Interop: XSafe Head Maintainer (#11458)

* WIP: Cross-Head Maintenance

* Add NextExecutingMessage ; Add ChainsDB Tests

* Add Tests for SafetyCheckers

* spelling

* correct test

* add safety_checkers_test.go

* Address Coments From Proto

op-supervisor/supervisor/backend/db/db.go

@@ -6,6 +6,9 @@ import (
 	"io"
 
 	"github.com/ethereum-optimism/optimism/op-service/eth"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/entrydb"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/heads"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/logs"
 	backendTypes "github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/types"
 	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/types"
 )
@@ -20,11 +23,18 @@ type LogStorage interface {
 	Rewind(newHeadBlockNum uint64) error
 	LatestBlockNum() uint64
 	ClosestBlockInfo(blockNum uint64) (uint64, backendTypes.TruncatedHash, error)
+	Contains(blockNum uint64, logIdx uint32, loghash backendTypes.TruncatedHash) (bool, entrydb.EntryIdx, error)
+	LastCheckpointBehind(entrydb.EntryIdx) (logs.Iterator, error)
+	NextExecutingMessage(logs.Iterator) (backendTypes.ExecutingMessage, error)
 }
 
 type HeadsStorage interface {
+	Current() *heads.Heads
+	Apply(op heads.Operation) error
 }
 
+// ChainsDB is a database that stores logs and heads for multiple chains.
+// it implements the ChainsStorage interface.
 type ChainsDB struct {
 	logDBs map[types.ChainID]LogStorage
 	heads  HeadsStorage
@@ -49,6 +59,79 @@ func (db *ChainsDB) Resume() error {
 	return nil
 }
 
+// UpdateCrossSafeHeads updates the cross-heads of all chains
+// this is an example of how to use the SafetyChecker to update the cross-heads
+func (db *ChainsDB) UpdateCrossSafeHeads() error {
+	checker := NewSafetyChecker(Safe, *db)
+	return db.UpdateCrossHeads(checker)
+}
+
+// UpdateCrossHeadsForChain updates the cross-head for a single chain.
+// the provided checker controls which heads are considered.
+// TODO: we should invert control and have the underlying logDB call their own update
+// for now, monolithic control is fine. There may be a stronger reason to refactor if the API needs it.
+func (db *ChainsDB) UpdateCrossHeadsForChain(chainID types.ChainID, checker SafetyChecker) error {
+	// start with the xsafe head of the chain
+	xHead := checker.CrossHeadForChain(chainID)
+	// advance as far as the local head
+	localHead := checker.LocalHeadForChain(chainID)
+	// get an iterator for the last checkpoint behind the x-head
+	i, err := db.logDBs[chainID].LastCheckpointBehind(xHead)
+	if err != nil {
+		return fmt.Errorf("failed to rewind cross-safe head for chain %v: %w", chainID, err)
+	}
+	// advance the logDB through all executing messages we can
+	// this loop will break:
+	// - when we reach the local head
+	// - when we reach a message that is not safe
+	// - if an error occurs
+	for {
+		exec, err := db.logDBs[chainID].NextExecutingMessage(i)
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return fmt.Errorf("failed to read next executing message for chain %v: %w", chainID, err)
+		}
+		// if we are now beyond the local head, stop
+		if i.Index() > localHead {
+			break
+		}
+		// use the checker to determine if this message is safe
+		safe := checker.Check(
+			types.ChainIDFromUInt64(uint64(exec.Chain)),
+			exec.BlockNum,
+			exec.LogIdx,
+			exec.Hash)
+		if !safe {
+			break
+		}
+		// if all is well, prepare the x-head update to this point
+		xHead = i.Index()
+	}
+
+	// have the checker create an update to the x-head in question, and apply that update
+	err = db.heads.Apply(checker.Update(chainID, xHead))
+	if err != nil {
+		return fmt.Errorf("failed to update cross-head for chain %v: %w", chainID, err)
+	}
+	return nil
+}
+
+// UpdateCrossSafeHeads updates the cross-heads of all chains
+// based on the provided SafetyChecker. The SafetyChecker is used to determine
+// the safety of each log entry in the database, and the cross-head associated with it.
+func (db *ChainsDB) UpdateCrossHeads(checker SafetyChecker) error {
+	currentHeads := db.heads.Current()
+	for chainID := range currentHeads.Chains {
+		if err := db.UpdateCrossHeadsForChain(chainID, checker); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// LatestBlockNum returns the latest block number that has been recorded to the logs db
+// for the given chain. It does not contain safety guarantees.
 func (db *ChainsDB) LatestBlockNum(chain types.ChainID) uint64 {
 	logDB, ok := db.logDBs[chain]
 	if !ok {

