eth: support bubbling up bad blocks from sync to the engine API (ethereum#25190)

* eth: support bubbling up bad blocks from sync to the engine API

* eth/catalyst: fix typo

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>

* eth/catalyst: fix typo

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>

* Update eth/catalyst/api.go

* eth/catalyst: when forgetting bad hashes, also forget descendants

* eth/catalyst: minor bad block tweaks for resilience

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>
Co-authored-by: Martin Holst Swende <martin@swende.se>

Author: 3 people

core/beacon/types.go

@@ -42,7 +42,7 @@ type payloadAttributesMarshaling struct {
 
 //go:generate go run github.com/fjl/gencodec -type ExecutableDataV1 -field-override executableDataMarshaling -out gen_ed.go
 
-// ExecutableDataV1 structure described at https://github.com/ethereum/execution-apis/src/engine/specification.md
+// ExecutableDataV1 structure described at https://github.com/ethereum/execution-apis/tree/main/src/engine/specification.md
 type ExecutableDataV1 struct {
 	ParentHash    common.Hash    `json:"parentHash"    gencodec:"required"`
 	FeeRecipient  common.Address `json:"feeRecipient"  gencodec:"required"`

eth/catalyst/api.go

@@ -50,12 +50,47 @@ func Register(stack *node.Node, backend *eth.Ethereum) error {
 	return nil
 }
 
+const (
+	// invalidBlockHitEviction is the number of times an invalid block can be
+	// referenced in forkchoice update or new payload before it is attempted
+	// to be reprocessed again.
+	invalidBlockHitEviction = 128
+
+	// invalidTipsetsCap is the max number of recent block hashes tracked that
+	// have lead to some bad ancestor block. It's just an OOM protection.
+	invalidTipsetsCap = 512
+)
+
 type ConsensusAPI struct {
-	eth          *eth.Ethereum
+	eth *eth.Ethereum
+
 	remoteBlocks *headerQueue  // Cache of remote payloads received
 	localBlocks  *payloadQueue // Cache of local payloads generated
-	// Lock for the forkChoiceUpdated method
-	forkChoiceLock sync.Mutex
+
+	// The forkchoice update and new payload method require us to return the
+	// latest valid hash in an invalid chain. To support that return, we need
+	// to track historical bad blocks as well as bad tipsets in case a chain
+	// is constantly built on it.
+	//
+	// There are a few important caveats in this mechanism:
+	//   - The bad block tracking is ephemeral, in-memory only. We must never
+	//     persist any bad block information to disk as a bug in Geth could end
+	//     up blocking a valid chain, even if a later Geth update would accept
+	//     it.
+	//   - Bad blocks will get forgotten after a certain threshold of import
+	//     attempts and will be retried. The rationale is that if the network
+	//     really-really-really tries to feed us a block, we should give it a
+	//     new chance, perhaps us being racey instead of the block being legit
+	//     bad (this happened in Geth at a point with import vs. pending race).
+	//   - Tracking all the blocks built on top of the bad one could be a bit
+	//     problematic, so we will only track the head chain segment of a bad
+	//     chain to allow discarding progressing bad chains and side chains,
+	//     without tracking too much bad data.
+	invalidBlocksHits map[common.Hash]int           // Emhemeral cache to track invalid blocks and their hit count
+	invalidTipsets    map[common.Hash]*types.Header // Ephemeral cache to track invalid tipsets and their bad ancestor
+	invalidLock       sync.Mutex                    // Protects the invalid maps from concurrent access
+
+	forkChoiceLock sync.Mutex // Lock for the forkChoiceUpdated method
 }
 
