[Patch] Complete bits necessary for making fcntl's file locking optional

[EdSchouten]

BPO	27764
Nosy	@EdSchouten
Files	patch-no-flock.txt: Patch for making flock() and lockf() optional

^{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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assignee = None
closed_at = None
created_at = <Date 2016-08-14.20:41:01.914>
labels = ['extension-modules']
title = "[Patch] Complete bits necessary for making fcntl's file locking optional"
updated_at = <Date 2016-08-14.20:41:01.914>
user = 'https://github.com/EdSchouten'

bugs.python.org fields:

activity = <Date 2016-08-14.20:41:01.914>
actor = 'EdSchouten'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2016-08-14.20:41:01.914>
creator = 'EdSchouten'
dependencies = []
files = ['44105']
hgrepos = []
issue_num = 27764
keywords = []
message_count = 1.0
messages = ['272692']
nosy_count = 1.0
nosy_names = ['EdSchouten']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
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type = None
url = 'https://bugs.python.org/issue27764'
versions = ['Python 3.6']

[EdSchouten]

Python's fcntl module already provides some support for making support for file locking optional. For example, constants like F_SETFL are only defined if present. Unfortunately, the accompanying functions 'flock()' and 'lockf()' are present unconditionally.

CloudABI, a sandboxed runtime environment for UNIX (https://mail.python.org/pipermail/python-dev/2016-July/145708.html), doesn't implement these ways of doing file locking, for the reason that these operations do not fit nicely within its process model. These locks are per-process; not per-descriptor. This means that processes cannot safely be composed or decomposed.

Attached is a relatively small patch to make flock() and lockf() optional, only compiling them when the necessary APIs are present. The LOCK_* constants are now also defined optionally.

[benjaminp]

CloudABI is no more.