[et] build and run commands disable RBE with a flag or when not available

tools/engine_tool/lib/src/build_utils.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 'dart:io' as io show Directory;
+
 import 'package:engine_build_configs/engine_build_configs.dart';
+import 'package:path/path.dart' as p;
 
 import 'environment.dart';
 import 'logger.dart';
@@ -63,13 +66,30 @@ void debugCheckBuilds(List<Build> builds) {
 }
 
 /// Build the build target in the environment.
-Future<int> runBuild(Environment environment, Build build) async {
+Future<int> runBuild(
+  Environment environment,
+  Build build, {
+  List<String> extraGnArgs = const <String>[],
+}) async {
+  // If RBE config files aren't in the tree, then disable RBE.
+  final String rbeConfigPath = p.join(
+    environment.engine.srcDir.path,
+    'flutter',
+    'build',
+    'rbe',
+  );
+  final List<String> gnArgs = <String>[
+    ...extraGnArgs,
+    if (!io.Directory(rbeConfigPath).existsSync()) '--no-rbe',
+  ];
+
   final BuildRunner buildRunner = BuildRunner(
     platform: environment.platform,
     processRunner: environment.processRunner,
     abi: environment.abi,
     engineSrcDir: environment.engine.srcDir,
     build: build,
+    extraGnArgs: gnArgs,
     runTests: false,
   );
 

tools/engine_tool/lib/src/commands/build_command.dart

@@ -31,6 +31,12 @@ final class BuildCommand extends CommandBase {
           config.name: config.gn.join(' '),
       },
     );
+    argParser.addFlag(
+      rbeFlag,
+      defaultsTo: true,
+      help: 'RBE is enabled by default when available. Use --no-rbe to '
+            'disable it.',
+    );
   }
 
   /// List of compatible builds.
@@ -45,14 +51,19 @@ final class BuildCommand extends CommandBase {
   @override
   Future<int> run() async {
     final String configName = argResults![configFlag] as String;
+    final bool useRbe = argResults![rbeFlag] as bool;
     final Build? build =
         builds.where((Build build) => build.name == configName).firstOrNull;
     if (build == null) {
       environment.logger.error('Could not find config $configName');
       return 1;
     }
 
+    final List<String> extraGnArgs = <String>[
+      if (!useRbe) '--no-rbe',
+    ];
+
     // TODO(loic-sharma): Fetch dependencies if needed.
-    return runBuild(environment, build);
+    return runBuild(environment, build, extraGnArgs: extraGnArgs);
   }
 }

tools/engine_tool/lib/src/commands/flags.dart

@@ -16,5 +16,6 @@ const String builderFlag = 'builder';
 const String configFlag = 'config';
 const String dryRunFlag = 'dry-run';
 const String quietFlag = 'quiet';
+const String rbeFlag = 'rbe';
 const String runTestsFlag = 'run-tests';
 const String verboseFlag = 'verbose';

tools/engine_tool/lib/src/commands/run_command.dart

@@ -37,6 +37,12 @@ final class RunCommand extends CommandBase {
           build.name: build.gn.join(' '),
       },
     );
+    argParser.addFlag(
+      rbeFlag,
+      defaultsTo: true,
+      help: 'RBE is enabled by default when available. Use --no-rbe to '
+            'disable it.',
+    );
   }
 
   /// List of compatible builds.
@@ -142,15 +148,20 @@ final class RunCommand extends CommandBase {
       return 1;
     }
 
+    final bool useRbe = argResults![rbeFlag] as bool;
+    final List<String> extraGnArgs = <String>[
+      if (!useRbe) '--no-rbe',
+    ];
+
     // First build the host.
-    int r = await runBuild(environment, hostBuild);
+    int r = await runBuild(environment, hostBuild, extraGnArgs: extraGnArgs);
     if (r != 0) {
       return r;
     }
 
     // Now build the target if it isn't the same.
     if (hostBuild.name != build.name) {
-      r = await runBuild(environment, build);
+      r = await runBuild(environment, build, extraGnArgs: extraGnArgs);
       if (r != 0) {
         return r;
       }