Not using errgroup
Mistake
This code correctly handles the aggregation of results with no data races, but has no solution for error handling.
func handler (ctx context.Context, circles []Circle) ([]Result, error) {
results := make([]Result, len(circles))
wg := sync.WaitGroup{}
wg.Add(len(results))
for i, circle := range circles {
// variable shadowing to avoid data race
i := i
circle := circle
go func() {
defer wg.Done()
result, err := foo(ctx, circle)
if err != nil {
// ??????
}
results[i] = result
}()
}
wg.Wait()
}
Fix
func handle(ctx context.Context, circles []Circle) ([]Result, error) {
results := make([]Result, len(circles))
g, ctx := errgroup.WithContext(ctx)
for i, circle := range circles {
i := i
circle := circle
g.Go(func() error {
result, err := foo(ctx, circle)
if err != nil {
return err
}
results[i] = result
return nil
})
}
err := g.Wait()
if err != nil {
return nil, err
}
return results, nil
}