Subrun product mixing with multiple files

[knoepfel]

This issue has been migrated from https://cdcvs.fnal.gov/redmine/issues/25864 (FNAL account required)
Originally created by @gaponenko on 2021-05-21 20:47:03

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Hello,

An event mixing module that uses SubRun information

https://github.com/Mu2e/Offline/blob/b9b85d90027482921b6ba60846b57c68b78b05d7/EventMixing/src/ResamplingMixer_module.cc

works fine if configured with a single secondary input file, even if
the file has to be wrapped. However if the fileNames sequence
contains more than one file name, there is an exception thrown at the
transition to the second file, like

%MSG-s ArtException:  PostEndJob 21-May-2021 15:20:24 CDT ModuleEndJob
---- EventProcessorFailure BEGIN
  EventProcessor: an exception occurred during current event processing
  ---- ScheduleExecutionFailure BEGIN
    Path: ProcessingStopped.
    ---- NotFound BEGIN
      NO_SUBRUN - Unable to find an entry in the SubRun tree corresponding to event ID run: 1 subRun: 1 event: 1 in secondary mixing input file sim.owner.g4test_stage0.ver.2.art.
      The above exception was thrown while processing module ResamplingMixer/rsmix run: 1 subRun: 0 event: 11
    ---- NotFound END
    Exception going through path p1
  ---- ScheduleExecutionFailure END
---- EventProcessorFailure END
%MSG

This looks like a bug to me.

To reproduce:

   mkdir newdir
   cd newdir
   setup mu2e
   setup muse
   muse link v00_00_00
   muse setup -q debug
   mkdir run
   cd run
   cp -p /mu2e/data/users/gandr/20210520-rsbug/run/*.fcl .
   cp -p /mu2e/data/users/gandr/20210520-rsbug/run/sim*.art .
   mu2e -c s1.fcl

There are 4 lines at the end of s1.fcl that can be (un)commented.

One can use files s0-*.fcl to re-create the two individual input files.