feat(process): allow zero-value task signals

Co-Authored-By: Virgil <virgil@lethean.io>

actions.go

@@ -66,6 +66,7 @@ type TaskProcessKill struct {
 }
 
 // TaskProcessSignal requests signalling a managed process by ID or PID through Core.PERFORM.
+// Signal 0 is allowed for liveness checks.
 //
 // Example:
 //

service.go

@@ -668,10 +668,6 @@ func (s *Service) handleTask(c *core.Core, task core.Task) core.Result {
 			return core.Result{Value: coreerr.E("Service.handleTask", "task process kill requires an id or pid", nil), OK: false}
 		}
 	case TaskProcessSignal:
-		if m.Signal == 0 {
-			return core.Result{Value: coreerr.E("Service.handleTask", "task process signal requires a signal", nil), OK: false}
-		}
-
 		switch {
 		case m.ID != "":
 			if err := s.Signal(m.ID, m.Signal); err != nil {

service_test.go

@@ -815,6 +815,30 @@ func TestService_OnStartup(t *testing.T) {
 		assert.Equal(t, StatusKilled, proc.Status)
 	})
 
+	t.Run("allows signal zero liveness checks", func(t *testing.T) {
+		svc, c := newTestService(t)
+
+		err := svc.OnStartup(context.Background())
+		require.NoError(t, err)
+
+		ctx, cancel := context.WithCancel(context.Background())
+		defer cancel()
+
+		proc, err := svc.Start(ctx, "sleep", "60")
+		require.NoError(t, err)
+
+		result := c.PERFORM(TaskProcessSignal{
+			ID:     proc.ID,
+			Signal: syscall.Signal(0),
+		})
+		require.True(t, result.OK)
+
+		assert.True(t, proc.IsRunning())
+
+		cancel()
+		<-proc.Done()
+	})
+
 	t.Run("registers process.wait task", func(t *testing.T) {
 		svc, c := newTestService(t)