Bug 952564 - Disallow empty lists in moz.build variables r=gps

--HG--
extra : rebase_source : 9a70b6a3150fe898a724a43815dc923bb0d70b89
extra : amend_source : 7aa800ef61427117e5dc461443f82ebd403d8164

dom/media/webspeech/synth/moz.build

@@ -41,8 +41,6 @@ if CONFIG['MOZ_WEBSPEECH']:
             'test/nsFakeSynthServices.cpp'
         ]
 
-    DIRS = []
-
     if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'windows':
         DIRS += ['windows']
 

python/mozbuild/mozbuild/frontend/sandbox.py

@@ -286,6 +286,10 @@ class Sandbox(dict):
             if key in self._context and self._context[key] is not value:
                 raise KeyError('global_ns', 'reassign', key)
 
+            if (key not in self._context and isinstance(value, (list, dict))
+               and not value):
+                raise KeyError('Variable %s assigned an empty value.' % key)
+
             self._context[key] = value
         else:
             dict.__setitem__(self, key, value)

python/mozbuild/mozbuild/test/frontend/data/reader-error-empty-list/moz.build

@@ -0,0 +1,5 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# Any copyright is dedicated to the Public Domain.
+# http://creativecommons.org/publicdomain/zero/1.0/
+
+DIRS = []

python/mozbuild/mozbuild/test/frontend/test_reader.py

@@ -11,8 +11,10 @@ import unittest
 from mozunit import main
 
 from mozbuild.frontend.context import BugzillaComponent
-from mozbuild.frontend.reader import BuildReaderError
+from mozbuild.frontend.reader import (
-from mozbuild.frontend.reader import BuildReader
+    BuildReaderError,
+    BuildReader,
+)
 
 from mozbuild.test.common import MockConfig
 
@@ -246,6 +248,15 @@ class TestBuildReader(unittest.TestCase):
 
         contexts = list(reader.read_topsrcdir())
 
+    def test_error_empty_list(self):
+        reader = self.reader('reader-error-empty-list')
+
+        with self.assertRaises(BuildReaderError) as bre:
+            list(reader.read_topsrcdir())
+
+        e = bre.exception
+        self.assertIn('Variable DIRS assigned an empty value.', str(e))
+
     def test_inheriting_variables(self):
         reader = self.reader('inheriting-variables')