set better defaults for TCPServer options

[giampaolo]

BPO	36003
Nosy	@taleinat, @giampaolo, @asvetlov, @1st1
PRs	bpo-36003: change socketserver.TCPServer reuse_addr and backlog default options #11875

^{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 = None
closed_at = None
created_at = <Date 2019-02-15.13:44:39.433>
labels = ['easy', '3.8', 'type-feature', 'library']
title = 'set better defaults for TCPServer options'
updated_at = <Date 2019-06-08.18:18:49.463>
user = 'https://github.com/giampaolo'

bugs.python.org fields:

activity = <Date 2019-06-08.18:18:49.463>
actor = 'taleinat'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-02-15.13:44:39.433>
creator = 'giampaolo.rodola'
dependencies = []
files = []
hgrepos = []
issue_num = 36003
keywords = ['patch', 'easy']
message_count = 3.0
messages = ['335612', '335614', '345058']
nosy_count = 5.0
nosy_names = ['taleinat', 'giampaolo.rodola', 'asvetlov', 'neologix', 'yselivanov']
pr_nums = ['11875']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36003'
versions = ['Python 3.8']

Linked PRs

• gh-80184: Set getattr(socket, "SOMAXCONN", 5) as the default queue size for TCPServer #134249

[giampaolo]

socketserver.TCPServer provides the following defaults:

allow_reuse_address = False
request_queue_size = 5

Proposal is to:

• have "allow_reuse_address = True" on POSIX in order to immediately reuse previous sockets which were bound on the same address and remained in TIME_WAIT state
• have "request_queue_size = None" so that it's up to socket.listen() to choose a default reasonable value (usually 128)

[giampaolo]

Update: because "request_queue_size" is passed to server_activate() method which can be subclassed, a better default for not breaking backward compatibility is 0 (not None).

[taleinat]

I'm not sure that changing the default value for "reuse address" justifies breaking backwards compatibility like this. Admittedly, I'm not an expert on networking, so perhaps there's a good reason that I'm unaware of.

As for the queue size, passing 0 to socket.listen() does seem like a more reasonable default than the rather arbitrary value of 5. Again, it's arguable whether it's worth changing this given that it has been like this for a long time, but in this case at least the potential for harm seems negligible.

[gpshead]

agreed, lets use 0 as the queue size but leave reuse address alone.

[gpshead]

(we're working out if we even want to that on the PR, see comments there)