Only force-submit withdrawal batches if block is fresh (#41)

op-batcher/batcher/channel_out.go

@@ -2,6 +2,7 @@ package batcher
 
 import (
 	"errors"
+	"time"
 
 	"github.com/ethereum-optimism/optimism/op-batcher/batcher"
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
@@ -12,6 +13,8 @@ import (
 
 var ErrWithdrawalDetected = errors.New("withdrawal detected")
 
+const minBlockFreshness = 10 * time.Second
+
 func NewChannelOut(cfg batcher.ChannelConfig, rollupCfg *rollup.Config) (derive.ChannelOut, error) {
 	co, err := batcher.NewChannelOut(cfg, rollupCfg)
 	if err != nil {
@@ -24,11 +27,18 @@ func NewChannelOut(cfg batcher.ChannelConfig, rollupCfg *rollup.Config) (derive.
 
 type channelOut struct {
 	derive.ChannelOut
-	fullErr error
+	fullErr            error
+	withdrawalDetected bool
 }
 
 func (c *channelOut) AddBlock(config *rollup.Config, block *types.Block) (*derive.L1BlockInfo, error) {
 	if block.Bloom().Test(predeploys.L2ToL1MessagePasserAddr.Bytes()) {
+		c.withdrawalDetected = true
+	}
+	// If this channel contains a withdrawal, and the block is recent, we can submit the batch immediately.
+	// We care about the block freshness because otherwise users could cause the batch submission to slow
+	// down significantly by submitting at least 1 withdrawal each block.
+	if c.withdrawalDetected && time.Unix(int64(block.Time()), 0).Add(minBlockFreshness).After(time.Now()) {
 		c.fullErr = ErrWithdrawalDetected
 	}
 	return c.ChannelOut.AddBlock(config, block)