New submission from Maxime Belanger:
At Dropbox, we use a custom fork of Python to deploy our Desktop Client onto
various platforms, including macOS. We currently use the Mac OS X 10.11 SDK,
but are considering updating to the macOS 10.12 SDK, which introduced new
low-level functions, such as `getentropy`. Apple uses a series of "availability
macros" with new warnings in Clang to consider the case when the
`MACOSX_DEPLOYMENT_TARGET` is below the introduction version for a given symbol.
This has caused significant issues for `autoconf`-based projects, as running
`configure` mistakes the built version into believing certain symbols are
available, when they're not. As a result, deploying a viable build to users on
older versions of Mac is rather complicated.
Current "solutions" (in recent versions of Xcode) include:
* Passing `-no_weak_imports` to the linker, which will fail any build using
weakly-linked symbols (a pretty big hammer).
* A new compiler warning `-Wunguarded-availability`, which detects uses that
contradict the `MACOSX_DEPLOYMENT_TARGET` using some AST tricks.
In our testing, neither had any effect on Python's `configure`, though these
solutions worked for some other projects. We've implemented a workaround based
on patching `pyconfig.h` after `configure` is complete, but ideally, Python's
`configure` should detect `MACOSX_DEPLOYMENT_TARGET` and act accordingly (e.g.
if `MACOSX_DEPLOYMENT_TARGET` is below 10.12, `getentropy` should not be
considered available).
The following symbols are affected:
New in 10.13 (Python 3 only):
utimensat HAVE_UTIMENSAT
futimens HAVE_FUTIMENS
New in 10.12:
getentropy HAVE_GETENTROPY (Python 2/3)
clock_gettime HAVE_CLOCK_GETTIME (Python 2/3)
clock_settime HAVE_CLOCK_SETTIME (Python 3)
clock_getres HAVE_CLOCK_GETRES (Python 3)
New in 10.10 (Python 3 only):
fstatat HAVE_FSTATAT
faccessat HAVE_FACCESSAT
fchmodat HAVE_FCHMODAT
fchownat HAVE_FCHOWNAT
linkat HAVE_LINKAT
fdopendir HAVE_FDOPENDIR
mkdirat HAVE_MKDIRAT
renameat HAVE_RENAMEAT
unlinkat HAVE_UNLINKAT
readlinkat HAVE_READLINKAT
symlinkat HAVE_SYMLINKAT
openat HAVE_OPENAT
----------
messages: 301432
nosy: Maxime Belanger
priority: normal
severity: normal
status: open
title: `configure` script incorrectly detects symbols as available on Mac w/
Xcode 8+
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue31359>
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