Add friendly error message when GenTests is missing or misspelled

unittests/test_test.py

Index: unittests/test_test.py
diff --git a/unittests/test_test.py b/unittests/test_test.py
index 9ef908f2cbc8e600a7c9041f5df46e74446cda12..9f7ac6638a46d27d75ef1db1c1e40ba19baf6936 100755
--- a/unittests/test_test.py
+++ b/unittests/test_test.py
@@ -70,10 +70,12 @@ class RecipeWriter(object):
         'DEPS = %r' % self.DEPS,
         '',
         'def RunSteps(api):',
-      ] + ['  %s' % l for l in self.RunStepsLines] + [
-        '',
-        'def GenTests(api):',
-      ] + ['  %s' % l for l in self.GenTestsLines]))
+      ] + ['  %s' % l for l in self.RunStepsLines] + ['']))
+      if self.GenTestsLines:
+        f.write('\n'.join([
+          '',
+          'def GenTests(api):',
+        ] + ['  %s' % l for l in self.GenTestsLines]))
     for test_name, test_contents in self.expectations.iteritems():
       name = ''.join('_' if c in '<>:"\\/|?*\0' else c for c in test_name)
       with open(os.path.join(self.expect_dir, '%s.json' % name), 'w') as f:
@@ -401,6 +403,17 @@ class TestTest(unittest.TestCase):
     self._run_recipes('test', 'run', '--json', self.json_path)
     self.assertEqual(self.json_generator.get(), self.json_contents)
 
+  def test_test_missing_gentests(self):
+    rw = RecipeWriter(os.path.join(self._root_dir, 'recipes'), 'foo')
+    rw.RunStepsLines = ['pass']
+    rw.GenTestsLines = None
+    rw.add_expectation('basic')
+    rw.write()
+    with self.assertRaises(subprocess.CalledProcessError) as cm:
+      self._run_recipes('test', 'run', '--json', self.json_path)
+    self.assertIn('Missing or misspelled GenTests function',
+                  cm.exception.output)
+
   def test_test_recipe_not_covered(self):
     rw = RecipeWriter(os.path.join(self._root_dir, 'recipes'), 'foo')
     rw.RunStepsLines = ['if False:', '  pass']