Fix crash when a writer is in the global namespace

[jmcarcell]

BEGINRELEASENOTES

• Fix crash when a writer is in the global namespace by adding a class that will manage all the writers and finish them before exiting

ENDRELEASENOTES

The following code crashes (either Writer or RNTupleWriter):

import podio
writer = podio.root_io.RNTupleWriter('rntuple.root')

The workaround is to add a del writer at the end. This happens because the writers do something when they are destructed but podio and/or ROOT may get destructed before. Error message:

Error in <TKey::Create>: Cannot allocate 658 bytes for ID =  Title =
SysError in <TFile::Seek>: cannot seek to position 0 in file rntuple.root, retpos=-1 Bad file descriptor
Fatal: !rv violated at line 1152 of `/root/tree/ntuple/v7/src/RMiniFile.cxx'
aborting

[tmadlener]

This does also affect all the readers, right? Could we the _all_writers.add into the BaseReaderMixin constructor?

[jmcarcell]

Hm nope, at least they don't crash. I've moved the management to base_writer.py, in lines of code it's the same since every writer has now to do super().__init__()

[tmadlener]

Not really sure about the AllWriters name, but I can't think of anything better. Since this is internal in any case we should be able to change this later if we come up with something better.

[jmcarcell]

WriterManager, WriterDeleter or WriterFinisher or even WriterDestroyer for maximum effect