Issue 3017 - Improve metadata file mapping precedence

packager.py

 # coding: utf-8
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # Note: These are the base functions common to all packagers, the actual
 # packagers are implemented in packagerGecko and packagerChrome.
 
 import sys, os, re, codecs, subprocess, json, zipfile
@@ skipping 77 matching lines @@
         return False
     return True
 
   def read(self, path, relpath='', skip=None):
     if os.path.isdir(path):
       for file in os.listdir(path):
         name = relpath + ('/' if relpath != '' else '') + file
         if (skip == None or file not in skip) and self.isIncluded(name):
           self.read(os.path.join(path, file), name)
     else:
-      file = open(path, 'rb')
-      if relpath in self:
-        print >>sys.stderr, 'Warning: File %s defined multiple times' % relpath
-      self[relpath] = file.read()
-      file.close()
+      with open(path, 'rb') as file:
+        if relpath in self:
+          print >>sys.stderr, 'Warning: File %s defined multiple times' % relpath
+        else:
+          self[relpath] = file.read()
 
   def readMappedFiles(self, mappings):
     for item in mappings:
       target, source = item
 
       # Make sure the file is inside an included directory
       if '/' in target and not self.isIncluded(target):
         continue
       parts = source.split('/')
       path = os.path.join(os.path.dirname(item.source), *parts)
@@ skipping 16 matching lines @@
     names = self.keys()
     names.sort(key=sortKey)
     for name in names:
       zip.writestr(name, self[name])
     zip.close()
 
   def zipToString(self, sortKey=None):
     buffer = StringIO()
     self.zip(buffer, sortKey=sortKey)
     return buffer.getvalue()