et generates {out}/{buildName} if missing on et {build|test|query} (flutter#52866)

Closes flutter#148442.

This restores functionality that existed prior in flutter/engine#52832:

- Splits `runGn` to `ensureBuildDir`, which is in practice what it does.

Author: matanlurey

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;
+
 import 'package:engine_build_configs/engine_build_configs.dart';
+import 'package:path/path.dart' as p;
 
 import 'environment.dart';
 import 'label.dart';
@@ -179,8 +182,41 @@ Future<int> runBuild(
   return buildResult ? 0 : 1;
 }
 
-/// Given a [Build] object, run only its GN step.
-Future<int> runGn(
+/// Run a [build]'s GN step if the output directory is missing.
+Future<bool> ensureBuildDir(
+  Environment environment,
+  Build build, {
+  List<String> extraGnArgs = const <String>[],
+  required bool enableRbe,
+}) async {
+  // TODO(matanlurey): https://github.com/flutter/flutter/issues/148442.
+  final io.Directory buildDir = io.Directory(
+    p.join(
+      environment.engine.outDir.path,
+      build.ninja.config,
+    ),
+  );
+  if (buildDir.existsSync()) {
+    return true;
+  }
+
+  final bool built = await _runGn(
+    environment,
+    build,
+    extraGnArgs: extraGnArgs,
+    enableRbe: enableRbe,
+  );
+  if (built && !buildDir.existsSync()) {
+    environment.logger.error(
+      'The specified build did not produce the expected output directory: '
+      '${buildDir.path}',
+    );
+    return false;
+  }
+  return built;
+}
+
+Future<bool> _runGn(
   Environment environment,
   Build build, {
   List<String> extraGnArgs = const <String>[],
@@ -203,12 +239,11 @@ Future<int> runGn(
     runTests: false,
   );
 
-  final bool buildResult = await buildRunner.run((RunnerEvent event) {
+  return buildRunner.run((RunnerEvent event) {
     switch (event) {
       case RunnerResult(ok: false):
         environment.logger.error(event);
       default:
     }
   });
-  return buildResult ? 0 : 1;
 }

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

@@ -76,11 +76,16 @@ et build //flutter/fml:fml_benchmarks  # Build a specific target in `//flutter/f
       if (useLto) '--lto' else '--no-lto',
     ];
 
+    final List<String> commandLineTargets = argResults!.rest;
+    if (commandLineTargets.isNotEmpty && !await ensureBuildDir(environment, build, enableRbe: useRbe)) {
+      return 1;
+    }
+
     // Builds only accept labels as arguments, so convert patterns to labels.
     // TODO(matanlurey): Can be optimized in cases where wildcards are not used.
     final Gn gn = Gn.fromEnvironment(environment);
     final Set<Label> allTargets = <Label>{};
-    for (final String pattern in argResults!.rest) {
+    for (final String pattern in commandLineTargets) {
       final TargetPattern target = TargetPattern.parse(pattern);
       final List<BuildTarget> targets = await gn.desc(
         'out/${build.ninja.config}',

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

@@ -195,6 +195,10 @@ et query targets //flutter/fml/...  # List all targets under `//flutter/fml`
       return 1;
     }
 
+    if (!await ensureBuildDir(environment, build, enableRbe: useRbe)) {
+      return 1;
+    }
+
     // Builds only accept labels as arguments, so convert patterns to labels.
     // TODO(matanlurey): Can be optimized in cases where wildcards are not used.
     final Gn gn = Gn.fromEnvironment(environment);

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

@@ -68,6 +68,10 @@ et test //flutter/fml:fml_benchmarks  # Run a single test target in `//flutter/f
       return 1;
     }
 
+    if (!await ensureBuildDir(environment, build, enableRbe: useRbe)) {
+      return 1;
+    }
+
     // Builds only accept labels as arguments, so convert patterns to labels.
     final Gn gn = Gn.fromEnvironment(environment);