[SuperSize] Ensure BulkForkAndCall() cleans up |pool| on exception.

For BulkForkAndCall(), the first exception encountered can get
re-raised. However, this bypasses code to clean |pool|. For the
upcoming work to convert SuperSize to Python 3, this causes

  parallel_test.py: testBulkForkAndCall_exception()

to hang, if another test is also run, and vpython3 (instead of python3)
is used.

This CL prevents the problem by ensuring that BulkForkAndCall() cleans
up |pool| when an exception is raised.

Also fix missing parameter to _ForkTestHelper() in
testBulkForkAndCall_exception().

Bug: 1054018
Change-Id: I2942438130700fa7ccf804064ae317159c2aa895
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107121Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751228}

tools/binary_size/libsupersize/parallel.py

@@ -209,12 +209,14 @@ def BulkForkAndCall(func, arg_tuples, **kwargs):
 
   pool = _MakeProcessPool(arg_tuples, **kwargs)
   wrapped_func = _FuncWrapper(func)
-  for result in pool.imap_unordered(wrapped_func, xrange(len(arg_tuples))):
-    _CheckForException(result)
-    yield result
-  pool.close()
-  pool.join()
-  _all_pools.remove(pool)
+  try:
+    for result in pool.imap_unordered(wrapped_func, xrange(len(arg_tuples))):
+      _CheckForException(result)
+      yield result
+  finally:
+    pool.close()
+    pool.join()
+    _all_pools.remove(pool)
 
 
 def CallOnThread(func, *args, **kwargs):

tools/binary_size/libsupersize/parallel_test.py

@@ -149,7 +149,7 @@ class ConcurrentTest(unittest.TestCase):
   def testBulkForkAndCall_exception(self):
     parent_pid = os.getpid()
     results = parallel.BulkForkAndCall(_ForkTestHelper,
-                                       [(1, 'a', self, parent_pid)])
+                                       [(1, 'a', None, self, parent_pid)])
     self.assertRaises(TypeError, results.next)