Failing concurrent test

learn_go_with_tests/my_sync/go.mod

@@ -0,0 +1,3 @@
+module my_sync
+
+go 1.23.1

learn_go_with_tests/my_sync/my_sync.go

@@ -0,0 +1,13 @@
+package my_sync
+
+type Counter struct {
+	count int
+}
+
+func (c *Counter) Inc() {
+	c.count++
+}
+
+func (c *Counter) Value() int {
+	return c.count
+}

learn_go_with_tests/my_sync/my_sync_test.go

@@ -0,0 +1,44 @@
+package my_sync
+
+import (
+	"sync"
+	"testing"
+)
+
+func assertCount(t testing.TB, got Counter, want int) {
+	t.Helper()
+	if got.Value() != want {
+		t.Errorf("got %d, want %d", got.Value(), want)
+	}
+}
+
+func TestCounter(t *testing.T) {
+	t.Run("Incrementing the counter 3 times leaves it at 3", func(t *testing.T) {
+		counter := Counter{}
+		counter.Inc()
+		counter.Inc()
+		counter.Inc()
+
+		assertCount(t, counter, 3)
+
+	})
+
+	t.Run("it runs safely concurrently", func(t *testing.T) {
+		wantedCount := 1000
+		counter := Counter{}
+
+		var wg sync.WaitGroup
+		wg.Add(wantedCount)
+
+		for i := 0; i < wantedCount; i++ {
+			go func() {
+				counter.Inc()
+				wg.Done()
+			}()
+		}
+		// blocking call to wait for all goroutines
+		wg.Wait()
+
+		assertCount(t, counter, wantedCount)
+	})
+}