sync2: add TryAcquire to Semaphore

go/sync2/semaphore.go

@@ -49,6 +49,17 @@ func (sem *Semaphore) Acquire() bool {
 	}
 }
 
+// TryAcquire acquires a semaphore if it's immediately available.
+// It returns false otherwise.
+func (sem *Semaphore) TryAcquire() bool {
+	select {
+	case <-sem.slots:
+		return true
+	default:
+		return false
+	}
+}
+
 // Release releases the acquired semaphore. You must
 // not release more than the number of semaphores you've
 // acquired.

go/sync2/semaphore_test.go

@@ -20,7 +20,7 @@ func TestSemaNoTimeout(t *testing.T) {
 	}()
 	s.Acquire()
 	if !released {
-		t.Errorf("want true, got false")
+		t.Errorf("release: false, want true")
 	}
 }
 
@@ -31,11 +31,25 @@ func TestSemaTimeout(t *testing.T) {
 		time.Sleep(10 * time.Millisecond)
 		s.Release()
 	}()
-	if ok := s.Acquire(); ok {
-		t.Errorf("want false, got true")
+	if s.Acquire() {
+		t.Errorf("Acquire: true, want false")
 	}
 	time.Sleep(10 * time.Millisecond)
-	if ok := s.Acquire(); !ok {
-		t.Errorf("want true, got false")
+	if !s.Acquire() {
+		t.Errorf("Acquire: false, want true")
+	}
+}
+
+func TestSemaTryAcquire(t *testing.T) {
+	s := NewSemaphore(1, 0)
+	if !s.TryAcquire() {
+		t.Errorf("TryAcquire: false, want true")
+	}
+	if s.TryAcquire() {
+		t.Errorf("TryAcquire: true, want false")
+	}
+	s.Release()
+	if !s.TryAcquire() {
+		t.Errorf("TryAcquire: false, want true")
 	}
 }