[hw-5] concurrency, graceful shutdown, concurrent tests

cart/internal/infra/errgroup/errgroup.go

@@ -0,0 +1,62 @@
+package errgroup
+
+import (
+	"context"
+	"sync"
+)
+
+type Group struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	wg sync.WaitGroup
+
+	firstErr chan error
+
+	// Used to limit number of goroutines (workers) for group.
+	workerLimCh chan struct{}
+}
+
+func WithContext(parent context.Context, workers int) (*Group, context.Context) {
+	ctx, cancel := context.WithCancel(parent)
+	return &Group{
+		ctx:         ctx,
+		cancel:      cancel,
+		firstErr:    make(chan error, 1),
+		workerLimCh: make(chan struct{}, workers),
+	}, ctx
+}
+
+func (g *Group) Go(f func(ctx context.Context) error) {
+	// Wait for worker to be freed.
+	g.workerLimCh <- struct{}{}
+
+	g.wg.Add(1)
+	go func() {
+		defer func() {
+			g.wg.Done()
+
+			<-g.workerLimCh // release worker
+		}()
+
+		if err := f(g.ctx); err != nil {
+			select {
+			case g.firstErr <- err:
+				g.cancel()
+			default: // got error from other goroutine
+			}
+		}
+	}()
+}
+
+func (g *Group) Wait() error {
+	g.wg.Wait()
+	g.cancel()
+
+	select {
+	case err := <-g.firstErr:
+		return err
+	default:
+		return nil
+	}
+}