Add support for LOCAL_ENGINE_HOST to Linux/Mac/Win builds. (#132579)

Partial work towards https://github.com/flutter/flutter/issues/132245.

I also couldn't help myself to do a very minor refactor and add some comments to `LocalEngineInfo` because I was getting confused myself implementing it.

packages/flutter_tools/lib/src/android/gradle.dart

@@ -390,20 +390,20 @@ class AndroidGradleBuilder implements AndroidBuilder {
     final LocalEngineInfo? localEngineInfo = _artifacts.localEngineInfo;
     if (localEngineInfo != null) {
       final Directory localEngineRepo = _getLocalEngineRepo(
-        engineOutPath: localEngineInfo.engineOutPath,
+        engineOutPath: localEngineInfo.targetOutPath,
         androidBuildInfo: androidBuildInfo,
         fileSystem: _fileSystem,
       );
       _logger.printTrace(
-          'Using local engine: ${localEngineInfo.engineOutPath}\n'
+          'Using local engine: ${localEngineInfo.targetOutPath}\n'
               'Local Maven repo: ${localEngineRepo.path}'
       );
       command.add('-Plocal-engine-repo=${localEngineRepo.path}');
       command.add('-Plocal-engine-build-mode=${buildInfo.modeName}');
-      command.add('-Plocal-engine-out=${localEngineInfo.engineOutPath}');
-      command.add('-Plocal-engine-host-out=${localEngineInfo.engineHostOutPath}');
+      command.add('-Plocal-engine-out=${localEngineInfo.targetOutPath}');
+      command.add('-Plocal-engine-host-out=${localEngineInfo.hostOutPath}');
       command.add('-Ptarget-platform=${_getTargetPlatformByLocalEnginePath(
-          localEngineInfo.engineOutPath)}');
+          localEngineInfo.targetOutPath)}');
     } else if (androidBuildInfo.targetArchs.isNotEmpty) {
       final String targetPlatforms = androidBuildInfo
           .targetArchs
@@ -716,18 +716,18 @@ class AndroidGradleBuilder implements AndroidBuilder {
     final LocalEngineInfo? localEngineInfo = _artifacts.localEngineInfo;
     if (localEngineInfo != null) {
       final Directory localEngineRepo = _getLocalEngineRepo(
-        engineOutPath: localEngineInfo.engineOutPath,
+        engineOutPath: localEngineInfo.targetOutPath,
         androidBuildInfo: androidBuildInfo,
         fileSystem: _fileSystem,
       );
       _logger.printTrace(
-        'Using local engine: ${localEngineInfo.engineOutPath}\n'
+        'Using local engine: ${localEngineInfo.targetOutPath}\n'
         'Local Maven repo: ${localEngineRepo.path}'
       );
       command.add('-Plocal-engine-repo=${localEngineRepo.path}');
       command.add('-Plocal-engine-build-mode=${buildInfo.modeName}');
-      command.add('-Plocal-engine-out=${localEngineInfo.engineOutPath}');
-      command.add('-Plocal-engine-host-out=${localEngineInfo.engineHostOutPath}');
+      command.add('-Plocal-engine-out=${localEngineInfo.targetOutPath}');
+      command.add('-Plocal-engine-host-out=${localEngineInfo.hostOutPath}');
 
       // Copy the local engine repo in the output directory.
       try {
@@ -742,7 +742,7 @@ class AndroidGradleBuilder implements AndroidBuilder {
         );
       }
       command.add('-Ptarget-platform=${_getTargetPlatformByLocalEnginePath(
-          localEngineInfo.engineOutPath)}');
+          localEngineInfo.targetOutPath)}');
     } else if (androidBuildInfo.targetArchs.isNotEmpty) {
       final String targetPlatforms = androidBuildInfo.targetArchs
           .map((AndroidArch e) => e.platformName).join(',');

packages/flutter_tools/lib/src/artifacts.dart

@@ -284,17 +284,40 @@ class EngineBuildPaths {
   final String? webSdk;
 }
 
-/// Information about a local engine build
+/// Information about a local engine build (i.e. `--local-engine[-host]=...`).
+///
+/// See https://github.com/flutter/flutter/wiki/The-flutter-tool#using-a-locally-built-engine-with-the-flutter-tool
+/// for more information about local engine builds.
 class LocalEngineInfo {
+  /// Creates a reference to a local engine build.
+  ///
+  /// The [targetOutPath] and [hostOutPath] are assumed to be resolvable
+  /// paths to the built engine artifacts for the target (device) and host
+  /// (build) platforms, respectively.
   const LocalEngineInfo({
-    required this.engineOutPath,
-    required this.engineHostOutPath,
-    required this.localEngineName,
+    required this.targetOutPath,
+    required this.hostOutPath,
   });
 
