sync: add tests

src/sync/once_test.go

@@ -0,0 +1,62 @@
+package sync_test
+
+import (
+	"sync"
+	"testing"
+)
+
+// TestOnceUncontended tests Once on a single goroutine.
+func TestOnceUncontended(t *testing.T) {
+	var once sync.Once
+	{
+		var ran bool
+		once.Do(func() {
+			ran = true
+		})
+		if !ran {
+			t.Error("first call to Do did not run")
+		}
+	}
+	{
+		var ran bool
+		once.Do(func() {
+			ran = true
+		})
+		if ran {
+			t.Error("second call to Do ran")
+		}
+	}
+}
+
+// TestOnceConcurrent tests multiple concurrent invocations of sync.Once.
+func TestOnceConcurrent(t *testing.T) {
+	var once sync.Once
+	var mu sync.Mutex
+	mu.Lock()
+	var ran bool
+	var ranTwice bool
+	once.Do(func() {
+		ran = true
+
+		// Start a goroutine and (approximately) wait for it to enter the call to Do.
+		var startWait sync.Mutex
+		startWait.Lock()
+		go func() {
+			startWait.Unlock()
+			once.Do(func() {
+				ranTwice = true
+			})
+			mu.Unlock()
+		}()
+		startWait.Lock()
+	})
+	if !ran {
+		t.Error("first call to Do did not run")
+	}
+
+	// Wait for the goroutine to finish.
+	mu.Lock()
+	if ranTwice {
+		t.Error("second concurrent call to Once also ran")
+	}
+}