[macOS] Generate universal gen_snapshots

[cbracken]

This reverts commit ac4c31a (#52913).
This relands commit 4e33c10 (#52885).

Previously, the gen_snapshot_arm64 and gen_snapshot_x64 binaries used by the tool were all built for x64 architecture. As such, developers building apps with Flutter rely on Rosetta translation with every build.

This refactors the gen_snapshot build rules on macOS hosts to consistently produce gen_snapshot_arm64 and gen_snapshot_x64 binaries with the target architecture of the build but with as universal binaries with both host architectures.

arm64 host build

Prior to this patch we emitted:

• gen_snapshot_arm64 (arch: x64, target_arch: simarm64)

After this patch, we emit:

• artifacts_x64/gen_snapshot_arm64 (arch: x64, target_arch: simarm64)
• artifacts_arm64/gen_snapshot_arm64 (arch: arm64, target_arch: arm64)
• gen_snapshot_arm64 (universal binary composed of both of the above)

x64 host build

Prior to this patch we emitted:

• gen_snapshot_x64 (arch: x64, target_arch: x64)

After this patch, we emit:

• artifacts_x64/gen_snapshot_x64 (arch: x64, target_arch: x64)
• artifacts_arm64/gen_snapshot_x64 (arch: arm64, target_arch: simx64)
• gen_snapshot_x64 (universal binary composed of both of the above)

Note that host builds on macOS currently default to a host architecture of x64 (can be overridden via --force-mac-arm64) regardless of host architecture and thus, the build itself relies on Rosetta translation when invoked on Apple Silicon arm64 hardware. This is to ensure a consistent build in CI regardless of bot architecture. See:

engine/tools/gn

Lines 502 to 505 in 6fa734d

	# TODO(cbracken):
	# https://github.com/flutter/flutter/issues/103386
	if get_host_os() == 'mac' and not args.force_mac_arm64:
	gn_args['host_cpu'] = 'x64'

Issue: flutter/flutter#101138
Issue: flutter/flutter#69157

Related issue: flutter/flutter#103386

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[flutter-dashboard]

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[stuartmorgan]

test-exempt: configuration change

[jmagman]

Passed in post-submit with longer timeout: https://ci.chromium.org/ui/p/flutter/builders/prod/Mac%20mac_host_engine/11165/overview

[jfahrenkrug]

@cbracken Thanks for this fix! Do we have an idea which Flutter version this will be released in?

[jmagman]

This commit is at flutter/flutter@c9f72d5, check out
https://github.com/flutter/flutter/blob/master/docs/releases/Where's-my-commit.md#finding-the-versions-that-contain-framework-commit-x

[zanderso]

AFAICT, the zip archives produced by the GN rules for the gen_snapshot archive aren't actually being uploaded to storage and pulled by the tool.

See the build config file here where a different script is used to produce gen_snapshot.zip files. Those are the ones that get uploaded to storage.

I think the fix is to just upload the gen_snapshot.zip files produced by the individual builds instead of using a global generator after the fact. However, I suspect that GN rule that creates the gen_snpashot.zip file will have to also embed an entitlements.txt file like create_macos_gen_snapshot.py does here.

[jmagman]

AFAICT, the zip archives produced by the GN rules for the gen_snapshot archive aren't actually being uploaded to storage and pulled by the tool.

@cbracken, when you take a look at this can you also add an arch check to the framework so we can make sure the right ones are getting pulled in?
https://github.com/flutter/flutter/blob/32c0c39ea5d81f0c9b3e51a513c76c4ca3d5d111/packages/flutter_tools/test/general.shard/artifacts_test.dart#L45

Here's an arch check example
https://github.com/flutter/flutter/blob/32c0c39ea5d81f0c9b3e51a513c76c4ca3d5d111/packages/flutter_tools/test/host_cross_arch.shard/ios_content_validation_test.dart#L314-L318