CollectionBase::getBuffers should be marked const

[tmadlener]

Once the Frame is merged and used CollectionBase::getBuffers should be marked const as it will be required for writing from multiple threads. In principle it should be rather straight forward to do.

This will only become possible once #287 is merged, because currently getBuffers is also used for reading and not only writing.

[hegner]

For this the handling of the UserDataCollections would need to change. Any plans for that?

[tmadlener]

There are effectively two options as I see it at the moment:

• Mark _vecPtr as mutable, such that we can still assign in a const function. This would potentially require to also introduce a mutex into the UserDataCollection to protect against race conditions.
• Give up on default move-ability, and properly implement non-default constructors and move-constructors that set _vecPtr during initialization, such that a const getBuffers becomes trivial.

I am leaning towards the second here (although I think I have a branch somewhere for the first), simply because I don't want the mutex in there and, I think we still need a const_cast with that.

[tmadlener]

After having a look into this, I realized this will require quite a bit of work and we decided to address this only after a v1.0 release, since it is not really user visible in any case (or at least it shouldn't be). Currently we explicitly const_cast the collections in the writers, e.g.:

podio/src/ROOTFrameWriter.cc

Lines 43 to 44 in ac26a12

	auto* coll = frame.getCollectionForWrite(name);
	collections.emplace_back(name, const_cast<podio::CollectionBase*>(coll));

Addressing this properly also requires changes in the CollectionWriteBuffers since they currently store non-const pointers, but if we mark getBuffers as const we would need a constructor from const pointers. The CollectionWriteBuffers should definitely only have const pointers, but to get things to that requires a bit of time.