Improve logging messages on group termination

We can report group termination status more accurately if we know that
the termination was requested by the engine. ExecContext() uses
SIGKILL to stop processes, which tends to result in a "killed" error
depending on who wins the errgroup race: this is expected, so it would
be best to report it as such.

Conversely, a successful exit status is unexpected if the engine did
not explicitly request termination.

engine.go

@@ -143,6 +143,7 @@ type groupContext struct {
 	cancel   context.CancelFunc
 	leases   []lease
 	group    *Group
+	canceled atomic.Bool
 }
 
 func newGroupContext(ttl time.Duration) *groupContext {
@@ -216,8 +217,6 @@ func (e *Engine) buildGroupFromRequest(gctx *groupContext, tx Transaction, req *
 		return err
 	}
 
-	//log.Printf("request: %+v", req)
-
 	// Create the group.
 	group, err := newGroup(req, e.registry)
 	if err != nil {
@@ -307,12 +306,12 @@ func (e *Engine) startGroup(gctx *groupContext, tx Transaction) {
 	go func() {
 		err := tx.Execute(gctx.ctx)
 		switch {
-		case errors.Is(err, context.Canceled):
+		case gctx.canceled.Load():
 			log.Printf("group %s stopped successfully", gid)
 		case err != nil:
 			log.Printf("group %s terminated with error: %v", gid, err)
 		default:
-			log.Printf("group %s terminated successfully", gid)
+			log.Printf("group %s terminated unexpectedly without error", gid)
 		}
 
 		e.mx.Lock()
@@ -343,8 +342,10 @@ func (e *Engine) StopGroup(req *StopGroupRequest) error {
 	if !ok {
 		return fmt.Errorf("group %s not found", id)
 	}
+
 	log.Printf("terminating group %s", id)
 	rg.cancel()
+	rg.canceled.Store(true)
 
 	return nil
 }