beacon/light/sync: print error log if checkpoint retrieval fails (#29532)

Co-authored-by: Felix Lange <fjl@twurst.com>

beacon/blsync/block_sync_test.go

@@ -28,8 +28,8 @@ import (
 )
 
 var (
-	testServer1 = "testServer1"
-	testServer2 = "testServer2"
+	testServer1 = testServer("testServer1")
+	testServer2 = testServer("testServer2")
 
 	testBlock1 = types.NewBeaconBlock(&deneb.BeaconBlock{
 		Slot: 123,
@@ -51,6 +51,12 @@ var (
 	})
 )
 
+type testServer string
+
+func (t testServer) Name() string {
+	return string(t)
+}
+
 func TestBlockSync(t *testing.T) {
 	ht := &testHeadTracker{}
 	blockSync := newBeaconBlockSync(ht)

beacon/light/api/api_server.go

@@ -73,8 +73,10 @@ func (s *ApiServer) SendRequest(id request.ID, req request.Request) {
 			r.Updates, r.Committees, err = s.api.GetBestUpdatesAndCommittees(data.FirstPeriod, data.Count)
 			resp = r
 		case sync.ReqHeader:
+			var r sync.RespHeader
 			log.Debug("Beacon API: requesting header", "reqid", id, "hash", common.Hash(data))
-			resp, err = s.api.GetHeader(common.Hash(data))
+			r.Header, r.Canonical, r.Finalized, err = s.api.GetHeader(common.Hash(data))
+			resp = r
 		case sync.ReqCheckpointData:
 			log.Debug("Beacon API: requesting checkpoint data", "reqid", id, "hash", common.Hash(data))
 			resp, err = s.api.GetCheckpointData(common.Hash(data))
@@ -101,3 +103,8 @@ func (s *ApiServer) Unsubscribe() {
 		s.unsubscribe = nil
 	}
 }
+
+// Name implements request.Server
+func (s *ApiServer) Name() string {
+	return s.api.url
+}

beacon/light/api/light_api.go

@@ -291,7 +291,9 @@ func decodeFinalityUpdate(enc []byte) (types.FinalityUpdate, error) {
 
 // GetHeader fetches and validates the beacon header with the given blockRoot.
 // If blockRoot is null hash then the latest head header is fetched.
-func (api *BeaconLightApi) GetHeader(blockRoot common.Hash) (types.Header, error) {
+// The values of the canonical and finalized flags are also returned. Note that
+// these flags are not validated.
+func (api *BeaconLightApi) GetHeader(blockRoot common.Hash) (types.Header, bool, bool, error) {
 	var blockId string
 	if blockRoot == (common.Hash{}) {
 		blockId = "head"
@@ -300,11 +302,12 @@ func (api *BeaconLightApi) GetHeader(blockRoot common.Hash) (types.Header, error
 	}
 	resp, err := api.httpGetf("/eth/v1/beacon/headers/%s", blockId)
 	if err != nil {
-		return types.Header{}, err
+		return types.Header{}, false, false, err
 	}
 
 	var data struct {
-		Data struct {
+		Finalized bool `json:"finalized"`
+		Data      struct {
 			Root      common.Hash `json:"root"`
 			Canonical bool        `json:"canonical"`
 			Header    struct {
@@ -314,16 +317,16 @@ func (api *BeaconLightApi) GetHeader(blockRoot common.Hash) (types.Header, error
 		} `json:"data"`
 	}
 	if err := json.Unmarshal(resp, &data); err != nil {
-		return types.Header{}, err
+		return types.Header{}, false, false, err
 	}
 	header := data.Data.Header.Message
 	if blockRoot == (common.Hash{}) {
 		blockRoot = data.Data.Root
 	}
 	if header.Hash() != blockRoot {
-		return types.Header{}, errors.New("retrieved beacon header root does not match")
+		return types.Header{}, false, false, errors.New("retrieved beacon header root does not match")
 	}
-	return header, nil
+	return header, data.Data.Canonical, data.Finalized, nil
 }
 
