buffer: propagate originating parent

When creating a slice, make sure to propagate the originating parent.
This is to prevent a buf.parent.parent.(etc) scenario.

Also speed up the constructor by preventing lookup of non-existant
properties by setting them beforehand in the prototype. (see
https://github.com/joyent/node/commit/7ce5a31#commitcomment-3332779)

lib/buffer.js

@@ -197,6 +197,11 @@ Buffer.dispose = function(obj) {
 };
 
 
+// pre-set for values that may exist in the future
+Buffer.prototype.length = undefined;
+Buffer.prototype.parent = undefined;
+
+
 // toString(encoding, start=0, end=buffer.length)
 Buffer.prototype.toString = function(encoding, start, end) {
   encoding = !!encoding ? (encoding + '').toLowerCase() : 'utf8';
@@ -387,7 +392,8 @@ Buffer.prototype.slice = function(start, end) {
     end = start;
 
   var buf = new Buffer();
-  buf.parent = sliceOnto(this, buf, start, end);
+  sliceOnto(this, buf, start, end);
+  buf.parent = this.parent === undefined ? this : this.parent;
   buf.length = end - start;
 
   return buf;

test/simple/test-buffer.js

@@ -23,6 +23,7 @@ var common = require('../common');
 var assert = require('assert');
 
 var Buffer = require('buffer').Buffer;
+var SlowBuffer = require('buffer').SlowBuffer;
 
 // counter to ensure unique value is always copied
 var cntr = 0;
@@ -271,6 +272,21 @@ for (var j = 0; j < 100; j++) {
 }
 
 
+// make sure only top level parent propagates from allocPool
+var b = new Buffer(5);
+var c = b.slice(0, 4);
+var d = c.slice(0, 2);
+assert.equal(b.parent, c.parent);
+assert.equal(b.parent, d.parent);
+
+// also from a non-pooled instance
+var b = new SlowBuffer(5);
+var c = b.slice(0, 4);
+var d = c.slice(0, 2);
+assert.equal(b, c.parent);
+assert.equal(b, d.parent);
+
+
 
 // Bug regression test
 var testValue = '\u00F6\u65E5\u672C\u8A9E'; // ö日本語