ensure nil step does not cause problems

engine/linter/linter.go

@@ -77,6 +77,9 @@ func checkPipeline(pipeline *resource.Pipeline, trusted bool) error {
 func checkSteps(pipeline *resource.Pipeline, trusted bool) error {
 	steps := append(pipeline.Services, pipeline.Steps...)
 	for _, step := range steps {
+		if step == nil {
+			return errors.New("linter: nil step")
+		}
 		if err := checkStep(step, trusted); err != nil {
 			return err
 		}

engine/resource/parser.go

@@ -39,6 +39,9 @@ func lint(pipeline *Pipeline) error {
 	// ensure pipeline steps are not unique.
 	names := map[string]struct{}{}
 	for _, step := range pipeline.Steps {
+		if step == nil {
+			return errors.New("Linter: detected nil step")
+		}
 		if step.Name == "" {
 			return errors.New("Linter: invalid or missing step name")
 		}

engine/resource/parser_test.go

@@ -105,6 +105,14 @@ func TestParseLintErr(t *testing.T) {
 	}
 }
 
+func TestParseLintNilStep(t *testing.T) {
+	_, err := manifest.ParseFile("testdata/nilstep.yml")
+	if err == nil {
+		t.Errorf("Expect linter returns error")
+		return
+	}
+}
+
 func TestParseNoMatch(t *testing.T) {
 	r := &manifest.RawResource{Kind: "pipeline", Type: "exec"}
 	_, match, _ := parse(r)

engine/resource/testdata/nilstep.yml

@@ -0,0 +1,9 @@
+---
+kind: pipeline
+type: docker
+name: test
+
+steps:
+- ~
+
+...