Bug 1236111 - part 2: fix mozbuild to use the file mode modifiers specified for opening when writing a FileAvoidWrite, r=gps,nalexander

--HG--
extra : commitid : J6mVFwKgMQh
extra : rebase_source : 8a8b0ba2c21cf561af8dd667e387106869a65124

python/mozbuild/mozbuild/util.py

@@ -166,7 +166,12 @@ class FileAvoidWrite(BytesIO):
                 existing.close()
 
         ensureParentDir(self.name)
-        with open(self.name, 'w') as file:
+        # Maintain 'b' if specified.  'U' only applies to modes starting with
+        # 'r', so it is dropped.
+        writemode = 'w'
+        if 'b' in self.mode:
+            writemode += 'b'
+        with open(self.name, writemode) as file:
             file.write(buf)
 
         if self._capture_diff: