Support --local-engine when for single CPU architecture AOT builds on macOS

[jason-simmons]

I'm seeing errors when I run flutter run --local-engine host_profile --profile on a Mac x64 host using the current engine and flutter_tools.

flutter_tools reports that it can't find gen_snapshot_x64 or gen_snapshot_arm64 and that the FlutterMacOS.framework binaries are x64 only.

This is related to some of the recent flutter_tools changes for enabling ARM64 support (a618ca2, fd3c34c)

@jmagman @cbracken

[jmagman]

@cbracken looks like macOS needs to be audited for local engine single-architecture issues.

flutter/packages/flutter_tools/lib/src/ios/xcode_build_settings.dart

Lines 189 to 201 in a618ca2

	if (!useMacOSConfig) {
	String arch;
	if (localEngineName.endsWith('_arm')) {
	arch = 'armv7';
	} else if (localEngineName.contains('_arm64')) {
	arch = 'arm64';
	} else if (localEngineName.contains('_sim')) {
	arch = 'x86_64';
	} else {
	arch = 'arm64';
	}
	xcodeBuildSettings.add('ARCHS=$arch');
	}

[jmagman]

-dDarwinArchs shouldn't hard-coded, it should come from ARCHS

flutter/packages/flutter_tools/bin/macos_assemble.sh

Line 73 in 71ec251

"-dDarwinArchs=x86_64 arm64"

- "-dDarwinArchs=x86_64 arm64" 
+ "-dDarwinArchs=${ARCHS}"

like this:

flutter/packages/flutter_tools/bin/xcode_backend.dart

Line 418 in 71ec251

'-dIosArchs=${environment['ARCHS'] ?? ''}',

[jmagman]

Jason's logs also show:

[        ] [   +2 ms] Target compile_macos_framework failed: ProcessException: Failed to find "path/to/engine/src/out/host_profile/./gen_snapshot_x64" in the search path.

out/host_profile/gen_snapshot_x64 does not exist, but out/host_profile/gen_snapshot does. Either need to generate as gen_snapshot_x64 in this case, or update the tool to handle the missing _x64 in a local engine.

[jmagman]

"-dDarwinArchs=x86_64 arm64" 

is also incorrect since it shouldn't build both in debug mode.

[jmagman]

#100811 and #100917 have merged. The last piece is to fix the gen_snapshot_x64 vs gen_snapshot.
So if you hit this issue on x64, the workaround for now is to symlink out/host_<variant>/gen_snapshot to out/host_<variant>/gen_snapshot_x64.

[cbracken]

I'll patch that bit in the engine build rather than in the tool, as we do for iOS.

[cbracken]

Patch is out at flutter/engine#32326.

That said, it's important to note that the correct local engine to point to for macOS builds is a local engine built with --mac flag; e.g. mac_release_unopt_arm64 etc. Host builds will likely work for x64 targets, but not currently for arm64, where an x64 gen_snapshot is built instead, since in host builds the target is the host.

FWIW it should work fine if you're building on and targeting x64, but I could imagine us tightening things up one day, so we should probably encourage people to use a Mac target build.

[jmagman]

That said, it's important to note that the correct local engine to point to for macOS builds is a local engine built with --mac flag; e.g. mac_release_unopt_arm64 etc. Host builds will likely work for x64 targets, but not currently for arm64, where an x64 gen_snapshot is built instead, since in host builds the target is the host.

@cbracken can you update https://github.com/flutter/flutter/wiki/Compiling-the-engine#compiling-for-macos-or-linux to make that clear?

[cbracken]

Will do. To be clear, flutter/engine#32326 will get things working with a host build (as well as target builds) right now, but long-term I could imagine things diverging enough that it's no longer the case.

[github-actions]

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