-  final String engineOutPath;
-  final String engineHostOutPath;
-  final String localEngineName;
+  /// The path to the engine artifacts for the target (device) platform.
+  ///
+  /// For example, if the target platform is Android debug, this would be a path
+  /// like `/path/to/engine/src/out/android_debug_unopt`. To retrieve just the
+  /// name (platform), see [localTargetName].
+  final String targetOutPath;
+
+  /// The path to the engine artifacts for the host (build) platform.
+  ///
+  /// For example, if the host platform is debug, this would be a path like
+  /// `/path/to/engine/src/out/host_debug_unopt`. To retrieve just the name
+  /// (platform), see [localHostName].
+  final String hostOutPath;
+
+  /// The name of the target (device) platform, i.e. `android_debug_unopt`.
+  String get localTargetName => globals.fs.path.basename(targetOutPath);
+
+  /// The name of the host (build) platform, e.g. `host_debug_unopt`.
+  String get localHostName => globals.fs.path.basename(hostOutPath);
 }
 
 // Manages the engine artifacts of Flutter.
@@ -824,9 +847,8 @@ class CachedLocalEngineArtifacts implements Artifacts {
   }) : _fileSystem = fileSystem,
        localEngineInfo =
          LocalEngineInfo(
-           engineOutPath: engineOutPath,
-           engineHostOutPath: _hostEngineOutPath,
-           localEngineName: fileSystem.path.basename(engineOutPath)
+           targetOutPath: engineOutPath,
+           hostOutPath: _hostEngineOutPath,
          ),
        _cache = cache,
        _processManager = processManager,
@@ -937,28 +959,28 @@ class CachedLocalEngineArtifacts implements Artifacts {
         return _flutterTesterPath(platform!);
       case Artifact.isolateSnapshotData:
       case Artifact.vmSnapshotData:
-        return _fileSystem.path.join(localEngineInfo.engineOutPath, 'gen', 'flutter', 'lib', 'snapshot', artifactFileName);
+        return _fileSystem.path.join(localEngineInfo.targetOutPath, 'gen', 'flutter', 'lib', 'snapshot', artifactFileName);
       case Artifact.icuData:
       case Artifact.flutterXcframework:
       case Artifact.flutterMacOSFramework:
-        return _fileSystem.path.join(localEngineInfo.engineOutPath, artifactFileName);
+        return _fileSystem.path.join(localEngineInfo.targetOutPath, artifactFileName);
       case Artifact.platformKernelDill:
         if (platform == TargetPlatform.fuchsia_x64 || platform == TargetPlatform.fuchsia_arm64) {
-          return _fileSystem.path.join(localEngineInfo.engineOutPath, 'flutter_runner_patched_sdk', artifactFileName);
+          return _fileSystem.path.join(localEngineInfo.targetOutPath, 'flutter_runner_patched_sdk', artifactFileName);
         }
         return _fileSystem.path.join(_getFlutterPatchedSdkPath(mode), artifactFileName);
       case Artifact.platformLibrariesJson:
         return _fileSystem.path.join(_getFlutterPatchedSdkPath(mode), 'lib', artifactFileName);
       case Artifact.flutterFramework:
         return _getIosEngineArtifactPath(
-            localEngineInfo.engineOutPath, environmentType, _fileSystem, _platform);
+            localEngineInfo.targetOutPath, environmentType, _fileSystem, _platform);
       case Artifact.flutterPatchedSdkPath:
         // When using local engine always use [BuildMode.debug] regardless of
         // what was specified in [mode] argument because local engine will
         // have only one flutter_patched_sdk in standard location, that
         // is happen to be what debug(non-release) mode is using.
         if (platform == TargetPlatform.fuchsia_x64 || platform == TargetPlatform.fuchsia_arm64) {
-          return _fileSystem.path.join(localEngineInfo.engineOutPath, 'flutter_runner_patched_sdk');
+          return _fileSystem.path.join(localEngineInfo.targetOutPath, 'flutter_runner_patched_sdk');
         }
         return _getFlutterPatchedSdkPath(BuildMode.debug);
       case Artifact.skyEnginePath:
@@ -967,11 +989,11 @@ class CachedLocalEngineArtifacts implements Artifacts {
         final String hostPlatform = getNameForHostPlatform(getCurrentHostPlatform());
         final String modeName = mode!.isRelease ? 'release' : mode.toString();
         final String dartBinaries = 'dart_binaries-$modeName-$hostPlatform';
-        return _fileSystem.path.join(localEngineInfo.engineOutPath, 'host_bundle', dartBinaries, 'kernel_compiler.dart.snapshot');
+        return _fileSystem.path.join(localEngineInfo.targetOutPath, 'host_bundle', dartBinaries, 'kernel_compiler.dart.snapshot');
       case Artifact.fuchsiaFlutterRunner:
         final String jitOrAot = mode!.isJit ? '_jit' : '_aot';
         final String productOrNo = mode.isRelease ? '_product' : '';
-        return _fileSystem.path.join(localEngineInfo.engineOutPath, 'flutter$jitOrAot${productOrNo}_runner-0.far');
+        return _fileSystem.path.join(localEngineInfo.targetOutPath, 'flutter$jitOrAot${productOrNo}_runner-0.far');
       case Artifact.fontSubset:
         return _fileSystem.path.join(_hostEngineOutPath, artifactFileName);
       case Artifact.constFinder:
@@ -999,11 +1021,11 @@ class CachedLocalEngineArtifacts implements Artifacts {
 
