ninja-mac is expanded to become a universal (“fat”) executable. The
existing x86_64 slice was taken from the x86_64 ninja-mac as it
previously existed in this location, and a new arm64 slice was added.

This is an aid to native development on arm64. In macOS, subprocesses
inherit their parent’s “binpref,” meaning that children of a translated
process will, by default, also be translated, if there’s a choice
between native and translated. Where ninja-mac is x86_64-only and runs
translated on arm64, all tools it invokes will also run translated, even
if native versions are available. This has an impact on any part of the
toolchain for which universal versions are available: in Chrome’s case,
tools such as ld64 are affected. In Crashpad’s build, which uses the
compiler provided by Xcode, it even affects the compiler, leading to
significant differences in build time on arm64.

mark@arm-and-hammer zsh% file $(which ninja-mac)
…/ninja-mac: Mach-O 64-bit executable x86_64
mark@arm-and-hammer zsh% time ninja -C out/release
ninja: Entering directory `out/release'
[1/1] Regenerating ninja files
[511/511] LINK ./crashpad_minidump_test
ninja -C out/release  1384.03s user 111.05s system 393% cpu 6:19.53 total

mark@arm-and-hammer zsh% file $(which ninja-mac)
…/ninja-mac: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64]
…/ninja-mac (for architecture x86_64):	Mach-O 64-bit executable x86_64
…/ninja-mac (for architecture arm64):	Mach-O 64-bit executable arm64
mark@arm-and-hammer zsh% time ninja -C out/release
ninja: Entering directory `out/release'
[1/1] Regenerating ninja files
[511/511] LINK ./crashpad_minidump_test
ninja -C out/release  606.80s user 34.71s system 701% cpu 1:31.41 total

This differs from the previous attempt, e64cd3ae
(https://chromium-review.googlesource.com/c/2288417) in that the arm64
version was built with Xcode 12.2b1 (12B5018i). The linker in this
version of Xcode produces linker-signed output for arm64 by default. As
of macOS 11.0db6 (20A5364e), all arm64 code must be signed. More
information at
https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11-universal-apps-beta-release-notes#Code-Signing.
In the previous attempt, an older Xcode was used and the output was not
signed at all. Because it did not run on systems upgraded to 11.0db6,
this version of ninja-mac was backed out in a070dd18
(https://chromium-review.googlesource.com/c/2393775).

To create this ninja-mac:

% git clone https://github.com/ninja-build/ninja.git
% cd ninja
% git checkout v1.8.2
% mkdir ../build
% cd ../build
% LDFLAGS=-Wl,-s ../ninja/configure.py --bootstrap
% lipo -create -output ninja-mac $(which ninja-mac) ninja

An explanation of the LDFLAGS: e64cd3ae used “strip” on the output,
matching the stripped x86_64 slice, but “strip” is not currently
compatible with linker-signed executables. This is documented at
https://developer.apple.com/documentation/xcode-release-notes/xcode-12_2-beta-release-notes#Apple-Clang-Compiler.
I was unable to use the recommended workaround in this case. Rather than
leave the output unstripped, I asked ld64 to strip symbols using its -s
option, which it believes is obsolete, but in fact provides the intended
behavior.

The x86_64 slice is unsigned, as it was previously. The new arm64 slice
is linker-signed with an ad-hoc signature. The x86_64 slice was built
with the macOS 10.12 SDK and runs on Mac OS X 10.6 or later. The arm64
slice was built with and has a minimum runtime requirement of macOS
11.0. In practice, the difference doesn’t matter, as the 11.0 SDK is
required to build for arm64, and no macOS version earlier than 11.0 will
run on arm64.

Bug: chromium:1103315
Change-Id: I9f201d0d6f476e187acc0008681c3fd2c1a72aa0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2427051Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>