Change legacy rando geth fork choice to be deterministic (#871)

* Change legacy rando geth fork choice to be deterministic, based on block hash.

* core: add forkchoice tests

* fix

---------

Co-authored-by: Manav Darji <manavdarji.india@gmail.com>

core/forkchoice.go

@@ -17,6 +17,7 @@
 package core
 
 import (
+	"bytes"
 	"errors"
 	"math/big"
 
@@ -114,7 +115,8 @@ func (f *ForkChoice) ReorgNeeded(current *types.Header, extern *types.Header) (b
 			currentPreserve, externPreserve = f.preserve(current), f.preserve(extern)
 		}
 
-		reorg = !currentPreserve && (externPreserve || f.rand.Float64() < 0.5)
+		// Compare hashes of block in case of tie breaker. Lexicographically larger hash wins.
+		reorg = !currentPreserve && (externPreserve || bytes.Compare(current.Hash().Bytes(), extern.Hash().Bytes()) < 0)
 	}
 
 	return reorg, nil

core/forkchoice_test.go

@@ -10,6 +10,8 @@ import (
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/params"
 	"github.com/ethereum/go-ethereum/trie"
+
+	"github.com/stretchr/testify/require"
 )
 
 // chainValidatorFake is a mock for the chain validator service
@@ -30,6 +32,60 @@ func newChainReaderFake(getTd func(hash common.Hash, number uint64) *big.Int) *c
 	return &chainReaderFake{getTd: getTd}
 }
 
+// nolint: tparallel
+func TestForkChoice(t *testing.T) {
+	t.Parallel()
+
+	// Create mocks for forker
+	getTd := func(hash common.Hash, number uint64) *big.Int {
+		if number <= 2 {
+			return big.NewInt(int64(number))
+		}
+
+		return big.NewInt(0)
+	}
+	mockChainReader := newChainReaderFake(getTd)
+	mockForker := NewForkChoice(mockChainReader, nil, nil)
+
+	createHeader := func(number int64, extra []byte) *types.Header {
+		return &types.Header{
+			Number: big.NewInt(number),
+			Extra:  extra,
+		}
+	}
+
+	// Create headers for different cases
+	headerA := createHeader(1, []byte("A"))
+	headerB := createHeader(2, []byte("B"))
+	headerC := createHeader(3, []byte("C"))
+	headerD := createHeader(4, []byte("D")) // 0x96b0f70c01f4d2b1ee2df5b0202c099776f24c9375ffc89d94b880007633961b (hash)
+	headerE := createHeader(4, []byte("E")) // 0xdc0acf54354ff86194baeaab983098a49a40218cffcc77a583726fc06c429685 (hash)
+
+	testCases := []struct {
+		name     string
+		current  *types.Header
+		incoming *types.Header
+		want     bool
+	}{
+		{"tdd(incoming) > tdd(current)", headerA, headerB, true},
+		{"tdd(current) > tdd(incoming)", headerB, headerA, false},
+		{"tdd(current) = tdd(incoming), number(incoming) > number(current)", headerC, headerD, false},
+		{"tdd(current) = tdd(incoming), number(current) > number(incoming)", headerD, headerC, true},
+		{"tdd(current) = tdd(incoming), number(current) = number(incoming), hash(current) > hash(incoming)", headerE, headerD, false},
+		{"tdd(current) = tdd(incoming), number(current) = number(incoming), hash(incoming) > hash(current)", headerD, headerE, true},
+	}
+
+	// nolint: paralleltest
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			res, err := mockForker.ReorgNeeded(tc.current, tc.incoming)
+			require.Equal(t, tc.want, res, tc.name)
+			require.NoError(t, err, tc.name)
+		})
+	}
+}
+
 func TestPastChainInsert(t *testing.T) {
 	t.Parallel()