LRU Cache with maxsize=None

[rhettinger]

BPO	10593
Nosy	@rhettinger, @ncoghlan
Files	cache2.diff: Support maxsize=None

^{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/ncoghlan'
closed_at = <Date 2010-12-01.03:46:15.204>
created_at = <Date 2010-12-01.01:01:44.122>
labels = ['type-feature', 'library']
title = 'LRU Cache with maxsize=None'
updated_at = <Date 2010-12-01.03:46:15.203>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2010-12-01.03:46:15.203>
actor = 'rhettinger'
assignee = 'ncoghlan'
closed = True
closed_date = <Date 2010-12-01.03:46:15.204>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2010-12-01.01:01:44.122>
creator = 'rhettinger'
dependencies = []
files = ['19887']
hgrepos = []
issue_num = 10593
keywords = ['patch']
message_count = 3.0
messages = ['122967', '122969', '122977']
nosy_count = 2.0
nosy_names = ['rhettinger', 'ncoghlan']
pr_nums = []
priority = 'low'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue10593'
versions = ['Python 3.2']

[rhettinger]

Nick, I may have found a straight-forward way to incorporate your idea for the cache to support maxsize=None. Let me know what you think.

[ncoghlan]

Nice!

You may still need to use the lock even for the simple unbounded case though - incrementing hits and misses isn't atomic, so the statistics may be miscounted if you get a hit or miss in different threads at the same time.

Alternatively, we could just document the hit/miss statistics as approximate figures for tuning purposes rather than guaranteeing 100% accuracy in all situations.

[rhettinger]

Applied in r86911.