Support buffer protocol with type specs

[scoder]

BPO	40724
Nosy	@pfmoore, @ncoghlan, @pitrou, @scoder, @vstinner, @encukou, @skrah, @miss-islington, @remilapeyre
PRs	bpo-40724: Support setting buffer slots from type specs #20648 [3.9] bpo-40724: Support setting buffer slots from type specs (GH-20648) #20683 bpo-40724: Fix return type of heapctypewithbuffer_releasebuffer() #20685 [3.9] bpo-40724: Fix return type of test helper function heapctypewithbuffer_releasebuffer() (GH-20685) #20690

^{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 2020-06-07.07:29:43.798>
created_at = <Date 2020-05-22.07:51:02.014>
labels = ['expert-C-API', 'type-feature', '3.9', '3.10']
title = 'Support buffer protocol with type specs'
updated_at = <Date 2020-06-07.07:29:43.797>
user = 'https://github.com/scoder'

bugs.python.org fields:

activity = <Date 2020-06-07.07:29:43.797>
actor = 'scoder'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-07.07:29:43.798>
closer = 'scoder'
components = ['C API']
creation = <Date 2020-05-22.07:51:02.014>
creator = 'scoder'
dependencies = []
files = []
hgrepos = []
issue_num = 40724
keywords = ['patch']
message_count = 5.0
messages = ['369561', '370194', '370849', '370878', '370879']
nosy_count = 9.0
nosy_names = ['paul.moore', 'ncoghlan', 'pitrou', 'scoder', 'vstinner', 'petr.viktorin', 'skrah', 'miss-islington', 'remi.lapeyre']
pr_nums = ['20648', '20683', '20685', '20690']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40724'
versions = ['Python 3.9', 'Python 3.10']

[scoder]

As far as I can see it in typeslots.h [1], the buffer protocol is still not supported when using type specs:

/* Disabled, see bpo-10181 */
#undef Py_bf_getbuffer
#undef Py_bf_releasebuffer

It seems that the discussion in bpo-10181 did not lead anywhere at the time (2010-2012).

I'm not talking about the limited API or the stable ABI here, I'm talking about using type specs to define an extension type. The work-around is to manually set "PyTypeObject->tp_as_buffer" after creating the type, but since PyType_Ready() then does not see that struct pointer, you also still have to make sure that the type correctly inherits the slots from the base type if you're not defining all of them yourself.

Can we just add support for the above slots?

[1]

cpython/Include/typeslots.h

Lines 2 to 4 in ca82910

	/* Disabled, see #10181 */
	#undef Py_bf_getbuffer
	#undef Py_bf_releasebuffer

[encukou]

Yes, it should be possible to wrap them in #if so they aren't part of the stable ABI.

[scoder]

New changeset f7c4e23 by scoder in branch 'master':
bpo-40724: Support setting buffer slots from type specs (GH-20648)
f7c4e23

[scoder]

New changeset 1e4fa91 by Miss Islington (bot) in branch '3.9':
bpo-40724: Support setting buffer slots from type specs (GH-20648) (GH-20683)
1e4fa91

[scoder]

New changeset 1d711f2 by Miss Islington (bot) in branch '3.9':
bpo-40724: Fix return type of test helper function heapctypewithbuffer_releasebuffer() (GH-20685) (GH-20690)
1d711f2