op-supervisor/supervisor/backend/db/init_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 
 	"github.com/ethereum-optimism/optimism/op-service/eth"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/entrydb"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/logs"
 	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/types"
 	"github.com/stretchr/testify/require"
 )
@@ -48,6 +50,14 @@ type stubLogStore struct {
 	rewoundTo          uint64
 }
 
+func (s *stubLogStore) Contains(blockNum uint64, logIdx uint32, loghash types.TruncatedHash) (bool, entrydb.EntryIdx, error) {
+	panic("not supported")
+}
+
+func (s *stubLogStore) LastCheckpointBehind(entrydb.EntryIdx) (logs.Iterator, error) {
+	panic("not supported")
+}
+
 func (s *stubLogStore) ClosestBlockInfo(blockNum uint64) (uint64, types.TruncatedHash, error) {
 	if s.closestBlockErr != nil {
 		return 0, types.TruncatedHash{}, s.closestBlockErr
@@ -55,6 +65,10 @@ func (s *stubLogStore) ClosestBlockInfo(blockNum uint64) (uint64, types.Truncate
 	return s.closestBlockNumber, types.TruncatedHash{}, nil
 }
 
+func (s *stubLogStore) NextExecutingMessage(logs.Iterator) (types.ExecutingMessage, error) {
+	panic("not supported")
+}
+
 func (s *stubLogStore) Rewind(headBlockNum uint64) error {
 	s.rewoundTo = headBlockNum
 	return nil

op-supervisor/supervisor/backend/db/logs/db.go

@@ -202,29 +202,42 @@ func (db *DB) ClosestBlockInfo(blockNum uint64) (uint64, types.TruncatedHash, er
 	return checkpoint.blockNum, entry.hash, nil
 }
 
+// Get returns the truncated hash of the log at the specified blockNum and logIdx,
+// or an error if the log is not found.
+func (db *DB) Get(blockNum uint64, logiIdx uint32) (types.TruncatedHash, error) {
+	db.rwLock.RLock()
+	defer db.rwLock.RUnlock()
+	hash, _, err := db.findLogInfo(blockNum, logiIdx)
+	return hash, err
+}
+
 // Contains return true iff the specified logHash is recorded in the specified blockNum and logIdx.
-// logIdx is the index of the log in the array of all logs the block.
+// If the log is found, the entry index of the log is returned, too.
+// logIdx is the index of the log in the array of all logs in the block.
 // This can be used to check the validity of cross-chain interop events.
-func (db *DB) Contains(blockNum uint64, logIdx uint32, logHash types.TruncatedHash) (bool, error) {
+func (db *DB) Contains(blockNum uint64, logIdx uint32, logHash types.TruncatedHash) (bool, entrydb.EntryIdx, error) {
 	db.rwLock.RLock()
 	defer db.rwLock.RUnlock()
 	db.log.Trace("Checking for log", "blockNum", blockNum, "logIdx", logIdx, "hash", logHash)
 
-	evtHash, _, err := db.findLogInfo(blockNum, logIdx)
+	evtHash, iter, err := db.findLogInfo(blockNum, logIdx)
 	if errors.Is(err, ErrNotFound) {
 		// Did not find a log at blockNum and logIdx
-		return false, nil
+		return false, 0, nil
 	} else if err != nil {
-		return false, err
+		return false, 0, err
 	}
 	db.log.Trace("Found initiatingEvent", "blockNum", blockNum, "logIdx", logIdx, "hash", evtHash)
 	// Found the requested block and log index, check if the hash matches
-	return evtHash == logHash, nil
+	if evtHash == logHash {
+		return true, iter.Index(), nil
+	}
+	return false, 0, nil
 }
 
