counter: handle racing situation with mutex

counter/counter.go

@@ -1,10 +1,15 @@
 package counter
 
+import "sync"
+
 type Counter struct {
 	count int
+	mutex sync.Mutex
 }
 
 func (c *Counter) Inc() {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
 	c.count++
 }
 

counter/counter_test.go

@@ -1,6 +1,9 @@
 package counter
 
-import "testing"
+import (
+	"sync"
+	"testing"
+)
 
 func assertCounter(t testing.TB, got Counter, want int) {
 	t.Helper()
@@ -19,4 +22,21 @@ func TestCounter(t *testing.T) {
 
 		assertCounter(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()
+			}()
+		}
+		wg.Wait()
+		assertCounter(t, counter, wantedCount)
+	})
 }