Re: macOS versioned sysroot

Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com> writes:
> src/tools/darwin_sysroot (previously in src/template/darwin) contains this:
> # [...] Using a version-specific sysroot seems
> # desirable, so if the path is a non-version-specific symlink, expand
> # it.

> This code has been whacked around quite a bit, so it's hard to find the
> origin of this. But I'm going to submit a vote for "seems *not* desirable".

The reasoning for this is in the commit log for 4823621db:

Also, "xcrun --show-sdk-path" may give a path that is valid but lacks
any OS version identifier. We don't really want that, since most
of the motivation for wiring -isysroot into the build flags at all
is to ensure that all parts of a PG installation are built against
the same SDK, even when considering extensions built later and/or on
a different machine. Insist on finding "N.N" in the directory name
before accepting the result. (Adding "--sdk macosx" to the xcrun
call seems to produce the same answer as xcodebuild, but usually
more quickly because it's cached, so we also try that as a fallback.)

The core reason why we don't want to use Xcode's default SDK in cases
like this is that Apple's technology for introducing new syscalls
does not play nice with Autoconf: for example, configure will think
that preadv/pwritev exist when using a Big Sur SDK, even when building
on an older macOS version where they don't exist. It'd be nice to
have a better solution to that problem, but this patch doesn't attempt
to fix that.

Discussion: https://postgr.es/m/ed3b8e5d-0da8-6ebd-fd1c-e0ac80a4b204@postgrespro.ru

Re-reading the linked discussion makes me quite loath to remove
the version dependency logic; we'd certainly just be reinventing
that wheel next time Apple adds a new syscall that we care about.

Perhaps it would be adequate if we could select MacOSX13.sdk instead
of MacOSX13.1.sdk given the available choices:

$ ll /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
total 0
drwxr-xr-x 7 root wheel 224 Nov 12 16:18 MacOSX.sdk/
lrwxr-xr-x 1 root wheel 10 Dec 14 10:51 MacOSX13.1.sdk@ -> MacOSX.sdk
lrwxr-xr-x 1 root wheel 10 Dec 14 10:51 MacOSX13.sdk@ -> MacOSX.sdk

It does not seem that xcrun/xcodebuild will offer that, but we
could contemplate putting in some ad-hoc pathname munging to
strip any dot-digits part.

regards, tom lane