Make json.tool soak up input before opening output for writing

[CharString]

BPO	45644
Nosy	@methane, @ambv, @miss-islington, @remilapeyre, @CharString
PRs	bpo-45644 Make json.tool soak up infile before writing to outfile #29269 bpo-45644: Make json.tool read infile before writing to outfile #29273 [3.10] bpo-45644: Make json.tool read infile before writing to outfile (GH-29273) #29445 [3.9] bpo-45644: Make json.tool read infile before writing to outfile (GH-29273) #29446 bpo-33927: Handle pyton -m json.tool --json-lines infile infile #29478 bpo-45644: json.tool: Use staged write for outfile. #30659

^{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 = <Date 2021-11-06.18:02:21.133>
created_at = <Date 2021-10-28.12:59:07.008>
labels = ['type-bug', '3.9', '3.10', '3.11']
title = 'Make json.tool soak up input before opening output for writing'
updated_at = <Date 2022-01-18.09:34:45.992>
user = 'https://github.com/CharString'

bugs.python.org fields:

activity = <Date 2022-01-18.09:34:45.992>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2021-11-06.18:02:21.133>
closer = 'lukasz.langa'
components = []
creation = <Date 2021-10-28.12:59:07.008>
creator = 'CharString'
dependencies = []
files = []
hgrepos = []
issue_num = 45644
keywords = ['patch']
message_count = 8.0
messages = ['405182', '405866', '405868', '405872', '405873', '405874', '405913', '405937']
nosy_count = 5.0
nosy_names = ['methane', 'lukasz.langa', 'miss-islington', 'remi.lapeyre', 'CharString']
pr_nums = ['29269', '29273', '29445', '29446', '29478', '30659']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45644'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[CharString]

json.tool is very cute and handy for making json readable.

But rewriting a file in place requires tools like sponge (on POSIX) or a tmpfile, because

$ python -m json.tool foo.json foo.json

results in an empty foo.json.

I propose soaking up the infile before opening the outfile for writing, to prevent that. Much like sort -o does, but without the explicit flag.
The patch I have prepared changes no behaviours, other than preventing an empty file... (still I see this as an enhancement and not a bug fix)

[ambv]

This is arguably a bugfix since truncating the infile cannot be construed as a useful feature in any sense. Thus I'm inclined to backport this to 3.10.1 and 3.9.9 too.

[ambv]

New changeset 815dad4 by Chris Wesseling in branch 'main':
bpo-45644: Make json.tool read infile before writing to outfile (GH-29273)
815dad4