.Lock(); counter++; mu.Unlock() }()

// FIX 2: atomic operation (faster for simple numeric ops)
var atomicCounter int64
go func() { atomic.AddInt64(&atomicCounter, 1) }()

// FIX 3: channel — one goroutine owns the counter
inc := make(chan struct{}, 100)
go func() { n := 0; for range inc { n++ } }()
inc <- struct{}{}  // safe

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