fix: preserve custom error handler continuations

mdl-examples/bug-tests/349-custom-error-handler-routing.mdl

@@ -0,0 +1,64 @@
+-- ============================================================================
+-- Bug #349: Custom error-handler routing during microflow roundtrip
+-- ============================================================================
+--
+-- Symptom (before fix):
+--   Custom error-handler bodies that did NOT explicitly return were
+--   rebuilt as DETACHED terminal paths — they got their own EndEvent and
+--   never rejoined the next activity. Empty custom handlers could either
+--   lose their error flow entirely or rejoin into statements that read
+--   an output variable absent on the error path.
+--
+-- Root cause:
+--   The microflow builder handled custom error handlers immediately at
+--   the source activity. It did not retain pending handler state until
+--   the next safe continuation was known, so a later handler could
+--   overwrite an earlier pending handler.
+--
+-- After fix:
+--   - Pending custom-handler state is queued.
+--   - Non-terminal handler bodies rejoin through a merge before the
+--     next safe continuation, instead of fabricating detached EndEvents.
+--   - Empty custom-handler error flows are preserved.
+--   - Output-producing handlers terminate before output-dependent
+--     continuation in void microflows.
+--
+-- Usage:
+--   mxcli exec mdl-examples/bug-tests/349-custom-error-handler-routing.mdl -p app.mpr
+--   mxcli -p app.mpr -c "describe microflow BugTest349.MF_Caller"
+--   `mx check` against the resulting MPR must report 0 errors and the
+--   non-terminal handler must rejoin the continuation activity.
+-- ============================================================================
+
+create module BugTest349;
+
+create microflow BugTest349.MF_RefreshData (
+  $Token: string
+)
+begin
+  log info node 'BugTest349' 'refreshed: ' + $Token;
+end;
+/
+
+create microflow BugTest349.MF_NextBatch ()
+begin
+  log info node 'BugTest349' 'next batch';
+end;
+/
+
+-- Caller with non-terminal custom error handler. The handler body logs
+-- but does not return, so its tail must rejoin the continuation
+-- (`call microflow ... MF_NextBatch ()`) instead of becoming a detached
+-- terminal path.
+create microflow BugTest349.MF_Caller (
+  $Token: string
+)
+begin
+  call microflow BugTest349.MF_RefreshData (Token = $Token)
+  on error without rollback {
+    log error node 'BugTest349' 'refresh failed';
+  };
+
+  call microflow BugTest349.MF_NextBatch ();
+end;
+/

mdl/ast/ast_microflow.go

@@ -249,6 +249,7 @@ type IfStmt struct {
 	Condition   Expression           // IF condition
 	ThenBody    []MicroflowStatement // THEN branch
 	ElseBody    []MicroflowStatement // ELSE branch (optional)
+	HasElse     bool                 // true when the source contained ELSE, even if the body is empty
 	Annotations *ActivityAnnotations // Optional @position, @caption, @color, @annotation
 }
 

mdl/executor/cmd_diff_mdl.go

@@ -423,7 +423,7 @@ func microflowStatementToMDL(ctx *ExecContext, stmt ast.MicroflowStatement, inde
 		for _, thenStmt := range s.ThenBody {
 			lines = append(lines, microflowStatementToMDL(ctx, thenStmt, indent+1)...)
 		}
-		if len(s.ElseBody) > 0 {
+		if s.HasElse || len(s.ElseBody) > 0 {
 			lines = append(lines, indentStr+"else")
 			for _, elseStmt := range s.ElseBody {
 				lines = append(lines, microflowStatementToMDL(ctx, elseStmt, indent+1)...)

mdl/executor/cmd_microflows_builder.go

@@ -23,8 +23,10 @@ type flowBuilder struct {
 	posY                int
 	baseY               int // Base Y position (for returning after ELSE branches)
 	spacing             int
-	returnValue         string            // Return value expression for RETURN statement (used by buildFlowGraph final EndEvent)
+	returnValue         string // Return value expression for RETURN statement (used by buildFlowGraph final EndEvent)
+	returnType          *ast.MicroflowReturnType
 	endsWithReturn      bool              // True if the flow already ends with EndEvent(s) from RETURN statements
+	lastReturnEndID     model.ID          // Last explicit RETURN EndEvent, used as a fallback error-handler target
 	varTypes            map[string]string // Variable name -> entity qualified name (for CHANGE statements)
 	declaredVars        map[string]string // Declared primitive variables: name -> type (e.g., "$IsValid" -> "Boolean")
 	errors              []string          // Validation errors collected during build
@@ -54,6 +56,18 @@ type flowBuilder struct {
 	nanoflowsCache        []*microflows.Nanoflow
 	nanoflowsCacheLoaded  bool
 	manualLoopBackTarget  model.ID
+	// Pending custom error-handler routing uses two representations: the
+	// currently active handler lives in the flat fields below, while handlers
+	// postponed across branch boundaries are queued in pendingErrorHandlers.
+	// Mutate this state through the helper methods in builder_flows.go so the
+	// active/queued invariant stays synchronized.
+	emptyErrorHandlerFrom    model.ID
+	errorHandlerTailFrom     model.ID
+	errorHandlerSource       model.ID
+	errorHandlerSkipVar      string
+	errorHandlerTailIsSource bool
+	errorHandlerReturnValue  string
+	pendingErrorHandlers     []pendingErrorHandlerState
 }
 