 // Executes checks if the log identified by the specific block number and log index, has an ExecutingMessage associated
 // with it that needs to be checked as part of interop validation.
-// logIdx is the index of the log in the array of all logs the block.
+// logIdx is the index of the log in the array of all logs in the block.
 // Returns the ExecutingMessage if it exists, or ExecutingMessage{} if the log is found but has no ExecutingMessage.
 // Returns ErrNotFound if the specified log does not exist in the database.
 func (db *DB) Executes(blockNum uint64, logIdx uint32) (types.ExecutingMessage, error) {
@@ -241,7 +254,7 @@ func (db *DB) Executes(blockNum uint64, logIdx uint32) (types.ExecutingMessage,
 	return execMsg, nil
 }
 
-func (db *DB) findLogInfo(blockNum uint64, logIdx uint32) (types.TruncatedHash, *iterator, error) {
+func (db *DB) findLogInfo(blockNum uint64, logIdx uint32) (types.TruncatedHash, Iterator, error) {
 	entryIdx, err := db.searchCheckpoint(blockNum, logIdx)
 	if errors.Is(err, io.EOF) {
 		// Did not find a checkpoint to start reading from so the log cannot be present.
@@ -477,6 +490,58 @@ func (db *DB) Rewind(headBlockNum uint64) error {
 	return nil
 }
 
+// NextExecutingMessage returns the next executing message in the log database.
+// it skips over any non-executing messages, and will return an error if encountered.
+// the iterator is modified in the process.
+func (db *DB) NextExecutingMessage(iter Iterator) (types.ExecutingMessage, error) {
+	db.rwLock.RLock()
+	defer db.rwLock.RUnlock()
+	// this for-loop will break:
+	// - when the iterator reaches the end of the log
+	// - when the iterator reaches an executing message
+	// - if an error occurs
+	for {
+		_, _, _, err := iter.NextLog()
+		if err != nil {
+			return types.ExecutingMessage{}, err
+		}
+		// if the log is not an executing message, both exec and err are empty
+		exec, err := iter.ExecMessage()
+		if err != nil {
+			return types.ExecutingMessage{}, fmt.Errorf("failed to get executing message: %w", err)
+		}
+		if exec != (types.ExecutingMessage{}) {
+			return exec, nil
+		}
+	}
+}
+
+// LastCheckpointBehind returns an iterator for the last checkpoint behind the specified entry index.
+// If the entry index is a search checkpoint, the iterator will start at that checkpoint.
+// After searching back long enough (the searchCheckpointFrequency), an error is returned,
+// as checkpoints are expected to be found within the frequency.
+func (db *DB) LastCheckpointBehind(entryIdx entrydb.EntryIdx) (Iterator, error) {
+	for attempts := 0; attempts < searchCheckpointFrequency; attempts++ {
+		// attempt to read the index entry as a search checkpoint
+		_, err := db.readSearchCheckpoint(entryIdx)
+		if err == nil {
+			return db.newIterator(entryIdx)
+		}
+		// ErrDataCorruption is the return value if the entry is not a search checkpoint
+		// if it's not that type of error, we should return it instead of continuing
+		if !errors.Is(err, ErrDataCorruption) {
+			return nil, err
+		}
+		// don't attempt to read behind the start of the data
+		if entryIdx == 0 {
+			break
+		}
+		// reverse if we haven't found it yet
+		entryIdx -= 1
+	}
+	return nil, fmt.Errorf("failed to find a search checkpoint in the last %v entries", searchCheckpointFrequency)
+}
+
 func (db *DB) readSearchCheckpoint(entryIdx entrydb.EntryIdx) (searchCheckpoint, error) {
 	data, err := db.store.Read(entryIdx)
 	if err != nil {