   @override
   String getEngineType(TargetPlatform platform, [ BuildMode? mode ]) {
-    return _fileSystem.path.basename(localEngineInfo.engineOutPath);
+    return _fileSystem.path.basename(localEngineInfo.targetOutPath);
   }
 
   String _getFlutterPatchedSdkPath(BuildMode? buildMode) {
-    return _fileSystem.path.join(localEngineInfo.engineOutPath,
+    return _fileSystem.path.join(localEngineInfo.targetOutPath,
         buildMode == BuildMode.release ? 'flutter_patched_sdk_product' : 'flutter_patched_sdk');
   }
 
@@ -1053,14 +1075,14 @@ class CachedLocalEngineArtifacts implements Artifacts {
   }
 
   String _getFlutterWebSdkPath() {
-    return _fileSystem.path.join(localEngineInfo.engineOutPath, 'flutter_web_sdk');
+    return _fileSystem.path.join(localEngineInfo.targetOutPath, 'flutter_web_sdk');
   }
 
   String _genSnapshotPath() {
     const List<String> clangDirs = <String>['.', 'clang_x64', 'clang_x86', 'clang_i386', 'clang_arm64'];
     final String genSnapshotName = _artifactToFileName(Artifact.genSnapshot, _platform)!;
     for (final String clangDir in clangDirs) {
-      final String genSnapshotPath = _fileSystem.path.join(localEngineInfo.engineOutPath, clangDir, genSnapshotName);
+      final String genSnapshotPath = _fileSystem.path.join(localEngineInfo.targetOutPath, clangDir, genSnapshotName);
       if (_processManager.canRun(genSnapshotPath)) {
         return genSnapshotPath;
       }
@@ -1070,11 +1092,11 @@ class CachedLocalEngineArtifacts implements Artifacts {
 
   String _flutterTesterPath(TargetPlatform platform) {
     if (_platform.isLinux) {
-      return _fileSystem.path.join(localEngineInfo.engineOutPath, _artifactToFileName(Artifact.flutterTester, _platform));
+      return _fileSystem.path.join(localEngineInfo.targetOutPath, _artifactToFileName(Artifact.flutterTester, _platform));
     } else if (_platform.isMacOS) {
-      return _fileSystem.path.join(localEngineInfo.engineOutPath, 'flutter_tester');
+      return _fileSystem.path.join(localEngineInfo.targetOutPath, 'flutter_tester');
     } else if (_platform.isWindows) {
-      return _fileSystem.path.join(localEngineInfo.engineOutPath, 'flutter_tester.exe');
+      return _fileSystem.path.join(localEngineInfo.targetOutPath, 'flutter_tester.exe');
     }
     throw Exception('Unsupported platform $platform.');
   }
@@ -1381,11 +1403,13 @@ class _TestArtifacts implements Artifacts {
 
 class _TestLocalEngine extends _TestArtifacts {
   _TestLocalEngine(
-      String engineOutPath, String engineHostOutPath, super.fileSystem)
-      : localEngineInfo = LocalEngineInfo(
-            engineOutPath: engineOutPath,
-            engineHostOutPath: engineHostOutPath,
-            localEngineName: fileSystem.path.basename(engineOutPath));
+    String engineOutPath,
+    String engineHostOutPath,
+    super.fileSystem,
+  ) : localEngineInfo = LocalEngineInfo(
+        targetOutPath: engineOutPath,
+        hostOutPath: engineHostOutPath,
+      );
 
