Avoid leaking linker flags into distutils: add PY_LDFLAGS_NODIST

[stratakis]

BPO	35257
Nosy	@vstinner, @ned-deily, @merwok, @dstufft, @stratakis, @hroncok, @miss-islington
PRs	bpo-35257: Avoid leaking the linker flags into distutils. #10900 [WIP] bpo-35502: Fix memory leak in xml.etree iterparse() #11169 [3.7] bpo-35257: Avoid leaking LTO linker flags into distutils (GH-10900) #11264 [3.6] bpo-35257: Avoid leaking LTO linker flags into distutils (GH-10900) #11265 bpo-35257: fix broken BLDSHARED - needs LDFLAGS too #11297 [3.7] bpo-35257: fix broken BLDSHARED - needs LDFLAGS too (GH-11297) #11298 [3.6] bpo-35257: fix broken BLDSHARED - needs LDFLAGS too (GH-11297) #11299
Files	demo.c setup.py

^{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 2018-12-20.20:39:51.124>
created_at = <Date 2018-11-15.16:14:01.289>
labels = ['extension-modules', '3.8', 'build', 'library', '3.7']
title = 'Avoid leaking linker flags into distutils: add PY_LDFLAGS_NODIST'
updated_at = <Date 2019-01-07.11:54:37.112>
user = 'https://github.com/stratakis'

bugs.python.org fields:

activity = <Date 2019-01-07.11:54:37.112>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-20.20:39:51.124>
closer = 'ned.deily'
components = ['Build', 'Distutils', 'Extension Modules']
creation = <Date 2018-11-15.16:14:01.289>
creator = 'cstratak'
dependencies = []
files = ['47976', '47977']
hgrepos = []
issue_num = 35257
keywords = ['patch']
message_count = 28.0
messages = ['329951', '329952', '331119', '331122', '331177', '331419', '331472', '331540', '331845', '331852', '332150', '332153', '332155', '332227', '332228', '332229', '332230', '332232', '332233', '332234', '332257', '332259', '332466', '332467', '332469', '332470', '332490', '333145']
nosy_count = 7.0
nosy_names = ['vstinner', 'ned.deily', 'eric.araujo', 'dstufft', 'cstratak', 'hroncok', 'miss-islington']
pr_nums = ['10900', '11169', '11264', '11265', '11297', '11298', '11299']
priority = None
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35257'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[stratakis]

Through acb8c52 the CFLAGS_NODIST variable was created, in order to place there compiler flags used by the interpreter, but not intended to be propagated to C extensions.

I saw a similar issue when working on backporting 67e997bcfdac55191033d57a16d1408aL1313 on python 3.6, where the -flto flag should be passed to CFLAGS_NODIST instead of BASECFLAGS, however even if that is fixed, the LDFLAGS will still be propagated to C extensions.

Thus in order to provide more flexibility in that regard, I propose to add the LDFLAGS_NODIST variable, which in a similar vein as CFLAGS_NODIST, will hold the LDFLAGS intended to be used only by the interpreter.

Thoughts or comments on this approach?

[stratakis]

Correction: The second commit is referring to #9908

[stratakis]

So to better illustrate the actual issue I'll be using an example from the python documentation [0][1].

Get the demo.c and the setup.py. Compile cpython first with --with-lto and then compile the demo.c with ./python3 setup.py build.

You will notice that various link time optimization linker flags are passed to the extension.

[0] https://docs.python.org/3/extending/extending.html#keyword-parameters-for-extension-functions
[1] https://docs.python.org/3/extending/building.html#building-c-and-c-extensions-with-distutils

[stratakis]

And here is the difference between compiling the extension with the current tip, comparing to applying my current draft PR:

Master branch with the linker flags propagated:

running build
running build_ext
building 'demo' extension
creating build
creating build/temp.linux-x86_64-3.8
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/home/Harris/dev/cpython/_install/include/python3.8m -c demo.c -o build/temp.linux-x86_64-3.8/demo.o
creating build/lib.linux-x86_64-3.8
gcc -pthread -shared -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none -g build/temp.linux-x86_64-3.8/demo.o -o build/lib.linux-x86_64-3.8/demo.cpython-38m-x86_64-linux-gnu.so

With introduction of the LDFLAGS_NODIST variable:

running build
running build_ext
building 'demo' extension
creating build
creating build/temp.linux-x86_64-3.8
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/home/Harris/dev/cpython/_install/include/python3.8m -c demo.c -o build/temp.linux-x86_64-3.8/demo.o
creating build/lib.linux-x86_64-3.8
gcc -pthread -shared build/temp.linux-x86_64-3.8/demo.o -o build/lib.linux-x86_64-3.8/demo.cpython-38m-x86_64-linux-gnu.so

[stratakis]

PR has been finalized.

[ned-deily]

Unfortunately, it appears this won't be resolved in time for 3.7.2rc1 and 3.6.8rc1 and it would not be appropriate to merge something this potentially disruptive without prior exposure. Since 3.6.8 is planned to be the final 3.6.x bugfix release, unless some critical problem is discovered prior to their final releases it's likely this will not be fixed for 3.6.

[ned-deily]

Unfortunately, it appears this won't be resolved in time for 3.7.2rc1 and 3.6.8rc1

An update: the cutoff for these releases has been extended until about 30 hours from now so there is perhaps a small chance that the PR for this could still be updated, reviewed, and merged in time. If not, it can wait.

[stratakis]

The PR is pending another round of review.