The race, and the fix in Go

An unguarded increment loses updates; the race detector finds it.

// racy increments the same int from many goroutines: run with -race.
func racy(times int) int {
    count := 0
    var wg sync.WaitGroup
    for range times {
        wg.Go(func() { count++ })
    }
    wg.Wait()
    return count
}

func safe(times int) int64 {
    var count atomic.Int64
    var wg sync.WaitGroup
    for range times {
        wg.Go(func() { count.Add(1) })
    }
    wg.Wait()
    return count.Load()
}

How it works

count++ is a read, an add and a write.
Run the tests with -race to have the runtime catch it.
The fix is a mutex or an atomic, not hope.

Keywords and builtins used here

for
func
int
int64
range
return
var

The run, in numbers

Lines: 20
Characters to type: 358
Tokens: 93
Three-star pace: 95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 59 seconds.

Type this snippet

Step 3 of 3 in Goroutines, step 3 of 25 in Concurrency.

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