   @override
   bool get isLocalEngine => true;

packages/flutter_tools/lib/src/build_info.dart

@@ -647,7 +647,7 @@ List<DarwinArch> defaultIOSArchsForEnvironment(
   // Handle single-arch local engines.
   final LocalEngineInfo? localEngineInfo = artifacts.localEngineInfo;
   if (localEngineInfo != null) {
-    final String localEngineName = localEngineInfo.localEngineName;
+    final String localEngineName = localEngineInfo.localTargetName;
     if (localEngineName.contains('_arm64')) {
       return <DarwinArch>[ DarwinArch.arm64 ];
     }
@@ -670,7 +670,7 @@ List<DarwinArch> defaultMacOSArchsForEnvironment(Artifacts artifacts) {
   // Handle single-arch local engines.
   final LocalEngineInfo? localEngineInfo = artifacts.localEngineInfo;
   if (localEngineInfo != null) {
-    if (localEngineInfo.localEngineName.contains('_arm64')) {
+    if (localEngineInfo.localTargetName.contains('_arm64')) {
       return <DarwinArch>[ DarwinArch.arm64 ];
     }
     return <DarwinArch>[ DarwinArch.x86_64 ];

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

@@ -181,12 +181,15 @@ Future<List<String>> _xcodeBuildSettingsLines({
 
   final LocalEngineInfo? localEngineInfo = globals.artifacts?.localEngineInfo;
   if (localEngineInfo != null) {
-    final String engineOutPath = localEngineInfo.engineOutPath;
+    final String engineOutPath = localEngineInfo.targetOutPath;
     xcodeBuildSettings.add('FLUTTER_ENGINE=${globals.fs.path.dirname(globals.fs.path.dirname(engineOutPath))}');
 
-    final String localEngineName = localEngineInfo.localEngineName;
+    final String localEngineName = localEngineInfo.localTargetName;
     xcodeBuildSettings.add('LOCAL_ENGINE=$localEngineName');
 
+    final String localEngineHostName = localEngineInfo.localHostName;
+    xcodeBuildSettings.add('LOCAL_ENGINE_HOST=$localEngineHostName');
+
     // Tell Xcode not to build universal binaries for local engines, which are
     // single-architecture.
     //

packages/flutter_tools/lib/src/linux/build_linux.dart

@@ -56,9 +56,11 @@ Future<void> buildLinux(
   environmentConfig['FLUTTER_TARGET'] = target;
   final LocalEngineInfo? localEngineInfo = globals.artifacts?.localEngineInfo;
   if (localEngineInfo != null) {
-    final String engineOutPath = localEngineInfo.engineOutPath;
-    environmentConfig['FLUTTER_ENGINE'] = globals.fs.path.dirname(globals.fs.path.dirname(engineOutPath));
-    environmentConfig['LOCAL_ENGINE'] = localEngineInfo.localEngineName;
+    final String targetOutPath = localEngineInfo.targetOutPath;
+    // $ENGINE/src/out/foo_bar_baz -> $ENGINE/src
+    environmentConfig['FLUTTER_ENGINE'] = globals.fs.path.dirname(globals.fs.path.dirname(targetOutPath));
+    environmentConfig['LOCAL_ENGINE'] = localEngineInfo.localTargetName;
+    environmentConfig['LOCAL_ENGINE_HOST'] = localEngineInfo.localHostName;
   }
   writeGeneratedCmakeConfig(Cache.flutterRoot!, linuxProject, buildInfo, environmentConfig, logger);
 

packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -247,9 +247,11 @@ void _writeGeneratedFlutterConfig(
   };
   final LocalEngineInfo? localEngineInfo = globals.artifacts?.localEngineInfo;
   if (localEngineInfo != null) {
-    final String engineOutPath = localEngineInfo.engineOutPath;
-    environment['FLUTTER_ENGINE'] = globals.fs.path.dirname(globals.fs.path.dirname(engineOutPath));
-    environment['LOCAL_ENGINE'] = localEngineInfo.localEngineName;
+    final String targetOutPath = localEngineInfo.targetOutPath;
+    // Get the engine source root $ENGINE/src/out/foo_bar_baz -> $ENGINE/src
+    environment['FLUTTER_ENGINE'] = globals.fs.path.dirname(globals.fs.path.dirname(targetOutPath));
+    environment['LOCAL_ENGINE'] = localEngineInfo.localTargetName;
+    environment['LOCAL_ENGINE_HOST'] = localEngineInfo.localHostName;
   }
   writeGeneratedCmakeConfig(Cache.flutterRoot!, windowsProject, buildInfo, environment, globals.logger);
 }

