Add friendly error message when GenTests is missing or misspelled

unittests/test_test.py

 #!/usr/bin/env python
 # Copyright 2017 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import json
 import os
 import shutil
 import subprocess
 import sys
@@ skipping 52 matching lines @@
     for d in dirs:
       if not os.path.exists(d):
         os.makedirs(d)
     with open(os.path.join(self.recipes_dir, '%s.py' % self.name), 'w') as f:
       f.write('\n'.join([
         'from recipe_engine import post_process',
         '',
         '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:
         json.dump(test_contents, f)
 
 
 class RecipeModuleWriter(object):
   """Helper to write a recipe module for tests."""
 
   def __init__(self, root_dir, name):
@@ skipping 307 matching lines @@
 
   def test_test_expectation_pass(self):
     rw = RecipeWriter(os.path.join(self._root_dir, 'recipes'), 'foo')
     rw.DEPS = ['recipe_engine/step']
     rw.RunStepsLines = ['api.step("test", ["echo", "bar"])']
     rw.add_expectation('basic', [{'cmd': ['echo', 'bar'], 'name': 'test'}])
     rw.write()
     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']
     rw.add_expectation('basic')
     rw.write()
     with self.assertRaises(subprocess.CalledProcessError) as cm:
       self._run_recipes('test', 'run', '--json', self.json_path)
     self.assertIn('FATAL: Insufficient coverage', cm.exception.output)
     self.assertNotIn('CHECK(FAIL)', cm.exception.output)
     self.assertNotIn('foo.basic failed', cm.exception.output)
@@ skipping 758 matching lines @@
             .check_failure('bar_check', 'bar_file', 2, 'bar_func', ['bar_args'])
             .internal_failure('bar_internal')
             .uncovered_module('bar_module')
             .unused_expectation('bar_expectation')
             .get(),
         self.json_contents)
 
 
 if __name__ == '__main__':
   sys.exit(unittest.main())