C Unpickler memory leak when unpickling big geojsons

Bug report

Bug description:

I noticed that the pickle.load method leaves more memory allocated than just the size of the returned object when unpickling geojson dictionaries.

I initially noticed the leak in my docker container running on x86. Here is a code snippet that reproduced the problem for me on M1 MacOS Sonoma (python 3.11.9).

import io
import pickle

import psutil

process = psutil.Process()

to_pickle = {
    "type": "FeatureCollection",
    "crs": {"properties": {"name": "EPSG:4326"}, "type": "name"},
    "features": [
        {
            "type": "Feature",
            "id": f"id{idx}",
            "properties": {
                "str_prop": f"str_var{idx}",
                "int_prop": idx,
                "bool_prop": bool(idx % 2),
                "none_prop": None,
                "float_prop": float(idx),
            },
        }
        for idx in range(9000)
    ],
}
b = pickle.dumps(to_pickle)
del to_pickle

print(
    f"Initial memory usage: {round(int(process.memory_info().rss / 1024) / 1024, 3)}MiB"
)
pickle.Unpickler(io.BytesIO(b)).load()
print(
    f"After 1st unpickle memory usage: {round(int(process.memory_info().rss / 1024) / 1024, 3)}MiB"
)
for i in range(1000):
    pickle.Unpickler(io.BytesIO(b)).load()

print(
    f"Final memory usage: {round(int(process.memory_info().rss / 1024) / 1024, 3)}MiB"
)

CPython versions tested on:

3.11

Operating systems tested on:

Linux, macOS

まず、提供された Python 再現プログラムを影響を受ける CPython バージョンで実行し、pickle.Unpickler(...).load() の呼び出しを繰り返した後のメモリを比較します。これらのネストされた辞書に使用される C Unpickler の経路を追跡します。繰り返し unpickling を行っても報告された過剰なメモリが割り当てられたままにならず、このケースの回帰テストが追加されれば完了です。