Use a linked list for the ceval pending calls.

[ericsnowcurrently]

BPO	35466
Nosy	@ericsnowcurrently, @serhiy-storchaka, @emilyemorehouse
PRs	bpo-35466: Use a linked list for the list of ceval pending calls. #11122

^{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/ericsnowcurrently'
closed_at = <Date 2019-03-02.00:03:46.438>
created_at = <Date 2018-12-11.22:10:27.063>
labels = ['interpreter-core', 'type-feature', '3.8']
title = 'Use a linked list for the ceval pending calls.'
updated_at = <Date 2019-03-02.00:03:46.437>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2019-03-02.00:03:46.437>
actor = 'eric.snow'
assignee = 'eric.snow'
closed = True
closed_date = <Date 2019-03-02.00:03:46.438>
closer = 'eric.snow'
components = ['Interpreter Core']
creation = <Date 2018-12-11.22:10:27.063>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 35466
keywords = ['patch']
message_count = 4.0
messages = ['331655', '331662', '331663', '336974']
nosy_count = 3.0
nosy_names = ['eric.snow', 'serhiy.storchaka', 'emilyemorehouse']
pr_nums = ['11122']
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35466'
versions = ['Python 3.8']

[ericsnowcurrently]

Currently the list of pending calls (see Include/internal/pycore_ceval.h) is implemented as a circular buffer. A linked list would be easier to understand and modify. It also allows for removing the restriction on the number of pending calls.

[serhiy-storchaka]

Was not a circular array used intentionally because this allows to avoid calling malloc()?

And the proposed code looks more complicated to me than the current code.

[ericsnowcurrently]

I suppose performance could have been a motivator originally. However, I don't see a benefit now that signals handlers are no longer pending calls. If it's a real problem then we can leverage a free list.

As to complexity, the motivator for this change was difficulty I had (along with another core dev) in quickly understanding the logic for manipulating the circular buffer. I find the linked list much easier to understand.

[ericsnowcurrently]

At this point I'm not terribly interested in this. :)