read: account for entries changed after _read

If this.entries is changed after _read() has been called, we will be
out of sync and try to access an invalid index of this.entries. When
the entry cannot be found, we emit end and close, which can drop files
from reading.

(At least partially) fixes npm/npm#5082.

Credit: @evanlucas
Reviewed-By: @othiym23
PR-URL: #50

lib/dir-reader.js

@@ -24,6 +24,7 @@ function DirReader (props) {
   }
 
   self.entries = null
+  self._entries = []
   self._index = -1
   self._paused = false
   self._length = -1
@@ -47,6 +48,7 @@ DirReader.prototype._getEntries = function () {
     if (er) return self.error(er)
 
     self.entries = entries
+    self._entries = entries.slice()
 
     self.emit('entries', entries)
     if (self._paused) self.once('resume', processEntries)
@@ -74,7 +76,7 @@ DirReader.prototype._read = function () {
   }
 
   self._index++
-  if (self._index >= self.entries.length) {
+  if (self._index >= self._entries.length) {
     if (!self._ended) {
       self._ended = true
       self.emit('end')
@@ -83,12 +85,14 @@ DirReader.prototype._read = function () {
     return
   }
 
-  // ok, handle this one, then.
-
   // save creating a proxy, by stat'ing the thing now.
-  var p = path.resolve(self._path, self.entries[self._index])
+  var nextEntry = self._entries[self._index]
+  if (!nextEntry) return this._read()
+
+  // ok, handle this one, then.
+  var p = path.resolve(self._path, nextEntry)
   assert(p !== self._path)
-  assert(self.entries[self._index])
+  assert(nextEntry)
 
   // set this to prevent trying to _read() again in the stat time.
   self._currentEntry = p