internal/ethapi, eth, eth/catalyst: gate eth_syncing on CL handshake

eth_syncing currently returns false as soon as the local downloader
believes the chain to be done. On a freshly started node this happens
before the consensus client has talked to it: the persisted head loads
into memory, no CL handshake has occurred, the downloader sees nothing
to do, Progress.Done() is true, eth_syncing reports synced.

That is wrong from an operator perspective. Load balancers (HAProxy,
NGINX), L2 supervisors and multi-node setups commonly gate routing on
eth_syncing. They start sending live traffic to a node that has not
actually learned about any new head yet, which surfaces as missing
state, stale reads, and unhealthy upstreams.

Maintainer-endorsed direction in the issue thread: "default geth to
'syncing' on startup and only switch to 'synced' once we learn about
a new block".

Implement that with a sticky atomic.Bool on *Ethereum, set the first
time the consensus layer drives the node via the Engine API
(ForkchoiceUpdated or NewPayload), and consulted from eth_syncing.

  - eth/backend.go: add Ethereum.clContacted with
    MarkConsensusContacted/ConsensusContacted helpers
  - eth/catalyst/api.go: call MarkConsensusContacted at the same point
    where lastForkchoiceUpdate / lastNewPayloadUpdate are stamped, so
    the gate flips on every CL message regardless of the response
    status (handshake recorded even when we reply STATUS_SYNCING)
  - internal/ethapi/backend.go: add ConsensusContacted() to the Backend
    interface and to the two test mocks (api_test.go testBackend,
    transaction_args_test.go backendMock; both default to true so
    existing tests keep their original semantics)
  - eth/api_backend.go: implement ConsensusContacted on EthAPIBackend
  - internal/ethapi/api.go: in EthereumAPI.Syncing, only short-circuit
    to "false" when both progress.Done() AND ConsensusContacted() are
    true; otherwise return the progress map as during an active sync

Adds dedicated tests in internal/ethapi/syncing_test.go covering:
  - the new gate (Done but no CL contact -> truthy progress)
  - normal post-handshake behavior (Done + CL contact -> false)
  - active-sync behavior is unchanged regardless of the gate

Refs #33687.

eth/api_backend.go

@@ -422,6 +422,12 @@ func (b *EthAPIBackend) SyncProgress(ctx context.Context) ethereum.SyncProgress
 	return prog
 }
 
+// ConsensusContacted reports whether the consensus layer has driven this node
+// via the Engine API at least once since process start.
+func (b *EthAPIBackend) ConsensusContacted() bool {
+	return b.eth.ConsensusContacted()
+}
+
 func (b *EthAPIBackend) SuggestGasTipCap(ctx context.Context) (*big.Int, error) {
 	return b.gpo.SuggestTipCap(ctx)
 }

eth/backend.go

@@ -25,6 +25,7 @@ import (
 	"math/big"
 	"runtime"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/ethereum/go-ethereum/accounts"
@@ -123,6 +124,26 @@ type Ethereum struct {
 	lock sync.RWMutex // Protects the variadic fields (e.g. gas price and etherbase)
 
 	shutdownTracker *shutdowncheck.ShutdownTracker // Tracks if and when the node has shutdown ungracefully
+
+	// clContacted records whether the consensus client has ever spoken to us
+	// via the Engine API. Until that happens, the node has not learned about
+	// any new head from the network and cannot truthfully report itself as
+	// "synced" — eth_syncing falls back to reporting an in-progress sync.
+	clContacted atomic.Bool
+}
+
+// MarkConsensusContacted records that the consensus layer has driven this node
+// at least once via the Engine API. The flag is sticky: once set, it stays set
+// for the lifetime of the process. eth_syncing uses it to avoid reporting a
+// freshly started node as "synced" before any CL handshake has occurred.
+func (s *Ethereum) MarkConsensusContacted() {
+	s.clContacted.Store(true)
+}
+
+// ConsensusContacted reports whether the consensus layer has ever driven this
+// node via the Engine API since process start.
+func (s *Ethereum) ConsensusContacted() bool {
+	return s.clContacted.Load()
 }
 
