[Hist] Backwards compatibility broken for THnSparseL in 6.32

[guitargeek]

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Description

Backwards compatibility for THnSparseL was broken in 6.32 by #8546, @ferdymercury @vepadulano.

Reproducer

Write a THnSparseL to a file e.g. with ROOT 6.30.06:

void write_thnsparse_l()
{
   std::unique_ptr<TFile> file{TFile::Open("hist.root", "RECREATE")};
   Int_t bins[2] = {10, 20};
   Double_t xmin[2] = {0., -5.};
   Double_t xmax[2] = {10., 5.};
   THnSparseL hist{"hist", "hist", 2, bins, xmin, xmax};
   file->WriteObject(&hist, "hist");
}

Try to read it with ROOT 6.32.00 (or master):

root hist.root -e 'hist'

You'll get this error:

Error in <TClingCallFunc::make_ctor_wrapper>: Failed to compile
  ==== SOURCE BEGIN ====
__attribute__((used)) extern "C" void __ctor_0(void** ret, void* arena, unsigned long nary)
{
   if (!arena) {
      if (!nary) {
         *ret = new THnSparseT<TArrayL>;
      }
      else {
         *ret = new THnSparseT<TArrayL>[nary];
      }
   }
   else {
      if (!nary) {
         *ret = new (arena) THnSparseT<TArrayL>;
      }
      else {
         *ret = new (arena) THnSparseT<TArrayL>[nary];
      }
   }
}

  ==== SOURCE END ====
Error in <TClingCallFunc::ExecDefaultConstructor>: Called with no wrapper, not implemented!
Error in <TClingClassInfo::New()>: Call of default constructor failed to return an object for class: THnSparseT<TArrayL>
Error in <TClass::New>: cannot create object of class THnSparseT<TArrayL>
Error in <TKey::ReadObj>: Cannot create new object of class THnSparseT<TArrayL>
IncrementalExecutor::executeFunction: symbol '_ZN10THnSparseTI7TArrayLE5ClassEv' unresolved while linking [cling interface function]!
You are probably missing the definition of THnSparseT<TArrayL>::Class()
Maybe you need to load the corresponding shared library?
IncrementalExecutor::executeFunction: symbol '_ZN10THnSparseTI7TArrayLE8StreamerER7TBuffer' unresolved while linking [cling interface function]!
You are probably missing the definition of THnSparseT<TArrayL>::Streamer(TBuffer&)
Maybe you need to load the corresponding shared library?

ROOT version

ROOT 6.32.00 upwards.

Additional context

Thanks do @philippe554 for catching this! This issue is important for project HighLO because we're using THnSparseL as a container for reduced data.

[pcanal]

Note: THnSparseT<TArrayL> was removed because its underlying type (long)'s size depends on the platforms (32 vs 64 bits).
However, it would have been much to either leave it as is or mark it as deprecated.

We need to fix this and make sure older files can be read.

In the long term, you should consider moving to the newer THnSparseT<TArrayL64> which guarantees the 64 bits on all platforms (we should also make sure that we update our code to that THnSparseT<TArrayL> can be read into a THnSparseT<TArrayL64> (by adding the relevant renaming rule if need be).

[ferdymercury]

Yep, I know. I warned about this problem up to 3 times in that PR, insisting that it had nothing to do with the new feature that I wanted to add:

#8546 (comment)
#8546 (comment)
#8546 (comment)

After 3y, I was forced to give in :s

[pcanal]

I also missed it at the time. Sorry.

To avoid this, we should also add test of the backward compatibility (especially the reading of older file).

[vepadulano]

It's enough to add back the dictionary for THnSparse<TArrayL> to read back the histogram. Now I'm also working on a trasparent conversion from THnSparseL written as THnSparse<TArrayL> to THnSparseL read as THnSparse<TArrayL64>, let's see if I can make that work.