Bug 1295486 - Decode YAML files to UTF-8 at read time; r=dustin

Before, we'd open files and feed bytes to yaml.load(). When a str
is fed to yaml.load(), it attempts to guess the encoding. It defaults
to UTF-8 unless somebody set us up the BOM. This is probably OK.
Except if the file isn't valid UTF-8, the exception will be raised
in the bowels of YAML parsing and it may not be obvious the failure
is due to invalid UTF-8 input versus say Python str/unicode
coercion foo.

We change all call sites that load YAML from a file to use
codecs.open() to open the file in UTF-8 and perform UTF-8
decoding/validation at file read time. This should make any UTF-8
failures more obvious. Furthermore, it reinforces that our YAML files
are UTF-8 and not some other encoding.

I discovered this issue as part of trying to get emoji symbols to
render on Treeherder. Unfortunately, it appears pyyaml detects
many emoji as unprintable characters and refuses to load them. This
makes me sad and makes me want to abandon pyyaml/YAML in favor of
something that supports emoji :P

MozReview-Commit-ID: AOvAruZFfnK

taskcluster/taskgraph/parameters.py

@@ -6,6 +6,7 @@
 
 from __future__ import absolute_import, print_function, unicode_literals
 
+import codecs
 import json
 import yaml
 from mozbuild.util import ReadOnlyDict
@@ -27,7 +28,7 @@ def load_parameters_file(options):
     filename = options['parameters']
     if not filename:
         return Parameters()
-    with open(filename) as f:
+    with codecs.open(filename, 'rb', 'utf-8') as f:
         if filename.endswith('.yml'):
             return Parameters(**yaml.safe_load(f))
         elif filename.endswith('.json'):