packages/flutter_tools/test/general.shard/artifacts_test.dart

@@ -11,7 +11,7 @@ import 'package:flutter_tools/src/build_info.dart';
 import 'package:flutter_tools/src/cache.dart';
 
 import '../src/common.dart';
-import '../src/fake_process_manager.dart';
+import '../src/context.dart';
 import '../src/fakes.dart';
 
 void main() {
@@ -545,4 +545,37 @@ void main() {
       );
     });
   });
+
+  group('LocalEngineInfo', () {
+    late FileSystem fileSystem;
+    late LocalEngineInfo localEngineInfo;
+
+    setUp(() {
+      fileSystem = MemoryFileSystem.test();
+    });
+
+    testUsingContext('determines the target device name from the path', () {
+      localEngineInfo = LocalEngineInfo(
+        targetOutPath: fileSystem.path.join(fileSystem.currentDirectory.path, 'out', 'android_debug_unopt'),
+        hostOutPath: fileSystem.path.join(fileSystem.currentDirectory.path, 'out', 'host_debug_unopt'),
+      );
+
+      expect(localEngineInfo.localTargetName, 'android_debug_unopt');
+    }, overrides: <Type, Generator>{
+      FileSystem: () => fileSystem,
+      ProcessManager: () => FakeProcessManager.any(),
+    });
+
+    testUsingContext('determines the target device name from the path when using a custom engine path', () {
+      localEngineInfo = LocalEngineInfo(
+        targetOutPath: fileSystem.path.join(fileSystem.currentDirectory.path, 'out', 'android_debug_unopt'),
+        hostOutPath: fileSystem.path.join(fileSystem.currentDirectory.path, 'out', 'host_debug_unopt'),
+      );
+
+      expect(localEngineInfo.localHostName, 'host_debug_unopt');
+    }, overrides: <Type, Generator>{
+      FileSystem: () => fileSystem,
+      ProcessManager: () => FakeProcessManager.any(),
+    });
+  });
 }

packages/flutter_tools/test/general.shard/build_info_test.dart

@@ -2,7 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:file/memory.dart';
+
 import 'package:flutter_tools/src/artifacts.dart';
+import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/logger.dart';
 import 'package:flutter_tools/src/build_info.dart';
 
@@ -101,7 +104,7 @@ void main() {
     expect(getNameForTargetPlatform(TargetPlatform.android), isNot(contains('ios')));
   });
 
-  testWithoutContext('defaultIOSArchsForEnvironment', () {
+  testUsingContext('defaultIOSArchsForEnvironment', () {
     expect(defaultIOSArchsForEnvironment(
       EnvironmentType.physical,
       Artifacts.testLocalEngine(localEngineHost: 'host_debug_unopt', localEngine: 'ios_debug_unopt'),
@@ -124,9 +127,12 @@ void main() {
     expect(defaultIOSArchsForEnvironment(
       EnvironmentType.simulator, Artifacts.test(),
     ), <DarwinArch>[ DarwinArch.x86_64, DarwinArch.arm64 ]);
+  }, overrides: <Type, Generator>{
+      FileSystem: () => MemoryFileSystem.test(),
+      ProcessManager: () => FakeProcessManager.any(),
     });
 
-  testWithoutContext('defaultMacOSArchsForEnvironment', () {
+  testUsingContext('defaultMacOSArchsForEnvironment', () {
     expect(defaultMacOSArchsForEnvironment(
       Artifacts.testLocalEngine(localEngineHost: 'host_debug_unopt', localEngine: 'host_debug_unopt'),
     ).single, DarwinArch.x86_64);
@@ -138,6 +144,9 @@ void main() {
     expect(defaultMacOSArchsForEnvironment(
       Artifacts.test(),
     ), <DarwinArch>[ DarwinArch.x86_64, DarwinArch.arm64 ]);
+  }, overrides: <Type, Generator>{
+      FileSystem: () => MemoryFileSystem.test(),
+      ProcessManager: () => FakeProcessManager.any(),
     });
 
   testWithoutContext('getIOSArchForName on Darwin arches', () {