op-supervisor/supervisor/backend/db/logs/db_test.go

@@ -548,7 +548,7 @@ func requireContains(t *testing.T, db *DB, blockNum uint64, logIdx uint32, logHa
 	require.LessOrEqual(t, len(execMsg), 1, "cannot have multiple executing messages for a single log")
 	m, ok := db.m.(*stubMetrics)
 	require.True(t, ok, "Did not get the expected metrics type")
-	result, err := db.Contains(blockNum, logIdx, types.TruncateHash(logHash))
+	result, _, err := db.Contains(blockNum, logIdx, types.TruncateHash(logHash))
 	require.NoErrorf(t, err, "Error searching for log %v in block %v", logIdx, blockNum)
 	require.Truef(t, result, "Did not find log %v in block %v with hash %v", logIdx, blockNum, logHash)
 	require.LessOrEqual(t, m.entriesReadForSearch, int64(searchCheckpointFrequency), "Should not need to read more than between two checkpoints")
@@ -564,7 +564,7 @@ func requireContains(t *testing.T, db *DB, blockNum uint64, logIdx uint32, logHa
 func requireNotContains(t *testing.T, db *DB, blockNum uint64, logIdx uint32, logHash common.Hash) {
 	m, ok := db.m.(*stubMetrics)
 	require.True(t, ok, "Did not get the expected metrics type")
-	result, err := db.Contains(blockNum, logIdx, types.TruncateHash(logHash))
+	result, _, err := db.Contains(blockNum, logIdx, types.TruncateHash(logHash))
 	require.NoErrorf(t, err, "Error searching for log %v in block %v", logIdx, blockNum)
 	require.Falsef(t, result, "Found unexpected log %v in block %v with hash %v", logIdx, blockNum, logHash)
 	require.LessOrEqual(t, m.entriesReadForSearch, int64(searchCheckpointFrequency), "Should not need to read more than between two checkpoints")
@@ -584,10 +584,10 @@ func requireExecutingMessage(t *testing.T, db *DB, blockNum uint64, logIdx uint3
 	require.NotZero(t, m.entriesReadForSearch, "Must read at least some entries to find the log")
 }
 
-func requireWrongHash(t *testing.T, db *DB, blockNum uint64, logIdx uint32, logHash common.Hash, execMsg types.ExecutingMessage) {
+func requireWrongHash(t *testing.T, db *DB, blockNum uint64, logIdx uint32, logHash common.Hash, _ types.ExecutingMessage) {
 	m, ok := db.m.(*stubMetrics)
 	require.True(t, ok, "Did not get the expected metrics type")
-	result, err := db.Contains(blockNum, logIdx, types.TruncateHash(logHash))
+	result, _, err := db.Contains(blockNum, logIdx, types.TruncateHash(logHash))
 	require.NoErrorf(t, err, "Error searching for log %v in block %v", logIdx, blockNum)
 	require.Falsef(t, result, "Found unexpected log %v in block %v with hash %v", logIdx, blockNum, logHash)
 

op-supervisor/supervisor/backend/db/logs/iterator.go

@@ -9,6 +9,12 @@ import (
 	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/types"
 )
 
+type Iterator interface {
+	NextLog() (blockNum uint64, logIdx uint32, evtHash types.TruncatedHash, outErr error)
+	Index() entrydb.EntryIdx
+	ExecMessage() (types.ExecutingMessage, error)
+}
+
 type iterator struct {
 	db           *DB
 	nextEntryIdx entrydb.EntryIdx
@@ -63,6 +69,10 @@ func (i *iterator) NextLog() (blockNum uint64, logIdx uint32, evtHash types.Trun
 	return
 }
 
