encoding: Never crash if unknown bytes in OS errors

Fixes #1599

docs/changelog.rst

@@ -22,6 +22,10 @@ Feature release.
 - MPD: Added Unix domain sockets for binding MPD to.
   (Fixes: :issue:`1531`, PR: :issue:`1629`)
 
+- Ensure that decoding of OS errors with unknown encoding never crashes, but
+  instead replaces unknown bytes with a replacement marker. (Fixes:
+  :issue:`1599`)
+
 
 v2.1.0 (2017-01-02)
 ===================

mopidy/internal/encoding.py

@@ -9,4 +9,4 @@ def locale_decode(bytestr):
     try:
         return compat.text_type(bytestr)
     except UnicodeError:
-        return bytes(bytestr).decode(locale.getpreferredencoding())
+        return bytes(bytestr).decode(locale.getpreferredencoding(), 'replace')

tests/internal/test_encoding.py

@@ -43,3 +43,10 @@ class LocaleDecodeTest(unittest.TestCase):
         encoding.locale_decode(b'abc')
 
         self.assertFalse(mock.called)
+
+    def test_replaces_unknown_bytes_instead_of_crashing(self, mock):
+        mock.return_value = 'US-ASCII'
+
+        result = encoding.locale_decode(b'abc\xc3def')
+
+        assert result == 'abc\ufffddef'