What is a data race in Go, how do you detect it, and what are the three main fixes?

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 behaviour: silent data corruption, non-deterministic results, or rare crashes.

// Classic data race — unsynchronised 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++  // DATA RACE: read-increment-write is not atomic
    }()
}
wg.Wait()
fmt.Println(counter) // may print anything less than 1000

// DETECT: go run -race main.go   OR   go test -race ./...
// Race detector output:
// ==================
// WARNING: DATA RACE
// Write at 0x... by goroutine 7:  main.main.func1() :11
// Previous write at 0x... by goroutine 6: main.main.func1() :11
// ==================

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

// FIX 2: atomic (faster for simple numeric ops)
var atomic64 int64
go func() { atomic.AddInt64(&atomic64, 1) }()

// FIX 3: channel — single goroutine owns the variable
type incMsg struct{}
ch := make(chan incMsg)
go func() {
    n := 0
    for range ch { n++ }
}()

The race detector adds 5–10× CPU overhead and uses ThreadSanitizer (TSan) under the hood. It reports every detected race with full stack traces for both the current and prior conflicting access — making the root cause straightforward to identify. Always run your test suite with -race.

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