+func (i *iterator) Index() entrydb.EntryIdx {
+	return i.nextEntryIdx - 1
+}
+
 func (i *iterator) ExecMessage() (types.ExecutingMessage, error) {
 	if !i.hasExecMsg {
 		return types.ExecutingMessage{}, nil

op-supervisor/supervisor/backend/db/safety_checkers.go

+package db
+
+import (
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/entrydb"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/heads"
+	backendTypes "github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/types"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/types"
+)
+
+const (
+	Unsafe    = "unsafe"
+	Safe      = "safe"
+	Finalized = "finalized"
+)
+
+// SafetyChecker is an interface for checking the safety of a log entry
+// and updating the local head for a chain.
+type SafetyChecker interface {
+	LocalHeadForChain(chainID types.ChainID) entrydb.EntryIdx
+	CrossHeadForChain(chainID types.ChainID) entrydb.EntryIdx
+	Check(chain types.ChainID, blockNum uint64, logIdx uint32, logHash backendTypes.TruncatedHash) bool
+	Update(chain types.ChainID, index entrydb.EntryIdx) heads.OperationFn
+}
+
+// unsafeChecker is a SafetyChecker that uses the unsafe head as the view into the database
+type unsafeChecker struct {
+	chainsDB ChainsDB
+}
+
+// safeChecker is a SafetyChecker that uses the safe head as the view into the database
+type safeChecker struct {
+	chainsDB ChainsDB
+}
+
+// finalizedChecker is a SafetyChecker that uses the finalized head as the view into the database
+type finalizedChecker struct {
+	chainsDB ChainsDB
+}
+
+// NewSafetyChecker creates a new SafetyChecker of the given type
+func NewSafetyChecker(t string, chainsDB ChainsDB) SafetyChecker {
+	switch t {
+	case Unsafe:
+		return &unsafeChecker{
+			chainsDB: chainsDB,
+		}
+	case Safe:
+		return &safeChecker{
+			chainsDB: chainsDB,
+		}
+	case Finalized:
+		return &finalizedChecker{
+			chainsDB: chainsDB,
+		}
+	default:
+		panic("unknown safety checker type")
+	}
+}
+
+// LocalHeadForChain returns the local head for the given chain
+// based on the type of SafetyChecker
+func (c *unsafeChecker) LocalHeadForChain(chainID types.ChainID) entrydb.EntryIdx {
+	heads := c.chainsDB.heads.Current().Get(chainID)
+	return heads.Unsafe
+}
+
+func (c *safeChecker) LocalHeadForChain(chainID types.ChainID) entrydb.EntryIdx {
+	heads := c.chainsDB.heads.Current().Get(chainID)
+	return heads.LocalSafe
+}
+
+func (c *finalizedChecker) LocalHeadForChain(chainID types.ChainID) entrydb.EntryIdx {
+	heads := c.chainsDB.heads.Current().Get(chainID)
+	return heads.LocalFinalized
+}
+
+// CrossHeadForChain returns the x-head for the given chain
+// based on the type of SafetyChecker
+func (c *unsafeChecker) CrossHeadForChain(chainID types.ChainID) entrydb.EntryIdx {
+	heads := c.chainsDB.heads.Current().Get(chainID)
