[et] Lookup output filesystem path, not label

Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Noticed while working on: flutter/flutter#147071

Author: cbracken

tools/engine_tool/lib/src/gn_utils.dart

@@ -77,30 +77,9 @@ final class BuildTarget {
 Future<Map<String, BuildTarget>> findTargets(
     Environment environment, Directory buildDir) async {
   final Map<String, BuildTarget> r = <String, BuildTarget>{};
-  final List<String> getBuildInfoCommandLine = <String>[
-    gnBinPath(environment),
-    'desc',
-    buildDir.path,
-    '*',
-    '--format=json',
-  ];
-
-  final ProcessRunnerResult result = await environment.processRunner.runProcess(
-      getBuildInfoCommandLine,
-      workingDirectory: environment.engine.srcDir,
-      failOk: true);
-
-  // Handle any process failures.
-  fatalIfFailed(environment, getBuildInfoCommandLine, result);
-
-  late final Map<String, Object?> jsonResult;
-  try {
-    jsonResult = jsonDecode(result.stdout) as Map<String, Object?>;
-  } catch (e) {
-    environment.logger.fatal(
-        'gn desc output could not be parsed:\nE=$e\nIN=${result.stdout}\n');
-  }
 
+  final Map<String, Object?> jsonResult =
+      await _gnDesc(buildDir.path, '*', environment);
   for (final MapEntry<String, Object?> targetEntry in jsonResult.entries) {
     final String label = targetEntry.key;
     if (targetEntry.value == null) {
@@ -120,7 +99,7 @@ Future<Map<String, BuildTarget>> findTargets(
     }
     final bool testOnly = getBool(properties, 'testonly');
     final List<String> outputs =
-        getListOfString(properties, 'outputs') ?? <String>[];
+        await _gnOutputs(buildDir.path, label, environment);
     File? executable;
     if (type == BuildTargetType.executable) {
       if (outputs.isEmpty) {
@@ -135,6 +114,48 @@ Future<Map<String, BuildTarget>> findTargets(
   return r;
 }
 
+Future<Map<String, Object?>> _gnDesc(
+    String buildDir, String label, Environment environment) async {
+  final List<String> getBuildInfoCommandLine = <String>[
+    gnBinPath(environment),
+    'desc',
+    buildDir,
+    label,
+    '--format=json',
+  ];
+
+  final ProcessRunnerResult result = await environment.processRunner.runProcess(
+      getBuildInfoCommandLine,
+      workingDirectory: environment.engine.srcDir,
+      failOk: true);
+
+  // Handle any process failures.
+  fatalIfFailed(environment, getBuildInfoCommandLine, result);
+
+  late final Map<String, Object?> jsonResult;
+  try {
+    jsonResult = jsonDecode(result.stdout) as Map<String, Object?>;
+  } catch (e) {
+    environment.logger.fatal(
+        'gn desc output could not be parsed:\nE=$e\nIN=${result.stdout}\n');
+  }
+  return jsonResult;
+}
+
+Future<List<String>> _gnOutputs(
+    String buildDir, String label, Environment environment) async {
+  final List<String> getOutputsCommandLine = <String>[
+    gnBinPath(environment),
+    'outputs',
+    buildDir,
+    label,
+  ];
+  final ProcessRunnerResult outputsResult = await environment.processRunner
+      .runProcess(getOutputsCommandLine,
+          workingDirectory: environment.engine.srcDir, failOk: true);
+  return outputsResult.stdout.split('\n');
+}
+
 /// Process selectors and filter allTargets for matches.
 ///
 /// We support:

tools/engine_tool/test/fixtures.dart

@@ -225,6 +225,10 @@ String attachedDevices() => '''
 ]
 ''';
 
+String gnOutputsDisplaylistUnittestsOutput() => '''
+display_list_unittests
+''';
+
 String gnDescOutput() => '''
 {
    "//flutter/display_list:display_list_unittests": {

tools/engine_tool/test/gn_utils_test.dart

@@ -39,6 +39,8 @@ void main() {
   final List<CannedProcess> cannedProcesses = <CannedProcess>[
     CannedProcess((List<String> command) => command.contains('desc'),
         stdout: fixtures.gnDescOutput()),
+    CannedProcess((List<String> command) => command.contains('outputs'),
+        stdout: fixtures.gnOutputsDisplaylistUnittestsOutput()),
   ];
 
   test('find test targets', () async {
@@ -85,11 +87,15 @@ void main() {
     try {
       final Environment env = testEnv.environment;
       final List<Build> builds = runnableBuilds(env, configs, true);
-      final Build? build = builds.where(
-        (Build build) => build.name == 'linux/host_debug',
-      ).firstOrNull;
+      final Build? build = builds
+          .where(
+            (Build build) => build.name == 'linux/host_debug',
+          )
+          .firstOrNull;
       final List<BuildTarget>? selectedTargets = await targetsFromCommandLine(
-        env, build!, <String>[],
+        env,
+        build!,
+        <String>[],
       );
       expect(selectedTargets, isNotNull);
       expect(selectedTargets, isEmpty);
@@ -105,11 +111,16 @@ void main() {
     try {
       final Environment env = testEnv.environment;
       final List<Build> builds = runnableBuilds(env, configs, true);
-      final Build? build = builds.where(
-        (Build build) => build.name == 'linux/host_debug',
-      ).firstOrNull;
+      final Build? build = builds
+          .where(
+            (Build build) => build.name == 'linux/host_debug',
+          )
+          .firstOrNull;
       final List<BuildTarget>? selectedTargets = await targetsFromCommandLine(
-        env, build!, <String>[], defaultToAll: true,
+        env,
+        build!,
+        <String>[],
+        defaultToAll: true,
       );
       expect(selectedTargets, isNotNull);
       expect(selectedTargets, isNotEmpty);