Bug 1053140 part 2 - Add a test for exported variable override. r=gps

python/mozbuild/mozbuild/test/frontend/data/inheriting-variables/foo/baz/moz.build

@@ -3,3 +3,5 @@
 # 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/.
+
+XPIDL_MODULE = 'baz'

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

@@ -242,7 +242,7 @@ class TestBuildReader(unittest.TestCase):
         self.assertEqual([sandbox['RELATIVEDIR'] for sandbox in sandboxes],
             ['', 'foo', 'foo/baz', 'bar'])
         self.assertEqual([sandbox['XPIDL_MODULE'] for sandbox in sandboxes],
-            ['foobar', 'foobar', 'foobar', 'foobar'])
+            ['foobar', 'foobar', 'baz', 'foobar'])
 
     def test_process_eval_callback(self):
         def strip_dirs(sandbox):

python/mozbuild/mozbuild/test/frontend/test_sandbox.py

@@ -180,6 +180,28 @@ class TestSandbox(unittest.TestCase):
         self.assertEqual(e.args[1], 'reassign')
         self.assertEqual(e.args[2], 'DIRS')
 
+    def test_exec_source_reassign_exported(self):
+        config = MockConfig()
+
+        exports = {'DIST_SUBDIR': 'browser'}
+
+        sandbox = MozbuildSandbox(config, '', metadata={'exports': exports})
+
+        self.assertEqual(sandbox['DIST_SUBDIR'], 'browser')
+
+        sandbox.exec_source('DIST_SUBDIR = "foo"', 'foo.py')
+        with self.assertRaises(SandboxExecutionError) as se:
+          sandbox.exec_source('DIST_SUBDIR = "bar"', 'foo.py')
+
+        self.assertEqual(sandbox['DIST_SUBDIR'], 'foo')
+        e = se.exception
+        self.assertIsInstance(e.exc_value, KeyError)
+
+        e = se.exception.exc_value
+        self.assertEqual(e.args[0], 'global_ns')
+        self.assertEqual(e.args[1], 'reassign')
+        self.assertEqual(e.args[2], 'DIST_SUBDIR')
+
     def test_add_tier_dir_regular_str(self):
         sandbox = self.sandbox()