fix: handle missing TaskPage key in workflow activity mutation

When setting PAGE on a workflow activity where the TaskPage BSON key
is absent (not just nil), dSet silently failed because it only updates
existing keys. Added replaceActivity helper that appends the key to
the activity document and replaces it in the BSON tree by  match.

Three cases now handled correctly:
- TaskPage exists with value: update Page field in place
- TaskPage exists with nil value: replace via dSet
- TaskPage absent: append key, replace activity in tree

Author: retran

mdl/backend/mpr/workflow_mutator.go

@@ -162,14 +162,21 @@ func (m *mprWorkflowMutator) SetActivityProperty(activityRef string, atPos int,
 	case "PAGE":
 		taskPage := dGetDoc(actDoc, "TaskPage")
 		if taskPage != nil {
+			// TaskPage exists and has a value — update the Page field in place.
 			dSet(taskPage, "Page", value)
+			return nil
+		}
+		pageRef := bson.D{
+			{Key: "$ID", Value: bsonutil.NewIDBsonBinary()},
+			{Key: "$Type", Value: "Workflows$PageReference"},
+			{Key: "Page", Value: value},
+		}
+		if dSet(actDoc, "TaskPage", pageRef) {
+			// TaskPage key exists (nil value) — replaced in place via dSet.
 		} else {
-			pageRef := bson.D{
-				{Key: "$ID", Value: bsonutil.NewIDBsonBinary()},
-				{Key: "$Type", Value: "Workflows$PageReference"},
-				{Key: "Page", Value: value},
-			}
-			dSet(actDoc, "TaskPage", pageRef)
+			// TaskPage key absent — append to activity and replace in BSON tree.
+			actDoc = append(actDoc, bson.E{Key: "TaskPage", Value: pageRef})
+			m.replaceActivity(actDoc)
 		}
 		return nil
 
@@ -555,6 +562,53 @@ func (m *mprWorkflowMutator) Save() error {
 // Internal helpers — activity search
 // ---------------------------------------------------------------------------
 
+// replaceActivity replaces an activity document in the workflow's BSON tree
+// by matching on $ID. This is needed when appending new keys to an activity
+// document, because the slice header returned by findActivityByCaption cannot
+// propagate appends back to the parent bson.A.
+func (m *mprWorkflowMutator) replaceActivity(updated bson.D) {
+	actID := extractBinaryIDFromDoc(dGet(updated, "$ID"))
+	if actID == "" {
+		return
+	}
+	flow := dGetDoc(m.rawData, "Flow")
+	if flow == nil {
+		return
+	}
+	replaceActivityRecursive(flow, actID, updated)
+}
+
+func replaceActivityRecursive(flow bson.D, actID string, updated bson.D) bool {
+	activitiesVal := dGet(flow, "Activities")
+	arr := toBsonA(activitiesVal)
+	if len(arr) == 0 {
+		return false
+	}
+	// Skip the int32 type marker at index 0 if present.
+	start := 0
+	if _, ok := arr[0].(int32); ok {
+		start = 1
+	} else if _, ok := arr[0].(int); ok {
+		start = 1
+	}
+	for i := start; i < len(arr); i++ {
+		actDoc, ok := arr[i].(bson.D)
+		if !ok {
+			continue
+		}
+		if extractBinaryIDFromDoc(dGet(actDoc, "$ID")) == actID {
+			arr[i] = updated
+			return true
+		}
+		for _, nestedFlow := range getNestedFlows(actDoc) {
+			if replaceActivityRecursive(nestedFlow, actID, updated) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // findActivityByCaption searches the workflow for an activity matching caption.
 func (m *mprWorkflowMutator) findActivityByCaption(caption string, atPosition int) (bson.D, error) {
 	flow := dGetDoc(m.rawData, "Flow")

mdl/backend/mpr/workflow_mutator_test.go

@@ -1136,8 +1136,7 @@ func TestWorkflowMutator_InsertBoundaryEvent_NoDelay(t *testing.T) {
 // ---------------------------------------------------------------------------
 
 func TestWorkflowMutator_SetActivityProperty_Page_New(t *testing.T) {
-	// Note: When TaskPage key doesn't pre-exist in BSON, dSet silently fails.
-	// The key must be present (even as nil) for PAGE to work on a new activity.
+	// TaskPage key present with nil value — should be replaced with a new PageReference.
 	act := makeWfActivity("Workflows$UserTask", "Review", "task1")
 	act = append(act, bson.E{Key: "TaskPage", Value: nil})
 	m := newMutator(makeWorkflowDoc(act))
@@ -1157,21 +1156,23 @@ func TestWorkflowMutator_SetActivityProperty_Page_New(t *testing.T) {
 }
 
 func TestWorkflowMutator_SetActivityProperty_Page_MissingKey(t *testing.T) {
-	// BUG: dSet silently fails when TaskPage key is absent — pageRef is lost.
+	// Regression test: dSet silently failed when TaskPage key was absent.
+	// Fixed by appending the key to the activity and replacing it in the BSON tree.
 	act := makeWfActivity("Workflows$UserTask", "Review", "task1")
 	// No TaskPage field at all
 	m := newMutator(makeWorkflowDoc(act))
 
-	// No error returned, but the set is silently lost
 	if err := m.SetActivityProperty("Review", 0, "PAGE", "MyModule.TaskPage"); err != nil {
 		t.Fatalf("SetActivityProperty PAGE failed: %v", err)
 	}
 
 	actDoc, _ := m.findActivityByCaption("Review", 0)
 	taskPage := dGetDoc(actDoc, "TaskPage")
-	// This documents the bug: TaskPage is nil because dSet can't create new keys
-	if taskPage != nil {
-		t.Log("BUG FIXED: TaskPage is now set even when key was absent")
+	if taskPage == nil {
+		t.Fatal("TaskPage should be set even when key was absent")
+	}
+	if got := dGetString(taskPage, "Page"); got != "MyModule.TaskPage" {
+		t.Errorf("Page = %q, want MyModule.TaskPage", got)
 	}
 }