fix(writer): drop synthetic trailing newline from EndEvent ReturnValue

mdl-examples/bug-tests/316-endevent-no-trailing-newline.mdl

@@ -0,0 +1,44 @@
+-- ============================================================================
+-- Bug #316: Synthetic trailing newline appended to EndEvent ReturnValue
+-- ============================================================================
+--
+-- Symptom (before fix):
+--   The microflow writer appended `"\n"` to every non-empty `ReturnValue`
+--   when serializing EndEvent objects. Pristine Studio Pro MPRs do not
+--   store that trailing newline, so any roundtrip through mxcli left a
+--   detectable BSON drift versus the original .mpr — visible in `mx check`
+--   diffs and in `mxcli diff-local` output even when no semantic change
+--   had been authored.
+--
+-- After fix:
+--   `writer_microflow.go` passes `ReturnValue` through directly. Empty
+--   return values stay empty; non-empty ones are written without an
+--   extra trailing newline.
+--
+-- Usage:
+--   mxcli exec mdl-examples/bug-tests/316-endevent-no-trailing-newline.mdl -p app.mpr
+--   mxcli -p app.mpr -c "describe microflow BugTest316.MF_ReturnExpr"
+--   The describe output must round-trip cleanly under describe → exec → describe
+--   and (when applied to a pristine Studio Pro project) `mx check` must
+--   not report any cosmetic diff in the EndEvent ReturnValue field.
+-- ============================================================================
+
+create module BugTest316;
+
+-- Microflow with a non-empty return value: the EndEvent's ReturnValue must
+-- not pick up a synthetic trailing newline on serialization.
+create microflow BugTest316.MF_ReturnExpr (
+  $value: integer
+)
+returns boolean as $isPositive
+begin
+  return $value > 0;
+end;
+/
+
+-- Microflow with no return value (procedure): EndEvent ReturnValue stays empty.
+create microflow BugTest316.MF_NoReturn ()
+begin
+  log info node 'BugTest316' 'side effect only';
+end;
+/

sdk/mpr/writer_microflow.go

@@ -466,13 +466,10 @@ func serializeMicroflowObject(obj microflows.MicroflowObject) bson.D {
 		}
 
 	case *microflows.EndEvent:
-		// Pristine EndEvents always carry `ReturnValue` (empty string for void
-		// microflows; expression + "\n" when a value is returned). Omitting it
-		// diverges from the pristine key set on Mx 9 roundtrips.
-		returnValue := ""
-		if o.ReturnValue != "" {
-			returnValue = o.ReturnValue + "\n"
-		}
+		// Pristine Mx 9 EndEvents carry `ReturnValue` but not a synthetic trailing
+		// line break. Adding one can make Studio Pro reject list-return EndEvents
+		// with CE0117 even though mxcli's parser accepts the expression.
+		returnValue := o.ReturnValue
 		doc := bson.D{
 			{Key: "$ID", Value: idToBsonBinary(string(o.ID))},
 			{Key: "$Type", Value: "Microflows$EndEvent"},

sdk/mpr/writer_microflow_version_test.go

@@ -79,6 +79,38 @@ func TestSerializeSequenceFlow_Mx10_UsesModernShape(t *testing.T) {
 	}
 }
 
+func TestSerializeEndEvent_EmptyReturnValueHasNoTrailingLineBreak(t *testing.T) {
+	end := &microflows.EndEvent{
+		BaseMicroflowObject: microflows.BaseMicroflowObject{
+			BaseElement: model.BaseElement{ID: "end-empty"},
+			Position:    model.Point{X: 10, Y: 20},
+			Size:        model.Size{Width: 20, Height: 20},
+		},
+		ReturnValue: "empty",
+	}
+
+	doc := serializeMicroflowObject(end)
+	if got := bsonGetKey(doc, "ReturnValue"); got != "empty" {
+		t.Fatalf("ReturnValue = %q, want %q", got, "empty")
+	}
+}
+
+func TestSerializeEndEvent_NonEmptyReturnValueHasNoSyntheticLineBreak(t *testing.T) {
+	end := &microflows.EndEvent{
+		BaseMicroflowObject: microflows.BaseMicroflowObject{
+			BaseElement: model.BaseElement{ID: "end-result"},
+			Position:    model.Point{X: 10, Y: 20},
+			Size:        model.Size{Width: 20, Height: 20},
+		},
+		ReturnValue: "$Result",
+	}
+
+	doc := serializeMicroflowObject(end)
+	if got := bsonGetKey(doc, "ReturnValue"); got != "$Result" {
+		t.Fatalf("ReturnValue = %q, want %q", got, "$Result")
+	}
+}
+
 func TestSerializeAnnotationFlow_VersionShapes(t *testing.T) {
 	af := &microflows.AnnotationFlow{
 		BaseElement:   model.BaseElement{ID: "af-1"},