fix(semaphore): panic on Unlock without a matching Lock

semaphore.go

@@ -26,9 +26,14 @@ func (s *semaphore) Lock() {
 	s.ch <- struct{}{}
 }
 
-// Unlock releases semaphore, can block if nothing acquired before.
+// Unlock releases semaphore. It panics if called without a matching Lock/TryLock,
+// matching sync.Mutex rather than blocking forever on an empty channel.
 func (s *semaphore) Unlock() {
-	<-s.ch
+	select {
+	case <-s.ch:
+	default:
+		panic("syncs: unlock of unlocked semaphore")
+	}
 }
 
 // TryLock acquires semaphore if possible, returns true if acquired, false otherwise.

semaphore_test.go

@@ -85,3 +85,18 @@ func TestSemaphore_TryLock(t *testing.T) {
 		})
 	}
 }
+
+func TestSemaphore_UnlockUnlocked(t *testing.T) {
+	sema := NewSemaphore(1)
+	assert.PanicsWithValue(t, "syncs: unlock of unlocked semaphore", func() {
+		sema.Unlock()
+	})
+
+	sema.Lock()
+	assert.NotPanics(t, func() {
+		sema.Unlock()
+	})
+	assert.PanicsWithValue(t, "syncs: unlock of unlocked semaphore", func() {
+		sema.Unlock()
+	})
+}