feat: auto-detect Engine API GetPayload version for Osaka fork

tac0turtle · claude · tac0turtle · commit ce758251727b · 2026-02-27T14:17:49.000+01:00
GetPayload now automatically selects between engine_getPayloadV4 (Prague)
and engine_getPayloadV5 (Osaka) by caching the last successful version and
retrying with the alternative on "Unsupported fork" errors (code -38005).

This handles Prague chains, Osaka-at-genesis chains, and time-based
Prague-to-Osaka upgrades with zero configuration. At most one extra
RPC call occurs at the fork transition point.

Co-Authored-By: Claude Opus 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/execution/evm/engine_rpc_client.go b/execution/evm/engine_rpc_client.go
@@ -2,16 +2,28 @@ package evm
 
 import (
 	"context"
+	"errors"
+	"sync/atomic"
 
 	"github.com/ethereum/go-ethereum/beacon/engine"
 	"github.com/ethereum/go-ethereum/rpc"
 )
 
+// engineErrUnsupportedFork is the Engine API error code for "Unsupported fork".
+// Defined in the Engine API specification.
+const engineErrUnsupportedFork = -38005
+
 var _ EngineRPCClient = (*engineRPCClient)(nil)
 
 // engineRPCClient is the concrete implementation wrapping *rpc.Client.
+// It auto-detects whether to use engine_getPayloadV4 (Prague) or
+// engine_getPayloadV5 (Osaka) by caching the last successful version
+// and falling back on "Unsupported fork" errors.
 type engineRPCClient struct {
 	client *rpc.Client
+	// useV5 tracks whether GetPayload should prefer V5 (Osaka).
+	// Starts false (V4/Prague). Flips automatically on unsupported-fork errors.
+	useV5 atomic.Bool
 }
 
 // NewEngineRPCClient creates a new Engine API client.
@@ -29,11 +41,31 @@ func (e *engineRPCClient) ForkchoiceUpdated(ctx context.Context, state engine.Fo
 }
 
 func (e *engineRPCClient) GetPayload(ctx context.Context, payloadID engine.PayloadID) (*engine.ExecutionPayloadEnvelope, error) {
+	method := "engine_getPayloadV4"
+	altMethod := "engine_getPayloadV5"
+	if e.useV5.Load() {
+		method = "engine_getPayloadV5"
+		altMethod = "engine_getPayloadV4"
+	}
+
 	var result engine.ExecutionPayloadEnvelope
-	err := e.client.CallContext(ctx, &result, "engine_getPayloadV4", payloadID)
+	err := e.client.CallContext(ctx, &result, method, payloadID)
+	if err == nil {
+		return &result, nil
+	}
+
+	if !isUnsupportedForkErr(err) {
+		return nil, err
+	}
+
+	// Primary method returned "Unsupported fork" -- try the other version.
+	err = e.client.CallContext(ctx, &result, altMethod, payloadID)
 	if err != nil {
 		return nil, err
 	}
+
+	// The alt method worked -- cache it for future calls.
+	e.useV5.Store(altMethod == "engine_getPayloadV5")
 	return &result, nil
 }
 
