Use fastRand instead of locked-rand in skiplist (#1173)

The math/rand package (https://golang.org/src/math/rand/rand.go) uses
a global lock to allow concurrent access to the rand object.

The PR replaces `math.Rand` with `ristretto/z.FastRand()`. `FastRand`
is much faster than `math.Rand` and `rand.New(..)` generators.

The change improves concurrent writes to skiplist by ~30%
```go
func BenchmarkWrite(b *testing.B) {
	value := newValue(123)
	l := NewSkiplist(int64((b.N + 1) * MaxNodeSize))
	defer l.DecrRef()
	b.ResetTimer()
	b.RunParallel(func(pb *testing.PB) {
		rng := rand.New(rand.NewSource(time.Now().UnixNano()))
		for pb.Next() {
			l.Put(randomKey(rng), y.ValueStruct{Value: value, Meta: 0, UserMeta: 0})
		}
	})
}
```
```
name      old time/op  new time/op  delta
Write-16   657ns ± 3%   441ns ± 1%  -32.94%  (p=0.000 n=9+10)
```

(cherry picked from commit 9d6512b)

skl/skl.go

@@ -34,11 +34,11 @@ package skl
 
 import (
 	"math"
-	"math/rand"
 	"sync/atomic"
 	"unsafe"
 
 	"github.com/dgraph-io/badger/v2/y"
+	"github.com/dgraph-io/ristretto/z"
 )
 
 const (
@@ -165,9 +165,9 @@ func (s *node) casNextOffset(h int, old, val uint32) bool {
 //	return n != nil && y.CompareKeys(key, n.key) > 0
 //}
 
-func randomHeight() int {
+func (s *Skiplist) randomHeight() int {
 	h := 1
-	for h < maxHeight && rand.Uint32() <= heightIncrease {
+	for h < maxHeight && z.FastRand() <= heightIncrease {
 		h++
 	}
 	return h
@@ -300,7 +300,7 @@ func (s *Skiplist) Put(key []byte, v y.ValueStruct) {
 	}
 
 	// We do need to create a new node.
-	height := randomHeight()
+	height := s.randomHeight()
 	x := newNode(s.arena, key, v, height)
 
 	// Try to increase s.height via CAS.

skl/skl_test.go

@@ -499,7 +499,7 @@ func BenchmarkReadWriteMap(b *testing.B) {
 			b.RunParallel(func(pb *testing.PB) {
 				rng := rand.New(rand.NewSource(time.Now().UnixNano()))
 				for pb.Next() {
-					if rand.Float32() < readFrac {
+					if rng.Float32() < readFrac {
 						mutex.RLock()
 						_, ok := m[string(randomKey(rng))]
 						mutex.RUnlock()
@@ -516,3 +516,16 @@ func BenchmarkReadWriteMap(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkWrite(b *testing.B) {
+	value := newValue(123)
+	l := NewSkiplist(int64((b.N + 1) * MaxNodeSize))
+	defer l.DecrRef()
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		rng := rand.New(rand.NewSource(time.Now().UnixNano()))
+		for pb.Next() {
+			l.Put(randomKey(rng), y.ValueStruct{Value: value, Meta: 0, UserMeta: 0})
+		}
+	})
+}

table/table.go

@@ -232,6 +232,7 @@ func OpenTable(fd *os.File, opts Options) (*Table, error) {
 	if err := t.initBiggestAndSmallest(); err != nil {
 		return nil, errors.Wrapf(err, "failed to initialize table")
 	}
+
 	if opts.ChkMode == options.OnTableRead || opts.ChkMode == options.OnTableAndBlockRead {
 		if err := t.VerifyChecksum(); err != nil {
 			_ = fd.Close()
@@ -320,6 +321,9 @@ func (t *Table) readIndex() error {
 	readPos -= 4
 	buf := t.readNoFail(readPos, 4)
 	checksumLen := int(y.BytesToU32(buf))
+	if checksumLen < 0 {
+		return errors.New("checksum length less than zero. Data corrupted")
+	}
 
 	// Read checksum.
 	expectedChk := &pb.Checksum{}

table/table_test.go

@@ -743,7 +743,10 @@ func TestTableChecksum(t *testing.T) {
 	f := buildTestTable(t, "k", 10000, opts)
 	fi, err := f.Stat()
 	require.NoError(t, err, "unable to get file information")
-	f.WriteAt(rb, rand.Int63n(fi.Size()))
+	// Write random bytes at random location.
+	n, err := f.WriteAt(rb, rand.Int63n(fi.Size()))
+	require.NoError(t, err)
+	require.Equal(t, n, len(rb))
 
 	_, err = OpenTable(f, opts)
 	if err == nil || !strings.Contains(err.Error(), "checksum") {