docstr: Test that SessionLog.START message discards expired events.
  
  This discard logic is preferred over the out-of-order step
  discard logic, but this logic can only be used for event protos
  which have the SessionLog enum, which was introduced to
  event.proto for file_version >= brain.Event:2.
code: def testSessionLogStartMessageDiscardsExpiredEvents(self):
      """Test that SessionLog.START message discards expired events.
  
      This discard logic is preferred over the out-of-order step
      discard logic, but this logic can only be used for event protos
      which have the SessionLog enum, which was introduced to
      event.proto for file_version >= brain.Event:2.
      """
      gen = _EventGenerator(self)
      acc = ea.EventAccumulator(gen)
      gen.AddEvent(
          event_pb2.Event(
              wall_time=0, step=1, file_version="brain.Event:2"
          )
      )
  
      gen.AddScalar("s1", wall_time=1, step=100, value=20)
      gen.AddScalar("s1", wall_time=1, step=200, value=20)
      gen.AddScalar("s1", wall_time=1, step=300, value=20)
      gen.AddScalar("s1", wall_time=1, step=400, value=20)
  
      gen.AddScalar("s2", wall_time=1, step=202, value=20)
      gen.AddScalar("s2", wall_time=1, step=203, value=20)
  
      slog = event_pb2.SessionLog(status=event_pb2.SessionLog.START)
      gen.AddEvent(
          event_pb2.Event(wall_time=2, step=201, session_log=slog)
      )
      acc.Reload()
      self.assertEqual([x.step for x in acc.Scalars("s1")], [100, 200])
      self.assertEqual([x.step for x in acc.Scalars("s2")], [])
  
imports: import os
  import numpy
  import tensorflow
  from tensorboard.backend.event_processing import event_accumulator
  from tensorboard.compat.proto import config_pb2
  from tensorboard.compat.proto import event_pb2
  from tensorboard.compat.proto import graph_pb2
  from tensorboard.compat.proto import meta_graph_pb2
  from tensorboard.compat.proto import summary_pb2
  from tensorboard.plugins.distribution import compressor
  from tensorboard.util import tensor_util
  from tensorboard.util import test_util
  from tensorboard.util import tb_logging
label_desc: Use image node_id 17 for calling the MockingEventAccumulatorTest obj's underlying member method code with example usage: obj.testSessionLogStartMessageDiscardsExpiredEvents() without return types