PEP-3118: remove obsolete write-locks

[skrah]

BPO	14203
Nosy	@ncoghlan, @pitrou, @skrah, @serhiy-storchaka
Files	bytearray_getbuffer.diff issue14203-2.diff issue14203-3.diff

^{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 2015-02-04.13:51:46.544>
created_at = <Date 2012-03-05.18:32:48.582>
labels = ['interpreter-core', 'performance']
title = 'PEP-3118: remove obsolete write-locks'
updated_at = <Date 2015-02-04.13:51:46.543>
user = 'https://github.com/skrah'

bugs.python.org fields:

activity = <Date 2015-02-04.13:51:46.543>
actor = 'skrah'
assignee = 'none'
closed = True
closed_date = <Date 2015-02-04.13:51:46.544>
closer = 'skrah'
components = ['Interpreter Core']
creation = <Date 2012-03-05.18:32:48.582>
creator = 'skrah'
dependencies = []
files = ['24738', '38003', '38004']
hgrepos = []
issue_num = 14203
keywords = ['patch', 'needs review']
message_count = 15.0
messages = ['154969', '154971', '154972', '219995', '235346', '235348', '235352', '235354', '235355', '235358', '235359', '235360', '235361', '235363', '235386']
nosy_count = 6.0
nosy_names = ['ncoghlan', 'pitrou', 'skrah', 'python-dev', 'serhiy.storchaka', 'kelleynnn']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue14203'
versions = ['Python 3.5']

[skrah]

bytearray_getbuffer() checks for view==NULL. But this has never been
allowed:

PEP: "The second argument is the address to a bufferinfo structure.
Both arguments must never be NULL."

DOCS (3.2): "view must point to an existing Py_buffer structure
allocated by the caller".

A quick grep through the source tree shows no instances where
the middle argument of either PyObject_GetBuffer of bf_getbuffer
is NULL or 0.

Patch attached, all tests pass. I wouldn't be comfortable to
commit it without review though (it's just too strange).

BTW, the next conditional in bytearray_getbuffer ...

    if (ret >= 0) {
        obj->ob_exports++;
    }

is also superfluous, since PyBuffer_FillInfo() cannot fail
if readonly==0.

[skrah]

array_buffer_getbuf does a similar thing:

if (view==NULL) goto finish;

finish:
self->ob_exports++; // ???
return 0

Where does this view==NULL thing come from?

[skrah]

I seems to be a feature to get a "lock" on an exporter
without the exporter filling in a buffer. It was there
from the beginning:

http://svn.python.org/view/python/branches/release30-maint/Objects/bytearrayobject.c?r1=56849&r2=57181

Last use that I can see is here:

http://hg.python.org/cpython/file/df3b2b5db900/Modules/posixmodule.c#l561

[kelleynnn]

I have verified that this feature is unused in the source tree; in fact, there are no internal calls to bytearray_getbuffer() at all. The only thing bytearray_getbuffer() does with its second arg is pass it to PyBuffer_FillInfo(), which immediately checks it and passes 0 up the call stack if it is NULL. (A comment in PyBuffer_FillInfo() asks why -1 is not passed up instead; it's probably to distinguish this feature from the error condition handled in the immediately following conditional block.)

There are potentially other issues stemming from this legacy feature in bytearray_getbuffer(), PyBuffer_FillInfo(), and elsewhere. The maintainers may see fit to open tickets on these issues as well.

There's more relevant commentary on the feature here: http://comments.gmane.org/gmane.comp.python.devel/130521

[skrah]

New patch with tests.

[python-dev]

New changeset e8fe32d43c96 by Stefan Krah in branch 'default':
Issue bpo-14203: Remove obsolete support for view==NULL in PyBuffer_FillInfo()
https://hg.python.org/cpython/rev/e8fe32d43c96

[skrah]

bytesiobuf_getbuffer() also still has this obsolete feature, so
BufferError should be raised if view==NULL.

I'm unsure if this plays well with the new SHARED_BUF(b) thing.

Should the exception be raised before or after?

[serhiy-storchaka]

I think before. It doesn't harm, but it doesn't make much sense to unshare the buffer if its address is not used.

[serhiy-storchaka]

E.g. the buffer should be unshared before incrementing b->exports, but if an exception is raised instead, there is no need to unshare the buffer before raising.

[skrah]

Ok, thanks! bpo-14203-3.diff should be correct, then.

[python-dev]

New changeset 0d5095a2422f by Stefan Krah in branch 'default':
Issue bpo-14203: Remove obsolete support for view==NULL in bytesiobuf_getbuffer()
https://hg.python.org/cpython/rev/0d5095a2422f

[serhiy-storchaka]

Compiling failed on Windows: http://buildbot.python.org/all/builders/AMD64%20Windows7%20SP1%203.x/builds/5650