Document that lru_cache uses hard references

[wouterdb]

BPO	44310
Nosy	@rhettinger, @serhiy-storchaka, @cryvate, @pablogsal, @miss-islington, @nanjekyejoannah, @@wouterdb, @Fidget-Spinner
PRs	bpo-44310: added docs to functools to warn for memory leaks #26528 bpo-44310: Note that lru_cache keep references to both arguments and results #26715 [3.10] bpo-44310: Note that lru_cache keep references to both arguments and results (GH-26715) #26716 bpo-44310: Add a FAQ entry for caching method calls #26731 [3.10] bpo-44310: Add a FAQ entry for caching method calls (GH-26731) #26777 [3.9] bpo-44310: Add a FAQ entry for caching method calls (GH-26731) #26778 bpo-44310: Remove dubious suggestion in cache example #26789
Superseder	bpo-19859: functools.lru_cache keeps objects alive forever

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2021-06-14.05:54:45.410>
created_at = <Date 2021-06-04.11:45:58.109>
labels = ['3.11', 'library', '3.9', '3.10', 'performance']
title = 'Document that lru_cache uses hard references'
updated_at = <Date 2021-06-20.15:11:01.892>
user = 'https://github.com/wouterdb'

bugs.python.org fields:

activity = <Date 2021-06-20.15:11:01.892>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2021-06-14.05:54:45.410>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2021-06-04.11:45:58.109>
creator = 'Wouter De Borger2'
dependencies = []
files = []
hgrepos = []
issue_num = 44310
keywords = ['patch']
message_count = 21.0
messages = ['395075', '395087', '395088', '395125', '395144', '395149', '395152', '395157', '395158', '395336', '395774', '395796', '395887', '395912', '396016', '396017', '396018', '396019', '396020', '396021', '396178']
nosy_count = 9.0
nosy_names = ['rhettinger', 'python-dev', 'serhiy.storchaka', 'cryvate', 'pablogsal', 'miss-islington', 'nanjekyejoannah', 'Wouter De Borger2', 'kj']
pr_nums = ['26528', '26715', '26716', '26731', '26777', '26778', '26789']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = '19859'
type = 'resource usage'
url = 'https://bugs.python.org/issue44310'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[wouterdb]

# Problem

the functools.lru_cache decorator locks all arguments to the function in memory (inclusing self), causing hard to find memory leaks.

# Expected

I had assumed that the lru_cache would keep weak-references and that when an object is garbage colected, all its cache entries expire as unreachable. This is not the case.

# Solutions

1. I think it is worth at least mentioning this behavior in de documentation.
2. I also think it would be good if the LRU cache actually uses weak references.

I will try to make a PR for this.