Copied a missing file from from jacobsa/bazilfuse@b378951.

internal/fuseshim/buffer.go

+package bazilfuse
+
+import "unsafe"
+
+// 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])
+}
+
+// reset clears out the contents of the buffer.
+func (w *buffer) reset() {
+	for i := range (*w)[:cap(*w)] {
+		(*w)[i] = 0
+	}
+	*w = (*w)[:0]
+}
+
+func newBuffer(extra uintptr) buffer {
+	const hdrSize = unsafe.Sizeof(outHeader{})
+	buf := make(buffer, hdrSize, hdrSize+extra)
+	return buf
+}