 // GetCheckpointData fetches and validates bootstrap data belonging to the given checkpoint.
@@ -446,7 +449,7 @@ func (api *BeaconLightApi) StartHeadListener(listener HeadEventListener) func()
 		defer wg.Done()
 
 		// Request initial data.
-		if head, err := api.GetHeader(common.Hash{}); err == nil {
+		if head, _, _, err := api.GetHeader(common.Hash{}); err == nil {
 			listener.OnNewHead(head.Slot, head.Hash())
 		}
 		if signedHead, err := api.GetOptimisticHeadUpdate(); err == nil {

beacon/light/request/scheduler.go

@@ -93,7 +93,9 @@ type (
 	// the modules that do not interact with them directly.
 	// In order to make module testing easier, Server interface is used in
 	// events and modules.
-	Server      any
+	Server interface {
+		Name() string
+	}
 	Request     any
 	Response    any
 	ID          uint64

beacon/light/request/scheduler_test.go

@@ -70,6 +70,10 @@ type testServer struct {
 	canRequest int
 }
 
+func (s *testServer) Name() string {
+	return ""
+}
+
 func (s *testServer) subscribe(eventCb func(Event)) {
 	s.eventCb = eventCb
 }

beacon/light/request/server.go

@@ -58,6 +58,7 @@ const (
 // EvResponse or EvFail. Additionally, it may also send application-defined
 // events that the Modules can interpret.
 type requestServer interface {
+	Name() string
 	Subscribe(eventCallback func(Event))
 	SendRequest(ID, Request)
 	Unsubscribe()
@@ -69,6 +70,7 @@ type requestServer interface {
 // limit the number of parallel in-flight requests and temporarily disable
 // new requests based on timeouts and response failures.
 type server interface {
+	Server
 	subscribe(eventCallback func(Event))
 	canRequestNow() bool
 	sendRequest(Request) ID
@@ -138,6 +140,11 @@ type serverWithTimeout struct {
 	lastID       ID
 }
 
+// Name implements request.Server
+func (s *serverWithTimeout) Name() string {
+	return s.parent.Name()
+}
+
 // init initializes serverWithTimeout
 func (s *serverWithTimeout) init(clock mclock.Clock) {
 	s.clock = clock

beacon/light/request/server_test.go

@@ -153,6 +153,7 @@ type testRequestServer struct {
 	eventCb func(Event)
 }
 
+func (rs *testRequestServer) Name() string                  { return "" }
 func (rs *testRequestServer) Subscribe(eventCb func(Event)) { rs.eventCb = eventCb }
 func (rs *testRequestServer) SendRequest(ID, Request)       {}
 func (rs *testRequestServer) Unsubscribe()                  {}

beacon/light/sync/head_sync_test.go

@@ -24,10 +24,10 @@ import (
 )
 
 var (
-	testServer1 = "testServer1"
-	testServer2 = "testServer2"
-	testServer3 = "testServer3"
-	testServer4 = "testServer4"
+	testServer1 = testServer("testServer1")
+	testServer2 = testServer("testServer2")
+	testServer3 = testServer("testServer3")
+	testServer4 = testServer("testServer4")
 
 	testHead0 = types.HeadInfo{}
 	testHead1 = types.HeadInfo{Slot: 123, BlockRoot: common.Hash{1}}
@@ -42,6 +42,12 @@ var (
 	testSHead4 = types.SignedHeader{SignatureSlot: 0x6444, Header: types.Header{Slot: 0x6443, StateRoot: common.Hash{4}}}
 )
 
+type testServer string
+
+func (t testServer) Name() string {
+	return string(t)
+}
+
 func TestValidatedHead(t *testing.T) {
 	chain := &TestCommitteeChain{}
 	ht := &TestHeadTracker{}

beacon/light/sync/test_helpers.go

@@ -75,7 +75,7 @@ func (ts *TestScheduler) Run(testIndex int, exp ...any) {
 		if count == 0 {
 			continue
 		}
-		ts.t.Errorf("Missing %d Server.Fail(s) from server %s in test case #%d", count, server.(string), testIndex)
+		ts.t.Errorf("Missing %d Server.Fail(s) from server %s in test case #%d", count, server.Name(), testIndex)
 	}
 
