util/sha256x: new package (#5337)

The hash.Hash provided by sha256.New is much more efficient
if we always provide it with data a multiple of the block size.
This avoids double-copying of data into the internal block
of sha256.digest.x. Effectively, we are managing a block ourselves
to ensure we only ever call hash.Hash.Write with full blocks.

Performance:

	name    old time/op    new time/op    delta
	Hash    33.5µs ± 1%    20.6µs ± 1%  -38.40%  (p=0.000 n=10+9)

The logic has gone through CPU-hours of fuzzing.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/sha256x/sha256_test.go

@@ -0,0 +1,149 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sha256x
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"hash"
+	"math/rand"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+// naiveHash is an obviously correct implementation of Hash.
+type naiveHash struct {
+	hash.Hash
+	scratch [8]byte
+}
+
+func newNaive() *naiveHash               { return &naiveHash{Hash: sha256.New()} }
+func (h *naiveHash) HashUint8(n uint8)   { h.Write(append(h.scratch[:0], n)) }
+func (h *naiveHash) HashUint16(n uint16) { h.Write(binary.LittleEndian.AppendUint16(h.scratch[:0], n)) }
+func (h *naiveHash) HashUint32(n uint32) { h.Write(binary.LittleEndian.AppendUint32(h.scratch[:0], n)) }
+func (h *naiveHash) HashUint64(n uint64) { h.Write(binary.LittleEndian.AppendUint64(h.scratch[:0], n)) }
+func (h *naiveHash) HashBytes(b []byte)  { h.Write(b) }
+
+var bytes = func() (out []byte) {
+	out = make([]byte, 130)
+	for i := range out {
+		out[i] = byte(i)
+	}
+	return out
+}()
+
+type hasher interface {
+	HashUint8(uint8)
+	HashUint16(uint16)
+	HashUint32(uint32)
+	HashUint64(uint64)
+	HashBytes([]byte)
+}
+
+func hashSuite(h hasher) {
+	for i := 0; i < 10; i++ {
+		for j := 0; j < 10; j++ {
+			h.HashUint8(0x01)
+			h.HashUint8(0x23)
+			h.HashUint32(0x456789ab)
+			h.HashUint8(0xcd)
+			h.HashUint8(0xef)
+			h.HashUint16(0x0123)
+			h.HashUint32(0x456789ab)
+			h.HashUint16(0xcdef)
+			h.HashUint8(0x01)
+			h.HashUint64(0x23456789abcdef01)
+			h.HashUint16(0x2345)
+			h.HashUint8(0x67)
+			h.HashUint16(0x89ab)
+			h.HashUint8(0xcd)
+		}
+		h.HashBytes(bytes[:(i+1)*13])
+	}
+}
+func Test(t *testing.T) {
+	c := qt.New(t)
+	h1 := New()
+	h2 := newNaive()
+	hashSuite(h1)
+	hashSuite(h2)
+	c.Assert(h1.Sum(nil), qt.DeepEquals, h2.Sum(nil))
+}
+
+func Fuzz(f *testing.F) {
+	f.Fuzz(func(t *testing.T, seed int64) {
+		c := qt.New(t)
+
+		execute := func(h hasher, r *rand.Rand) {
+			for i := 0; i < r.Intn(256); i++ {
+				switch r.Intn(5) {
+				case 0:
+					n := uint8(r.Uint64())
+					h.HashUint8(n)
+				case 1:
+					n := uint16(r.Uint64())
+					h.HashUint16(n)
+				case 2:
+					n := uint32(r.Uint64())
+					h.HashUint32(n)
+				case 3:
+					n := uint64(r.Uint64())
+					h.HashUint64(n)
+				case 4:
+					b := make([]byte, r.Intn(256))
+					r.Read(b)
+					h.HashBytes(b)
+				}
+			}
+		}
+
+		r1 := rand.New(rand.NewSource(seed))
+		r2 := rand.New(rand.NewSource(seed))
+
+		h1 := New()
+		h2 := newNaive()
+
+		execute(h1, r1)
+		execute(h2, r2)
+
+		c.Assert(h1.Sum(nil), qt.DeepEquals, h2.Sum(nil))
+
+		execute(h1, r1)
+		execute(h2, r2)
+
+		c.Assert(h1.Sum(nil), qt.DeepEquals, h2.Sum(nil))
+
+		h1.Reset()
+		h2.Reset()
+
+		execute(h1, r1)
+		execute(h2, r2)
+
+		c.Assert(h1.Sum(nil), qt.DeepEquals, h2.Sum(nil))
+	})
+}
+
+func Benchmark(b *testing.B) {
+	var sum [sha256.Size]byte
+	b.Run("Hash", func(b *testing.B) {
+		b.ReportAllocs()
+		h := New()
+		for i := 0; i < b.N; i++ {
+			h.Reset()
+			hashSuite(h)
+			h.Sum(sum[:0])
+		}
+	})
+	b.Run("Naive", func(b *testing.B) {
+		b.ReportAllocs()
+		h := newNaive()
+		for i := 0; i < b.N; i++ {
+			h.Reset()
+			hashSuite(h)
+			h.Sum(sum[:0])
+		}
+	})
+}