 // NewConsensusAPI creates a new consensus api for the given backend.
@@ -64,11 +99,16 @@ func NewConsensusAPI(eth *eth.Ethereum) *ConsensusAPI {
 	if eth.BlockChain().Config().TerminalTotalDifficulty == nil {
 		log.Warn("Engine API started but chain not configured for merge yet")
 	}
-	return &ConsensusAPI{
-		eth:          eth,
-		remoteBlocks: newHeaderQueue(),
-		localBlocks:  newPayloadQueue(),
+	api := &ConsensusAPI{
+		eth:               eth,
+		remoteBlocks:      newHeaderQueue(),
+		localBlocks:       newPayloadQueue(),
+		invalidBlocksHits: make(map[common.Hash]int),
+		invalidTipsets:    make(map[common.Hash]*types.Header),
 	}
+	eth.Downloader().SetBadBlockCallback(api.setInvalidAncestor)
+
+	return api
 }
 
 // ForkchoiceUpdatedV1 has several responsibilities:
@@ -96,6 +136,10 @@ func (api *ConsensusAPI) ForkchoiceUpdatedV1(update beacon.ForkchoiceStateV1, pa
 	// reason.
 	block := api.eth.BlockChain().GetBlockByHash(update.HeadBlockHash)
 	if block == nil {
+		// If this block was previously invalidated, keep rejecting it here too
+		if res := api.checkInvalidAncestor(update.HeadBlockHash, update.HeadBlockHash); res != nil {
+			return beacon.ForkChoiceResponse{PayloadStatus: *res, PayloadID: nil}, nil
+		}
 		// If the head hash is unknown (was not given to us in a newPayload request),
 		// we cannot resolve the header, so not much to do. This could be extended in
 		// the future to resolve from the `eth` network, but it's an unexpected case
@@ -266,6 +310,10 @@ func (api *ConsensusAPI) NewPayloadV1(params beacon.ExecutableDataV1) (beacon.Pa
 		hash := block.Hash()
 		return beacon.PayloadStatusV1{Status: beacon.VALID, LatestValidHash: &hash}, nil
 	}
+	// If this block was rejected previously, keep rejecting it
+	if res := api.checkInvalidAncestor(block.Hash(), block.Hash()); res != nil {
+		return *res, nil
+	}
 	// If the parent is missing, we - in theory - could trigger a sync, but that
 	// would also entail a reorg. That is problematic if multiple sibling blocks
 	// are being fed to us, and even more so, if some semi-distant uncle shortens
@@ -293,7 +341,7 @@ func (api *ConsensusAPI) NewPayloadV1(params beacon.ExecutableDataV1) (beacon.Pa
 	}
 	if block.Time() <= parent.Time() {
 		log.Warn("Invalid timestamp", "parent", block.Time(), "block", block.Time())
-		return api.invalid(errors.New("invalid timestamp"), parent), nil
+		return api.invalid(errors.New("invalid timestamp"), parent.Header()), nil
 	}
 	// Another cornercase: if the node is in snap sync mode, but the CL client
 	// tries to make it import a block. That should be denied as pushing something
@@ -310,7 +358,13 @@ func (api *ConsensusAPI) NewPayloadV1(params beacon.ExecutableDataV1) (beacon.Pa
 	log.Trace("Inserting block without sethead", "hash", block.Hash(), "number", block.Number)
 	if err := api.eth.BlockChain().InsertBlockWithoutSetHead(block); err != nil {
 		log.Warn("NewPayloadV1: inserting block failed", "error", err)
-		return api.invalid(err, parent), nil
+
+		api.invalidLock.Lock()
+		api.invalidBlocksHits[block.Hash()] = 1
+		api.invalidTipsets[block.Hash()] = block.Header()
+		api.invalidLock.Unlock()
+
+		return api.invalid(err, parent.Header()), nil
 	}
 	// We've accepted a valid payload from the beacon client. Mark the local
 	// chain transitions to notify other subsystems (e.g. downloader) of the
@@ -339,8 +393,13 @@ func computePayloadId(headBlockHash common.Hash, params *beacon.PayloadAttribute
 // delayPayloadImport stashes the given block away for import at a later time,
 // either via a forkchoice update or a sync extension. This method is meant to
 // be called by the newpayload command when the block seems to be ok, but some
-// prerequisite prevents it from being processed (e.g. no parent, or nap sync).
+// prerequisite prevents it from being processed (e.g. no parent, or snap sync).
 func (api *ConsensusAPI) delayPayloadImport(block *types.Block) (beacon.PayloadStatusV1, error) {
+	// Sanity check that this block's parent is not on a previously invalidated
+	// chain. If it is, mark the block as invalid too.
+	if res := api.checkInvalidAncestor(block.ParentHash(), block.Hash()); res != nil {
+		return *res, nil
+	}
 	// Stash the block away for a potential forced forkchoice update to it
 	// at a later time.
 	api.remoteBlocks.put(block.Hash(), block.Header())
@@ -360,14 +419,70 @@ func (api *ConsensusAPI) delayPayloadImport(block *types.Block) (beacon.PayloadS
 	return beacon.PayloadStatusV1{Status: beacon.ACCEPTED}, nil
 }
 