 	if !reflect.DeepEqual(ts.sent[testIndex], expReqs) {
@@ -104,7 +104,7 @@ func (ts *TestScheduler) Send(server request.Server, req request.Request) reques
 
 func (ts *TestScheduler) Fail(server request.Server, desc string) {
 	if ts.expFail[server] == 0 {
-		ts.t.Errorf("Unexpected Fail from server %s in test case #%d: %s", server.(string), ts.testIndex, desc)
+		ts.t.Errorf("Unexpected Fail from server %s in test case #%d: %s", server.Name(), ts.testIndex, desc)
 		return
 	}
 	ts.expFail[server]--

beacon/light/sync/types.go

@@ -36,7 +36,11 @@ type (
 		Updates    []*types.LightClientUpdate
 		Committees []*types.SerializedSyncCommittee
 	}
-	ReqHeader         common.Hash
+	ReqHeader  common.Hash
+	RespHeader struct {
+		Header               types.Header
+		Canonical, Finalized bool
+	}
 	ReqCheckpointData common.Hash
 	ReqBeaconBlock    common.Hash
 )

beacon/light/sync/update_sync.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/beacon/light"
 	"github.com/ethereum/go-ethereum/beacon/light/request"
+	"github.com/ethereum/go-ethereum/beacon/params"
 	"github.com/ethereum/go-ethereum/beacon/types"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/log"
@@ -42,6 +43,31 @@ type CheckpointInit struct {
 	checkpointHash common.Hash
 	locked         request.ServerAndID
 	initialized    bool
+	// per-server state is used to track the state of requesting checkpoint header
+	// info. Part of this info (canonical and finalized state) is not validated
+	// and therefore it is requested from each server separately after it has
+	// reported a missing checkpoint (which is also not validated info).
+	serverState map[request.Server]serverState
+	// the following fields are used to determine whether the checkpoint is on
+	// epoch boundary. This information is validated and therefore stored globally.
+	parentHash                  common.Hash
+	hasEpochInfo, epochBoundary bool
+	cpSlot, parentSlot          uint64
+}
+
+const (
+	ssDefault         = iota // no action yet or checkpoint requested
+	ssNeedHeader             // checkpoint req failed, need cp header
+	ssHeaderRequested        // cp header requested
+	ssNeedParent             // cp header slot %32 != 0, need parent to check epoch boundary
+	ssParentRequested        // cp parent header requested
+	ssPrintStatus            // has all necessary info, print log message if init still not successful
+	ssDone                   // log message printed, no more action required
+)
+
+type serverState struct {
+	state                           int
+	hasHeader, canonical, finalized bool // stored per server because not validated
 }
 
 // NewCheckpointInit creates a new CheckpointInit.
@@ -49,40 +75,109 @@ func NewCheckpointInit(chain committeeChain, checkpointHash common.Hash) *Checkp
 	return &CheckpointInit{
 		chain:          chain,
 		checkpointHash: checkpointHash,
+		serverState:    make(map[request.Server]serverState),
 	}
 }
 
