fix: Skip empty withParam tasks. Fixes #6834

test/e2e/functional/dag-empty-param.yaml

@@ -0,0 +1,39 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Workflow
+metadata:
+  generateName: dag-param-result-
+spec:
+  entrypoint: dag-param-result-example
+  templates:
+  - name: dag-param-result-example
+    dag:
+      tasks:
+      - name: generate
+        template: gen-number-list
+      - name: sleep
+        template: sleep-n-sec
+        arguments:
+          parameters:
+          - name: seconds
+            value: "{{item}}"
+        withParam: "{{tasks.generate.outputs.result}}"
+        dependencies:
+        - generate
+
+  - name: gen-number-list
+    script:
+      image: python:alpine3.6
+      command: [python]
+      source: |
+        import json
+        import sys
+        json.dump([i for i in range(0, -1)], sys.stdout)
+
+  - name: sleep-n-sec
+    inputs:
+      parameters:
+      - name: seconds
+    container:
+      image: argoproj/argosay:v1
+      command: [sh, -c]
+      args: ["echo sleeping for {{inputs.parameters.seconds}} seconds; sleep {{inputs.parameters.seconds}}; echo done"]

test/e2e/functional_test.go

@@ -401,6 +401,23 @@ func (s *FunctionalSuite) TestLoopEmptyParam() {
 		})
 }
 
+func (s *FunctionalSuite) TestDAGEmptyParam() {
+	s.Given().
+		Workflow("@functional/dag-empty-param.yaml").
+		When().
+		SubmitWorkflow().
+		WaitForWorkflow().
+		Then().
+		ExpectWorkflow(func(t *testing.T, _ *metav1.ObjectMeta, status *wfv1.WorkflowStatus) {
+			assert.Equal(t, wfv1.WorkflowSucceeded, status.Phase)
+			if assert.Len(t, status.Nodes, 3) {
+				nodeStatus := status.Nodes.FindByDisplayName("sleep")
+				assert.Equal(t, wfv1.NodeSkipped, nodeStatus.Phase)
+				assert.Equal(t, "Skipped, empty params", nodeStatus.Message)
+			}
+		})
+}
+
 // 128M is for argo executor
 func (s *FunctionalSuite) TestPendingRetryWorkflow() {
 	s.Given().

workflow/controller/dag.go

@@ -437,7 +437,12 @@ func (woc *wfOperationCtx) executeDAGTask(ctx context.Context, dagCtx *dagContex
 	// For example, if we had task A with withItems of ['foo', 'bar'] which expanded to ['A(0:foo)', 'A(1:bar)'], we still
 	// need to create a node for A.
 	if task.ShouldExpand() {
-		if taskGroupNode == nil {
+		// DAG task with empty withParams list should be skipped
+		if len(expandedTasks) == 0 {
+			skipReason := "Skipped, empty params"
+			woc.initializeNode(nodeName, wfv1.NodeTypeSkipped, dagTemplateScope, task, dagCtx.boundaryID, wfv1.NodeSkipped, skipReason)
+			connectDependencies(nodeName)
+		} else if taskGroupNode == nil {
 			connectDependencies(nodeName)
 			taskGroupNode = woc.initializeNode(nodeName, wfv1.NodeTypeTaskGroup, dagTemplateScope, task, dagCtx.boundaryID, wfv1.NodeRunning, "")
 		}