Add some benchmarks of moving a bunch of data through a pipeline

Add some benchmarks that move MB-scale data through pipelines
consisting of alternating commands and functions, one in small writes,
and one buffered into larger writes, then processing it one line at a
time. This is not so efficient, because every transition from
`Function` → `Command` requires an extra (hidden) goroutine that
copies the data from an `io.Reader` to a `*os.File`.

We can make this faster!

pipe/pipeline_test.go

@@ -935,6 +935,97 @@ func BenchmarkTenMixedStages(b *testing.B) {
 	}
 }
 
+func BenchmarkMoreDataUnbuffered(b *testing.B) {
+	ctx := context.Background()
+
+	cp := func(_ context.Context, _ pipe.Env, stdin io.Reader, stdout io.Writer) error {
+		_, err := io.Copy(stdout, stdin)
+		return err
+	}
+
+	for i := 0; i < b.N; i++ {
+		count := 0
+		p := pipe.New()
+		p.Add(
+			pipe.Function(
+				"seq",
+				func(ctx context.Context, _ pipe.Env, stdin io.Reader, stdout io.Writer) error {
+					for i := 1; i <= 100000; i++ {
+						fmt.Fprintln(stdout, i)
+					}
+					return nil
+				},
+			),
+			pipe.Command("cat"),
+			pipe.Function("copy2", cp),
+			pipe.Command("cat"),
+			pipe.Function("copy3", cp),
+			pipe.Command("cat"),
+			pipe.Function("copy4", cp),
+			pipe.Command("cat"),
+			pipe.Function("copy5", cp),
+			pipe.Command("cat"),
+			pipe.LinewiseFunction(
+				"count",
+				func(ctx context.Context, _ pipe.Env, line []byte, stdout *bufio.Writer) error {
+					count++
+					return nil
+				},
+			),
+		)
+		err := p.Run(ctx)
+		if assert.NoError(b, err) {
+			assert.EqualValues(b, 100000, count)
+		}
+	}
+}
+
+func BenchmarkMoreDataBuffered(b *testing.B) {
+	ctx := context.Background()
+
+	cp := func(_ context.Context, _ pipe.Env, stdin io.Reader, stdout io.Writer) error {
+		_, err := io.Copy(stdout, stdin)
+		return err
+	}
+
+	for i := 0; i < b.N; i++ {
+		count := 0
+		p := pipe.New()
+		p.Add(
+			pipe.Function(
+				"seq",
+				func(ctx context.Context, _ pipe.Env, stdin io.Reader, stdout io.Writer) error {
+					out := bufio.NewWriter(stdout)
+					for i := 1; i <= 1000000; i++ {
+						fmt.Fprintln(out, i)
+					}
+					return out.Flush()
+				},
+			),
+			pipe.Command("cat"),
+			pipe.Function("copy2", cp),
+			pipe.Command("cat"),
+			pipe.Function("copy3", cp),
+			pipe.Command("cat"),
+			pipe.Function("copy4", cp),
+			pipe.Command("cat"),
+			pipe.Function("copy5", cp),
+			pipe.Command("cat"),
+			pipe.LinewiseFunction(
+				"count",
+				func(ctx context.Context, _ pipe.Env, line []byte, stdout *bufio.Writer) error {
+					count++
+					return nil
+				},
+			),
+		)
+		err := p.Run(ctx)
+		if assert.NoError(b, err) {
+			assert.EqualValues(b, 1000000, count)
+		}
+	}
+}
+
 func genErr(err error) pipe.StageFunc {
 	return func(_ context.Context, _ pipe.Env, _ io.Reader, _ io.Writer) error {
 		return err