add arena skip list

zongxingye · Jan 13, 2022 · 3c46506 · 3c46506
1 parent 0e4f376
commit 3c46506
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diff --git a/arenaskl/arena.go b/arenaskl/arena.go
@@ -0,0 +1,114 @@
+/*
+ * Copyright 2017 Dgraph Labs, Inc. and Contributors
+ * Modifications copyright (C) 2017 Andy Kimball and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package arenaskl
+
+import (
+	"errors"
+	"math"
+	"sync/atomic"
+	"unsafe"
+)
+
+var (
+	// ErrArenaFull allocation failed because arena is full.
+	ErrArenaFull = errors.New("allocation failed because arena is full")
+)
+
+type Align uint8
+
+const (
+	Align1 = 0
+	Align8 = 7
+)
+
+// Arena should be lock-free.
+type Arena struct {
+	n   uint64
+	buf []byte
+}
+
+// NewArena allocates a new arena of the specified size and returns it.
+func NewArena(size uint32) *Arena {
+	// Don't store data at position 0 in order to reserve offset=0 as a kind
+	// of nil pointer.
+	out := &Arena{
+		n:   1,
+		buf: make([]byte, size),
+	}
+	return out
+}
+
+func (a *Arena) Size() uint32 {
+	s := atomic.LoadUint64(&a.n)
+	if s > math.MaxUint32 {
+		// Saturate at MaxUint32.
+		return math.MaxUint32
+	}
+	return uint32(s)
+}
+
+func (a *Arena) Cap() uint32 {
+	return uint32(len(a.buf))
+}
+
+func (a *Arena) Reset() {
+	atomic.StoreUint64(&a.n, 1)
+}
+
+func (a *Arena) Alloc(size, overflow uint32, align Align) (uint32, error) {
+	// Verify that the arena isn't already full.
+	origSize := atomic.LoadUint64(&a.n)
+	if int(origSize) > len(a.buf) {
+		return 0, ErrArenaFull
+	}
+
+	// Pad the allocation with enough bytes to ensure the requested alignment.
+	padded := uint32(size) + uint32(align)
+
+	// Use 64-bit arithmetic to protect against overflow.
+	newSize := atomic.AddUint64(&a.n, uint64(padded))
+	if int(newSize)+int(overflow) > len(a.buf) {
+		// Doubles as a check against newSize > math.MaxUint32.
+		return 0, ErrArenaFull
+	}
+
+	// Return the aligned offset.
+	offset := (uint32(newSize) - padded + uint32(align)) & ^uint32(align)
+	return offset, nil
+}
+
+func (a *Arena) GetBytes(offset uint32, size uint32) []byte {
+	if offset == 0 {
+		return nil
+	}
+	return a.buf[offset : offset+size]
+}
+
+func (a *Arena) GetPointer(offset uint32) unsafe.Pointer {
+	if offset == 0 {
+		return nil
+	}
+	return unsafe.Pointer(&a.buf[offset])
+}
+
+func (a *Arena) GetPointerOffset(ptr unsafe.Pointer) uint32 {
+	if ptr == nil {
+		return 0
+	}
+	return uint32(uintptr(ptr) - uintptr(unsafe.Pointer(&a.buf[0])))
+}
diff --git a/arenaskl/arena_test.go b/arenaskl/arena_test.go
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2017 Dgraph Labs, Inc. and Contributors
+ * Modifications copyright (C) 2017 Andy Kimball and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package arenaskl
+
+import (
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestArenaSizeOverflow tests that large allocations do not cause Arena's
+// internal size accounting to overflow and produce incorrect results.
+func TestArenaSizeOverflow(t *testing.T) {
+	a := NewArena(math.MaxUint32)
+
+	// Allocating under the limit throws no error.
+	offset, err := a.Alloc(math.MaxUint16, 0, Align1)
+	require.Nil(t, err)
+	require.Equal(t, uint32(1), offset)
+	require.Equal(t, uint32(math.MaxUint16)+1, a.Size())
+
+	// Allocating over the limit could cause an accounting
+	// overflow if 32-bit arithmetic was used. It shouldn't.
+	_, err = a.Alloc(math.MaxUint32, 0, Align1)
+	require.Equal(t, ErrArenaFull, err)
+	require.Equal(t, uint32(math.MaxUint32), a.Size())
+
+	// Continuing to allocate continues to throw an error.
+	_, err = a.Alloc(math.MaxUint16, 0, Align1)
+	require.Equal(t, ErrArenaFull, err)
+	require.Equal(t, uint32(math.MaxUint32), a.Size())
+}