presubmit: don't die when skipping unknown checks

If someone doesn't want to run a check, and that check doesn't
exist... great! We won't be running that check anyway.

R=iannucci

Bug: 770408, 828154
Change-Id: I74478cac9988e21a1fadfb2c9d23dac1697aaa46
Reviewed-on: https://chromium-review.googlesource.com/991093Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Sean McCullough <seanmccullough@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

presubmit_support.py

@@ -1513,17 +1513,14 @@ def load_files(options, args):
   return change_class, files
 
 
-class NonexistantCannedCheckFilter(Exception):
-  pass
-
-
 @contextlib.contextmanager
 def canned_check_filter(method_names):
   filtered = {}
   try:
     for method_name in method_names:
       if not hasattr(presubmit_canned_checks, method_name):
-        raise NonexistantCannedCheckFilter(method_name)
+        logging.warn('Skipping unknown "canned" check %s' % method_name)
+        continue
       filtered[method_name] = getattr(presubmit_canned_checks, method_name)
       setattr(presubmit_canned_checks, method_name, lambda *_a, **_kw: [])
     yield
@@ -1692,10 +1689,6 @@ def main(argv=None):
           gerrit_obj,
           options.dry_run)
     return not results.should_continue()
-  except NonexistantCannedCheckFilter, e:
-    print >> sys.stderr, (
-      'Attempted to skip nonexistent canned presubmit check: %s' % e.message)
-    return 2
   except PresubmitFailure, e:
     print >> sys.stderr, e
     print >> sys.stderr, 'Maybe your depot_tools is out of date?'

tests/presubmit_unittest.py

@@ -175,7 +175,7 @@ class PresubmitUnittest(PresubmitTestsBase):
       'AffectedFile', 'Change', 'DoPostUploadExecuter', 'DoPresubmitChecks',
       'GetPostUploadExecuter', 'GitAffectedFile', 'CallCommand', 'CommandData',
       'GitChange', 'InputApi', 'ListRelevantPresubmitFiles', 'main',
-      'NonexistantCannedCheckFilter', 'OutputApi', 'ParseFiles',
+      'OutputApi', 'ParseFiles',
       'PresubmitFailure', 'PresubmitExecuter', 'PresubmitOutput', 'ScanSubDirs',
       'ast', 'auth', 'cPickle', 'cpplint', 'cStringIO', 'contextlib',
       'canned_check_filter', 'fix_encoding', 'fnmatch', 'gclient_utils',
@@ -198,15 +198,6 @@ class PresubmitUnittest(PresubmitTestsBase):
       self.assertEqual(canned.CheckOwners(None, None), [])
     self.assertEqual(canned.CheckOwners, orig)
 
-  def testCannedCheckFilterFail(self):
-    canned = presubmit.presubmit_canned_checks
-    orig = canned.CheckOwners
-    def failAttempt():
-      with presubmit.canned_check_filter(['CheckOwners', 'Spazfleem']):
-        pass
-    self.assertRaises(presubmit.NonexistantCannedCheckFilter, failAttempt)
-    self.assertEqual(canned.CheckOwners, orig)
-
   def testListRelevantPresubmitFiles(self):
     files = [
       'blat.cc',