Actually fix stray threads in cq_stats tests

infra/tools/cq_stats/cq_stats.py

 #!/usr/bin/python
 # 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.
 '''Generate stats of CQ usage.'''
 
 import argparse
 import calendar
 import collections
 import copy
@@ skipping 577 matching lines @@
   # Fetch and process each patchset log
   def get_patch_stats(patch_id):
     return derive_patch_stats(args, begin_date, end_date, patch_id)
 
   if args.seq or not args.thread_pool:
     iterable = map(get_patch_stats, raw_patches)
   else:
     pool = ThreadPool(min(args.thread_pool, len(raw_patches)))
     iterable = pool.imap_unordered(get_patch_stats, raw_patches)
     pool.close()
+    pool.join()
 
   patches, issues = set(), set()
   for patch_id, pstats in iterable:
     # The pragma-no-cover statement below is needed due to some bug in coverage
     # engine. When adding
     #   print 'foo'
     # before continue-statement, coverage engine reports full coverage, while
     # without it it reports missing branch coverage for if-statement and missing
     # line coverage for continue-statement.
     # TODO(sergiyb): Fix coverage engine and remove this pragma.
@@ skipping 442 matching lines @@
       }
 
     return try_job_stats
 
   if args.seq or not args.thread_pool:
     iterable = map(get_flakiness_stats, stats['patch_stats'].keys())
   else:
     pool = ThreadPool(min(args.thread_pool, len(stats['patch_stats'].keys())))
     iterable = pool.imap_unordered(
         get_flakiness_stats, stats['patch_stats'].keys())
+    pool.close()
+    pool.join()
 
   try_job_stats = {}
   for result in iterable:
     for master, builder in result.iterkeys():
       keys = (
         'total',
         'flakes',
         'infra_failures',
         'compile_failures',
         'test_failures',
@@ skipping 196 matching lines @@
 
   return stats
 
 def main():
   args = parse_args()
   configure_session(args)
   logger = logging.getLogger()
   infra_libs.logs.process_argparse_options(args, logger)
   stats = acquire_stats(args)
   print_stats(args, stats)