Error log: https://pantheon.corp.google.com/logs/viewer?project=findit-for-me&key1=default&key2=alpha&minLogLevel=0&expandAll=false&timestamp=2018-04-30T16:49:55.742000000Z&customFacets=&limitCustomFacetWidth=true&dateRangeStart=2018-04-23T16:49:55.993Z&dateRangeEnd=2018-04-30T16:49:55.993Z&interval=P7D&resource=gae_app%2Fmodule_id%2Fwaterfall-backend%2Fversion_id%2F15192-6d645d6&logName=projects%2Ffindit-for-me%2Flogs%2Fappengine.googleapis.com%252Frequest_log&filters=text:%22cross-group%20transaction%20need%20to%20be%20explicitly%20specified%22&scrollTimestamp=2018-04-27T23:39:05.399925000Z

Most important traceback:
  File "/base/data/home/apps/s~findit-for-me/waterfall-backend:15192-6d645d6.409329331433507053/services/flake_failure/data_point_util.py", line 181, in UpdateAnalysisDataPoints
    analysis.build_number, commit_position)
  File "/base/data/home/apps/s~findit-for-me/waterfall-backend:15192-6d645d6.409329331433507053/services/step_util.py", line 112, in GetValidBoundingBuildsForStep
    upper_bound_build_number)
  File "/base/data/home/apps/s~findit-for-me/waterfall-backend:15192-6d645d6.409329331433507053/waterfall/build_util.py", line 103, in GetBuildInfo
    build_number)
  File "/base/data/home/apps/s~findit-for-me/waterfall-backend:15192-6d645d6.409329331433507053/waterfall/build_util.py", line 67, in DownloadBuildData
    build = WfBuild.Get(master_name, builder_name, build_number)
  File "/base/data/home/apps/s~findit-for-me/waterfall-backend:15192-6d645d6.409329331433507053/model/wf_build.py", line 29, in Get
    return WfBuild._CreateKey(master_name, builder_name, build_number).get()

UpdateAnalysisDataPoints is in a transaction, so getting WfBuild caused cross-group transaction error.