Memory leak in EnumerateLibrary

[jose-mr]

Describe the bug
Calling Chem.rdChemReactions.EnumerateLibrary in a loop increases the memory usage even if there are no future references to the EnumerationLibrary object.

To Reproduce

from rdkit import Chem
from rdkit.Chem import rdChemReactions

@profile
def run_once(rxn, mols):
    library = Chem.rdChemReactions.EnumerateLibrary(rxn, [mols, ])
  
@profile
def run_10000(rxn, mols):
    for _ in range(10000):
        library = Chem.rdChemReactions.EnumerateLibrary(rxn, [mols, ])
  
rxn = rdChemReactions.ReactionFromSmarts("[C:1]-[C:2]>>[C:1].[C:2]")
mols = [Chem.MolFromSmarts("[C]-[C]")]*10000
run_once(rxn, mols)
run_10000(rxn, mols)

Expected behavior
I would expect the memory footprint of both functions to be the same, but they are not. Here is the output of running this code with memory_profiler:

python -m memory_profiler minimal_example.py

Line # Mem usage Increment Occurences Line Contents

 5   74.211 MiB   74.211 MiB           1   @profile
 6                                         def run_once(rxn, mols):
 7   75.699 MiB    1.488 MiB           1       library = Chem.rdChemReactions.EnumerateLibrary(rxn, [mols, ])

Filename: minimal_example.py

Line # Mem usage Increment Occurences Line Contents

 9   75.699 MiB   75.699 MiB           1   @profile
10                                         def run_10000(rxn, mols):
11 6261.906 MiB    0.000 MiB       10001       for _ in range(10000):
12 6261.906 MiB 6186.207 MiB       10000           library = Chem.rdChemReactions.EnumerateLibrary(rxn, [mols, ])

The system monitor also shows an increased memory usage.

Configuration (please complete the following information):

• RDKit version: 2020.09.3
• OS: Ubuntu 18.04
• Python version (if relevant): Python 3.8.2
• Are you using conda? yes
• If you are using conda, which channel did you install the rdkit from? conda-forge

Additional context
I have noticed this in a much more complicated piece of code, but this minimal example seems to reproduce the problem.
Thank you for any help!

[bp-kelley]

@jose-mr Thanks for the bug report! I'll look into it.