Initial commit

vendor/github.com/cockroachdb/pebble/internal/arenaskl/arena.go

@@ -0,0 +1,125 @@
+/*
+ * 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 (
+	"sync/atomic"
+	"unsafe"
+
+	"github.com/cockroachdb/errors"
+	"github.com/cockroachdb/pebble/internal/constants"
+	"github.com/cockroachdb/pebble/internal/invariants"
+)
+
+// Arena is lock-free.
+type Arena struct {
+	n   atomic.Uint64
+	buf []byte
+}
+
+const nodeAlignment = 4
+
+var (
+	// ErrArenaFull indicates that the arena is full and cannot perform any more
+	// allocations.
+	ErrArenaFull = errors.New("allocation failed because arena is full")
+)
+
+// NewArena allocates a new arena using the specified buffer as the backing
+// store.
+func NewArena(buf []byte) *Arena {
+	if len(buf) > constants.MaxUint32OrInt {
+		if invariants.Enabled {
+			panic(errors.AssertionFailedf("attempting to create arena of size %d", len(buf)))
+		}
+		buf = buf[:constants.MaxUint32OrInt]
+	}
+	a := &Arena{
+		buf: buf,
+	}
+	// We don't store data at position 0 in order to reserve offset=0 as a kind of
+	// nil pointer.
+	a.n.Store(1)
+	return a
+}
+
+// Size returns the number of bytes allocated by the arena.
+func (a *Arena) Size() uint32 {
+	s := a.n.Load()
+	if s > constants.MaxUint32OrInt {
+		// The last failed allocation can push the size higher than len(a.buf).
+		// Saturate at the maximum representable offset.
+		return constants.MaxUint32OrInt
+	}
+	return uint32(s)
+}
+
+// Capacity returns the capacity of the arena.
+func (a *Arena) Capacity() uint32 {
+	return uint32(len(a.buf))
+}
+
+// alloc allocates a buffer of the given size and with the given alignment
+// (which must be a power of 2).
+//
+// If overflow is not 0, it also ensures that many bytes after the buffer are
+// inside the arena (this is used for structures that are larger than the
+// requested size but don't use those extra bytes).
+func (a *Arena) alloc(size, alignment, overflow uint32) (uint32, uint32, error) {
+	if invariants.Enabled && (alignment&(alignment-1)) != 0 {
+		panic(errors.AssertionFailedf("invalid alignment %d", alignment))
+	}
+	// Verify that the arena isn't already full.
+	origSize := a.n.Load()
+	if int(origSize) > len(a.buf) {
+		return 0, 0, ErrArenaFull
+	}
+
+	// Pad the allocation with enough bytes to ensure the requested alignment.
+	padded := uint64(size) + uint64(alignment) - 1
+
+	newSize := a.n.Add(padded)
+	if newSize+uint64(overflow) > uint64(len(a.buf)) {
+		return 0, 0, ErrArenaFull
+	}
+
+	// Return the aligned offset.
+	offset := (uint32(newSize) - size) & ^(alignment - 1)
+	return offset, uint32(padded), nil
+}
+
+func (a *Arena) getBytes(offset uint32, size uint32) []byte {
+	if offset == 0 {
+		return nil
+	}
+	return a.buf[offset : offset+size : 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])))
+}