Support multithreading in Geant4EDM4hepOutput plugin

[wdconinc]

This PR adds multithreading support to Geant4EDM4hepOutput. This is approached by:

• ensuring the m_file unique_ptr to podio::ROOTWriter is only made once,
• keeping track of the use count of the file in an atomic_size_t,
• ensuring the G4AutoLock is acquired in scope before each m_file access,
• closing the file when the last file user gives up access.

Additional notes:

• context()->run() fails because runPtr() is null in multithreaded running; this leads to missing runs and meta frames. EDIT: this is not consistent... not sure why not...
• After moving m_particles into the frame, the collection is left with a null m_storageMtx mutex pointer; this is avoided by not using clear() but moving assigning a clean new collection in place (Line 322). I suspect the clear() was added in analogy to the clear() calls below, but those are on maps, not podio collections.

Testing:

• There is currently no trivial way to test this out (this is a prereq for Add multithreading support to ddsim #1240), so this was tested with DG4/examples/SiDSim_MT.py with the following diff:

index 2a340914..736f190e 100644
--- a/DDG4/examples/SiDSim_MT.py
+++ b/DDG4/examples/SiDSim_MT.py
@@ -53,8 +53,7 @@ def setupWorker(geant4):
   logger.info("\n#PYTHON:  Configure I/O\n")
   # evt_lcio = geant4.setupLCIOOutput('LcioOutput','CLICSiD_'+time.strftime('%Y-%m-%d_%H-%M'))
   # evt_lcio.OutputLevel = Output.ERROR
-
-  geant4.setupROOTOutput('RootOutput', 'CLICSiD_' + time.strftime('%Y-%m-%d_%H-%M'))
+  geant4.setupEDM4hepOutput('EDM4hepOutput', 'CLICSiD_' + time.strftime('%Y-%m-%d_%H-%M') + ".edm4hep.root")
 
   gen = DDG4.GeneratorAction(kernel, "Geant4GeneratorActionInit/GenerationInit")
   kernel.generatorAction().adopt(gen)

TODO:

• I'd like to add this to one of the tests, but neither SiDSim.py nor SiDSim_MT.py is already used for tests. Suggestions welcome.

BEGINRELEASENOTES

• Support multithreading in Geant4EDM4hepOutput plugin

ENDRELEASENOTES

[github-actions]

Test Results

   16 files     16 suites   6h 13m 12s ⏱️
  370 tests   370 ✅ 0 💤 0 ❌
2 887 runs  2 887 ✅ 0 💤 0 ❌

Results for commit d6d8030.

♻️ This comment has been updated with latest results.

[MarkusFrankATcernch]

@tmadlener Thomas, could you please verify?
Thanks a lot!

[MarkusFrankATcernch]

@tmadlener Ooops sorry, I did not realize that the change is actually trivial before asking you. Sorry.