SoA layout with std::array type column crashes PoolOutputModule

[thomreis]

When a PortableCollection of an SoA with a layout that contains a column of std::array type is written to an EDM ROOT file the PoolOutputModule crashes (Observed in CMSSW_13_0_0_pre4).
This problem might be related to this ROOT issue root-project/root#12007 . ROOT seems to not support branchs of type std::array<T, N>[nElem].

Below is an example of a layout that is problematic. Note that the using is needed to not confuse the macro that interprets the layout with commas.

using Array = std::array<uint32_t, 10>;

GENERATE_SOA_LAYOUT(ExampleSoALayout,
  SOA_COLUMN(Array, data)
)
using ExampleSoA = ExampleSoALayout<>;

A workaround that avoids the crash is to wrap the std::array in a struct and use that as the column type. In addition the struct needs to be declared in the classes_def.xml file.

Below is an example of a working layout with the array inside the struct and some functions to make this more transparent when accessing the array data.

template <class T, std::size_t N>
struct StdArrayStruct {
  std::array<T, N> array;

  T* data() { return array.data(); };
  const T* data() const { return array.data(); };
  T& operator[](size_t n) { return array[n]; };
  const T& operator[](size_t n) const { return array[n]; };
};

using DataArrayStruct = StdArrayStruct<uint32_t, 10>;

GENERATE_SOA_LAYOUT(ExampleSoALayout,
  SOA_COLUMN(DataArrayStruct, data)
)
using ExampleSoA = ExampleSoALayout<>;

[cmsbuild]

A new Issue was created by @thomreis Thomas Reis.

@Dr15Jones, @perrotta, @dpiparo, @rappoccio, @makortel, @smuzaffar can you please review it and eventually sign/assign? Thanks.

cms-bot commands are listed here

[thomreis]

assign heterogeneous

[cmsbuild]

New categories assigned: heterogeneous

@fwyzard,@makortel you have been requested to review this Pull request/Issue and eventually sign? Thanks

[makortel]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@smuzaffar,@makortel you have been requested to review this Pull request/Issue and eventually sign? Thanks

[makortel]

@pcanal Could you take a look?

[makortel]

#43076 provides a workaround