improve compression size estimation in shadow compression

op-batcher/compressor/shadow_compressor.go

@@ -7,6 +7,17 @@ import (
 	"github.com/ethereum-optimism/optimism/op-node/rollup/derive"
 )
 
+const (
+	// safeCompressionOverhead is the largest potential blow-up in bytes we expect to see when
+	// compressing arbitrary (e.g. random) data.  Here we account for a 2 byte header, 4 byte
+	// digest, 5 byte EOF indicator, and then 5 byte flate block header for each 16k of potential
+	// data. Assuming frames are max 128k size (the current max blob size) this is 2+4+5+(5*8) = 51
+	// bytes.  If we start using larger frames (e.g. should max blob size increase) a larger blowup
+	// might be possible, but it would be highly unlikely, and the system still works if our
+	// estimate is wrong -- we just end up writing one more tx for the overflow.
+	safeCompressionOverhead = 51
+)
+
 type ShadowCompressor struct {
 	config Config
 
@@ -17,6 +28,8 @@ type ShadowCompressor struct {
 	shadowCompress *zlib.Writer
 
 	fullErr error
+
+	bound uint64 // best known upperbound on the size of the compressed output
 }
 
 // NewShadowCompressor creates a new derive.Compressor implementation that contains two
@@ -41,19 +54,30 @@ func NewShadowCompressor(config Config) (derive.Compressor, error) {
 		return nil, err
 	}
 
+	c.bound = safeCompressionOverhead
 	return c, nil
 }
 
 func (t *ShadowCompressor) Write(p []byte) (int, error) {
+	if t.fullErr != nil {
+		return 0, t.fullErr
+	}
 	_, err := t.shadowCompress.Write(p)
 	if err != nil {
 		return 0, err
 	}
+	newBound := t.bound + uint64(len(p))
+	cap := t.config.TargetFrameSize * uint64(t.config.TargetNumFrames)
+	if newBound > cap {
+		// Do not flush the buffer unless there's some chance we will be over the size limit.
+		// This reduces CPU but more importantly it makes the shadow compression ratio more
+		// closely reflect the ultimate compression ratio.
 		err = t.shadowCompress.Flush()
 		if err != nil {
 			return 0, err
 		}
-	if uint64(t.shadowBuf.Len()) > t.config.TargetFrameSize*uint64(t.config.TargetNumFrames) {
+		newBound = uint64(t.shadowBuf.Len()) + 4 // + 4 is to account for the digest written on close()
+		if newBound > cap {
 			t.fullErr = derive.CompressorFullErr
 			if t.Len() > 0 {
 				// only return an error if we've already written data to this compressor before
@@ -61,6 +85,8 @@ func (t *ShadowCompressor) Write(p []byte) (int, error) {
 				return 0, t.fullErr
 			}
 		}
+	}
+	t.bound = newBound
 	return t.compress.Write(p)
 }
 
@@ -78,6 +104,7 @@ func (t *ShadowCompressor) Reset() {
 	t.shadowBuf.Reset()
 	t.shadowCompress.Reset(&t.shadowBuf)
 	t.fullErr = nil
+	t.bound = safeCompressionOverhead
 }
 
 func (t *ShadowCompressor) Len() int {

op-batcher/compressor/shadow_compressor_test.go

-package compressor_test
+package compressor
 
 import (
 	"bytes"
 	"compress/zlib"
-	"crypto/rand"
 	"io"
+	"math/rand"
 	"testing"
 
-	"github.com/ethereum-optimism/optimism/op-batcher/compressor"
 	"github.com/ethereum-optimism/optimism/op-node/rollup/derive"
 	"github.com/stretchr/testify/require"
 )
 
+var r *rand.Rand
+
+func init() {
+	r = rand.New(rand.NewSource(99))
+}
+
 func randomBytes(t *testing.T, length int) []byte {
 	b := make([]byte, length)
-	_, err := rand.Read(b)
+	_, err := r.Read(b)
 	require.NoError(t, err)
 	return b
 }
@@ -52,9 +57,9 @@ func TestShadowCompressor(t *testing.T) {
 		fullErr:         derive.CompressorFullErr,
 	}, {
 		name:            "random data",
-		targetFrameSize: 1200,
+		targetFrameSize: 1 << 17,
 		targetNumFrames: 1,
-		data:            [][]byte{randomBytes(t, 512), randomBytes(t, 512), randomBytes(t, 512)},
+		data:            [][]byte{randomBytes(t, (1<<17)-1000), randomBytes(t, 512), randomBytes(t, 512)},
 		errs:            []error{nil, nil, derive.CompressorFullErr},
 		fullErr:         derive.CompressorFullErr,
 	}}
@@ -64,7 +69,7 @@ func TestShadowCompressor(t *testing.T) {
 			t.Parallel()
 			require.Equal(t, len(test.errs), len(test.data), "invalid test case: len(data) != len(errs)")
 
-			sc, err := compressor.NewShadowCompressor(compressor.Config{
+			sc, err := NewShadowCompressor(Config{
 				TargetFrameSize: test.targetFrameSize,
 				TargetNumFrames: test.targetNumFrames,
 			})
@@ -88,6 +93,7 @@ func TestShadowCompressor(t *testing.T) {
 
 			err = sc.Close()
 			require.NoError(t, err)
+			require.LessOrEqual(t, uint64(sc.Len()), sc.(*ShadowCompressor).bound)
 
 			buf, err := io.ReadAll(sc)
 			require.NoError(t, err)
@@ -110,3 +116,28 @@ func TestShadowCompressor(t *testing.T) {
 		})
 	}
 }
+
+// TestBoundInaccruateForLargeRandomData documents where our bounding heuristic starts to fail
+// (writing at least 128k of random data)
+func TestBoundInaccurateForLargeRandomData(t *testing.T) {
+	var sizeLimit int = 1 << 17
+
+	sc, err := NewShadowCompressor(Config{
+		TargetFrameSize: uint64(sizeLimit + 100),
+		TargetNumFrames: 1,
+	})
+	require.NoError(t, err)
+
+	_, err = sc.Write(randomBytes(t, sizeLimit+1))
+	require.NoError(t, err)
+	err = sc.Close()
+	require.NoError(t, err)
+	require.Greater(t, uint64(sc.Len()), sc.(*ShadowCompressor).bound)
+
+	sc.Reset()
+	_, err = sc.Write(randomBytes(t, sizeLimit))
+	require.NoError(t, err)
+	err = sc.Close()
+	require.NoError(t, err)
+	require.LessOrEqual(t, uint64(sc.Len()), sc.(*ShadowCompressor).bound)
+}