@@ -45,3 +77,10 @@ func (e *engineRPCClient) NewPayload(ctx context.Context, payload *engine.Execut
 	}
 	return &result, nil
 }
+
+// isUnsupportedForkErr reports whether err is an Engine API "Unsupported fork"
+// JSON-RPC error (code -38005).
+func isUnsupportedForkErr(err error) bool {
+	var rpcErr rpc.Error
+	return errors.As(err, &rpcErr) && rpcErr.ErrorCode() == engineErrUnsupportedFork
+}
diff --git a/execution/evm/engine_rpc_client_test.go b/execution/evm/engine_rpc_client_test.go
@@ -0,0 +1,259 @@
+package evm
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/beacon/engine"
+	"github.com/ethereum/go-ethereum/rpc"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// jsonRPCRequest is a minimal JSON-RPC request for test inspection.
+type jsonRPCRequest struct {
+	Method string            `json:"method"`
+	Params []json.RawMessage `json:"params"`
+	ID     json.RawMessage   `json:"id"`
+}
+
+// fakeEngineServer returns an httptest.Server that responds to engine_getPayloadV4
+// and engine_getPayloadV5 according to the provided handler. The handler receives
+// the method name and returns (result JSON, error code, error message).
+// If errorCode is 0, a success response is sent.
+func fakeEngineServer(t *testing.T, handler func(method string) (resultJSON string, errCode int, errMsg string)) *httptest.Server {
+	t.Helper()
+
+	var mu sync.Mutex
+	return httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		mu.Lock()
+		defer mu.Unlock()
+
+		var req jsonRPCRequest
+		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+			t.Logf("failed to decode request: %v", err)
+			http.Error(w, "bad request", http.StatusBadRequest)
+			return
+		}
+
+		resultJSON, errCode, errMsg := handler(req.Method)
+
+		w.Header().Set("Content-Type", "application/json")
+		if errCode != 0 {
+			resp := fmt.Sprintf(`{"jsonrpc":"2.0","id":%s,"error":{"code":%d,"message":"%s"}}`,
+				req.ID, errCode, errMsg)
+			_, _ = w.Write([]byte(resp))
+		} else {
+			resp := fmt.Sprintf(`{"jsonrpc":"2.0","id":%s,"result":%s}`, req.ID, resultJSON)
+			_, _ = w.Write([]byte(resp))
+		}
+	}))
+}
+
+// minimalPayloadEnvelopeJSON is a minimal valid ExecutionPayloadEnvelope JSON
+// that go-ethereum can unmarshal without error.
+const minimalPayloadEnvelopeJSON = `{
+	"executionPayload": {
+		"parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
+		"feeRecipient": "0x0000000000000000000000000000000000000000",
+		"stateRoot": "0x0000000000000000000000000000000000000000000000000000000000000001",
+		"receiptsRoot": "0x0000000000000000000000000000000000000000000000000000000000000000",
+		"logsBloom": "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
+		"prevRandao": "0x0000000000000000000000000000000000000000000000000000000000000000",
+		"blockNumber": "0x1",
+		"gasLimit": "0x1000000",
+		"gasUsed": "0x0",
+		"timestamp": "0x1",
+		"extraData": "0x",
+		"baseFeePerGas": "0x1",
+		"blockHash": "0x0000000000000000000000000000000000000000000000000000000000000002",
+		"transactions": [],
+		"blobGasUsed": "0x0",
+		"excessBlobGas": "0x0"
+	},
+	"blockValue": "0x0",
+	"blobsBundle": {
+		"commitments": [],
+		"proofs": [],
+		"blobs": []
+	},
+	"executionRequests": [],
+	"shouldOverrideBuilder": false
+}`
+
+func dialTestServer(t *testing.T, serverURL string) *rpc.Client {
+	t.Helper()
+	client, err := rpc.Dial(serverURL)
+	require.NoError(t, err)
+	return client
+}
+
+func TestGetPayload_PragueChain_UsesV4(t *testing.T) {
+	var calledMethods []string
+	var mu sync.Mutex
+
+	srv := fakeEngineServer(t, func(method string) (string, int, string) {
+		mu.Lock()
+		calledMethods = append(calledMethods, method)
+		mu.Unlock()
+
+		if method == "engine_getPayloadV4" {
+			return minimalPayloadEnvelopeJSON, 0, ""
+		}
+		return "", -38005, "Unsupported fork"
+	})
+	defer srv.Close()
+
+	client := NewEngineRPCClient(dialTestServer(t, srv.URL))
+	ctx := context.Background()
+
+	// First call -- should use V4 directly, succeed.
