fix(server): guard DAG cycle check against filtered-out optional dependencies (#6915)

pipeline/frontend/yaml/compiler/dag.go

@@ -75,6 +75,16 @@ func (c dagCompiler) compileByDependsOn() ([]*backend_types.Stage, error) {
 }
 
 func dfsVisit(steps map[string]*dagCompilerStep, name string, visited map[string]struct{}, path []string) error {
+	step, exists := steps[name]
+	if !exists {
+		// `name` is an optional dependency on a step that was filtered out. convertDAGToStages
+		// drops those edges, but it resolves one step at a time and runs this walk in the same
+		// loop, so the walk can reach a step whose dependsOn is not resolved yet and follow an
+		// edge to a filtered-out step. There is nothing to traverse, and a missing *required*
+		// dependency is still reported by convertDAGToStages when it reaches that step.
+		return nil
+	}
+
 	if _, ok := visited[name]; ok {
 		return &ErrStepDependencyCycle{path: path}
 	}
@@ -82,7 +92,7 @@ func dfsVisit(steps map[string]*dagCompilerStep, name string, visited map[string
 	visited[name] = struct{}{}
 	path = append(path, name)
 
-	for _, dep := range steps[name].dependsOn {
+	for _, dep := range step.dependsOn {
 		if err := dfsVisit(steps, dep.Name, visited, path); err != nil {
 			return err
 		}

pipeline/frontend/yaml/compiler/dag_test.go

@@ -162,6 +162,42 @@ func TestOptionalStepDependency(t *testing.T) {
 		assert.Len(t, stages, 2, "should produce 2 stages (build then deploy)")
 	})
 
+	t.Run("missing optional step dep on a step that has dependents", func(t *testing.T) {
+		// convertDAGToStages resolves one step at a time and runs the cycle check in the
+		// same loop, so if it reaches `notify` before `deploy`, the walk follows deploy's
+		// still-unresolved edge to the absent `lint`. Map iteration order decides, so run
+		// this enough times to cover both orders.
+		for range 100 {
+			steps := map[string]*dagCompilerStep{
+				"build": {
+					position: 0,
+					name:     "build",
+					step:     &backend_types.Step{Name: "build"},
+				},
+				"deploy": {
+					position: 1,
+					name:     "deploy",
+					step:     &backend_types.Step{Name: "deploy"},
+					dependsOn: constraint.DependsOn{
+						{Name: "build"},
+						{Name: "lint", Optional: true},
+					},
+				},
+				"notify": {
+					position: 2,
+					name:     "notify",
+					step:     &backend_types.Step{Name: "notify"},
+					dependsOn: constraint.DependsOn{
+						{Name: "deploy"},
+					},
+				},
+			}
+			stages, err := convertDAGToStages(steps)
+			assert.NoError(t, err)
+			assert.Len(t, stages, 3, "build, then deploy, then notify")
+		}
+	})
+
 	t.Run("missing required step dep still errors", func(t *testing.T) {
 		steps := map[string]*dagCompilerStep{
 			"deploy": {