 // addError records a validation error during flow building.
@@ -71,6 +85,10 @@ func (fb *flowBuilder) GetErrors() []string {
 	return fb.errors
 }
 
+func (fb *flowBuilder) hasDeclaredReturnValue() bool {
+	return fb.returnType != nil && fb.returnType.Type.Kind != ast.TypeVoid
+}
+
 // errorExampleDeclareVariable returns an example for declaring a variable.
 func errorExampleDeclareVariable(varName string) string {
 	// Remove $ prefix if present for cleaner display

mdl/executor/cmd_microflows_builder_actions.go

@@ -131,12 +131,7 @@ func (fb *flowBuilder) addCreateObjectAction(s *ast.CreateObjectStmt) model.ID {
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.Variable)
 
 	return activity.ID
 }
@@ -167,12 +162,7 @@ func (fb *flowBuilder) addCommitAction(s *ast.MfCommitStmt) model.ID {
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, "")
 
 	return activity.ID
 }
@@ -201,12 +191,7 @@ func (fb *flowBuilder) addDeleteAction(s *ast.DeleteObjectStmt) model.ID {
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, "")
 
 	return activity.ID
 }
@@ -468,12 +453,7 @@ func (fb *flowBuilder) addRetrieveAction(s *ast.RetrieveStmt) model.ID {
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.Variable)
 
 	return activity.ID
 }

mdl/executor/cmd_microflows_builder_annotations.go

@@ -225,6 +225,7 @@ func (fb *flowBuilder) addEndEventWithReturn(s *ast.ReturnStmt) model.ID {
 
 	fb.objects = append(fb.objects, endEvent)
 	fb.endsWithReturn = true
+	fb.lastReturnEndID = endEvent.ID
 	fb.posX += fb.spacing / 2
 	return endEvent.ID
 }

mdl/executor/cmd_microflows_builder_calls.go

@@ -158,12 +158,7 @@ func (fb *flowBuilder) addCallMicroflowAction(s *ast.CallMicroflowStmt) model.ID
 		fb.registerResultVariableType(s.OutputVariable, fb.lookupMicroflowReturnType(mfQN))
 	}
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -218,12 +213,7 @@ func (fb *flowBuilder) addCallNanoflowAction(s *ast.CallNanoflowStmt) model.ID {
 		fb.registerResultVariableType(s.OutputVariable, fb.lookupNanoflowReturnType(nfQN))
 	}
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -325,12 +315,7 @@ func (fb *flowBuilder) addCallJavaActionAction(s *ast.CallJavaActionStmt) model.
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -427,12 +412,7 @@ func (fb *flowBuilder) addCallJavaScriptActionAction(s *ast.CallJavaScriptAction
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -478,12 +458,7 @@ func (fb *flowBuilder) addCallExternalActionAction(s *ast.CallExternalActionStmt
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -978,12 +953,7 @@ func (fb *flowBuilder) addRestCallAction(s *ast.RestCallStmt) model.ID {
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -1187,12 +1157,7 @@ func (fb *flowBuilder) addExecuteDatabaseQueryAction(s *ast.ExecuteDatabaseQuery
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	// Build custom error handler flow if present
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -1259,11 +1224,7 @@ func (fb *flowBuilder) addImportFromMappingAction(s *ast.ImportFromMappingStmt)
 		}
 	}
 
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, s.OutputVariable)
 
 	return activity.ID
 }
@@ -1295,11 +1256,7 @@ func (fb *flowBuilder) addTransformJsonAction(s *ast.TransformJsonStmt) model.ID
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, "")
 
 	return activity.ID
 }
@@ -1333,11 +1290,7 @@ func (fb *flowBuilder) addExportToMappingAction(s *ast.ExportToMappingStmt) mode
 	fb.objects = append(fb.objects, activity)
 	fb.posX += fb.spacing
 
-	if s.ErrorHandling != nil && len(s.ErrorHandling.Body) > 0 {
-		errorY := fb.posY + VerticalSpacing
-		mergeID := fb.addErrorHandlerFlow(activity.ID, activityX, s.ErrorHandling.Body)
-		fb.handleErrorHandlerMerge(mergeID, activity.ID, errorY)
-	}
+	fb.finishCustomErrorHandler(activity.ID, activityX, s.ErrorHandling, "")
 
 	return activity.ID
 }