RootReader symbols in podioDict

[vvolkl]

When using the podio master branch running anything that is linked to podio::podioDict prints the following warnings (errors?):

~$ python
Python 3.9.10 (main, Mar 29 2022, 00:07:05) 
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ROOT
Warning in <TInterpreter::ReadRootmapFile>: class  edm4hep::ObjectID found in libedm4hepDict.so  is already in libedm4drDict.so 
>>> ROOT.gSystem.Load('libpodioDict')
cling::DynamicLibraryManager::loadLibrary(): /afs/cern.ch/user/v/vavolkl/repo/podio/install/lib64/libpodioDict.so: undefined symbol: _ZTVN5podio10ROOTReaderE
-1
>>> 

Can be fixed by linking to podio::podioRootIO as well, but should probably not happen in the first place.

Must have been introduced after v00-14-01, as that runs the above without issues.

[tmadlener]

The edm4hep::ObjectID warning is due to it being defined a second time in the dual-readout EDM: https://github.com/HEP-FCC/dual-readout/blob/master/edm4dr.yaml#L9 This should be fairly safe to ignore, and eventually be handled with something that fixes #263

The missing ROOTReader symbol in the podio dictionary seems to be a bug. I will have to check which change introduced that.

[tmadlener]

The behavior was introduced in #254. Specifically what seems to trigger this is defining the ~ROOTReader destructor in the ROOTReader.h header file, either via ~ROOTReader() = default or via ~ROOTReader() {}. Only declaring it in the header file and defining an empty destructor in the ROOTReader.cc file fixes this, i.e.:

diff --git a/include/podio/ROOTReader.h b/include/podio/ROOTReader.h
index e91bde3..b9afe9f 100644
--- a/include/podio/ROOTReader.h
+++ b/include/podio/ROOTReader.h
@@ -35,7 +35,7 @@ class ROOTReader : public IReader {
 
 public:
   ROOTReader() = default;
-  ~ROOTReader() = default;
+  ~ROOTReader();
 
   // non-copyable
   ROOTReader(const ROOTReader&) = delete;
diff --git a/src/ROOTReader.cc b/src/ROOTReader.cc
index 8c1a294..436df53 100644
--- a/src/ROOTReader.cc
+++ b/src/ROOTReader.cc
@@ -16,6 +16,9 @@
 
 namespace podio {
 
+ROOTReader::~ROOTReader() {
+}
+
 std::pair<TTree*, unsigned> ROOTReader::getLocalTreeAndEntry(const std::string& treename) {
   auto localEntry = m_chain->LoadTree(m_eventNumber);
   auto* tree = static_cast<TTree*>(m_chain->GetFile()->Get(treename.c_str()));

I am not entirely sure why that makes a difference for ROOT. I will try and see if there is a way around this or if actually linking against libpodioRootIO is the right thing to do.

[vvolkl]

Created the most straightforward fix here, but I think either linking podioRootIO to the dict or adding all the root sources to it should not be a problem - ROOT is required in any case to create the dict in the first place.