consensus/ethash: implement faster difficulty calculators (#21976)

This PR adds re-written difficulty calculators, which are based on uint256. It also adds a fuzzer + oss-fuzz integration for the new fuzzer. It does differential fuzzing between the new and old calculators.

Note: this PR does not actually enable the new calculators.

tests/fuzzers/difficulty/difficulty-fuzz.go

@@ -0,0 +1,145 @@
+// Copyright 2020 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 difficulty
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/consensus/ethash"
+	"github.com/ethereum/go-ethereum/core/types"
+)
+
+type fuzzer struct {
+	input     io.Reader
+	exhausted bool
+	debugging bool
+}
+
+func (f *fuzzer) read(size int) []byte {
+	out := make([]byte, size)
+	if _, err := f.input.Read(out); err != nil {
+		f.exhausted = true
+	}
+	return out
+}
+
+func (f *fuzzer) readSlice(min, max int) []byte {
+	var a uint16
+	binary.Read(f.input, binary.LittleEndian, &a)
+	size := min + int(a)%(max-min)
+	out := make([]byte, size)
+	if _, err := f.input.Read(out); err != nil {
+		f.exhausted = true
+	}
+	return out
+}
+
+func (f *fuzzer) readUint64(min, max uint64) uint64 {
+	if min == max {
+		return min
+	}
+	var a uint64
+	if err := binary.Read(f.input, binary.LittleEndian, &a); err != nil {
+		f.exhausted = true
+	}
+	a = min + a%(max-min)
+	return a
+}
+func (f *fuzzer) readBool() bool {
+	return f.read(1)[0]&0x1 == 0
+}
+
+// The function must return
+// 1 if the fuzzer should increase priority of the
+//    given input during subsequent fuzzing (for example, the input is lexically
+//    correct and was parsed successfully);
+// -1 if the input must not be added to corpus even if gives new coverage; and
+// 0  otherwise
+// other values are reserved for future use.
+func Fuzz(data []byte) int {
+	f := fuzzer{
+		input:     bytes.NewReader(data),
+		exhausted: false,
+	}
+	return f.fuzz()
+}
+
+var minDifficulty = big.NewInt(0x2000)
+
+type calculator func(time uint64, parent *types.Header) *big.Int
+
+func (f *fuzzer) fuzz() int {
+	// A parent header
+	header := &types.Header{}
+	if f.readBool() {
+		header.UncleHash = types.EmptyUncleHash
+	}
+	// Difficulty can range between 0x2000 (2 bytes) and up to 32 bytes
+	{
+		diff := new(big.Int).SetBytes(f.readSlice(2, 32))
+		if diff.Cmp(minDifficulty) < 0 {
+			diff.Set(minDifficulty)
+		}
+		header.Difficulty = diff
+	}
+	// Number can range between 0 and up to 32 bytes (but not so that the child exceeds it)
+	{
+		// However, if we use astronomic numbers, then the bomb exp karatsuba calculation
+		// in the legacy methods)
+		// times out, so we limit it to fit within reasonable bounds
+		number := new(big.Int).SetBytes(f.readSlice(0, 4)) // 4 bytes: 32 bits: block num max 4 billion
+		header.Number = number
+	}
+	// Both parent and child time must fit within uint64
+	var time uint64
+	{
+		childTime := f.readUint64(1, 0xFFFFFFFFFFFFFFFF)
+		//fmt.Printf("childTime: %x\n",childTime)
+		delta := f.readUint64(1, childTime)
+		//fmt.Printf("delta: %v\n", delta)
+		pTime := childTime - delta
+		header.Time = pTime
+		time = childTime
+	}
+	// Bomb delay will never exceed uint64
+	bombDelay := new(big.Int).SetUint64(f.readUint64(1, 0xFFFFFFFFFFFFFFFe))
+
+	if f.exhausted {
+		return 0
+	}
+
+	for i, pair := range []struct {
+		bigFn  calculator
+		u256Fn calculator
+	}{
+		{ethash.FrontierDifficultyCalulator, ethash.CalcDifficultyFrontierU256},
+		{ethash.HomesteadDifficultyCalulator, ethash.CalcDifficultyHomesteadU256},
+		{ethash.DynamicDifficultyCalculator(bombDelay), ethash.MakeDifficultyCalculatorU256(bombDelay)},
+	} {
+		want := pair.bigFn(time, header)
+		have := pair.u256Fn(time, header)
+		if want.Cmp(have) != 0 {
+			panic(fmt.Sprintf("pair %d: want %x have %x\nparent.Number: %x\np.Time: %x\nc.Time: %x\nBombdelay: %v\n", i, want, have,
+				header.Number, header.Time, time, bombDelay))
+		}
+	}
+	return 1
+}