def __init__(self):
# We periodically emit newly gathered output from tool invocations.
self._emitter_thread = \
PeriodicThread(target=self.flush, name='output-emitter', period_secs=0.5)
self._emitter_thread.daemon = True
# Map from workunit id to workunit.
self._workunits = {}
# We report to these reporters.
self._reporters = {} # name -> Reporter instance.
# We synchronize on this, to support parallel execution.
self._lock = threading.Lock()