+// setInvalidAncestor is a callback for the downloader to notify us if a bad block
+// is encountered during the async sync.
+func (api *ConsensusAPI) setInvalidAncestor(invalid *types.Header, origin *types.Header) {
+	api.invalidLock.Lock()
+	defer api.invalidLock.Unlock()
+
+	api.invalidTipsets[origin.Hash()] = invalid
+	api.invalidBlocksHits[invalid.Hash()]++
+}
+
+// checkInvalidAncestor checks whether the specified chain end links to a known
+// bad ancestor. If yes, it constructs the payload failure response to return.
+func (api *ConsensusAPI) checkInvalidAncestor(check common.Hash, head common.Hash) *beacon.PayloadStatusV1 {
+	api.invalidLock.Lock()
+	defer api.invalidLock.Unlock()
+
+	// If the hash to check is unknown, return valid
+	invalid, ok := api.invalidTipsets[check]
+	if !ok {
+		return nil
+	}
+	// If the bad hash was hit too many times, evict it and try to reprocess in
+	// the hopes that we have a data race that we can exit out of.
+	badHash := invalid.Hash()
+
+	api.invalidBlocksHits[badHash]++
+	if api.invalidBlocksHits[badHash] >= invalidBlockHitEviction {
+		log.Warn("Too many bad block import attempt, trying", "number", invalid.Number, "hash", badHash)
+		delete(api.invalidBlocksHits, badHash)
+
+		for descendant, badHeader := range api.invalidTipsets {
+			if badHeader.Hash() == badHash {
+				delete(api.invalidTipsets, descendant)
+			}
+		}
+		return nil
+	}
+	// Not too many failures yet, mark the head of the invalid chain as invalid
+	if check != head {
+		log.Warn("Marked new chain head as invalid", "hash", head, "badnumber", invalid.Number, "badhash", badHash)
+		for len(api.invalidTipsets) >= invalidTipsetsCap {
+			for key := range api.invalidTipsets {
+				delete(api.invalidTipsets, key)
+				break
+			}
+		}
+		api.invalidTipsets[head] = invalid
+	}
+	failure := "links to previously rejected block"
+	return &beacon.PayloadStatusV1{
+		Status:          beacon.INVALID,
+		LatestValidHash: &invalid.ParentHash,
+		ValidationError: &failure,
+	}
+}
+
 // invalid returns a response "INVALID" with the latest valid hash supplied by latest or to the current head
 // if no latestValid block was provided.
-func (api *ConsensusAPI) invalid(err error, latestValid *types.Block) beacon.PayloadStatusV1 {
+func (api *ConsensusAPI) invalid(err error, latestValid *types.Header) beacon.PayloadStatusV1 {
 	currentHash := api.eth.BlockChain().CurrentBlock().Hash()
 	if latestValid != nil {
 		// Set latest valid hash to 0x0 if parent is PoW block
 		currentHash = common.Hash{}
-		if latestValid.Difficulty().BitLen() == 0 {
+		if latestValid.Difficulty.BitLen() == 0 {
 			// Otherwise set latest valid hash to parent hash
 			currentHash = latestValid.Hash()
 		}

