Added a Buffer type.

internal/buffer/buffer.go

-package fuseshim
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// 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 buffer
 
 import (
+	"reflect"
 	"unsafe"
 
 	"github.com/jacobsa/fuse/internal/fusekernel"
 )
 
-// Buffer provides a mechanism for constructing a message from multiple
-// segments.
-type Buffer []byte
-
-// alloc allocates size bytes and returns a pointer to the new
-// segment.
-func (w *Buffer) Alloc(size uintptr) unsafe.Pointer {
-	s := int(size)
-	if len(*w)+s > cap(*w) {
-		old := *w
-		*w = make([]byte, len(*w), 2*cap(*w)+s)
-		copy(*w, old)
-	}
-	l := len(*w)
-	*w = (*w)[:l+s]
-	return unsafe.Pointer(&(*w)[l])
+// Buffer provides a mechanism for constructing a single contiguous fuse
+// message from multiple segments, where the first segment is always a
+// fusekernel.OutHeader message.
+//
+// Must be created with New.
+type Buffer struct {
+	slice []byte
 }
 
-// reset clears out the contents of the buffer.
-func (w *Buffer) reset() {
-	for i := range (*w)[:cap(*w)] {
-		(*w)[i] = 0
-	}
-	*w = (*w)[:0]
+// Create a new buffer whose initial contents are a zeroed fusekernel.OutHeader
+// message, and with room enough to grow by extra bytes.
+func New(extra uintptr) (b Buffer) {
+	const headerSize = unsafe.Sizeof(fusekernel.OutHeader{})
+	b.slice = make([]byte, headerSize, headerSize+extra)
+	return
 }
 
-func NewBuffer(extra uintptr) (buf Buffer) {
-	const hdrSize = unsafe.Sizeof(fusekernel.OutHeader{})
-	buf = make(Buffer, hdrSize, hdrSize+extra)
+// Return a pointer to the header at the start of the buffer.
+func (b *Buffer) OutHeader() (h *fusekernel.OutHeader) {
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&b.slice))
+	h = (*fusekernel.OutHeader)(unsafe.Pointer(sh.Data))
 	return
 }
+
+// Grow the buffer by the supplied number of bytes, returning a pointer to the
+// start of the new segment. The sum of the arguments given to Grow must not
+// exceed the argument given to New when creating the buffer.
+func (b *Buffer) Grow(size uintptr) (p unsafe.Pointer) {
+	sh := (*reflect.SliceHeader)(unsafe.Pointer(&b.slice))
+	p = unsafe.Pointer(sh.Data + uintptr(sh.Len))
+	b.slice = b.slice[:len(b.slice)+int(size)]
+	return
+}
+
+// Return a reference to the current contents of the buffer.
+func (b *Buffer) Bytes() []byte {
+	return b.slice
+}