 // New creates a new Ethereum object (including the initialisation of the common Ethereum object),

eth/catalyst/api.go

@@ -251,6 +251,9 @@ func (api *ConsensusAPI) forkchoiceUpdated(ctx context.Context, update engine.Fo
 	}
 	// Stash away the last update to warn the user if the beacon client goes offline
 	api.lastForkchoiceUpdate.Store(time.Now().Unix())
+	// Record that the consensus layer has driven us at least once. eth_syncing
+	// uses this to avoid claiming "synced" before any CL handshake has occurred.
+	api.eth.MarkConsensusContacted()
 
 	// Check whether we have the block yet in our database or not. If not, we'll
 	// need to either trigger a sync, or to reject this forkchoice update for a
@@ -846,6 +849,9 @@ func (api *ConsensusAPI) newPayload(ctx context.Context, params engine.Executabl
 	}
 	// Stash away the last update to warn the user if the beacon client goes offline
 	api.lastNewPayloadUpdate.Store(time.Now().Unix())
+	// Record that the consensus layer has driven us at least once. eth_syncing
+	// uses this to avoid claiming "synced" before any CL handshake has occurred.
+	api.eth.MarkConsensusContacted()
 
 	// If we already have the block locally, ignore the entire execution and just
 	// return a fake success.

internal/ethapi/api.go

@@ -153,11 +153,20 @@ func (api *EthereumAPI) BlobBaseFee(ctx context.Context) *hexutil.Big {
 // - highestBlock:  block number of the highest block header this node has received from peers
 // - pulledStates:  number of state entries processed until now
 // - knownStates:   number of known state entries that still need to be pulled
+//
+// Until the consensus layer has driven the node at least once via the Engine
+// API, the node has not actually learned about any new chain head and cannot
+// truthfully report itself as "synced". In that case Syncing returns the
+// progress object regardless of whether progress.Done() would be true.
 func (api *EthereumAPI) Syncing(ctx context.Context) (interface{}, error) {
 	progress := api.b.SyncProgress(ctx)
 
-	// Return not syncing if the synchronisation already completed
+	// Return not syncing if the synchronisation already completed AND we have
-	if progress.Done() {
+	// observed at least one Engine API call from the consensus layer. The CL
+	// gate prevents a freshly started node from being advertised as synced
+	// before any CL handshake has happened (a common operational footgun for
+	// load balancers and L2 stacks gating on eth_syncing).
+	if progress.Done() && api.b.ConsensusContacted() {
 		return false, nil
 	}
 	// Otherwise gather the block sync stats

internal/ethapi/api_test.go

@@ -493,6 +493,7 @@ func newTestBackend(t *testing.T, n int, gspec *core.Genesis, engine consensus.E
 func (b testBackend) SyncProgress(ctx context.Context) ethereum.SyncProgress {
 	return ethereum.SyncProgress{}
 }
+func (b testBackend) ConsensusContacted() bool { return true }
 func (b testBackend) SuggestGasTipCap(ctx context.Context) (*big.Int, error) {
 	return big.NewInt(0), nil
 }

internal/ethapi/backend.go

@@ -42,6 +42,10 @@ import (
 type Backend interface {
 	// General Ethereum API
 	SyncProgress(ctx context.Context) ethereum.SyncProgress
+	// ConsensusContacted reports whether the consensus layer has driven this
+	// node via the Engine API at least once since process start. eth_syncing
+	// uses it to avoid reporting "synced" before any CL handshake has happened.
+	ConsensusContacted() bool
 
