asyncio.set_running_loop() cache running loop holder

[tontinton]

BPO	41247
Nosy	@1st1, @miss-islington, @tontinton
PRs	bpo-41247: asyncio.set_running_loop() cache running loop holder #21401 [3.9] bpo-41247: asyncio.set_running_loop() cache running loop holder (GH-21401) #21402 [3.8] bpo-41247: asyncio.set_running_loop() cache running loop holder (GH-21401) #21403 [3.8] bpo-41247: asyncio.set_running_loop() cache running loop holder #21406

^{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 2020-07-08.19:30:41.924>
created_at = <Date 2020-07-08.19:00:14.846>
labels = []
title = 'asyncio.set_running_loop() cache running loop holder'
updated_at = <Date 2020-07-09.17:39:05.169>
user = 'https://github.com/tontinton'

bugs.python.org fields:

activity = <Date 2020-07-09.17:39:05.169>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-08.19:30:41.924>
closer = 'yselivanov'
components = []
creation = <Date 2020-07-08.19:00:14.846>
creator = 'tontinton'
dependencies = []
files = []
hgrepos = []
issue_num = 41247
keywords = ['patch']
message_count = 5.0
messages = ['373333', '373336', '373337', '373340', '373409']
nosy_count = 3.0
nosy_names = ['yselivanov', 'miss-islington', 'tontinton']
pr_nums = ['21401', '21402', '21403', '21406']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue41247'
versions = []

[tontinton]

There is a cache variable for the running loop holder, but once set_running_loop is called the variable was set to NULL so the next time get_running_loop would have to query a dictionary to receive the running loop holder.

I thought why not always cache the latest set_running_loop?

The only issue I thought of here is in the details of the implementation: I have too little experience in python to know if there could be a context switch to get_running_loop while set_running_loop is running.

If a context switch is possible there then this issue would be way harder to solve, but it is still solvable.

[miss-islington]

New changeset 529f426 by Tony Solomonik in branch 'master':
bpo-41247: asyncio.set_running_loop() cache running loop holder (GH-21401)
529f426

[1st1]

n python to know if there could be a context switch to get_running_loop while set_running_loop is running.

No, it's protected by the GIL.

Good catch, and merged.

[miss-islington]

New changeset fbd71f6 by Miss Islington (bot) in branch '3.9':
bpo-41247: asyncio.set_running_loop() cache running loop holder (GH-21401)
fbd71f6

[1st1]

New changeset 0b6169e by Tony Solomonik in branch '3.8':
bpo-41247: asyncio.set_running_loop() cache running loop holder (bpo-21406)
0b6169e