Segmentation fault when only some of the events have subset collections

[tmadlener]

This was originally reported by @dudarboh and it took us some debugging to understand what is going on. Technically it also not a bug in k4MarlinWrapper, but this is where it manifests as a bug. The actual issue is in LCIO, resp. a missing feature in LCIO (see iLCSoft/LCIO#199).

The issue

The PatchCollections processor can be used to patch collections into the LCEvent on the fly. It effectively creates an empty collection (of a given type) if that collection is not already present in the event. This is useful because EDM4hep output (more specifically the ROOT backend of podio) expects all events to have the same "collection content". An issue arises if the collection is a subset collection if it's present in the event. In that case events that have the collection (without patching) will try to store a subset collection, while events that have the collection via patching will try to store it as a normal collection. This leads to a segmentation fault because the branch structure that podio creates for the two cases is different and we try to write data into an unsuitable branch.