+	return heads.CrossUnsafe
+}
+
+func (c *safeChecker) CrossHeadForChain(chainID types.ChainID) entrydb.EntryIdx {
+	heads := c.chainsDB.heads.Current().Get(chainID)
+	return heads.CrossSafe
+}
+
+func (c *finalizedChecker) CrossHeadForChain(chainID types.ChainID) entrydb.EntryIdx {
+	heads := c.chainsDB.heads.Current().Get(chainID)
+	return heads.CrossFinalized
+}
+
+// check checks if the log entry is safe, provided a local head for the chain
+// it is used by the individual SafetyCheckers to determine if a log entry is safe
+func check(
+	chainsDB ChainsDB,
+	localHead entrydb.EntryIdx,
+	chain types.ChainID,
+	blockNum uint64,
+	logIdx uint32,
+	logHash backendTypes.TruncatedHash) bool {
+
+	// for the Check to be valid, the log must:
+	// exist at the blockNum and logIdx
+	// have a hash that matches the provided hash (implicit in the Contains call), and
+	// be less than or equal to the local head for the chain
+	exists, index, err := chainsDB.logDBs[chain].Contains(blockNum, logIdx, logHash)
+	if err != nil {
+		return false
+	}
+	return exists && index <= localHead
+}
+
+// Check checks if the log entry is safe, provided a local head for the chain
+// it passes on the local head this checker is concerned with, along with its view of the database
+func (c *unsafeChecker) Check(chain types.ChainID, blockNum uint64, logIdx uint32, logHash backendTypes.TruncatedHash) bool {
+	return check(c.chainsDB, c.LocalHeadForChain(chain), chain, blockNum, logIdx, logHash)
+}
+func (c *safeChecker) Check(chain types.ChainID, blockNum uint64, logIdx uint32, logHash backendTypes.TruncatedHash) bool {
+	return check(c.chainsDB, c.LocalHeadForChain(chain), chain, blockNum, logIdx, logHash)
+}
+func (c *finalizedChecker) Check(chain types.ChainID, blockNum uint64, logIdx uint32, logHash backendTypes.TruncatedHash) bool {
+	return check(c.chainsDB, c.LocalHeadForChain(chain), chain, blockNum, logIdx, logHash)
+}
+
+// Update creates an Operation that updates the x-head for the chain, given an index to set it to
+func (c *unsafeChecker) Update(chain types.ChainID, index entrydb.EntryIdx) heads.OperationFn {
+	return func(heads *heads.Heads) error {
+		chainHeads := heads.Get(chain)
+		chainHeads.CrossUnsafe = index
+		heads.Put(chain, chainHeads)
+		return nil
+	}
+}
+
+func (c *safeChecker) Update(chain types.ChainID, index entrydb.EntryIdx) heads.OperationFn {
+	return func(heads *heads.Heads) error {
+		chainHeads := heads.Get(chain)
+		chainHeads.CrossSafe = index
+		heads.Put(chain, chainHeads)
+		return nil
+	}
+}
+
+func (c *finalizedChecker) Update(chain types.ChainID, index entrydb.EntryIdx) heads.OperationFn {
+	return func(heads *heads.Heads) error {
+		chainHeads := heads.Get(chain)
+		chainHeads.CrossFinalized = index
+		heads.Put(chain, chainHeads)
+		return nil
+	}
+}

