Guard against empty pipeline in Pipeline.Start()

Pipeline.Start() indexes stageStarters[0] and p.stages[len-1]
unconditionally — zero stages causes an index out of bounds
panic. Return nil early when no stages have been added.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: znull

pipe/pipeline.go

@@ -268,6 +268,10 @@ func (p *Pipeline) Start(ctx context.Context) error {
 	atomic.StoreUint32(&p.started, 1)
 	ctx, p.cancel = context.WithCancel(ctx)
 
+	if len(p.stages) == 0 {
+		return nil
+	}
+
 	// We need to decide how to start the stages, especially what
 	// pipes to use to connect adjacent stages (`os.Pipe()` vs.
 	// `io.Pipe()`) based on the two stages' preferences.

pipe/pipeline_test.go

@@ -26,6 +26,12 @@ func TestMain(m *testing.M) {
 	goleak.VerifyTestMain(m)
 }
 
+func TestPipelineEmpty(t *testing.T) {
+	t.Parallel()
+	p := pipe.New()
+	assert.NoError(t, p.Run(context.Background()))
+}
+
 func TestPipelineFirstStageFailsToStart(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()