 // Process implements request.Module.
 func (s *CheckpointInit) Process(requester request.Requester, events []request.Event) {
+	if s.initialized {
+		return
+	}
 	for _, event := range events {
-		if !event.IsRequestEvent() {
-			continue
-		}
-		sid, req, resp := event.RequestInfo()
-		if s.locked == sid {
-			s.locked = request.ServerAndID{}
-		}
-		if resp != nil {
-			if checkpoint := resp.(*types.BootstrapData); checkpoint.Header.Hash() == common.Hash(req.(ReqCheckpointData)) {
-				s.chain.CheckpointInit(*checkpoint)
-				s.initialized = true
-				return
+		switch event.Type {
+		case request.EvResponse, request.EvFail, request.EvTimeout:
+			sid, req, resp := event.RequestInfo()
+			if s.locked == sid {
+				s.locked = request.ServerAndID{}
 			}
-
-			requester.Fail(event.Server, "invalid checkpoint data")
+			if event.Type == request.EvTimeout {
+				continue
+			}
+			switch s.serverState[sid.Server].state {
+			case ssDefault:
+				if resp != nil {
+					if checkpoint := resp.(*types.BootstrapData); checkpoint.Header.Hash() == common.Hash(req.(ReqCheckpointData)) {
+						s.chain.CheckpointInit(*checkpoint)
+						s.initialized = true
+						return
+					}
+					requester.Fail(event.Server, "invalid checkpoint data")
+				}
+				s.serverState[sid.Server] = serverState{state: ssNeedHeader}
+			case ssHeaderRequested:
+				if resp == nil {
+					s.serverState[sid.Server] = serverState{state: ssPrintStatus}
+					continue
+				}
+				newState := serverState{
+					hasHeader: true,
+					canonical: resp.(RespHeader).Canonical,
+					finalized: resp.(RespHeader).Finalized,
+				}
+				s.cpSlot, s.parentHash = resp.(RespHeader).Header.Slot, resp.(RespHeader).Header.ParentRoot
+				if s.cpSlot%params.EpochLength == 0 {
+					s.hasEpochInfo, s.epochBoundary = true, true
+				}
+				if s.hasEpochInfo {
+					newState.state = ssPrintStatus
+				} else {
+					newState.state = ssNeedParent
+				}
+				s.serverState[sid.Server] = newState
+			case ssParentRequested:
+				s.parentSlot = resp.(RespHeader).Header.Slot
+				s.hasEpochInfo, s.epochBoundary = true, s.cpSlot/params.EpochLength > s.parentSlot/params.EpochLength
+				newState := s.serverState[sid.Server]
+				newState.state = ssPrintStatus
+				s.serverState[sid.Server] = newState
+			}
+		case request.EvUnregistered:
+			delete(s.serverState, event.Server)
 		}
 	}
 	// start a request if possible
-	if s.initialized || s.locked != (request.ServerAndID{}) {
-		return
+	for _, server := range requester.CanSendTo() {
+		switch s.serverState[server].state {
+		case ssDefault:
+			if s.locked == (request.ServerAndID{}) {
+				id := requester.Send(server, ReqCheckpointData(s.checkpointHash))
+				s.locked = request.ServerAndID{Server: server, ID: id}
+			}
+		case ssNeedHeader:
+			requester.Send(server, ReqHeader(s.checkpointHash))
+			newState := s.serverState[server]
+			newState.state = ssHeaderRequested
+			s.serverState[server] = newState
+		case ssNeedParent:
+			requester.Send(server, ReqHeader(s.parentHash))
+			newState := s.serverState[server]
+			newState.state = ssParentRequested
+			s.serverState[server] = newState
+		}
 	}
-	cs := requester.CanSendTo()
-	if len(cs) == 0 {
-		return
+	// print log message if necessary
+	for server, state := range s.serverState {
+		if state.state != ssPrintStatus {
+			continue
+		}
+		switch {
+		case !state.hasHeader:
+			log.Error("blsync: checkpoint block is not available, reported as unknown", "server", server.Name())
+		case !state.canonical:
+			log.Error("blsync: checkpoint block is not available, reported as non-canonical", "server", server.Name())
+		case !s.hasEpochInfo:
+			// should be available if hasHeader is true and state is ssPrintStatus
+			panic("checkpoint epoch info not available when printing retrieval status")
+		case !s.epochBoundary:
+			log.Error("blsync: checkpoint block is not first of epoch", "slot", s.cpSlot, "parent", s.parentSlot, "server", server.Name())
+		case !state.finalized:
+			log.Error("blsync: checkpoint block is reported as non-finalized", "server", server.Name())
+		default:
+			log.Error("blsync: checkpoint not available, but reported as finalized; specified checkpoint hash might be too old", "server", server.Name())
+		}
+		s.serverState[server] = serverState{state: ssDone}
 	}
-	server := cs[0]
-	id := requester.Send(server, ReqCheckpointData(s.checkpointHash))
-	s.locked = request.ServerAndID{Server: server, ID: id}
 }
 
 // ForwardUpdateSync implements request.Module; it fetches updates between the