[Findit] fix exception in endpoint for alerts-dispatcher.

appengine/findit/findit_api.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """This module is to provide Findit service APIs through Cloud Endpoints:
 
 Current APIs include:
 1. Analysis of compile/test failures in Chromium waterfalls.
    Analyzes failures and detects suspected CLs.
 2. Analysis of flakes on Commit Queue.
@@ skipping 155 matching lines @@
     if not cl:
       return None, None
 
     master_name = buildbot.GetMasterNameFromUrl(build.master_url)
     builder_name = build.builder_name
     current_build = build.build_number
 
     # If the CL is found by a try job, only the first failure will be recorded.
     # So we might need to go to the first failure to get CL information.
     build_info = cl.GetBuildInfo(master_name, builder_name, current_build)
-    first_build_info = cl.GetBuildInfo(
+    first_build_info = None if not reference_build_key else cl.GetBuildInfo(
         *build_util.GetBuildInfoFromId(reference_build_key))
     return suspected_cl_util.GetSuspectedCLConfidenceScoreAndApproach(
         confidences, build_info, first_build_info)
 
   def _GenerateBuildFailureAnalysisResult(
       self, build, suspected_cls_in_result, step_name, first_failure, test_name,
       analysis_approach, confidences, try_job_status, is_flaky_test,
       reference_build_key):
 
     suspected_cls = []
@@ skipping 215 matching lines @@
 
     for failure in heuristic_analysis.result['failures']:
       step_name = failure['step_name']
       if failure.get('tests'):  # Test-level analysis.
         for test in failure['tests']:
           test_name = test['test_name']
           reference_build_key, swarming_task, try_job = (
               self._GetSwarmingTaskAndTryJobForFailure(
                   step_name, test_name, heuristic_analysis.failure_result_map,
                   swarming_tasks, try_jobs))
-
           self._PopulateResult(
               results, build, heuristic_analysis.failure_type, test,
               step_name, confidences, reference_build_key, swarming_task,
               try_job, test_name=test_name)
       else:
         reference_build_key, swarming_task, try_job = (
             self._GetSwarmingTaskAndTryJobForFailure(
                 step_name, None, heuristic_analysis.failure_result_map,
                 swarming_tasks, try_jobs))
         self._PopulateResult(
@@ skipping 87 matching lines @@
 
     try:
       _AsyncProcessFlakeReport(flake_analysis_request, user_email, is_admin)
       queued = True
     except Exception:
       # Ignore the report when fail to queue it for async processing.
       queued = False
       logging.exception('Failed to queue flake report for async processing')
 
     return _FlakeAnalysis(queued=queued)