Dict memory leak in Python 3.12.x / msgpack 1.0.x

[amachanic]

Hello,

I think I've found a second Python 3.12 memory leak (having read the issue for the other one, and tried upgrading to msgpack 1.0.8 to fix).

This issue occurs with dicts, and key uniqueness seems to be a major consideration. (A bunch of dicts with the exact same keys don't cause the problem -- some kind of key caching issue maybe?)

Here's a repro:

import psutil
import msgpack

# Set up a list of packed dicts
p = [
    msgpack.packb(
        {f'{p}.{r}': True for r in range(1000)}
    )
    for p in range(1000)
]

# Check initial state
print(f'initial mem: {psutil.Process().memory_info().rss}')

# Loop and unpack
i = 0
for x in p:
    y = msgpack.unpackb(x)
    i += 1
    if i % 100 == 0:
        print(f'subsequent mem: {psutil.Process().memory_info().rss}')

Output, msgpack 1.0.8 on Python 3.12.3:

initial mem: 24571904
subsequent mem: 34029568
subsequent mem: 42627072
subsequent mem: 47493120
subsequent mem: 60088320
subsequent mem: 64684032
subsequent mem: 69550080
subsequent mem: 89878528
subsequent mem: 94744576
subsequent mem: 99340288
subsequent mem: 104206336

Output, msgpack 0.6.2 on Python 3.12.3 or msgpack 1.0.8 on Python 3.11.9 (slightly different numbers but same effect):

initial mem: 22761472
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152
subsequent mem: 23089152

p.s. apologies for using psutil in the script, I didn't remember until later that it isn't a Python built-in.