Add friendly error message when GenTests is missing or misspelled

recipe_engine/test.py

 # 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.
 
 from __future__ import print_function
 
 import bdb
 import cStringIO
 import contextlib
 import copy
@@ skipping 397 matching lines @@
         # Run recipe loading under coverage context. This ensures we collect
         # coverage of all definitions and globals.
         recipe = _UNIVERSE_VIEW.load_recipe(recipe_name)
         test_api = loader.create_test_api(recipe.LOADED_DEPS, _UNIVERSE_VIEW)
 
         root, name = os.path.split(recipe_path)
         name = os.path.splitext(name)[0]
         # TODO(phajdan.jr): move expectation tree outside of the recipe tree.
         expect_dir = os.path.join(root, '%s.expected' % name)
 
+        if not recipe.gen_tests:
+          raise Exception('Missing or misspelled GenTests function.')
+
         # Immediately convert to list to force running the generator under
         # coverage context. Otherwise coverage would only report executing
         # the function definition, not GenTests body.
         recipe_tests = list(recipe.gen_tests(test_api))
       coverage_data.update(cov.get_data())
 
       for test_data in recipe_tests:
         # Put the test data in shared cache. This way it can only be generated
         # once. We do this primarily for _correctness_ , for example in case
         # a weird recipe generates tests non-deterministically. The recipe
@@ skipping 513 matching lines @@
 
   # Prevent flakiness caused by stale pyc files.
   package.cleanup_pyc(package_deps.root_package.recipes_dir)
 
   global _UNIVERSE_VIEW
   _UNIVERSE_VIEW = universe_view
   global _ENGINE_FLAGS
   _ENGINE_FLAGS = engine_flags
 
   return args.subfunc(args)