eth/catalyst/api_test.go

@@ -773,16 +773,16 @@ func TestTrickRemoteBlockCache(t *testing.T) {
 		if err != nil {
 			panic(err)
 		}
-		if status.Status == beacon.INVALID {
-			panic("success")
+		if status.Status == beacon.VALID {
+			t.Error("invalid status: VALID on an invalid chain")
 		}
 		// Now reorg to the head of the invalid chain
 		resp, err := apiB.ForkchoiceUpdatedV1(beacon.ForkchoiceStateV1{HeadBlockHash: payload.BlockHash, SafeBlockHash: payload.BlockHash, FinalizedBlockHash: payload.ParentHash}, nil)
 		if err != nil {
 			t.Fatal(err)
 		}
 		if resp.PayloadStatus.Status == beacon.VALID {
-			t.Errorf("invalid status: expected INVALID got: %v", resp.PayloadStatus.Status)
+			t.Error("invalid status: VALID on an invalid chain")
 		}
 		time.Sleep(100 * time.Millisecond)
 	}

eth/downloader/beaconsync.go

@@ -137,6 +137,13 @@ func (b *beaconBackfiller) setMode(mode SyncMode) {
 	b.resume()
 }
 
+// SetBadBlockCallback sets the callback to run when a bad block is hit by the
+// block processor. This method is not thread safe and should be set only once
+// on startup before system events are fired.
+func (d *Downloader) SetBadBlockCallback(onBadBlock badBlockFn) {
+	d.badBlock = onBadBlock
+}
+
 // BeaconSync is the post-merge version of the chain synchronization, where the
 // chain is not downloaded from genesis onward, rather from trusted head announces
 // backwards.

eth/downloader/downloader.go

@@ -85,6 +85,10 @@ var (
 // peerDropFn is a callback type for dropping a peer detected as malicious.
 type peerDropFn func(id string)
 
+// badBlockFn is a callback for the async beacon sync to notify the caller that
+// the origin header requested to sync to, produced a chain with a bad block.
+type badBlockFn func(invalid *types.Header, origin *types.Header)
+
 // headerTask is a set of downloaded headers to queue along with their precomputed
 // hashes to avoid constant rehashing.
 type headerTask struct {
@@ -113,6 +117,7 @@ type Downloader struct {
 
 	// Callbacks
 	dropPeer peerDropFn // Drops a peer for misbehaving
+	badBlock badBlockFn // Reports a block as rejected by the chain
 
 	// Status
 	// SYSCOIN
@@ -1538,7 +1543,7 @@ func (d *Downloader) importBlockResults(results []*fetchResult) error {
 		return errCancelContentProcessing
 	default:
 	}
-	// Retrieve the a batch of results to import
+	// Retrieve a batch of results to import
 	first, last := results[0].Header, results[len(results)-1].Header
 	log.Debug("Inserting downloaded chain", "items", len(results),
 		"firstnum", first.Number, "firsthash", first.Hash(),
@@ -1554,6 +1559,16 @@ func (d *Downloader) importBlockResults(results []*fetchResult) error {
 	if index, err := d.blockchain.InsertChain(blocks); err != nil {
 		if index < len(results) {
 			log.Debug("Downloaded item processing failed", "number", results[index].Header.Number, "hash", results[index].Header.Hash(), "err", err)
+
+			// In post-merge, notify the engine API of encountered bad chains
+			if d.badBlock != nil {
+				head, _, err := d.skeleton.Bounds()
+				if err != nil {
+					log.Error("Failed to retrieve beacon bounds for bad block reporting", "err", err)
+				} else {
+					d.badBlock(blocks[index].Header(), head)
+				}
+			}
 		} else {
 			// The InsertChain method in blockchain.go will sometimes return an out-of-bounds index,
 			// when it needs to preprocess blocks to import a sidechain.