event_log: turn id generation from a generator to a func call

Running lots of sync processes in parallel can hit the failure:
Fetching projects:  23% (124/523)Exception in thread Thread-201:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/local/src/repo/subcmds/sync.py", line 278, in _FetchProjectList
    success = self._FetchHelper(opt, project, *args, **kwargs)
  File "/usr/local/src/repo/subcmds/sync.py", line 357, in _FetchHelper
    start, finish, success)
  File "/usr/local/src/repo/event_log.py", line 104, in AddSync
    event = self.Add(project.relpath, task_name, start, finish, success)
  File "/usr/local/src/repo/event_log.py", line 74, in Add
    'id': (kind, next(self._next_id)),
ValueError: generator already executing

It looks like, while we lock the multiprocessing value correctly, the
generator that wraps the value isn't parallel safe.  Since we don't
have a way of doing that (as it's part of the language), turn it into
a plain function call instead.

Bug: https://crbug.com/gerrit/10293
Change-Id: I0db03601986ca0370a1699bab32adb03e7b2910a

event_log.py

@@ -51,7 +51,6 @@ class EventLog(object):
   def __init__(self):
     """Initializes the event log."""
     self._log = []
-    self._next_id = _EventIdGenerator()
     self._parent = None
 
   def Add(self, name, task_name, start, finish=None, success=None,
@@ -71,7 +70,7 @@ class EventLog(object):
       A dictionary of the event added to the log.
     """
     event = {
-        'id': (kind, next(self._next_id)),
+        'id': (kind, _NextEventId()),
         'name': name,
         'task_name': task_name,
         'start_time': start,
@@ -162,16 +161,16 @@ class EventLog(object):
         f.write('\n')
 
 
-def _EventIdGenerator():
-  """Returns multi-process safe iterator that generates locally unique id.
+# An integer id that is unique across this invocation of the program.
+_EVENT_ID = multiprocessing.Value('i', 1)
 
-  Yields:
+def _NextEventId():
+  """Helper function for grabbing the next unique id.
+
+  Returns:
     A unique, to this invocation of the program, integer id.
   """
-  eid = multiprocessing.Value('i', 1)
-
-  while True:
-    with eid.get_lock():
-      val = eid.value
-      eid.value += 1
-    yield val
+  with _EVENT_ID.get_lock():
+    val = _EVENT_ID.value
+    _EVENT_ID.value += 1
+  return val