op-supervisor/supervisor/backend/db/safety_checkers_test.go

+package db
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/entrydb"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/db/heads"
+	backendTypes "github.com/ethereum-optimism/optimism/op-supervisor/supervisor/backend/types"
+	"github.com/ethereum-optimism/optimism/op-supervisor/supervisor/types"
+	"github.com/stretchr/testify/require"
+)
+
+// TestHeadsForChain tests the heads for a chain,
+// confirming the Unsafe, Safe and Finalized all return the correct head for the chain.
+// and confirming that the chainID matters when finding the value
+func TestHeadsForChain(t *testing.T) {
+	h := heads.NewHeads()
+	chainHeads := heads.ChainHeads{
+		Unsafe:         entrydb.EntryIdx(1),
+		CrossUnsafe:    entrydb.EntryIdx(2),
+		LocalSafe:      entrydb.EntryIdx(3),
+		CrossSafe:      entrydb.EntryIdx(4),
+		LocalFinalized: entrydb.EntryIdx(5),
+		CrossFinalized: entrydb.EntryIdx(6),
+	}
+	h.Put(types.ChainIDFromUInt64(1), chainHeads)
+	chainsDB := NewChainsDB(nil, &stubHeadStorage{h})
+	tcases := []struct {
+		name          string
+		chainID       types.ChainID
+		checkerType   string
+		expectedLocal entrydb.EntryIdx
+		expectedCross entrydb.EntryIdx
+	}{
+		{
+			"Unsafe Head",
+			types.ChainIDFromUInt64(1),
+			Unsafe,
+			entrydb.EntryIdx(1),
+			entrydb.EntryIdx(2),
+		},
+		{
+			"Safe Head",
+			types.ChainIDFromUInt64(1),
+			Safe,
+			entrydb.EntryIdx(3),
+			entrydb.EntryIdx(4),
+		},
+		{
+			"Finalized Head",
+			types.ChainIDFromUInt64(1),
+			Finalized,
+			entrydb.EntryIdx(5),
+			entrydb.EntryIdx(6),
+		},
+		{
+			"Incorrect Chain",
+			types.ChainIDFromUInt64(100),
+			Safe,
+			entrydb.EntryIdx(0),
+			entrydb.EntryIdx(0),
+		},
+	}
+
+	for _, c := range tcases {
+		t.Run(c.name, func(t *testing.T) {
+			checker := NewSafetyChecker(c.checkerType, *chainsDB)
+			localHead := checker.LocalHeadForChain(c.chainID)
+			crossHead := checker.CrossHeadForChain(c.chainID)
+			require.Equal(t, c.expectedLocal, localHead)
+			require.Equal(t, c.expectedCross, crossHead)
+		})
+	}
+}
+
+func TestCheck(t *testing.T) {
+	h := heads.NewHeads()
+	chainHeads := heads.ChainHeads{
+		Unsafe:         entrydb.EntryIdx(6),
+		CrossUnsafe:    entrydb.EntryIdx(5),
+		LocalSafe:      entrydb.EntryIdx(4),
+		CrossSafe:      entrydb.EntryIdx(3),
+		LocalFinalized: entrydb.EntryIdx(2),
+		CrossFinalized: entrydb.EntryIdx(1),
+	}
+	h.Put(types.ChainIDFromUInt64(1), chainHeads)
+
+	// the logStore contains just a single stubbed log DB
+	logDB := &stubLogDB{}
+	logsStore := map[types.ChainID]LogStorage{
+		types.ChainIDFromUInt64(1): logDB,
+	}
+
+	chainsDB := NewChainsDB(logsStore, &stubHeadStorage{h})
+
+	tcases := []struct {
+		name             string
+		checkerType      string
+		chainID          types.ChainID
+		blockNum         uint64
+		logIdx           uint32
+		loghash          backendTypes.TruncatedHash
+		containsResponse containsResponse
+		expected         bool
+	}{
+		{
+			// confirm that checking Unsafe uses the unsafe head,
+			// and that we can find logs even *at* the unsafe head index
+			"Unsafe Log at Head",
+			Unsafe,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{true, entrydb.EntryIdx(6), nil},
+			true,
+		},
+		{
+			// confirm that checking the Safe head works
+			"Safe Log",
+			Safe,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{true, entrydb.EntryIdx(3), nil},
+			true,
+		},
+		{
+			// confirm that checking the Finalized head works
+			"Finalized Log",
+			Finalized,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{true, entrydb.EntryIdx(1), nil},
+			true,
+		},
+		{
+			// confirm that when exists is false, we return false
+			"Does not Exist",
+			Safe,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{false, entrydb.EntryIdx(1), nil},
+			false,
+		},
+		{
+			// confirm that when a head is out of range, we return false
+			"Unsafe Out of Range",
+			Unsafe,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{true, entrydb.EntryIdx(100), nil},
+			false,
+		},
+		{
+			// confirm that when a head is out of range, we return false
+			"Safe Out of Range",
+			Safe,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{true, entrydb.EntryIdx(5), nil},
+			false,
+		},
+		{
+			// confirm that when a head is out of range, we return false
+			"Finalized Out of Range",
+			Finalized,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{true, entrydb.EntryIdx(3), nil},
+			false,
+		},
+		{
+			// confirm that when Contains returns an error, we return false
+			"Error",
+			Safe,
+			types.ChainIDFromUInt64(1),
+			1,
+			1,
+			backendTypes.TruncatedHash{1, 2, 3},
+			containsResponse{false, entrydb.EntryIdx(0), fmt.Errorf("error")},
+			false,
+		},
+	}
+
+	for _, c := range tcases {
+		t.Run(c.name, func(t *testing.T) {
+			// rig the logStore to return the expected response
+			logDB.containsResponse = c.containsResponse
+			checker := NewSafetyChecker(c.checkerType, *chainsDB)
+			r := checker.Check(c.chainID, c.blockNum, c.logIdx, c.loghash)
+			// confirm that the expected outcome is correct
+			require.Equal(t, c.expected, r)
+		})
+	}
+}