 	SuggestGasTipCap(ctx context.Context) (*big.Int, error)
 	FeeHistory(ctx context.Context, blockCount uint64, lastBlock rpc.BlockNumber, rewardPercentiles []float64) (*big.Int, [][]*big.Int, []*big.Int, []float64, []*big.Int, []float64, error)

internal/ethapi/syncing_test.go

@@ -0,0 +1,96 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package ethapi
+
+import (
+	"context"
+	"testing"
+
+	"github.com/ethereum/go-ethereum"
+)
+
+// syncingBackend is a minimal Backend embedding that only implements the two
+// methods Syncing calls. Embedding the interface avoids pulling in the full
+// testBackend setup just to flip a single bool.
+type syncingBackend struct {
+	Backend
+	progress  ethereum.SyncProgress
+	contacted bool
+}
+
+func (b *syncingBackend) SyncProgress(_ context.Context) ethereum.SyncProgress { return b.progress }
+func (b *syncingBackend) ConsensusContacted() bool                             { return b.contacted }
+
+// TestSyncingReportsBeforeConsensusContact verifies that eth_syncing returns a
+// truthy progress object until the consensus layer has driven the node via the
+// Engine API at least once, even when the local downloader believes itself to
+// be done.
+func TestSyncingReportsBeforeConsensusContact(t *testing.T) {
+	api := NewEthereumAPI(&syncingBackend{
+		// progress.Done() returns true on a zero-valued struct because all
+		// remaining counters are zero and CurrentBlock >= HighestBlock.
+		progress:  ethereum.SyncProgress{},
+		contacted: false,
+	})
+	res, err := api.Syncing(context.Background())
+	if err != nil {
+		t.Fatalf("Syncing returned error: %v", err)
+	}
+	if v, ok := res.(bool); ok && !v {
+		t.Fatal("expected truthy syncing payload before CL handshake, got false")
+	}
+}
+
+// TestSyncingReportsFalseAfterConsensusContact verifies that once the
+// consensus layer has handshaken at least once and progress.Done() is true,
+// eth_syncing reports false.
+func TestSyncingReportsFalseAfterConsensusContact(t *testing.T) {
+	api := NewEthereumAPI(&syncingBackend{
+		progress:  ethereum.SyncProgress{},
+		contacted: true,
+	})
+	res, err := api.Syncing(context.Background())
+	if err != nil {
+		t.Fatalf("Syncing returned error: %v", err)
+	}
+	v, ok := res.(bool)
+	if !ok || v {
+		t.Fatalf("expected false after CL handshake when sync is done, got %v", res)
+	}
+}
+
+// TestSyncingReportsActiveSyncEvenWithoutConsensusContact verifies that when
+// the downloader is actively syncing, eth_syncing returns the progress map
+// regardless of the CL gate. This preserves the legacy semantics for the case
+// the issue thread did not affect.
+func TestSyncingReportsActiveSyncEvenWithoutConsensusContact(t *testing.T) {
+	api := NewEthereumAPI(&syncingBackend{
+		progress: ethereum.SyncProgress{
+			StartingBlock: 100,
+			CurrentBlock:  150,
+			HighestBlock:  200, // CurrentBlock < HighestBlock => Done()=false
+		},
+		contacted: false,
+	})
+	res, err := api.Syncing(context.Background())
+	if err != nil {
+		t.Fatalf("Syncing returned error: %v", err)
+	}
+	if _, ok := res.(map[string]interface{}); !ok {
+		t.Fatalf("expected progress map during active sync, got %T", res)
+	}
+}

internal/ethapi/transaction_args_test.go

@@ -326,6 +326,7 @@ func (b *backendMock) ChainConfig() *params.ChainConfig { return b.config }
 func (b *backendMock) SyncProgress(ctx context.Context) ethereum.SyncProgress {
 	return ethereum.SyncProgress{}
 }
+func (b *backendMock) ConsensusContacted() bool { return true }
 func (b *backendMock) FeeHistory(ctx context.Context, blockCount uint64, lastBlock rpc.BlockNumber, rewardPercentiles []float64) (*big.Int, [][]*big.Int, []*big.Int, []float64, []*big.Int, []float64, error) {
 	return nil, nil, nil, nil, nil, nil, nil
 }