Subset collections of datatypes with VectorMembers do not work about podio

[tmadlener]

Another issue with subset collections of datatypes with a VectorMember is that they cannot be written if they have been read from file, i.e.

auto reader = podio::ROOTFrameReader();
reader.openFile("inputfile.root");

auto writer = podio::ROOTFrameWriter("outputfile.root");

auto event = podio::Frame(reader.readEntry("events", 0));
// assume there is a subset collection with a subset collection
// of a datatype with a VectorMember, e.g. ExampleWithVectorMember

writer.writeFrame(event, "events");

The final call here will crash with something like:

#6  0x00007ff214836552 in ExampleWithVectorMemberCollectionData::getCollectionBuffers (this=0x5566b4c044a8, isSubsetColl=true) at /home/tmadlener/work/AIDASoft/podio/tests/src/ExampleWithVectorMemberCollectionData.cc:52
#7  0x00007ff214833cea in ExampleWithVectorMemberCollection::getBuffers (this=<optimized out>) at /home/tmadlener/work/AIDASoft/podio/tests/src/ExampleWithVectorMemberCollection.cc:155
#8  0x00007ff2148f1779 in podio::ROOTFrameWriter::initBranches (this=<optimized out>, catInfo=..., collections=..., parameters=...) at /home/tmadlener/work/AIDASoft/podio/src/ROOTFrameWriter.cc:77
#9  0x00007ff2148f3450 in podio::ROOTFrameWriter::writeFrame (this=this, entry=0x7ffd08b1aab0, frame=..., category="other_events", collsToWrite=std::vector of length 24, capacity 24 = {...}) at /home/tmadlener/work/AIDASoft/podio/src/ROOTFrameWriter.cc:52
#10 0x00007ff2148f3539 in podio::ROOTFrameWriter::writeFrame (this=this, entry=0x7ffd08b1aab0, frame=..., category="other_events") at /home/tmadlener/work/AIDASoft/podio/src/ROOTFrameWriter.cc:25

This is because, we try to access elements of a vector that do not exist in this case, here:

podio/python/templates/CollectionData.cc.jinja2

Lines 84 to 92 in 65b58ea

	podio::CollectionWriteBuffers {{ class_type }}::getCollectionBuffers(bool isSubsetColl) {
	{% if VectorMembers %}
	// Make sure these point to the right place, even if a collection has been
	// moved since it has been created
	{% for member in VectorMembers %}
	m_vecmem_info[{{ loop.index0 }}].second = &m_vec_{{ member.name }};
	{% endfor %}
	{% endif -%}

The access to the vector members need to be guarded by an if(!isSubsetColl)