+	_, err := client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+
+	mu.Lock()
+	assert.Equal(t, []string{"engine_getPayloadV4"}, calledMethods, "should call V4 only")
+	calledMethods = nil
+	mu.Unlock()
+
+	// Second call -- still V4 (cached).
+	_, err = client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+
+	mu.Lock()
+	assert.Equal(t, []string{"engine_getPayloadV4"}, calledMethods, "should still use V4")
+	mu.Unlock()
+}
+
+func TestGetPayload_OsakaChain_FallsBackToV5(t *testing.T) {
+	var calledMethods []string
+	var mu sync.Mutex
+
+	srv := fakeEngineServer(t, func(method string) (string, int, string) {
+		mu.Lock()
+		calledMethods = append(calledMethods, method)
+		mu.Unlock()
+
+		if method == "engine_getPayloadV5" {
+			return minimalPayloadEnvelopeJSON, 0, ""
+		}
+		return "", -38005, "Unsupported fork"
+	})
+	defer srv.Close()
+
+	client := NewEngineRPCClient(dialTestServer(t, srv.URL))
+	ctx := context.Background()
+
+	// First call -- V4 fails with -38005, falls back to V5.
+	_, err := client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+
+	mu.Lock()
+	assert.Equal(t, []string{"engine_getPayloadV4", "engine_getPayloadV5"}, calledMethods,
+		"should try V4 then fall back to V5")
+	calledMethods = nil
+	mu.Unlock()
+
+	// Second call -- should go directly to V5 (cached).
+	_, err = client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+
+	mu.Lock()
+	assert.Equal(t, []string{"engine_getPayloadV5"}, calledMethods,
+		"should use cached V5 without trying V4")
+	mu.Unlock()
+}
+
+func TestGetPayload_ForkUpgrade_SwitchesV4ToV5(t *testing.T) {
+	var mu sync.Mutex
+	osakaActive := false
+
+	srv := fakeEngineServer(t, func(method string) (string, int, string) {
+		mu.Lock()
+		active := osakaActive
+		mu.Unlock()
+
+		if active {
+			// Post-Osaka: V5 works, V4 rejected
+			if method == "engine_getPayloadV5" {
+				return minimalPayloadEnvelopeJSON, 0, ""
+			}
+			return "", -38005, "Unsupported fork"
+		}
+		// Pre-Osaka: V4 works, V5 rejected
+		if method == "engine_getPayloadV4" {
+			return minimalPayloadEnvelopeJSON, 0, ""
+		}
+		return "", -38005, "Unsupported fork"
+	})
+	defer srv.Close()
+
+	client := NewEngineRPCClient(dialTestServer(t, srv.URL))
+	ctx := context.Background()
+
+	// Pre-upgrade: V4 works.
+	_, err := client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+
+	// Simulate fork activation.
+	mu.Lock()
+	osakaActive = true
+	mu.Unlock()
+
+	// First post-upgrade call: V4 fails, falls back to V5, caches.
+	_, err = client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+
+	// Subsequent calls: V5 directly.
+	_, err = client.GetPayload(ctx, engine.PayloadID{})
+	require.NoError(t, err)
+}
+
+func TestGetPayload_NonForkError_Propagated(t *testing.T) {
+	srv := fakeEngineServer(t, func(method string) (string, int, string) {
+		// Return a different error (e.g., unknown payload)
+		return "", -38001, "Unknown payload"
+	})
+	defer srv.Close()
+
+	client := NewEngineRPCClient(dialTestServer(t, srv.URL))
+	ctx := context.Background()
+
+	_, err := client.GetPayload(ctx, engine.PayloadID{})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "Unknown payload")
+}
+
+func TestIsUnsupportedForkErr(t *testing.T) {
+	tests := []struct {
+		name     string
+		err      error
+		expected bool
+	}{
+		{"nil error", nil, false},
+		{"generic error", fmt.Errorf("something went wrong"), false},
+		{"unsupported fork code", &testRPCError{code: -38005, msg: "Unsupported fork"}, true},
+		{"different code", &testRPCError{code: -38001, msg: "Unknown payload"}, false},
+		{"wrapped unsupported fork", fmt.Errorf("call failed: %w", &testRPCError{code: -38005, msg: "Unsupported fork"}), true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.expected, isUnsupportedForkErr(tt.err))
+		})
+	}
+}
+
+// testRPCError implements rpc.Error for testing.
+type testRPCError struct {
+	code int
+	msg  string
+}
+
+func (e *testRPCError) Error() string  { return e.msg }
+func (e *testRPCError) ErrorCode() int { return e.code }
